diff --git a/Makefile b/Makefile
index 967faec4b..85764f6d3 100644
--- a/Makefile
+++ b/Makefile
@@ -11,3 +11,15 @@ test:
 	for dir in $(DIRS); do \
 		$(MAKE) -C $$dir $@ || exit 1; \
 	done
+
+# Cross-branch STL regression check. Renders every example at `master`
+# and at HEAD, then compares STL outputs. Use before landing a change
+# that touches core rendering or SDF code to confirm no unrelated
+# example's geometry was disturbed. Override the base/head refs with
+# e.g. `make stldiff BASE=v1.0 HEAD=feature-branch`.
+BASE ?= master
+HEAD ?= HEAD
+stldiff:
+	./tools/stldiff/run.sh $(BASE) $(HEAD)
+
+.PHONY: all clean hash test stldiff
diff --git a/examples/3dp_nutbolt/SHA1SUM b/examples/3dp_nutbolt/SHA1SUM
index de75d34a2..d818aa1d6 100644
--- a/examples/3dp_nutbolt/SHA1SUM
+++ b/examples/3dp_nutbolt/SHA1SUM
@@ -1,4 +1,4 @@
-62730f847b96546e44f172cf6ff10418d2e7c768  metric_bolt.stl
-b6b872684fb42428e41a7968ca44ce766ddcd431  inch_nut.stl
-7692568d9d23c14b2ccd87671c32a90ed240520b  metric_nut.stl
-fb6b9a563e035acf30595bf26123c3aaff7ed8db  inch_bolt.stl
+2cfc14602640e6f7218815a411c21a68c93053c3  inch_nut.stl
+72511cb417650cba23f292e02bdaca03b46272dc  metric_nut.stl
+c088d7cb69cc55cfcd4ab34d3767ac6a4ed7f87e  inch_bolt.stl
+5fe74df283a275721c340c0bb17d8261600f0111  metric_bolt.stl
diff --git a/examples/angle/SHA1SUM b/examples/angle/SHA1SUM
index 67a3e412f..46dea0903 100644
--- a/examples/angle/SHA1SUM
+++ b/examples/angle/SHA1SUM
@@ -1 +1 @@
-6bb50c749489c6acad9d340abd616e3c3477fd2b  angle.stl
+6e15dba6d73714cadec736cadb9563f7bc6a3beb  angle.stl
diff --git a/examples/arrow/SHA1SUM b/examples/arrow/SHA1SUM
index c30c526ce..cd0596f3a 100644
--- a/examples/arrow/SHA1SUM
+++ b/examples/arrow/SHA1SUM
@@ -1,4 +1,4 @@
-ae5d00f9a29b0277a92650a8f8ed27e55e11c425  axes3.stl
-c68ef109b8e87c2f6d070b24434b2f32fc7d163c  axes1.stl
-921c2e7a1856002d66d9427801b4d060b92cb0bc  axes2.stl
-e27c2b82a6ad881a341ab53fae2f0e513ec1c539  arrow1.stl
+6aa2c8429e2698f523aad615c89a3b47b69df1a8  axes2.stl
+b4ba50b3e24b6f2d994f1a261b4a88a8ade6192b  axes3.stl
+cf7db3eee35aff1460600223b19baa74c89e3c19  axes1.stl
+2d6bc6ba3482b72868fa67825c190075ad8f5635  arrow1.stl
diff --git a/examples/axochord/SHA1SUM b/examples/axochord/SHA1SUM
index 2bbce5cdc..db0fd7658 100644
--- a/examples/axochord/SHA1SUM
+++ b/examples/axochord/SHA1SUM
@@ -1,3 +1,3 @@
-b8949b06966e37c461fe3049fa2048c339cf7deb  lower.stl
-e2f27841e47f74a1be3deecf551713ccbe77a92b  upper.stl
-e8e7ffd3272cf1971354199052aa43967e65f506  plate.dxf
+69962775e80b423e3a94f6449ba254477d7f22a0  upper.stl
+3865a8683916bb46852be8a39cc8fe72fd5a0115  lower.stl
+03272781c811f263aaa989c7081eb7e0650b3745  plate.dxf
diff --git a/examples/axoloti/SHA1SUM b/examples/axoloti/SHA1SUM
index d34c593f0..2fb307bc0 100644
--- a/examples/axoloti/SHA1SUM
+++ b/examples/axoloti/SHA1SUM
@@ -1,3 +1,3 @@
-fdd337d5bef744b283e18539ed8e1307bd4e3fc6  panel_and_base.stl
-ef1bfc7f9ff7929afe5e11cffbe14249728cb35f  base.stl
-8787e69b1f60d12fc3747ab7f7e9032dee8781ad  panel.stl
+ba8f79eb17dc6426f0542fd3a216b891821f624d  panel_and_base.stl
+8ec9aa4e318c1d49429e5394fccac779ead38e00  panel.stl
+0e091f490bc9658d2afb7f9495e09197a29d0607  base.stl
diff --git a/examples/beehive/SHA1SUM b/examples/beehive/SHA1SUM
index c9fe362d2..d130e1bfa 100644
--- a/examples/beehive/SHA1SUM
+++ b/examples/beehive/SHA1SUM
@@ -1,4 +1,4 @@
-84e8331b9c7ea2bb7b4e3431318718a133b55605  wheel.stl
-feb2edecb01188777216d15a00af662231a0ec34  antcap.stl
-f395c2d4885321096c16963979bb963f7bf7add2  retainer.stl
-6db55018eb7f1f7d72656d06e14972530e9b84bd  reducer.stl
+fd6dd8039a9528e1d55236d625f221f657cf53b6  antcap.stl
+398cc07c271ce64a8632135ea127b42f59b91ee0  wheel.stl
+df8a98c56e4138c9d820ea6867b75448f9358c5e  retainer.stl
+1c5329d9d07ebca7728cecb8418ecc3e7b5280cf  reducer.stl
diff --git a/examples/bezier/SHA1SUM b/examples/bezier/SHA1SUM
index 01ddb934f..9fd396825 100644
--- a/examples/bezier/SHA1SUM
+++ b/examples/bezier/SHA1SUM
@@ -1,6 +1,6 @@
-c9ba248df3904f9946a0c310d36bed0a87b3e809  bowlingpin.stl
-9f82530ef50026fcfaa6941bc21e15d1b06d66b2  shape.stl
+bc60b19af39eb17b1009b491dafa4d56964aadad  bowl.stl
+624f8655b11249544e82fb9ca6d93a411b6bd2f2  egg1.stl
 adddacde85dabbb082ed4aabc756cbeeebf1e3a2  egg2.stl
-1d89995aa46c3495e08993df787eb2ad803a860c  vase.stl
-b51816e709434b4457e038806b34880f5fc3e11e  bowl.stl
-3ed59663d04b5d2d86e56c3c394962a4b2b2be57  egg1.stl
+3ac488440597769a4bd2ec5efad68e23dca2104f  shape.stl
+e06726376a6fbb77c4a81c873b3b2fbc80adb342  vase.stl
+ac44df145f92f9b1d85971c4d4118e6fb2e89ab8  bowlingpin.stl
diff --git a/examples/birdhouse/SHA1SUM b/examples/birdhouse/SHA1SUM
index aaa339894..c9d53ed7e 100644
--- a/examples/birdhouse/SHA1SUM
+++ b/examples/birdhouse/SHA1SUM
@@ -1 +1 @@
-322d5fab83e578ccf5eb8b7fc74e3dc99365a393  birdhouse.stl
+c0e7bc46ce0388fe950a049c4feed7134747fac0  birdhouse.stl
diff --git a/examples/bjj/SHA1SUM b/examples/bjj/SHA1SUM
index e81130935..04b8a34ba 100644
--- a/examples/bjj/SHA1SUM
+++ b/examples/bjj/SHA1SUM
@@ -1,3 +1,3 @@
-69030ed0289a09f47470f04b29264d20e40f7ea0  bushing.stl
-0e64b5ac2910539fd8315cb1bf33526be6029239  plate.stl
-0ddedf9cb322fb860b144c36b1cc0e5bad2f34f6  gear.stl
+760353c8059792b55f4dbb0b4ada953528331354  gear.stl
+339d730965fc6d1fb89bf0dabb19923fde6d08ba  bushing.stl
+e11fd387aa686d5c71b6a570dfb090ec2c93f1e8  plate.stl
diff --git a/examples/bolt_container/SHA1SUM b/examples/bolt_container/SHA1SUM
index 0a5c80fe0..c49a33eca 100644
--- a/examples/bolt_container/SHA1SUM
+++ b/examples/bolt_container/SHA1SUM
@@ -1 +1 @@
-cf46d9d726cc5ac8605fb80bc7a786e2630f8901  container.stl
+e27eee531a9aa5f76bcf702b7978df6f246567ac  container.stl
diff --git a/examples/bucky/SHA1SUM b/examples/bucky/SHA1SUM
index f582d1e15..2ccab46a5 100644
--- a/examples/bucky/SHA1SUM
+++ b/examples/bucky/SHA1SUM
@@ -1 +1 @@
-7794b3356063098862ae85487169769df56cbd15  icosahedron.stl
+02fa09a9707bfc966f95a64a066dbbc4980dcf89  icosahedron.stl
diff --git a/examples/camshaft/SHA1SUM b/examples/camshaft/SHA1SUM
index f8892329c..a4c5b87c6 100644
--- a/examples/camshaft/SHA1SUM
+++ b/examples/camshaft/SHA1SUM
@@ -1 +1 @@
-50c80de94a736b0f68d51479424a5228a411db8a  camshaft.stl
+c260bd58b718dd7be6cde73293f00ebdfe997907  camshaft.stl
diff --git a/examples/cap/SHA1SUM b/examples/cap/SHA1SUM
index 1cf55ece9..8ea5152b3 100644
--- a/examples/cap/SHA1SUM
+++ b/examples/cap/SHA1SUM
@@ -1 +1 @@
-cf4553902f2d381224be06c99f7d5689bd8d17f6  cap.stl
+2d7d3e4e392d397b7ba5affd713e61eeeb647fac  cap.stl
diff --git a/examples/carburetor/SHA1SUM b/examples/carburetor/SHA1SUM
index 05c707ecf..2670ec657 100644
--- a/examples/carburetor/SHA1SUM
+++ b/examples/carburetor/SHA1SUM
@@ -1,2 +1,2 @@
-f022c40c4661c7a2b0c3ace90eaf8aaba3e15858  air.stl
-70e3044b059d284c335f9fa4d612ce8c032aa793  plate.stl
+65981160ade147447a7ad54d18b463a20fa00c5a  air.stl
+8a8b66558de59c87bc94f514096534962889f6d9  plate.stl
diff --git a/examples/challenge/SHA1SUM b/examples/challenge/SHA1SUM
index 22a51a7c3..6a0d91a68 100644
--- a/examples/challenge/SHA1SUM
+++ b/examples/challenge/SHA1SUM
@@ -1,5 +1,5 @@
-19eefd74e170f626b5cfabf704ca48116d19ff24  cc16a.stl
-c46acf64c98c70940845c1b93839ce00fe95167f  cc18c.stl
-6af88b0f55e0a7be1c0954f4e2acad4ee319f4a5  cc18b.stl
-ec17b150d1c94799f3c7ec405faba8c30c89c2bf  cc16b.stl
-4162385649041bde9b6871869f2f6e661986be36  cc18a.dxf
+f16fe753e52814fc449e7242e546964cbb331b1a  cc16a.stl
+b0e894f5e4daf9ed264a6b558f375b9e1837d70c  cc16b.stl
+c52766743c8809a84a57dba8622f43edc509b1af  cc18b.stl
+bb4772d8ee1aa5904a3e5dea4d7439503afdcea3  cc18c.stl
+09f690d3d08e97bac663c17156a1c1541124b736  cc18a.dxf
diff --git a/examples/cylinder_head/SHA1SUM b/examples/cylinder_head/SHA1SUM
index 82351d16b..91f708b3c 100644
--- a/examples/cylinder_head/SHA1SUM
+++ b/examples/cylinder_head/SHA1SUM
@@ -1 +1 @@
-0c9dfaf4cea0e8bea14770e86fe73d6a97f33a51  head.stl
+698a6cd42312597265772785d68fcd2bb7aceb11  head.stl
diff --git a/examples/dc2test/SHA1SUM b/examples/dc2test/SHA1SUM
index 055600c3a..48cb8c33b 100644
--- a/examples/dc2test/SHA1SUM
+++ b/examples/dc2test/SHA1SUM
@@ -1 +1 @@
-dee150f4f56fce0a473c82b704ca22ea2b2d62eb  output.dxf
+1e435a651ce855fd0d0b0387aa865548ad0416e3  output.dxf
diff --git a/examples/delta/SHA1SUM b/examples/delta/SHA1SUM
index c7ccc6082..99d294d25 100644
--- a/examples/delta/SHA1SUM
+++ b/examples/delta/SHA1SUM
@@ -1,5 +1,5 @@
