[read-fonts] Make FloatItemDelta float accessors public#1954
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Downstream user: harfbuzz/harfrust#403 |
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| /// Returns the (unrounded) delta value as a 32-bit float. | ||
| pub fn to_f32(self) -> f32 { | ||
| self.0 as f32 | ||
| } |
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is this genuinely useful, versus just exposing to_f64 and letting the caller convert if they want?
Expose FloatItemDelta::to_f32 (new) and to_f64 (was pub(crate)) so downstream crates can read the unrounded item-variation delta. Previously the only reachable accessor was the integer-rounded ItemVariationStore::compute_delta. Callers that need HarfBuzz-compatible positioning must keep the delta fractional and round only once, after scaling to output units; that requires the raw float value from compute_float_delta. Assisted by Claude Opus 4.8 (1M context)
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GPOS Device/VariationIndex deltas -- both value