-97ef663a5758b7445a22144ad5c334c92e3083b4  servomount.stl
-5d9f0d05b27ff33eaa94a64d19ee4eed09420c68  base.stl
-f3507391bc64d4526e2e482c26ee6c7ac97ee45d  platform.stl
-f4cea8dc2e93a4159834718c241537c0b7670cfc  rodend.stl
-0543b2f185d241c96dffc733f10bb2bc74387ea5  arm.stl
+9f34d7cc96e9df6e45637eaa48e3c9c63168e308  servomount.stl
+9f2c059e603ff0dd17fdca573d09a9a0c697e157  arm.stl
+9760406667b09dcb6e555a39cbe25f3ea3f7dc46  platform.stl
+167279f5c471a9a2a603796485f0f2472b791023  base.stl
+fa1853678241ab065da0e57ac4148a91ff6961fc  rodend.stl
diff --git a/examples/devo/SHA1SUM b/examples/devo/SHA1SUM
index ccb042a33..7c612cdb5 100644
--- a/examples/devo/SHA1SUM
+++ b/examples/devo/SHA1SUM
@@ -1 +1 @@
-5baf90c8014d247e1c39cdea33b088a8df0d5c47  energy_dome.stl
+83aa46ae352b1cbb22d890f15369cb5656517c78  energy_dome.stl
diff --git a/examples/draincover/SHA1SUM b/examples/draincover/SHA1SUM
index 8d49de6ea..6028de9a7 100644
--- a/examples/draincover/SHA1SUM
+++ b/examples/draincover/SHA1SUM
@@ -1,4 +1,4 @@
-61b8572f71a7a0023e797ae2e1a85520f203f2fa  drain6.stl
-43c77edc723d281660a542d09b75b33911c9be4b  vent2.stl
-39404c947113643c8ebd27c84dc3a8a34513798c  drain12.stl
-393f550896847c97efc2756b635f8b9ae18a0209  drain4.stl
+892b04f7b9447846e2b967bd560f7d35e8b4274c  vent2.stl
+4f5592ab08d454ab6805a7734251a1032c1dbaff  drain6.stl
+7c14b4d50e55658b00d57a36c15ab2985098b287  drain4.stl
+3ead10c9a1e22693a42d1182eeb988403d575810  drain12.stl
diff --git a/examples/drone/SHA1SUM b/examples/drone/SHA1SUM
index 3a36758ed..8b02391c3 100644
--- a/examples/drone/SHA1SUM
+++ b/examples/drone/SHA1SUM
@@ -1,2 +1,2 @@
-8cb1b6d115b4a98c3f23d8bf92b1a5cf93db2076  socket.stl
-cce17e34e80a8324403df0b20728ad2cb967e7e6  arm.stl
+3c845c0c5ca0d635de38a5d091bc263924b5d54c  arm.stl
+7461960f28509c6dc4f5716164c31f0fceb0a46c  socket.stl
diff --git a/examples/dust_collection/SHA1SUM b/examples/dust_collection/SHA1SUM
index 2d8f2795e..7ec1886a7 100644
--- a/examples/dust_collection/SHA1SUM
+++ b/examples/dust_collection/SHA1SUM
@@ -1,3 +1,3 @@
-5a26014d78ea3aaae3fbd954d4beeea1977b3cd8  mvh25_mpvc.stl
-bea8fa61ef57965207486603da1b9e494e359317  fdd_mpvc.stl
-f460f403225b08cde92414e0f9628ef03effcd5c  fdd_fvh25.stl
+984cff4aba61a54e0f08ffb14b3560406fed60db  fdd_mpvc.stl
+4444122bf47971bf11a4245434c446826bef82a4  mvh25_mpvc.stl
+9cfa522157f1351eaf142c44cd2ac9360a823349  fdd_fvh25.stl
diff --git a/examples/eurorack/SHA1SUM b/examples/eurorack/SHA1SUM
index 227fe5c62..9def216e5 100644
--- a/examples/eurorack/SHA1SUM
+++ b/examples/eurorack/SHA1SUM
@@ -1,6 +1,6 @@
-7882f266bf3fcc23a2ed182ea5bc714a3807e150  psu_mount.stl
-d564aa123ace80c138337110f8e13f9463ee21a5  pwr_mount.stl
-78ecb6999434e8a2f5461746dc60b3cc1881be9e  ar_panel.stl
-68167fd3dd643898752fa9db919a54f7a145da14  bb_panel.stl
-3fb5e0e24fbee00b087ed0cf03d16a9cd224ea31  pwr_panel.stl
-4d787958166ca82c714c0ab268daf2fc4199c12a  pwr_panel_routing.stl
+f44467d40791e51bf136dce632292962f4806d24  psu_mount.stl
+70db01e1c4b61eaf847a53cb828ab70402a175e2  pwr_panel_routing.stl
+2f0b66a758fecbd138d329849a6fcd76e0e30d33  pwr_panel.stl
+97486eb990d0d0627d2acfc921e6f18edfde7a40  pwr_mount.stl
+7991f9c83e5a323b5ab8a8c885ddf4db19ecd38c  bb_panel.stl
+0dda4526b038c0e4b5c055e5c5c2d4a3eaf1bfde  ar_panel.stl
diff --git a/examples/extrusion/SHA1SUM b/examples/extrusion/SHA1SUM
index 5ec2a1551..e5becdca7 100644
--- a/examples/extrusion/SHA1SUM
+++ b/examples/extrusion/SHA1SUM
@@ -1,2 +1,2 @@
-6a8cc1a21d2b2d43bed7ee1ed38081d868f65be6  extrude2.stl
-251f7fa0df5a1f16857c4ffb8fde55348c2b3dff  extrude1.stl
+eff84e7a47647642364e05281f9f965db4cfe9cd  extrude2.stl
+c1135247694e6f853dab11ab1eb7c62554a7291e  extrude1.stl
diff --git a/examples/fidget/SHA1SUM b/examples/fidget/SHA1SUM
index 557ca771c..f3234bf68 100644
--- a/examples/fidget/SHA1SUM
+++ b/examples/fidget/SHA1SUM
@@ -1,6 +1,6 @@
-9a5bd09c2d908028445a838beea1e10d1b57b170  cap_double_female.stl
-75160f66179c8669ebad5938e981f7ba4c7db417  cap_single.stl
-9e9f5b74d30f704eff968b7a861804cee2ebb219  body2.stl
-08391b73c438609c5e9e8e7ffbc4279383f5320a  cap_double_male.stl
-3b655d9d212ff0643cd37a30f713739efe2eb133  body1.stl
-535b7b8a297996ae16da4d76ed2adc9cfb353e8e  washer.stl
+3f89f6be239637ba5729677960f99a8ea43167c5  cap_double_male.stl
+188db987067420ace393933c7fdc4e01af61bf35  cap_double_female.stl
+8a97acd27677c5bfc1f50d4e49e43032fe221e6c  body2.stl
+5722fad6fe86022421f5ac7e4367446724e8602a  cap_single.stl
+6b5aac156b1162cd4a0210189e0848389cc4f137  body1.stl
+cd9e1b40bcfd7c0e7ab87aef25e98e78d3509150  washer.stl
diff --git a/examples/finial/SHA1SUM b/examples/finial/SHA1SUM
index 94c71126a..c1d1c8e45 100644
--- a/examples/finial/SHA1SUM
+++ b/examples/finial/SHA1SUM
@@ -1,2 +1,2 @@
-a7084874984ecddd96c14f47d1a82c94cd30779d  f1.stl
-7c6ded3a75f97d46be86e449fd8e62f6b99a4051  f2.stl
+5078fe034c2a1ac515491efb451e8d52a9920f82  f1.stl
+4eb43d9ed32883b97a610b15b88154e4b8478249  f2.stl
diff --git a/examples/flask/SHA1SUM b/examples/flask/SHA1SUM
index 31aed9b51..410bc498c 100644
--- a/examples/flask/SHA1SUM
+++ b/examples/flask/SHA1SUM
@@ -1,6 +1,6 @@
-2ee697f28873349477261d23b0e80d6d6b10d543  flask_250.stl
-76f19c5797eaf11cbb31e9a51e0f996b7dc68d8e  flask_150.stl
-3d5b0437f8f66101d2af4a545e42f903063b70d7  odd_side.stl
-8607cb0bcaeff18dae6f0a248fd8b28a48030b7f  flask_200.stl
-81b4bcfca85f4a7ae0e7f5417fdc6ca83e118e50  flask_300.stl
-db1261c6c479f6b0a61e1bcca1ebfffbf9300329  pins.stl
+0d144d415bd1c304c6a7a930b86e717bc6149538  flask_250.stl
+138662ec3599cae5a9f20076a8c95abb945169bd  flask_150.stl
+4260ef35a1cbc9ed4a393aab1ca4ea20a07a1579  pins.stl
+28262e0051ec5237ae26259a998657223eff75f7  flask_200.stl
+3fe89e95af1993dc4574b5f916f65111897fac96  flask_300.stl
+f57a9bac30cd05d46e709c053e6af5d806597ac1  odd_side.stl
diff --git a/examples/gas_cap/SHA1SUM b/examples/gas_cap/SHA1SUM
index 1eb8adda8..bcd4208cb 100644
--- a/examples/gas_cap/SHA1SUM
+++ b/examples/gas_cap/SHA1SUM
@@ -1 +1 @@
-ed1b97a8196c354418b1a286358d9b0673f9ea74  cap.stl
+e2fd73686ac8a64c340e5dc9706452e48c901aeb  cap.stl
diff --git a/examples/gears/SHA1SUM b/examples/gears/SHA1SUM
index 626a30b4c..14ef52da5 100644
--- a/examples/gears/SHA1SUM
+++ b/examples/gears/SHA1SUM
@@ -1 +1 @@
-170f81e55b74b24995ac4f38e4eebba385b12020  gear.stl
+9e2d4908749580432703c07646dd4c28c21a4dfd  gear.stl
diff --git a/examples/geneva/SHA1SUM b/examples/geneva/SHA1SUM
index ac1740c4f..2e91318cd 100644
--- a/examples/geneva/SHA1SUM
+++ b/examples/geneva/SHA1SUM
@@ -1,3 +1,3 @@
-296e89913ae93e4a7bc016cbfe98fb3ee475b1b0  driver.stl
-a631a9bb063e8128ec3c16118d69816d9cae9ce3  geneva.stl
-095429735fe9f68a0e84ac95938c26c2fa72ef60  driven.stl
+2649cadaac9b697f98ae0bbd119bb16c9a76c439  driver.stl
+fdf0d70d32fa4056856efdaf669226c0387cd6cc  driven.stl
+b18706de7733a247885ed2995ab6991705f7fbe2  geneva.stl
diff --git a/examples/gridfinity/SHA1SUM b/examples/gridfinity/SHA1SUM
index 5680ff8a3..ef63e5175 100644
--- a/examples/gridfinity/SHA1SUM
+++ b/examples/gridfinity/SHA1SUM
@@ -1,3 +1,3 @@
-55d9c91181b8f0fbd9a87c5b00b470ab57187e6b  base_4x4.stl
-ba2f2d8c9950c2a5dca960ab476c24d05db2afa5  body_1x1x3.stl
-26b282a6f550b7a552b75c735fb285b44a577ffc  body_1x2x1.stl
+30d49a3379941e634f7e96d4e2fdd1b879681138  body_1x2x1.stl
+fe21ad97b1846a22fddc37ace76d98c56cc7fedf  base_4x4.stl
+8f13ba46ac7e05f53c2048464316b93ecb9fc83e  body_1x1x3.stl
diff --git a/examples/gyroid/SHA1SUM b/examples/gyroid/SHA1SUM
index b6b029aa5..c8c2a538e 100644
--- a/examples/gyroid/SHA1SUM
+++ b/examples/gyroid/SHA1SUM
@@ -1,4 +1,4 @@
-f8ee93ac572b2f5cb6f9809c3af0d6fb7036b350  gyroid_teapot.stl
-2df88d6594aec5f8258ba4157d09185234356322  gyroid_cube.stl
-f8ee93ac572b2f5cb6f9809c3af0d6fb7036b350  gyroid_teapot_mesh.stl
-6b7f3136512def0513e84e1561e4c180b1c6accd  gyroid_surface.stl
+d0b97eecc4afe7b1656ce89deed94f2128930b60  gyroid_teapot.stl
+62092daf78fdadc26497632ba1a1994b88a19c01  gyroid_surface.stl
+a0f0df27e19562db3390158734aeda8abff29ce1  gyroid_cube.stl
+d0b97eecc4afe7b1656ce89deed94f2128930b60  gyroid_teapot_mesh.stl
diff --git a/examples/hole_patterns/SHA1SUM b/examples/hole_patterns/SHA1SUM
index ee0b8cad3..0df953e40 100644
--- a/examples/hole_patterns/SHA1SUM
+++ b/examples/hole_patterns/SHA1SUM
@@ -1 +1 @@
-32ec3b01f08847b08586caef7be43091212760b9  holes.dxf
+2f6b9a865e01286d0348fbb51462fa792a38613e  holes.dxf
diff --git a/examples/holes/SHA1SUM b/examples/holes/SHA1SUM
index 0b461953a..7a6612efe 100644
--- a/examples/holes/SHA1SUM
+++ b/examples/holes/SHA1SUM
@@ -1 +1 @@
-d2784c9846d50e7efeed831fa099659b13668a41  test_holes.stl
+7117424bff60639d1411aaeec5d1525b8318d4e4  test_holes.stl
diff --git a/examples/hollowing_stl/SHA1SUM b/examples/hollowing_stl/SHA1SUM
index a0ce6dcbb..b91110e9e 100644
--- a/examples/hollowing_stl/SHA1SUM
+++ b/examples/hollowing_stl/SHA1SUM
@@ -1 +1 @@
-875dd9994d5a2dc95dfb3ac85228b5caf27516c1  inside-carved-out.stl
+85cc33f5171d73955a4a5d6326e65025865a4df7  inside-carved-out.stl
diff --git a/examples/hpe/SHA1SUM b/examples/hpe/SHA1SUM
index e0e4fb2c9..af063c362 100644
--- a/examples/hpe/SHA1SUM
+++ b/examples/hpe/SHA1SUM
@@ -1,3 +1,4 @@
-3684e54340bd7d1d223fe2ddf5c8b76cc4e158ed  ap725.stl
-c05d996c6bd7eec45b73ff314af535973cdd58e4  ap723h.stl
-cfb33fac10848c216685087ba042ec3d0bdaaa52  ap745.stl
+cc4fae3a0fedc3ec280c8f2d19c6363cf57ce544  ap723h.stl
+e2816ba152aa300635c59dcd402b10f3bbe05b68  ap745.stl
+ac4a5164135f2379d29dadf8edddf8f9b2047d4c  ap723h_support.stl
+f34aedf6334b712e2c9005df6e3c60176157a083  ap725.stl
diff --git a/examples/inlet_hood/SHA1SUM b/examples/inlet_hood/SHA1SUM
index 4c0652013..5b8b595c7 100644
--- a/examples/inlet_hood/SHA1SUM
+++ b/examples/inlet_hood/SHA1SUM
@@ -1 +1 @@
-f23a71c6858b8a2532786d579a14cb7c43a73b24  hood.stl
+d88c0bae4c26b6c15da229490c5de564bafd7b34  hood.stl
diff --git a/examples/joko/SHA1SUM b/examples/joko/SHA1SUM
index 5dc3fd269..b4ca25754 100644
--- a/examples/joko/SHA1SUM
+++ b/examples/joko/SHA1SUM
@@ -1 +1 @@
-65a4b51616cc641f60a4d10ea8da28dff9173c2d  part.stl
+21e0817c8e33d32f43ae88cff813ef17215ef7ec  part.stl
diff --git a/examples/keycap/SHA1SUM b/examples/keycap/SHA1SUM
index 5baca1ed1..be28b4ee5 100644
--- a/examples/keycap/SHA1SUM
+++ b/examples/keycap/SHA1SUM
@@ -1 +1 @@
-ebb8225b51eb566d2521c235eaae4faf9e6f82bc  round_cap.stl
+728f39761ba3670e5baab5432882849bda925dcf  round_cap.stl
diff --git a/examples/loadcell/SHA1SUM b/examples/loadcell/SHA1SUM
index 14170d909..deb14b2e0 100644
--- a/examples/loadcell/SHA1SUM
+++ b/examples/loadcell/SHA1SUM
@@ -1 +1 @@
-916dd7c097a641db5f550ccb7b4a34b52fe15e0a  holder.stl
+d93ddfbcbcb89806218964137660ef90741d38c7  holder.stl
diff --git a/examples/maestro/SHA1SUM b/examples/maestro/SHA1SUM
index a269219b3..8540c726f 100644
--- a/examples/maestro/SHA1SUM
+++ b/examples/maestro/SHA1SUM
@@ -1 +1 @@
-3612ad2a8f95bd3877c51f989626165299794189  mm18.stl
+41f1579f616ffe262d6f7ab8235fad2e9f6be0f6  mm18.stl
diff --git a/examples/maixgo/SHA1SUM b/examples/maixgo/SHA1SUM
index 8d5ce7819..c8b7b191d 100644
--- a/examples/maixgo/SHA1SUM
+++ b/examples/maixgo/SHA1SUM
@@ -1 +1 @@
-7d9cbed1270d525c6b6d2b0ca90e4e24f39d9c97  bezel.stl
+63ff6a95256a7fad5c05033f05cdea8c34090e75  bezel.stl
diff --git a/examples/mcg/SHA1SUM b/examples/mcg/SHA1SUM
index 08a03988c..65c8c7f89 100644
--- a/examples/mcg/SHA1SUM
+++ b/examples/mcg/SHA1SUM
@@ -1 +1 @@
-3a827b3342c857a7f216a5a68996b47b18a1c02f  mcg.stl
+b2f71caa62ce75151b24a9322493cd7b51de26a8  mcg.stl
diff --git a/examples/mesh_test/SHA1SUM b/examples/mesh_test/SHA1SUM
index 49c77a6db..64218d889 100644
--- a/examples/mesh_test/SHA1SUM
+++ b/examples/mesh_test/SHA1SUM
@@ -1 +1 @@
-ff39b42ac5767309e2ab9d81beb6b98665c81293  test.dxf
+5c9cafc45232bf948a66d499c6110ec5be9e6157  test.dxf
diff --git a/examples/midget/SHA1SUM b/examples/midget/SHA1SUM
index 799a207e8..10c5bf0d3 100644
--- a/examples/midget/SHA1SUM
+++ b/examples/midget/SHA1SUM
@@ -1,2 +1,2 @@
-c5dd4356bc02b9022a27e0038234d6fc8252cd0b  crankcase_front.stl
-7d8d39a9ead1a1e30acf3e3917cce9e1a7d9f87e  cylinder_pattern.stl
+c72a64733d1ca328eaa97ca2bdba0f879ccbaa56  crankcase_front.stl
+3c6c2ea02ecf35937937fc623be840bb33a89ad3  cylinder_pattern.stl
diff --git a/examples/monkey_hat/SHA1SUM b/examples/monkey_hat/SHA1SUM
index 46ff55122..d183c5e3f 100644
--- a/examples/monkey_hat/SHA1SUM
+++ b/examples/monkey_hat/SHA1SUM
@@ -1 +1 @@
-f7649befff327283a12099b2f71fcb68df6c1a83  monkey-out.stl
+6d2a4913933a8abd14181e6d0f9c663454bb7a1b  monkey-out.stl
diff --git a/examples/msquare/SHA1SUM b/examples/msquare/SHA1SUM
index 9e5090fa7..7046017d0 100644
--- a/examples/msquare/SHA1SUM
+++ b/examples/msquare/SHA1SUM
@@ -1,3 +1,3 @@
-48fd5b9b918af4e361f98ac81c9b718a2951756a  ms6_upper.stl
-482b7e2692f703cc7d84cb88edd183302b49459e  ms6_lower.stl
-ed30d6ad716af8199dd1b0b4d20151d17e383a53  ms6.stl
+aed79c8c77bcac7eba63fc9bb020dff9846489bd  ms6.stl
+975e6f17a5aeaa4d4dc256c677a5e0dc2b3270a3  ms6_upper.stl
+17bca2041941d36dbb8baa444dd695639a36c7cd  ms6_lower.stl
diff --git a/examples/nordic/SHA1SUM b/examples/nordic/SHA1SUM
index 778962bed..b87d4b4e9 100644
--- a/examples/nordic/SHA1SUM
+++ b/examples/nordic/SHA1SUM
@@ -1,2 +1,2 @@
-9d554e73113456b5678aa7f1ce374f16e4ef1c64  nrf52dk.stl
-2773c2b035c4dd8e490a7373d08fa0597f921b31  nrf52833dk.stl
+228866d28e7cf769a0421ab2caa3719b39e43991  nrf52dk.stl
+4f33a790a06b9729e6b0ba80cd0d23ed19bc584c  nrf52833dk.stl
diff --git a/examples/nutcover/SHA1SUM b/examples/nutcover/SHA1SUM
index 8ab948f1c..e4b1cf94d 100644
--- a/examples/nutcover/SHA1SUM
+++ b/examples/nutcover/SHA1SUM
@@ -1 +1 @@
-ec616ec540309101ad01d025e107e937f349bbe6  cover.stl
+a7f8e8ed3f74cccae1b5b29d10644160cd259c08  cover.stl
diff --git a/examples/nutsandbolts/SHA1SUM b/examples/nutsandbolts/SHA1SUM
index acc1748cc..8e8d792d9 100644
--- a/examples/nutsandbolts/SHA1SUM
+++ b/examples/nutsandbolts/SHA1SUM
@@ -1 +1 @@
-89aa1acd441b79ca7a773a96ca7d28f4deacf135  nutandbolt.stl
+5c6fa9fdc196190b7cfbdac1b711afcf512154cc  nutandbolt.stl
diff --git a/examples/offset_box/SHA1SUM b/examples/offset_box/SHA1SUM
index 9393801c6..f1d81b261 100644
--- a/examples/offset_box/SHA1SUM
+++ b/examples/offset_box/SHA1SUM
@@ -1,2 +1,2 @@
-e3c7f02fb285edba70fb5a013983fddbf7500828  top.stl
-8200f6f80feee18dae64c1ea0e74f5e730801a85  base.stl
+311e3ab9dcc32ee1f4912c8c138aba64706859ea  top.stl
+ca57914e658cab5937255f20e31b1a5f688967cc  base.stl
diff --git a/examples/opengate/SHA1SUM b/examples/opengate/SHA1SUM
index 9ffc97d18..b8e6956ad 100644
--- a/examples/opengate/SHA1SUM
+++ b/examples/opengate/SHA1SUM
@@ -1 +1 @@
-cc8076715f0e645b2ee127b7414d7cdefaaba92e  main_board.stl
+562c374b4fe5f30e1feef63671908507f7e9e910  main_board.stl
diff --git a/examples/panel_box/SHA1SUM b/examples/panel_box/SHA1SUM
index 4be1a5008..82737e319 100644
--- a/examples/panel_box/SHA1SUM
+++ b/examples/panel_box/SHA1SUM
@@ -1,3 +1,3 @@
-d98110d092bb1e3a564d7e74e40b65bd2fe1af29  top.stl
-535d69110164ae7b75de5d9e354cc2c1293e07fc  bottom.stl
-a8db3ca28d49f0b8012c20aaa24b421dfb6049df  panel.stl
+ea9c0c5a86ab1c4d93ef11deb0e58548b81602ba  bottom.stl
+9ee3a6425b0926ed9c79d53481743391827d8a03  top.stl
+f450ac3944c306eadb5d4b415aeb6d729e51eb0e  panel.stl
diff --git a/examples/phone/SHA1SUM b/examples/phone/SHA1SUM
index 07bd2a1b7..efc02eca3 100644
--- a/examples/phone/SHA1SUM
+++ b/examples/phone/SHA1SUM
@@ -1,2 +1,2 @@
-9033aa45e4462fc013f6cc0a6dceda60e050bf4c  clip.stl
-3ab48d7cdc00764ca65c90fe80746449de7b96e7  holder.stl
+da7461fe57b8c71626ef553edfc9c688754101d7  holder.stl
+257974fd64f10b1de2e072e9428a897163ed588b  clip.stl
diff --git a/examples/pico_cnc/SHA1SUM b/examples/pico_cnc/SHA1SUM
index 7bec0a0ce..4785207f3 100644
--- a/examples/pico_cnc/SHA1SUM
+++ b/examples/pico_cnc/SHA1SUM
@@ -1,4 +1,4 @@
-033255dd88475e5820b81998863930c0f4d538e2  pen_holder.stl
-4d9d7f989da039576d40b14554ee6cad5372168a  serial.stl
-12a5d3b6ab2644f42ae5b2b6c8f279abd601935b  pico_cnc.stl
-b557553f1a0940c3a76178605501eaa306b52640  keypad_panel.stl
+f2db7ac03d9274449f2d43357721db9483d38ccc  keypad_panel.stl
+679670d48e3a3f36106cab2fbbcda917372aaac7  serial.stl
+74eba348d81e96ade37687513b875c369bb5ca96  pico_cnc.stl
+ca3bac984a710630f031373d8a602cf14aeddd6a  pen_holder.stl
diff --git a/examples/picorx/SHA1SUM b/examples/picorx/SHA1SUM
index ed8e48cf2..ec8f6e2e6 100644
--- a/examples/picorx/SHA1SUM
+++ b/examples/picorx/SHA1SUM
@@ -1,5 +1,5 @@
-7d66353a36a46c6a465dd0ec53b08543ca0a8a65  pcb_mount1.stl
-c86c28b126c302438484f9f7b25bd6f40b33a60d  lhs.stl
-ebc5de4e0b35ee9c29a9671020b422f888155b0b  rhs.stl
-67a18cda7fb2c50a162c45f89c7bb0072c41755b  pcb_mount0.stl
-3bc39f6053b1a73573ff7dba1486727e1a029031  bezel.stl
+f478553062801be01a466f14c6e5ace19fe15b43  pcb_mount1.stl
+a66250edc73fb7ffa78f7849aac8305238bc422a  pcb_mount0.stl
+aeb1e0a6d8a0d12128ea652dbf267abbc257174c  lhs.stl
+9520c819612262cc76e827d21d3c67e0f5366498  rhs.stl
+7575d01481335cdc503f4d042ee33a5198401e57  bezel.stl
diff --git a/examples/pillar_holder/SHA1SUM b/examples/pillar_holder/SHA1SUM
index eee5cc108..638ee8f7d 100644
--- a/examples/pillar_holder/SHA1SUM
+++ b/examples/pillar_holder/SHA1SUM
@@ -1 +1 @@
-52e30172cc63e86759bc67b1a23ee9daf8d3b4be  holder.stl
+e0b3d2776edd0fe929f286c5c8283b189a437123  holder.stl
diff --git a/examples/pipe_connectors/SHA1SUM b/examples/pipe_connectors/SHA1SUM
index fa84f8373..b39bff087 100644
--- a/examples/pipe_connectors/SHA1SUM
+++ b/examples/pipe_connectors/SHA1SUM
@@ -1,7 +1,7 @@
-95d6a38a0d672c6552cc77f40681348455b5dac6  pipe_connector_3a.stl
-4ad6edcfaab468408021848841ece53669e421c9  pipe_connector_5a.stl
-c108916ffeb0e2e00d285a5a3f206c2ecc782392  pipe_connector_4b.stl
-cd1f5fa4443691a6bd7c03267f0f4475f065b6fd  pipe_connector_4a.stl
-33bfd9b569ca66648a191c3f711d6ff6541403f2  pipe_connector_2a.stl
-18f46c177d4759d51e98070ff08bfc92faf5d052  pipe_connector_3b.stl
-09db51ea4ecf3a9c2a3c3e4101068694994c5749  pipe_connector_2b.stl
+daa6a28348413f2c4922b59f63e52c0924dcc7d3  pipe_connector_3a.stl
+0b1e252a5cf7c1f6585e8e0b9553b48fb8fecc9c  pipe_connector_3b.stl
+2daa7e6ff219ea5f82132ff2b3b76d070aaaf434  pipe_connector_2b.stl
+37d397a64c85a77192353f5c38290b057db7776d  pipe_connector_2a.stl
+0bad8e7be66d59c24a45b3e4cc5a829e98bb23ee  pipe_connector_5a.stl
+cbc0047ea2750f099a3b8f1419becc4127c837f7  pipe_connector_4a.stl
+b7a2de6eb370f7dab2d214aa6d4c95223bb6e490  pipe_connector_4b.stl
diff --git a/examples/pool/SHA1SUM b/examples/pool/SHA1SUM
index b5e889227..f26283022 100644
--- a/examples/pool/SHA1SUM
+++ b/examples/pool/SHA1SUM
@@ -1 +1 @@
-7dff183b3fafdb8540d8369b20915f9349e01a94  pool1.stl
+9bdfd1d51deb6d539b4d5e6975ab96541e7fdf55  pool1.stl
diff --git a/examples/pottery_wheel/SHA1SUM b/examples/pottery_wheel/SHA1SUM
index 8d5fdbe7e..286ed38c7 100644
--- a/examples/pottery_wheel/SHA1SUM
+++ b/examples/pottery_wheel/SHA1SUM
@@ -1,3 +1,3 @@
-4980cc758dc2569b7d5bb74314e0e95a944f8cd5  wheel.stl
-0568472581e3833567bda218a768e4f00f51f31c  core_box.stl
-d78b394cdced7576787ea604a0e24a46eab8e1e2  wheel.dxf
+7eb765113540c672839e06bc381917a7bc06cfb5  wheel.stl
+b680f974d424b1dca0425da72542c31402632645  core_box.stl
+c35b30ccabb7f0f311d5a2aaa60f49d47531f0ab  wheel.dxf
diff --git a/examples/radio/SHA1SUM b/examples/radio/SHA1SUM
index 692888bfb..b73416faa 100644
--- a/examples/radio/SHA1SUM
+++ b/examples/radio/SHA1SUM
@@ -1,3 +1,3 @@
-27aab2d05eb39290fedce423bbef0294eca13ca3  vc_mount.stl
-d4358341055f3514cbf1aada26da8e4dad7f0693  vc_knob.stl
-f45c689c7c34333a7a5d5f1b6eae8011cb23f8a8  fr_mount.stl
+eaf460aaee16a3776925893905b6f49323941612  fr_mount.stl
+1e3162621df8091667c643d1b71d7a7d3d3d21a4  vc_mount.stl
+9c51b103110335d909204374c37102de3b791ee0  vc_knob.stl
diff --git a/examples/ringnut_tool/SHA1SUM b/examples/ringnut_tool/SHA1SUM
index 7fb8056b0..15ee03ea2 100644
--- a/examples/ringnut_tool/SHA1SUM
+++ b/examples/ringnut_tool/SHA1SUM
@@ -1 +1 @@
-077cfbc13946409efecb92385474631926fb3582  tool.stl
+c97a6e640a05e44aba6808cc692254e10f82b3bb  tool.stl
diff --git a/examples/rpi/SHA1SUM b/examples/rpi/SHA1SUM
index 35718860f..12e6caaa3 100644
--- a/examples/rpi/SHA1SUM
+++ b/examples/rpi/SHA1SUM
@@ -1 +1 @@
-bf655073bd79cd23dfbf4b15bdf3f155125b0825  display_stand.stl
+f1060ddaa3ec90b6094f99610fb2b6a7d0fdd116  display_stand.stl
diff --git a/examples/screw_assortment/Makefile b/examples/screw_assortment/Makefile
new file mode 100644
index 000000000..d6053c1f9
--- /dev/null
+++ b/examples/screw_assortment/Makefile
@@ -0,0 +1,2 @@
+TOP = ../..
+include $(TOP)/mk/example.mk
diff --git a/examples/screw_assortment/SHA1SUM b/examples/screw_assortment/SHA1SUM
new file mode 100644
index 000000000..883ae4b60
--- /dev/null
+++ b/examples/screw_assortment/SHA1SUM
@@ -0,0 +1 @@
+e9687c98362308d5580688d9569bef7776dfda8e  screws.stl
diff --git a/examples/screw_assortment/main.go b/examples/screw_assortment/main.go
new file mode 100644
index 000000000..90310b612
--- /dev/null
+++ b/examples/screw_assortment/main.go
@@ -0,0 +1,183 @@
+// Screw assortment: renders an array of screw configurations covering all
+// thread profiles (ISO, ACME, buttress, plastic-buttress), multi-start,
+// left-hand, tapered, and extreme dimensions. Verifies all octree-rendered
+// meshes are watertight (zero boundary edges).
+//
+// Watertightness at extreme/low-resolution configurations is also covered
+// by the unit tests in render/screw_test.go; this example focuses on a
+// clean visual showcase of the full Screw3D configuration space.
+//
+// Usage:
+//
+//	go run main.go
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/deadsy/sdfx/render"
+	"github.com/deadsy/sdfx/sdf"
+)
+
+const meshCells = 300
+
+type config struct {
+	name                  string
+	radius, pitch, length float64
+	taperDeg              float64
+	starts                int
+	profile               string // "iso", "iso-int", "acme", "buttress", "plastic-buttress"
+}
+
+func makeThread(c config) sdf.SDF2 {
+	var thread sdf.SDF2
+	var err error
+	switch c.profile {
+	case "iso-int":
+		thread, err = sdf.ISOThread(c.radius, c.pitch, false)
+	case "acme":
+		thread, err = sdf.AcmeThread(c.radius, c.pitch)
+	case "buttress":
+		thread, err = sdf.ANSIButtressThread(c.radius, c.pitch)
+	case "plastic-buttress":
+		thread, err = sdf.PlasticButtressThread(c.radius, c.pitch)
+	default:
+		thread, err = sdf.ISOThread(c.radius, c.pitch, true)
+	}
+	if err != nil {
+		log.Fatal(err)
+	}
+	return thread
+}
+
+func buildScrew(c config) sdf.SDF3 {
+	thread := makeThread(c)
+	taperRad := sdf.DtoR(c.taperDeg)
+	screw, err := sdf.Screw3D(thread, c.length, taperRad, c.pitch, c.starts)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return screw
+}
+
+func allConfigs() []config {
+	return []config{
+		// --- Straight screws ---
+		// ISO external
+		{"M10x2", 5, 2, 20, 0, 1, "iso"},
+		{"M5x3", 2.5, 3, 10, 0, 1, "iso"},
+		{"coarse_M10x5", 5, 5, 20, 0, 1, "iso"},
+		{"steep_M3x3", 1.5, 3, 10, 0, 1, "iso"},
+		{"extreme_M2x3", 1.0, 3, 6, 0, 1, "iso"},
+		{"short_1pitch", 5, 2, 2, 0, 1, "iso"},
+		// Multi-start
+		{"dual_start", 5, 2, 20, 0, 2, "iso"},
+		{"triple_start", 5, 2, 20, 0, 3, "iso"},
+		{"multi8", 5, 2, 20, 0, 8, "iso"},
+		{"multi16", 5, 2, 20, 0, 16, "iso"},
+		{"multi8_coarse", 5, 5, 20, 0, 8, "iso"},
+		// Left-hand
+		{"left_M10x2", 5, 2, 20, 0, -1, "iso"},
+		{"left_8start", 5, 2, 20, 0, -8, "iso"},
+		{"left_steep_M3x3", 1.5, 3, 10, 0, -1, "iso"},
+		// ISO internal
+		{"internal_M10x2", 5, 2, 20, 0, 1, "iso-int"},
+		{"internal_M5x3", 2.5, 3, 10, 0, 1, "iso-int"},
+		// ACME
+		{"acme_M10x2", 5, 2, 20, 0, 1, "acme"},
+		{"acme_steep_M5x3", 2.5, 3, 10, 0, 1, "acme"},
+		// Buttress
+		{"buttress_M10x2", 5, 2, 20, 0, 1, "buttress"},
+		{"buttress_steep", 2.5, 3, 10, 0, 1, "buttress"},
+		{"buttress_left", 5, 2, 20, 0, -1, "buttress"},
+		{"buttress_multi4", 5, 2, 20, 0, 4, "buttress"},
+		// Plastic buttress
+		{"plastic_butt_M10", 5, 2, 20, 0, 1, "plastic-buttress"},
+		{"plastic_butt_left", 5, 2, 20, 0, -1, "plastic-buttress"},
+		{"plastic_butt_multi4", 5, 2, 20, 0, 4, "plastic-buttress"},
+		// Fine thread
+		{"fine_M20x0.5", 10, 0.5, 20, 0, 1, "iso"},
+		// Sub-pitch length
+		{"sub_pitch_len", 5, 3, 2, 0, 1, "iso"},
+
+		// --- Tapered screws ---
+		{"taper_1.8_NPT", 5, 2, 20, 1.79, 1, "iso"},
+		{"taper_5", 5, 2, 20, 5, 1, "iso"},
+		{"taper_15", 5, 2, 20, 15, 1, "iso"},
+		{"taper_30", 5, 2, 20, 30, 1, "iso"},
+		{"taper_45", 5, 2, 20, 45, 1, "iso"},
+		{"taper_30_4start", 5, 2, 20, 30, 4, "iso"},
+		{"taper_30_coarse", 5, 5, 20, 30, 1, "iso"},
+		{"taper_15_steep", 2.5, 3, 10, 15, 1, "iso"},
+		{"taper_30_left", 5, 2, 20, 30, -1, "iso"},
+		{"taper_15_internal", 5, 2, 20, 15, 1, "iso-int"},
+		{"taper_15_acme", 5, 2, 20, 15, 1, "acme"},
+		{"taper_30_buttress", 5, 2, 20, 30, 1, "buttress"},
+		{"taper_15_plastic_butt", 5, 2, 20, 15, 1, "plastic-buttress"},
+		{"taper_30_left_buttress", 5, 2, 20, 30, -1, "buttress"},
+		{"taper_15_multi4_buttress", 5, 2, 20, 15, 4, "buttress"},
+
+		// --- Oversized (placed last so it doesn't dominate the layout) ---
+		{"large_M64x6", 32, 6, 60, 0, 1, "iso"},
+	}
+}
+
+func main() {
+	configs := allConfigs()
+	allPassed := true
+
+	// Lay out screws in a row along X. Gap scales with the larger of the
+	// neighboring radii so oversized screws don't crowd small ones.
+	var allTris []*sdf.Triangle3
+	xOffset := 0.0
+	prevR := 0.0
+
+	for i, c := range configs {
+		screw := buildScrew(c)
+		bb := screw.BoundingBox()
+		r := bb.Max.X
+
+		tris := render.CollectTriangles(screw, render.NewMarchingCubesOctree(meshCells))
+		be := render.CountBoundaryEdges(tris)
+
+		status := "PASS"
+		if be > 0 {
+			status = "FAIL"
+			allPassed = false
+		}
+
+		fmt.Printf("%-28s r=%5.1f p=%3.1f taper=%5.1f° starts=%3d → %6d tris, %4d boundary edges [%s]\n",
+			c.name, c.radius, c.pitch, c.taperDeg, c.starts, len(tris), be, status)
+
+		if i > 0 {
+			gap := 3.0
+			if s := max(prevR, r) * 0.5; s > gap {
+				gap = s
+			}
+			xOffset += gap
+		}
+
+		for j := range tris {
+			t := tris[j]
+			t[0].X += xOffset + r
+			t[1].X += xOffset + r
+			t[2].X += xOffset + r
+			allTris = append(allTris, &t)
+		}
+		xOffset += r * 2
+		prevR = r
+	}
+
+	fmt.Println()
+	if allPassed {
+		fmt.Println("ALL PASSED: all octree meshes are watertight")
+	} else {
+		fmt.Println("SOME FAILED: octree meshes have holes (boundary edges)")
+	}
+
+	fmt.Printf("\nrendering screws.stl (%d triangles)\n", len(allTris))
+	if err := render.SaveSTL("screws.stl", allTris); err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/examples/servo/SHA1SUM b/examples/servo/SHA1SUM
index 964297d11..a299be79a 100644
--- a/examples/servo/SHA1SUM
+++ b/examples/servo/SHA1SUM
@@ -1 +1 @@
-ca5c01c247dd5a480714318930ef37ba0043f194  servos.stl
+96ee8a76288eeb418c11f1dfb3b521340daa018b  servos.stl
diff --git a/examples/spiral/SHA1SUM b/examples/spiral/SHA1SUM
index 592ed60e5..abad45cb8 100644
--- a/examples/spiral/SHA1SUM
+++ b/examples/spiral/SHA1SUM
@@ -1 +1 @@
-e49bc93d1dbcd097e4b2b3270600a52f1c0baec5  spiral.dxf
+2790f439b1e5a5faf8bdd54dad505fb9c280f529  spiral.dxf
diff --git a/examples/sprue/SHA1SUM b/examples/sprue/SHA1SUM
index 65e936616..d10534bf8 100644
--- a/examples/sprue/SHA1SUM
+++ b/examples/sprue/SHA1SUM
@@ -1 +1 @@
-55b3439fabeac744238d2052fef624d0fca9cd08  sprue.stl
+0bad329ab778797007e9fdcbe28162a0dd5f347d  sprue.stl
diff --git a/examples/square_flange/SHA1SUM b/examples/square_flange/SHA1SUM
index d85f3642d..b892a6e2f 100644
--- a/examples/square_flange/SHA1SUM
+++ b/examples/square_flange/SHA1SUM
@@ -1 +1 @@
-ddf54e3477b00ac380b961ee08de526cfe1c8333  flange.stl
+5b2378ee028bb1c57c33b6f99ede7e7713cf0fff  flange.stl
diff --git a/examples/tabbox/SHA1SUM b/examples/tabbox/SHA1SUM
index f24594af1..40ce7dd84 100644
--- a/examples/tabbox/SHA1SUM
+++ b/examples/tabbox/SHA1SUM
@@ -1,4 +1,4 @@
-f874b064006446cc8eb7530dc2da078d8d1fa92c  box0_upper.stl
-6c16b9628d3747b3e9092aef6e3ea1a27024809b  box1_lower.stl
-d868f85aae9ffa662f651cbf6181cd8e4cffc19d  box0_lower.stl
-015656ba1ed2562e2c50d1afae7db0c8c6da4679  box1_upper.stl
+3cd3906cb72a401f42fc9773414990abb12ac664  box1_lower.stl
+ee66a5dace683e110d1055cfabb31a1c6ef77055  box0_upper.stl
+da12ae7aad27657e21e8445de8b2d5b22e88230f  box0_lower.stl
+7d532010873308f772380f69ec5e09810aa306f2  box1_upper.stl
diff --git a/examples/tacho_bracket/SHA1SUM b/examples/tacho_bracket/SHA1SUM
index f6a866bc6..6a5ca0fbb 100644
--- a/examples/tacho_bracket/SHA1SUM
+++ b/examples/tacho_bracket/SHA1SUM
@@ -1 +1 @@
-c0887daaa070e07bce4f2bc9ad82c828bd81ca79  tacho.stl
+d5b8c7121609d780d3cc4204af269d13c8caa7d0  tacho.stl
diff --git a/examples/tapers/SHA1SUM b/examples/tapers/SHA1SUM
index 8fe55e841..8daedc5c2 100644
--- a/examples/tapers/SHA1SUM
+++ b/examples/tapers/SHA1SUM
@@ -1,2 +1,2 @@
-86e337c963e5878e7568c7c8dace8e5bf31e477e  taper2.stl
-f36cbf39c34dd2488c214fdf572bce8e69d77fb4  taper1.stl
+41696199e4158060f63bf478a8a6ff948864aba4  taper1.stl
+b1cf744fb06cab08750d87f888ee2623b90266c5  taper2.stl
diff --git a/examples/test/SHA1SUM b/examples/test/SHA1SUM
index 08d3dcc9d..344dee61e 100644
--- a/examples/test/SHA1SUM
+++ b/examples/test/SHA1SUM
@@ -1,42 +1,42 @@
-11162d0b9af736ee2e8fd106a37a86709b13363a  ellipsoid_egg.stl
-397ef43b3d047a16623c9f8def0ed5d98689b2d6  test6.stl
-e7657822e5acc11956d1bde0e7d131696c006742  test19.stl
-690d13892bdf3953ee3a2163a898ba4494bad08a  test26.stl
-4f02179179e7a6db6a6e8e4d1d7533008b7d9d3a  cut2d.stl
-cec10c8de98312f30da55028632f00435a233107  test30.stl
-d8c9d09a64da8b3cae851d21c90e153e64c0d537  cam1.stl
-ec884aec363798a0dc964196bc82d7ff490d4545  test13.stl
-5c9f8a124aa62eb17b877af4999d27b0155c3053  test3.stl
-ebfa594bcf5e63ff85b4effbd31d14ccc69da0e6  test16.stl
-b288710d07d20d4f61839b4253c6bdc5f1e13a8f  cam2.stl
-9db2812997ec4009fa791c7284c478c40bc99f2b  test2.stl
-1bcf50afa2d9495946f31ae441cef6582672ffe3  driver.stl
-639d8c3005ff4511b3337282ed9902a87b59134f  test14.stl
-87ad9a20e61afbf13c094479596a1734537c8b16  test1.stl
-9c4a86bafa823d60ea7d19a107e77ac795bcd5d9  test5.stl
-be5fdbfd6eb216e3b664e0174216ad7d15e852a4  test9.stl
+3d269fcbbb16b8b832c78e30dc2629dcec438557  test29.stl
+eaae66e9d5621f4652949cc80225b8e2ea5028ee  test15.stl
+3d1d2cf7a31bde27457e6c5d5ce4fb68839cdd61  test14.stl
+327448f6750bbd45cfb5cbd39559896c83cf2c82  test28.stl
+16a2f0f19a98c634ca58458b46eb600740eeca43  test16.stl
+ddd997b1949cb02a6700e97ce995e0203ab724eb  rounded_box.stl
+018b78652ee757c621f92cdfd22d19184c4a89a6  test17.stl
+bfd55946311480153f4903b4135cdf4fc38e3148  test13.stl
+c56d242041eb50d7f54bbc04189730432e81e4c7  test12.stl
+2815a07bd35f29ebaa3e6fd55579f70ccdd3f3e7  test10.stl
+85a99e342f1fcf7addbae440f948f1adc02c5616  test11.stl
+f071a72f5debe3244eac48037efe3a054a8415b3  test6.stl
+92e0c416725f0d7621e88422cf5b72dbb921c76b  screw.stl
+5deb8078c54e7190e145628d16a7cc3a9153619b  test7.stl
+c5bacb1fc9f66132a85c649ccdcb3998062807eb  cam2.stl
+91126234f9252d093fa3ed0b52eb032019667bff  cam0.stl
+e9e4da00282745212852491cabe83b1304f39ce2  flange.stl
+b0b3c0f1a8a464ee2f4700a018754073b956dcf2  loft.stl
+dac3cb434be0695a2043509d95194221065c94ff  test5.stl
+0cf650a1f4b62d35247072db41da764ce09f812f  test4.stl
+4cf5e2af7f5b6b696227b20bbd17558f4a9b6f9c  cam1.stl
+00fcba0bc543b7a7f5246ec0ff9291a7691586e2  test1.stl
+b25848bdd68c36d661870983e3e2b29c41400143  test3.stl
+8d384f6974f5396990f42125b3a4471c5c2212c0  driver.stl
+0f5f7064422e6522a59741c2745f3e868bcf1fda  test2.stl
+26b0d1fcefe39def718ea33a725c417a15d25983  ellipsoid_egg.stl
+db72c05e8493ecf220da307b8390130328788139  cut2d.stl
+4e0857d95996fbcf94cf5fe23484116e1307a751  rotate_copy.stl
+27a507a2f56be9a1ac9cdb2016fd2e5fb5e9a623  standard_pipe.stl
+c0d5620aa345916016c4d3430ff3e58249a65996  test9.stl
+f70e902b5a064bcec42a0f6f38f67023d80ff329  driven.stl
+560c81c73f57cae84003acdf71f70e99efc5f7f6  test20.stl
+83cfe52152f482eb9e3ee64970705dc13630389e  washer.stl
+c6b9a9b397c1bde286e10c21b91334bc5544aaaf  test21.stl
 730b024c8df2519a1bff56f58e14e977dd662463  test22.stl
-c8730de7c2fb10c77a39ac5bb286bdb264f12311  test12.stl
-fa9a5be2c035d98be4678e836fc1e6a75b7eed24  test10.stl
-cd0f60e64f827b228c34caa70f4ddf422e874193  test29.stl
-8c051b63eabb8b6f92cdec7d61a0d1a6a1873292  test4.stl
-175303fe7975a092691501d805c07f3809cfbc3c  rotate_copy.stl
-6971de9e278943f7584abc645cb84804c04fce49  test20.stl
-1196fc3a56e4f9f26c451ed21d216d6c29b1a8e8  test31.stl
-47a29b2948d43486c8d0ce78cad4d490a8542ac3  rounded_box.stl
-03cd9a89a0f77b30aef8af20310793c3dede7b97  test17.stl
-a9e78227521256d037889ca759424947d0232c87  flange.stl
-9a4de589d71433afb4cb14dcd975c97a2a334fe5  washer.stl
-64a8189b1fd324b23dd9252d669fe2e4ee5e5eca  test11.stl
-c62e1ed7bca22326719549ecc7317f9c37693870  test7.stl
-3fe68164f0fe09dc54208e0cc6bb9694ff6e7465  test21.stl
-4ff18a7cd49ea9e6f0408676e2f580bf9c57e460  test18.stl
-e84b6cd56d21272eec42a8f175305391eddd9135  standard_pipe.stl
-48fb28668c31ddf581bf5098635a4ff610d7be87  test15.stl
-11a9d8c84a86858a181ed220db2f873f0e55eb36  screw.stl
-1465a0a7b47a38de51830b3501cabd19143846a9  test28.stl
-d0c2d209a8b24ec3cbe18796a402de42fe1da797  loft.stl
-cc4aeae854f0f70d12542d3567075b3cc38810cc  cam0.stl
-a5a82faeaf72a02285533701cf0c4584ef1aa97f  test27.stl
-a27adffab3c8ca6136ef80fffb275530c977e046  driven.stl
-d1b3f78f93d7dcb90140928dd22b52c928cdd347  circle_2d.dxf
+6b7ee756f98b62c6c93ad988e8e9eab6c0c73349  test26.stl
+008bf06fdbbbc4d916035363f05b14c64cecb3fc  test27.stl
+ba9d31578d91a1f49d75ac038a03b2b26a745868  test31.stl
+0b8a2e537dbe03f7965379a5122ec20816e76b10  test19.stl
+6af10b39495a7c8eb39b37417d741133320d1dfb  test18.stl
+8a3fb773d6f267da2f34423e3111e83cd25b7051  test30.stl
+a8061c7c9f9bf4ac34a6b9da8a23206bc0106d92  circle_2d.dxf
diff --git a/examples/text/SHA1SUM b/examples/text/SHA1SUM
index 18ca66a48..b358fc07a 100644
--- a/examples/text/SHA1SUM
+++ b/examples/text/SHA1SUM
@@ -1,3 +1,3 @@
-ca4e4bfa168559104edb837f9e4727cb9eee42a2  shape.stl
+db3b02e36e8624a00476c82543f8cebb0ccd53c1  shape.stl
 5321bf047989f75d1de44db3751641c9b052403f  shape.svg
-bd950896148d9b2a8efba9a67b61ba868eb7171c  shape.dxf
+828e99c7d8200f6ed155ad71498d1ec735014de1  shape.dxf
diff --git a/render/mesh.go b/render/mesh.go
new file mode 100644
index 000000000..a5b7d1d9b
--- /dev/null
+++ b/render/mesh.go
@@ -0,0 +1,114 @@
+//-----------------------------------------------------------------------------
+/*
+
+Mesh Analysis Utilities
+
+Functions for collecting rendered triangles and checking mesh quality
+(watertightness, boundary edges).
+
+*/
+//-----------------------------------------------------------------------------
+
+package render
+
+import (
+	"math"
+	"sync"
+
+	"github.com/deadsy/sdfx/sdf"
+)
+
+//-----------------------------------------------------------------------------
+
+// CollectTriangles renders an SDF3 with the given renderer and returns all
+// triangles as a flat slice.
+func CollectTriangles(s sdf.SDF3, r Render3) []sdf.Triangle3 {
+	ch := make(chan []*sdf.Triangle3)
+	var tris []sdf.Triangle3
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for batch := range ch {
+			for _, t := range batch {
+				tris = append(tris, *t)
+			}
+		}
+	}()
+	r.Render(s, sdf.NewTriangle3Buffer(ch))
+	close(ch)
+	wg.Wait()
+	return tris
+}
+
+//-----------------------------------------------------------------------------
+
+// meshVertex is a quantized 3D position for vertex deduplication.
+// Quantized to 1e-4 resolution — well below meaningful geometry scale
+// but avoids overflow in int32 for typical part sizes (up to ~200mm).
+type meshVertex struct {
+	x, y, z int32
+}
+
+func quantizeVertex(x, y, z float64) meshVertex {
+	return meshVertex{int32(x * 1e4), int32(y * 1e4), int32(z * 1e4)}
+}
+
+// meshEdge is an unordered pair of vertices.
+type meshEdge struct {
+	a, b meshVertex
+}
+
+// makeMeshEdge returns a canonical (sorted) edge so (a,b) == (b,a).
+func makeMeshEdge(a, b meshVertex) meshEdge {
+	if a.x < b.x || (a.x == b.x && a.y < b.y) || (a.x == b.x && a.y == b.y && a.z < b.z) {
+		return meshEdge{a, b}
+	}
+	return meshEdge{b, a}
+}
+
+//-----------------------------------------------------------------------------
+
+// CountBoundaryEdges returns the number of edges shared by only one triangle.
+// A watertight mesh has zero boundary edges — every edge borders exactly
+// two triangles.
+func CountBoundaryEdges(tris []sdf.Triangle3) int {
+	edgeCount := make(map[meshEdge]int, len(tris)*3)
+	for _, t := range tris {
+		v0 := quantizeVertex(t[0].X, t[0].Y, t[0].Z)
+		v1 := quantizeVertex(t[1].X, t[1].Y, t[1].Z)
+		v2 := quantizeVertex(t[2].X, t[2].Y, t[2].Z)
+		edgeCount[makeMeshEdge(v0, v1)]++
+		edgeCount[makeMeshEdge(v1, v2)]++
+		edgeCount[makeMeshEdge(v2, v0)]++
+	}
+	boundary := 0
+	for _, count := range edgeCount {
+		if count == 1 {
+			boundary++
+		}
+	}
+	return boundary
+}
+
+// IsWatertight returns true if the rendered mesh has no boundary edges.
+func IsWatertight(tris []sdf.Triangle3) bool {
+	return CountBoundaryEdges(tris) == 0
+}
+
+//-----------------------------------------------------------------------------
+
+// MaxZ returns the maximum Z coordinate across all triangle vertices.
+func MaxZ(tris []sdf.Triangle3) float64 {
+	z := -math.MaxFloat64
+	for _, t := range tris {
+		for _, v := range t {
+			if v.Z > z {
+				z = v.Z
+			}
+		}
+	}
+	return z
+}
+
+//-----------------------------------------------------------------------------
diff --git a/render/screw_test.go b/render/screw_test.go
new file mode 100644
index 000000000..726ddb738
--- /dev/null
+++ b/render/screw_test.go
@@ -0,0 +1,178 @@
+package render
+
+import (
+	"testing"
+
+	"github.com/deadsy/sdfx/sdf"
+)
+
+// screwTestConfig defines a screw configuration for watertightness testing.
+type screwTestConfig struct {
+	name                  string
+	radius, pitch, length float64
+	taperDeg              float64
+	starts, cells         int
+	profile               string // "iso", "iso-int", "acme", "buttress", "plastic-buttress"
+}
+
+func makeTestThread(t *testing.T, profile string, radius, pitch float64) sdf.SDF2 {
+	t.Helper()
+	var thread sdf.SDF2
+	var err error
+	switch profile {
+	case "iso-int":
+		thread, err = sdf.ISOThread(radius, pitch, false)
+	case "acme":
+		thread, err = sdf.AcmeThread(radius, pitch)
+	case "buttress":
+		thread, err = sdf.ANSIButtressThread(radius, pitch)
+	case "plastic-buttress":
+		thread, err = sdf.PlasticButtressThread(radius, pitch)
+	default:
+		thread, err = sdf.ISOThread(radius, pitch, true)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	return thread
+}
+
+func buildScrew(t *testing.T, c screwTestConfig) sdf.SDF3 {
+	t.Helper()
+	thread := makeTestThread(t, c.profile, c.radius, c.pitch)
+	taperRad := sdf.DtoR(c.taperDeg)
+	screw, err := sdf.Screw3D(thread, c.length, taperRad, c.pitch, c.starts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return screw
+}
+
+func straightScrewConfigs() []screwTestConfig {
+	return []screwTestConfig{
+		// ISO external — various pitch/radius ratios and resolutions
+		{"M10x2_200", 5, 2, 20, 0, 1, 200, "iso"},
+		{"M10x2_100", 5, 2, 20, 0, 1, 100, "iso"},
+		{"M5x3_100", 2.5, 3, 10, 0, 1, 100, "iso"},
+		{"coarse_M10x5", 5, 5, 20, 0, 1, 100, "iso"},
+		{"steep_M3x3", 1.5, 3, 10, 0, 1, 100, "iso"},
+		{"steep_M3x3_50", 1.5, 3, 10, 0, 1, 50, "iso"},
+		{"extreme_M2x3", 1.0, 3, 6, 0, 1, 100, "iso"},
+		{"extreme_M2x3_50", 1.0, 3, 6, 0, 1, 50, "iso"},
+		{"M3x3_25cells", 1.5, 3, 10, 0, 1, 25, "iso"},
+		{"short_1pitch", 5, 2, 2, 0, 1, 100, "iso"},
+		{"large_M64x6", 32, 6, 60, 0, 1, 200, "iso"},
+		// Multi-start
+		{"multi8", 5, 2, 20, 0, 8, 100, "iso"},
+		{"multi16", 5, 2, 20, 0, 16, 100, "iso"},
+		{"multi8_coarse", 5, 5, 20, 0, 8, 100, "iso"},
+		// Left-hand
+		{"left_M10x2", 5, 2, 20, 0, -1, 100, "iso"},
+		{"left_8start", 5, 2, 20, 0, -8, 100, "iso"},
+		{"left_steep_M3x3", 1.5, 3, 10, 0, -1, 100, "iso"},
+		// ISO internal
+		{"internal_M10x2", 5, 2, 20, 0, 1, 100, "iso-int"},
+		{"internal_M5x3", 2.5, 3, 10, 0, 1, 100, "iso-int"},
+		// ACME
+		{"acme_M10x2", 5, 2, 20, 0, 1, 100, "acme"},
+		{"acme_steep_M5x3", 2.5, 3, 10, 0, 1, 100, "acme"},
+		// Buttress
+		{"buttress_M10x2", 5, 2, 20, 0, 1, 100, "buttress"},
+		{"buttress_steep", 2.5, 3, 10, 0, 1, 100, "buttress"},
+		{"buttress_left", 5, 2, 20, 0, -1, 100, "buttress"},
+		{"buttress_multi4", 5, 2, 20, 0, 4, 100, "buttress"},
+		{"buttress_25cells", 2.5, 3, 10, 0, 1, 25, "buttress"},
+		// Plastic buttress
+		{"plastic_butt_M10", 5, 2, 20, 0, 1, 100, "plastic-buttress"},
+		{"plastic_butt_left", 5, 2, 20, 0, -1, 100, "plastic-buttress"},
+		{"plastic_butt_multi4", 5, 2, 20, 0, 4, 100, "plastic-buttress"},
+		// Dual/triple start (common real-world)
+		{"dual_start", 5, 2, 20, 0, 2, 100, "iso"},
+		{"triple_start", 5, 2, 20, 0, 3, 100, "iso"},
+		// Very fine thread (low stretch factor, opposite extreme)
+		{"fine_M20x0.5", 10, 0.5, 20, 0, 1, 100, "iso"},
+		// Sub-pitch length
+		{"sub_pitch_len", 5, 3, 2, 0, 1, 100, "iso"},
+	}
+}
+
+func taperedScrewConfigs() []screwTestConfig {
+	return []screwTestConfig{
+		{"taper_1.8_NPT", 5, 2, 20, 1.79, 1, 100, "iso"},
+		{"taper_5", 5, 2, 20, 5, 1, 100, "iso"},
+		{"taper_15", 5, 2, 20, 15, 1, 100, "iso"},
+		{"taper_30", 5, 2, 20, 30, 1, 100, "iso"},
+		{"taper_45", 5, 2, 20, 45, 1, 100, "iso"},
+		{"taper_30_4start", 5, 2, 20, 30, 4, 100, "iso"},
+		{"taper_30_coarse", 5, 5, 20, 30, 1, 100, "iso"},
+		{"taper_15_steep", 2.5, 3, 10, 15, 1, 100, "iso"},
+		{"taper_30_50cells", 5, 2, 20, 30, 1, 50, "iso"},
+		{"taper_30_left", 5, 2, 20, 30, -1, 100, "iso"},
+		{"taper_15_internal", 5, 2, 20, 15, 1, 100, "iso-int"},
+		{"taper_15_acme", 5, 2, 20, 15, 1, 100, "acme"},
+		{"taper_30_buttress", 5, 2, 20, 30, 1, 100, "buttress"},
+		{"taper_15_plastic_butt", 5, 2, 20, 15, 1, 100, "plastic-buttress"},
+		{"taper_30_left_buttress", 5, 2, 20, 30, -1, 100, "buttress"},
+		{"taper_15_multi4_buttress", 5, 2, 20, 15, 4, 100, "buttress"},
+	}
+}
+
+func Test_Screw_Watertight_Straight(t *testing.T) {
+	for _, c := range straightScrewConfigs() {
+		t.Run(c.name, func(t *testing.T) {
+			screw := buildScrew(t, c)
+			tris := CollectTriangles(screw, NewMarchingCubesOctree(c.cells))
+			be := CountBoundaryEdges(tris)
+			if be != 0 {
+				t.Errorf("octree mesh has %d boundary edges (want 0 for watertight)", be)
+			}
+			t.Logf("%d tris, %d boundary edges", len(tris), be)
+		})
+	}
+}
+
+func Test_Screw_Watertight_Tapered(t *testing.T) {
+	for _, c := range taperedScrewConfigs() {
+		t.Run(c.name, func(t *testing.T) {
+			screw := buildScrew(t, c)
+			tris := CollectTriangles(screw, NewMarchingCubesOctree(c.cells))
+			be := CountBoundaryEdges(tris)
+			if be != 0 {
+				t.Errorf("octree mesh has %d boundary edges (want 0 for watertight)", be)
+			}
+			t.Logf("%d tris, %d boundary edges", len(tris), be)
+		})
+	}
+}
+
+func Test_Screw_EndCap_Position(t *testing.T) {
+	// Verify octree and uniform renderers agree on end-cap Z positions.
+	configs := straightScrewConfigs()
+	for _, c := range configs {
+		t.Run(c.name, func(t *testing.T) {
+			screw := buildScrew(t, c)
+			halfLen := c.length / 2.0
+			cubeSize := c.length / float64(c.cells)
+
+			octTris := CollectTriangles(screw, NewMarchingCubesOctree(c.cells))
+			uniTris := CollectTriangles(screw, NewMarchingCubesUniform(c.cells))
+
+			octMaxZ := MaxZ(octTris)
+			uniMaxZ := MaxZ(uniTris)
+
+			if halfLen-octMaxZ > cubeSize {
+				t.Errorf("octree maxZ=%.4f too far from halfLen=%.2f (err=%.4f, cubeSize=%.4f)",
+					octMaxZ, halfLen, halfLen-octMaxZ, cubeSize)
+			}
+			if halfLen-uniMaxZ > cubeSize {
+				t.Errorf("uniform maxZ=%.4f too far from halfLen=%.2f (err=%.4f, cubeSize=%.4f)",
+					uniMaxZ, halfLen, halfLen-uniMaxZ, cubeSize)
+			}
+			delta := octMaxZ - uniMaxZ
+			if delta > cubeSize || delta < -cubeSize {
+				t.Errorf("octree/uniform disagree: octMaxZ=%.4f uniMaxZ=%.4f delta=%.4f (cubeSize=%.4f)",
+					octMaxZ, uniMaxZ, delta, cubeSize)
+			}
+		})
+	}
+}
diff --git a/sdf/line.go b/sdf/line.go
index b72785d81..f18411230 100644
--- a/sdf/line.go
+++ b/sdf/line.go
@@ -101,7 +101,7 @@ func (a *Line2) IntersectLine(b *Line2) []v2.Vec {
 		if x != nil {
 			// collinear, intersecting
 			p0 := p.Add(r.MulScalar(x[0]))
-			if x[0] == x[1] {
+			if math.Abs(x[0]-x[1]) <= tolerance {
 				return []v2.Vec{p0}
 			}
 			p1 := p.Add(r.MulScalar(x[1]))
diff --git a/sdf/screw.go b/sdf/screw.go
index 488c2458d..eb2cc9768 100644
--- a/sdf/screw.go
+++ b/sdf/screw.go
@@ -306,6 +306,14 @@ func ISOThread(
 // ANSIButtressThread returns the 2d profile for an ANSI 45/7 buttress thread.
 // https://en.wikipedia.org/wiki/Buttress_thread
 // AMSE B1.9-1973
+//
+// The polygon spans x ∈ [-pitch, +pitch] and is evaluated by Screw3D within
+// the central pitch period x ∈ [-pitch/2, +pitch/2]. The extensions on both
+// sides reproduce the periodic thread structure (45° flank descending toward
+// next/previous valley) so the SDF is continuous across the SawTooth wrap
+// boundary at x = ±pitch/2 — required because buttress profiles are
+// asymmetric and a flat crest extension would create a wrap discontinuity
+// that breaks octree marching cubes.
 func ANSIButtressThread(
 	radius float64, // radius of thread
 	pitch float64, // thread to thread distance
@@ -318,20 +326,29 @@ func ANSIButtressThread(
 	h1 := ((b / 2.0) * pitch) + (0.5 * h0)
 	hp := pitch / 2.0
 
+	xV := t0*h0 - hp       // valley root x
+	x7 := hp - (h0-h1)*t1  // 7° flank top x (start of right-side crest)
+	x45 := (h0-h1)*t0 - hp // 45° flank top x (end of left-side crest)
+	yEdge := radius + x45  // y on 45° flank at x=±pitch (slope -1, x45<0)
+
 	tp := NewPolygon()
 	tp.Add(pitch, 0)
-	tp.Add(pitch, radius)
-	tp.Add(hp-((h0-h1)*t1), radius)
-	tp.Add(t0*h0-hp, radius-h1).Smooth(0.0714*pitch, 5)
-	tp.Add((h0-h1)*t0-hp, radius)
-	tp.Add(-pitch, radius)
+	tp.Add(pitch, yEdge)
+	tp.Add(x45+pitch, radius)
+	tp.Add(x7, radius)
+	tp.Add(xV, radius-h1).Smooth(0.0714*pitch, 5)
+	tp.Add(x45, radius)
+	tp.Add(x7-pitch, radius)
+	tp.Add(xV-pitch, radius-h1).Smooth(0.0714*pitch, 5)
+	tp.Add(-pitch, yEdge)
 	tp.Add(-pitch, 0)
 
 	return Polygon2D(tp.Vertices())
 }
 
 // PlasticButtressThread returns the 2d profile for a screw top style plastic buttress thread.
-// Similar to ANSI 45/7 - but with more corner rounding
+// Similar to ANSI 45/7 - but with more corner rounding.
+// See ANSIButtressThread for an explanation of the 2-period polygon shape.
 func PlasticButtressThread(
 	radius float64, // radius of thread
 	pitch float64, // thread to thread distance
@@ -344,13 +361,21 @@ func PlasticButtressThread(
 	h1 := ((b / 2.0) * pitch) + (0.5 * h0)
 	hp := pitch / 2.0
 
+	xV := t0*h0 - hp
+	x7 := hp - (h0-h1)*t1
+	x45 := (h0-h1)*t0 - hp
+	yEdge := radius + x45
+
 	tp := NewPolygon()
 	tp.Add(pitch, 0)
-	tp.Add(pitch, radius)
-	tp.Add(hp-((h0-h1)*t1), radius).Smooth(0.05*pitch, 5)
-	tp.Add(t0*h0-hp, radius-h1).Smooth(0.15*pitch, 5)
-	tp.Add((h0-h1)*t0-hp, radius).Smooth(0.15*pitch, 5)
-	tp.Add(-pitch, radius)
+	tp.Add(pitch, yEdge)
+	tp.Add(x45+pitch, radius).Smooth(0.15*pitch, 5)
+	tp.Add(x7, radius).Smooth(0.05*pitch, 5)
+	tp.Add(xV, radius-h1).Smooth(0.15*pitch, 5)
+	tp.Add(x45, radius).Smooth(0.15*pitch, 5)
+	tp.Add(x7-pitch, radius).Smooth(0.05*pitch, 5)
+	tp.Add(xV-pitch, radius-h1).Smooth(0.15*pitch, 5)
+	tp.Add(-pitch, yEdge)
 	tp.Add(-pitch, 0)
 
 	return Polygon2D(tp.Vertices())
@@ -360,16 +385,48 @@ func PlasticButtressThread(
 
 // ScrewSDF3 is a 3d screw form.
 type ScrewSDF3 struct {
-	thread SDF2    // 2D thread profile
-	pitch  float64 // thread to thread distance
-	lead   float64 // distance per turn (starts * pitch)
-	length float64 // total length of screw
-	taper  float64 // thread taper angle
-	starts int     // number of thread starts
-	bb     Box3    // bounding box
+	thread      SDF2    // 2D thread profile
+	pitch       float64 // thread to thread distance
+	lead        float64 // distance per turn (starts * pitch)
+	leadOver2π  float64 // lead / (2π), precomputed for the helical correction
+	length      float64 // total length of screw
+	taper       float64 // thread taper angle
+	tanTaper    float64 // tan(taper), precomputed
+	tan2Taper   float64 // tan²(taper), precomputed
+	threadDepth float64 // radial extent of thread profile (max.Y - min.Y)
+	starts      int     // number of thread starts
+	bb          Box3    // bounding box
 }
 
 // Screw3D returns a screw SDF3.
+//
+// The thread SDF2 represents the 2D thread profile, which is helically swept
+// to form the screw. Screw3D's evaluation maps any 3D point back to a 2D
+// coordinate via SawTooth-wrapping the helical position into [-pitch/2,
+// +pitch/2] and queries thread.Evaluate at that point.
+//
+// For the resulting SDF to be continuous (and therefore usable with the
+// octree marching cubes renderer, which prunes "empty" cubes by SDF
+// magnitude), the thread profile must satisfy:
+//
+//	thread.Evaluate(+pitch/2, y) == thread.Evaluate(-pitch/2, y)  for all y
+//
+// at least up to floating-point tolerance. In other words, the profile must
+// describe the periodic shape such that what's just to the left of x=+pitch/2
+// matches what's just to the right of x=-pitch/2 — including any flanks,
+// crests, or other features near the boundary.
+//
+// This is automatic for symmetric profiles like ISO and ACME, where the
+// polygon is mirror-symmetric in x and the wrap point sits in a region of
+// constant curvature. For asymmetric profiles like buttress threads, the
+// polygon must be authored to extend past ±pitch/2 with the actual periodic
+// continuation (next/previous flank descending into next/previous valley)
+// rather than flat scaffolding. See ANSIButtressThread for an example.
+//
+// Screw3D verifies wrap continuity at construction time and returns an error
+// if the profile is discontinuous. The uniform renderer is forgiving of
+// discontinuities (it samples on a regular grid), but the octree renderer
+// will silently produce holes — fail-fast at construction is preferable.
 func Screw3D(
 	thread SDF2, // 2D thread profile
 	length float64, // length of screw
@@ -392,42 +449,123 @@ func Screw3D(
 	if pitch <= 0 {
 		return nil, ErrMsg("pitch <= 0")
 	}
+	if err := checkThreadWrapContinuous(thread, pitch); err != nil {
+		return nil, err
+	}
 	s := ScrewSDF3{}
 	s.thread = thread
 	s.pitch = pitch
 	s.length = length / 2
 	s.taper = taper
 	s.lead = -pitch * float64(starts)
+	s.leadOver2π = s.lead / Tau
+	s.tanTaper = math.Tan(taper)
+	s.tan2Taper = s.tanTaper * s.tanTaper
 	// Work out the bounding box.
 	// The max-y axis of the sdf2 bounding box is the radius of the thread.
 	bb := s.thread.BoundingBox()
+	s.threadDepth = bb.Max.Y - bb.Min.Y
 	r := bb.Max.Y
 	// add the taper increment
-	r += s.length * math.Tan(taper)
+	r += s.length * s.tanTaper
 	s.bb = Box3{v3.Vec{-r, -r, -s.length}, v3.Vec{r, r, s.length}}
 	return &s, nil
 }
 
+// checkThreadWrapContinuous asserts that the thread profile's SDF agrees at
+// x=+pitch/2 and x=-pitch/2 — the SawTooth wrap boundary — at sample radii
+// across the profile. A mismatch indicates the profile doesn't represent the
+// actual periodic shape near the wrap and will produce holes when rendered
+// with the octree marching cubes renderer.
+func checkThreadWrapContinuous(thread SDF2, pitch float64) error {
+	bb := thread.BoundingBox()
+	hp := pitch / 2
+	yMin, yMax := bb.Min.Y, bb.Max.Y
+	// Sample at several radii spanning the profile. The crest level is the
+	// region that matters most (the surface is there); valleys are inside the
+	// material so a small wrap mismatch deep below the crest is harmless,
+	// but anywhere near or above the surface a discontinuity is fatal for
+	// the octree's empty-cube check.
+	const tol = 1e-6
+	for _, frac := range []float64{0.5, 0.7, 0.9, 1.0} {
+		y := yMin + frac*(yMax-yMin)
+		dPlus := thread.Evaluate(v2.Vec{X: hp, Y: y})
+		dMinus := thread.Evaluate(v2.Vec{X: -hp, Y: y})
+		if math.Abs(dPlus-dMinus) > tol {
+			return ErrMsg(fmt.Sprintf(
+				"thread profile is discontinuous at the SawTooth wrap boundary "+
+					"x=±pitch/2 (pitch=%g): SDF(+pitch/2,%g)=%g vs SDF(-pitch/2,%g)=%g, "+
+					"|Δ|=%g. Asymmetric profiles must extend past ±pitch/2 with the "+
+					"actual periodic continuation; see ANSIButtressThread for an example.",
+				pitch, y, dPlus, y, dMinus, math.Abs(dPlus-dMinus)))
+		}
+	}
+	return nil
+}
+
 // Evaluate returns the minimum distance to a 3d screw form.
 func (s *ScrewSDF3) Evaluate(p v3.Vec) float64 {
-	// map the 3d point back to the xy space of the profile
+	// Map the 3d point back to the 2d thread profile space.
 	p0 := v2.Vec{}
-	// the distance from the 3d z-axis maps to the 2d y-axis
-	p0.Y = math.Sqrt(p.X*p.X + p.Y*p.Y)
+	// The distance from the z-axis maps to the 2d y-axis (radial coordinate).
+	r := math.Sqrt(p.X*p.X + p.Y*p.Y)
+	p0.Y = r
 	if s.taper != 0 {
-		p0.Y += p.Z * math.Atan(s.taper)
+		// Adjust radial position for taper: the thread radius changes linearly
+		// with z. tanTaper converts the taper angle to a slope (rise/run).
+		p0.Y += p.Z * s.tanTaper
 	}
-	// the x/y angle and the z-height map to the 2d x-axis
-	// ie: the position along thread pitch
+	// The x/y angle and z-height map to the 2d x-axis: this unwraps the
+	// helical path into a linear coordinate along the thread pitch.
 	theta := math.Atan2(p.Y, p.X)
 	z := p.Z + s.lead*theta/Tau
 	p0.X = SawTooth(z, s.pitch)
-	// get the thread profile distance
+	// Get the thread profile distance in the 2d mapped space.
 	d0 := s.thread.Evaluate(p0)
-	// create a region for the screw length
+	// Create a region for the screw length.
 	d1 := math.Abs(p.Z) - s.length
-	// return the intersection
-	return math.Max(d0, d1)
+	// Intersect the thread with the length constraint.
+	d := math.Max(d0, d1)
+	// The 3D→2D mapping stretches distances: a unit step in 3D maps to
+	// a larger step in the unwrapped 2D space. Without correction, the
+	// SDF overestimates distances, causing the octree renderer's isEmpty
+	// check to skip cubes that contain the surface (producing holes).
+	//
+	// The Jacobian of the mapping (r,θ,z) → (p0.X, p0.Y) in physical
+	// cylindrical coordinates is:
+	//   J = | 0      k      1      |   where k = lead/(2πr)
+	//       | 1      0      tan(t) |   and t = taper angle
+	//
+	// The maximum stretch factor is the largest singular value of J,
+	// i.e. sqrt(λ_max) of J·Jᵀ:
+	//   | 1+k²     tan(t)      |
+	//   | tan(t)    1+tan²(t)  |
+	//
+	// When taper=0 this reduces to sqrt(1+k²).
+	//
+	// Dividing by this factor gives a correct distance bound (Lipschitz-1).
+	// Applied to the final intersection result rather than d0 alone to
+	// preserve end cap surface positions (d=0 stays at 0).
+	//
+	// The stretch factor varies with r (larger near the axis). The octree
+	// isEmpty check evaluates at cube centers, but the nearest surface may
+	// be at a smaller r where the stretch is larger. Using r reduced by the
+	// thread depth gives a conservative bound: the nearest surface point
+	// on the thread can be at most threadDepth closer to the axis.
+	if r > 0 {
+		rEff := r - s.threadDepth
+		if rEff < r*0.5 {
+			rEff = r * 0.5
+		}
+		k2 := s.leadOver2π / rEff
+		k2 *= k2
+		a := 1 + k2
+		c := 1 + s.tan2Taper
+		disc := (a-c)*(a-c) + 4*s.tan2Taper
+		stretch2 := (a + c + math.Sqrt(disc)) * 0.5
+		d /= math.Sqrt(stretch2)
+	}
+	return d
 }
 
 // BoundingBox returns the bounding box for a 3d screw form.
diff --git a/sdf/screw_test.go b/sdf/screw_test.go
new file mode 100644
index 000000000..7f5aa3c18
--- /dev/null
+++ b/sdf/screw_test.go
@@ -0,0 +1,130 @@
+package sdf
+
+import (
+	"math"
+	"testing"
+
+	v2 "github.com/deadsy/sdfx/vec/v2"
+	v3 "github.com/deadsy/sdfx/vec/v3"
+)
+
+// Test_Buttress_WrapContinuity verifies that asymmetric buttress thread
+// profiles produce the same SDF at x = +pitch/2 and x = -pitch/2 — the
+// SawTooth wrap boundary. A discontinuity here would cause octree marching
+// cubes to skip cubes straddling the boundary and produce mesh holes; the
+// 2-period polygon design is what makes the SDF wrap-continuous.
+func Test_Buttress_WrapContinuity(t *testing.T) {
+	cases := []struct {
+		name   string
+		make   func(r, p float64) (SDF2, error)
+		radius float64
+		pitch  float64
+	}{
+		{"ANSI", ANSIButtressThread, 5, 2},
+		{"Plastic", PlasticButtressThread, 5, 2},
+		{"ANSI_steep", ANSIButtressThread, 2.5, 3},
+		{"Plastic_steep", PlasticButtressThread, 2.5, 3},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			s, err := c.make(c.radius, c.pitch)
+			if err != nil {
+				t.Fatal(err)
+			}
+			hp := c.pitch / 2
+			// Sample a vertical line of points at the wrap boundary, comparing
+			// SDF(+pitch/2, y) vs SDF(-pitch/2, y) at radii covering the
+			// thread region. Tolerance is generous enough to absorb numerical
+			// noise from polygon edge intersections, but tight enough to
+			// catch the ~0.4mm gap that the periodic union previously masked.
+			tol := 1e-6
+			for _, y := range []float64{c.radius - 0.1, c.radius - 0.3, c.radius - 0.5, c.radius} {
+				dR := s.Evaluate(v2.Vec{X: hp, Y: y})
+				dL := s.Evaluate(v2.Vec{X: -hp, Y: y})
+				if math.Abs(dR-dL) > tol {
+					t.Errorf("y=%.3f: SDF discontinuous at wrap: dR(%+.4f)=%+.6f dL(%+.4f)=%+.6f delta=%.6f",
+						y, hp, dR, -hp, dL, dR-dL)
+				}
+			}
+		})
+	}
+}
+
+// Test_Screw3D_RejectsDiscontinuousProfile verifies that Screw3D refuses to
+// build a screw from a thread profile whose SDF is discontinuous at the
+// SawTooth wrap boundary (x=±pitch/2). Such profiles silently produce holes
+// in octree-rendered meshes, so we fail fast at construction. A user-defined
+// asymmetric profile with flat scaffolding extensions (which is exactly the
+// shape the original buttress profiles had) is the canonical example.
+func Test_Screw3D_RejectsDiscontinuousProfile(t *testing.T) {
+	// Asymmetric profile shaped like a buttress: notch at x=+pitch/4 with a
+	// near-vertical right flank and a 45° left flank, with flat-crest
+	// scaffolding extending from the period boundaries to ±pitch. SDF at
+	// x=+pitch/2 sees the near-vertical flank just to the left; SDF at
+	// x=-pitch/2 sees nothing nearby — so the wrap is discontinuous.
+	pitch := 2.0
+	radius := 5.0
+	depth := 0.6
+	tp := NewPolygon()
+	tp.Add(pitch, 0)
+	tp.Add(pitch, radius)
+	tp.Add(pitch/2+0.05, radius)  // gentle (right) flank top
+	tp.Add(pitch/4, radius-depth) // valley root, asymmetric position
+	tp.Add(pitch/4-depth, radius) // 45° (left) flank top
+	tp.Add(-pitch, radius)
+	tp.Add(-pitch, 0)
+	bad, err := Polygon2D(tp.Vertices())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = Screw3D(bad, 10, 0, pitch, 1)
+	if err == nil {
+		t.Fatalf("Screw3D accepted a discontinuous thread profile; expected an error")
+	}
+	t.Logf("got expected rejection: %s", err)
+}
+
+// Test_Screw_TaperSlope verifies that Evaluate uses tan(taper) not atan(taper)
+// for the taper slope computation. At 30° the error is 16.5%.
+func Test_Screw_TaperSlope(t *testing.T) {
+	taperAngle := DtoR(30)
+	slope := math.Tan(taperAngle)
+	slopeAtan := math.Atan(taperAngle)
+
+	thread, err := ISOThread(5, 2, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	screw, err := Screw3D(thread, 20, taperAngle, 2, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// At z=-8 (pitch multiple), SawTooth maps to 0 (thread crest center).
+	// The tan-predicted crest radius is larger than the atan-predicted one.
+	testZ := -8.0
+	rCrestTan := 5 + math.Abs(testZ)*slope
+	rCrestAtan := 5 + math.Abs(testZ)*slopeAtan
+
+	// A point between the two predicted radii should be inside if tan is used.
+	testR := (rCrestTan + rCrestAtan) / 2
+	d := screw.Evaluate(v3.Vec{X: testR, Y: 0, Z: testZ})
+	if d > 0 {
+		t.Errorf("point at r=%.3f (between tan=%.3f and atan=%.3f) is outside (d=%+.6f); expected inside with tan slope",
+			testR, rCrestTan, rCrestAtan, d)
+	}
+
+	// Verify surface is near the tan-predicted radius, not the atan-predicted one.
+	dInside := screw.Evaluate(v3.Vec{X: rCrestTan - 0.5, Y: 0, Z: testZ})
+	dOutside := screw.Evaluate(v3.Vec{X: rCrestTan + 0.5, Y: 0, Z: testZ})
+	if dInside >= 0 {
+		t.Errorf("r_crest-0.5: expected inside (d<0), got d=%+.4f", dInside)
+	}
+	if dOutside <= 0 {
+		t.Errorf("r_crest+0.5: expected outside (d>0), got d=%+.4f", dOutside)
+	}
+
+	t.Logf("taper=30° slope_tan=%.4f slope_atan=%.4f (%.1f%% error)",
+		slope, slopeAtan, 100*(slope-slopeAtan)/slope)
+}
diff --git a/tools/stldiff/main.go b/tools/stldiff/main.go
new file mode 100644
index 000000000..4f2c9be13
--- /dev/null
+++ b/tools/stldiff/main.go
@@ -0,0 +1,179 @@
+// stldiff: compare two binary STL files.
+// Prints IDENTICAL / MINOR / MATERIAL plus metrics.
+package main
+
+import (
+	"crypto/sha1"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+	"os"
+	"sort"
+)
+
+type vec3 struct{ x, y, z float32 }
+
+type tri struct{ a, b, c vec3 }
+
+func loadSTL(path string) ([]tri, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	var header [80]byte
+	if _, err := io.ReadFull(f, header[:]); err != nil {
+		return nil, err
+	}
+	var count uint32
+	if err := binary.Read(f, binary.LittleEndian, &count); err != nil {
+		return nil, err
+	}
+	tris := make([]tri, count)
+	for i := uint32(0); i < count; i++ {
+		var buf [12]float32
+		if err := binary.Read(f, binary.LittleEndian, &buf); err != nil {
+			return nil, err
+		}
+		var attr uint16
+		if err := binary.Read(f, binary.LittleEndian, &attr); err != nil {
+			return nil, err
+		}
+		tris[i] = tri{
+			a: vec3{buf[3], buf[4], buf[5]},
+			b: vec3{buf[6], buf[7], buf[8]},
+			c: vec3{buf[9], buf[10], buf[11]},
+		}
+	}
+	return tris, nil
+}
+
+func lessVec(a, b vec3) bool {
+	if a.x != b.x {
+		return a.x < b.x
+	}
+	if a.y != b.y {
+		return a.y < b.y
+	}
+	return a.z < b.z
+}
+
+func sortTriVerts(t tri) tri {
+	vs := [3]vec3{t.a, t.b, t.c}
+	sort.Slice(vs[:], func(i, j int) bool { return lessVec(vs[i], vs[j]) })
+	return tri{vs[0], vs[1], vs[2]}
+}
+
+func canonicalHash(tris []tri) string {
+	sorted := make([]tri, len(tris))
+	for i, t := range tris {
+		sorted[i] = sortTriVerts(t)
+	}
+	sort.Slice(sorted, func(i, j int) bool {
+		a, b := sorted[i], sorted[j]
+		if a.a != b.a {
+			return lessVec(a.a, b.a)
+		}
+		if a.b != b.b {
+			return lessVec(a.b, b.b)
+		}
+		return lessVec(a.c, b.c)
+	})
+	h := sha1.New()
+	buf := make([]byte, 4)
+	for _, t := range sorted {
+		for _, v := range [3]vec3{t.a, t.b, t.c} {
+			for _, f := range [3]float32{v.x, v.y, v.z} {
+				binary.LittleEndian.PutUint32(buf, math.Float32bits(f))
+				h.Write(buf)
+			}
+		}
+	}
+	return fmt.Sprintf("%x", h.Sum(nil))[:12]
+}
+
+type metrics struct {
+	count    int
+	min, max vec3
+	hash     string
+}
+
+func computeMetrics(tris []tri) metrics {
+	m := metrics{count: len(tris)}
+	if len(tris) == 0 {
+		return m
+	}
+	m.min = tris[0].a
+	m.max = tris[0].a
+	for _, t := range tris {
+		for _, v := range [3]vec3{t.a, t.b, t.c} {
+			if v.x < m.min.x {
+				m.min.x = v.x
+			}
+			if v.y < m.min.y {
+				m.min.y = v.y
+			}
+			if v.z < m.min.z {
+				m.min.z = v.z
+			}
+			if v.x > m.max.x {
+				m.max.x = v.x
+			}
+			if v.y > m.max.y {
+				m.max.y = v.y
+			}
+			if v.z > m.max.z {
+				m.max.z = v.z
+			}
+		}
+	}
+	m.hash = canonicalHash(tris)
+	return m
+}
+
+func bboxDelta(a, b metrics) float64 {
+	ax, ay, az := float64(a.max.x-a.min.x), float64(a.max.y-a.min.y), float64(a.max.z-a.min.z)
+	bx, by, bz := float64(b.max.x-b.min.x), float64(b.max.y-b.min.y), float64(b.max.z-b.min.z)
+	return math.Abs(ax-bx) + math.Abs(ay-by) + math.Abs(az-bz)
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+func main() {
+	if len(os.Args) != 3 {
+		fmt.Fprintln(os.Stderr, "usage: stldiff <a.stl> <b.stl>")
+		os.Exit(2)
+	}
+	a, err := loadSTL(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "A:", err)
+		os.Exit(2)
+	}
+	b, err := loadSTL(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "B:", err)
+		os.Exit(2)
+	}
+	ma, mb := computeMetrics(a), computeMetrics(b)
+	if ma.hash == mb.hash {
+		fmt.Printf("IDENTICAL  tris=%d hash=%s\n", ma.count, ma.hash)
+		return
+	}
+	bb := bboxDelta(ma, mb)
+	triDelta := mb.count - ma.count
+	bboxSize := float64(ma.max.x-ma.min.x) + float64(ma.max.y-ma.min.y) + float64(ma.max.z-ma.min.z)
+	relBbox := bb / bboxSize
+	triRel := float64(abs(triDelta)) / float64(ma.count+1)
+	status := "MATERIAL"
+	if relBbox < 1e-4 && triRel < 0.01 {
+		status = "MINOR   "
+	}
+	fmt.Printf("%s  tris=%d→%d (Δ%+d, %.2f%%)  bbox-Δ=%.3e (%.2f%%)  hashA=%s hashB=%s\n",
+		status, ma.count, mb.count, triDelta, triRel*100, bb, relBbox*100, ma.hash, mb.hash)
+}
diff --git a/tools/stldiff/run.sh b/tools/stldiff/run.sh
new file mode 100755
index 000000000..75240cb69
--- /dev/null
+++ b/tools/stldiff/run.sh
@@ -0,0 +1,86 @@
+#!/bin/bash
+# Cross-branch STL regression check.
+#
+# Renders every example/*/ at BASE and HEAD revs, then compares the STL
+# outputs. Reports per-file: IDENTICAL (canonical hash match), MINOR (tiny
+# float drift), or MATERIAL (geometry genuinely changed). Intended as a
+# sanity check that a core-library change didn't inadvertently alter
+# unrelated examples.
+#
+# Usage:  run.sh [BASE_REF] [HEAD_REF]   (defaults: master, HEAD)
+
+set -u
+
+BASE_REF="${1:-master}"
+HEAD_REF="${2:-HEAD}"
+
+REPO_ROOT=$(cd "$(dirname "$0")/../.." && pwd)
+TOOL_DIR="$REPO_ROOT/tools/stldiff"
+BASE_TREE=$(mktemp -d -t sdfx-stldiff-base-XXXXXX)
+HEAD_TREE=$(mktemp -d -t sdfx-stldiff-head-XXXXXX)
+LOG=$(mktemp -t sdfx-stldiff-results-XXXXXX.txt)
+TIMEOUT=120
+
+cleanup() {
+  git -C "$REPO_ROOT" worktree remove --force "$BASE_TREE" 2>/dev/null
+  git -C "$REPO_ROOT" worktree remove --force "$HEAD_TREE" 2>/dev/null
+}
+trap cleanup EXIT
+
+git -C "$REPO_ROOT" worktree add --detach "$BASE_TREE" "$BASE_REF" >/dev/null
+git -C "$REPO_ROOT" worktree add --detach "$HEAD_TREE" "$HEAD_REF" >/dev/null
+
+( cd "$TOOL_DIR" && go build -o "$TOOL_DIR/stldiff" . )
+
+run_with_timeout() {
+  local d=$1 n=$2
+  ( cd "$d" && rm -f *.stl && make all >/dev/null 2>&1 && ./"$n" >/dev/null 2>&1 ) &
+  local pid=$!
+  ( sleep $TIMEOUT && kill -9 $pid 2>/dev/null ) &
+  local watcher=$!
+  wait $pid 2>/dev/null
+  local rc=$?
+  kill -9 $watcher 2>/dev/null
+  wait $watcher 2>/dev/null
+  return $rc
+}
+
+: > "$LOG"
+
+for dir in "$HEAD_TREE"/examples/*/; do
+  name=$(basename "$dir")
+  basedir="$BASE_TREE/examples/$name"
+  headdir="$HEAD_TREE/examples/$name"
+
+  [ -d "$basedir" ] || { echo "[$name] SKIP (not in base)" | tee -a "$LOG"; continue; }
+  [ -f "$basedir/main.go" ] || { echo "[$name] SKIP (no main.go)" | tee -a "$LOG"; continue; }
+
+  run_with_timeout "$basedir" "$name"; bstatus=$?
+  run_with_timeout "$headdir" "$name"; hstatus=$?
+  if [ $bstatus -ne 0 ] || [ $hstatus -ne 0 ]; then
+    echo "[$name] BUILD/RUN FAILED (base=$bstatus head=$hstatus)" | tee -a "$LOG"
+    continue
+  fi
+
+  any=0
+  for bstl in "$basedir"/*.stl; do
+    [ -f "$bstl" ] || continue
+    any=1
+    fname=$(basename "$bstl")
+    hstl="$headdir/$fname"
+    if [ ! -f "$hstl" ]; then
+      echo "[$name/$fname] MISSING on head" | tee -a "$LOG"
+      continue
+    fi
+    result=$("$TOOL_DIR/stldiff" "$bstl" "$hstl" 2>&1)
+    echo "[$name/$fname] $result" | tee -a "$LOG"
+  done
+  [ $any -eq 0 ] && echo "[$name] no STL produced" | tee -a "$LOG"
+done
+
+echo "---" | tee -a "$LOG"
+echo "Summary (base=$BASE_REF head=$HEAD_REF):" | tee -a "$LOG"
+echo "  IDENTICAL: $(grep -c IDENTICAL "$LOG")" | tee -a "$LOG"
+echo "  MINOR:     $(grep -c 'MINOR ' "$LOG")" | tee -a "$LOG"
+echo "  MATERIAL:  $(grep -c MATERIAL "$LOG")" | tee -a "$LOG"
+echo "  Results log: $LOG"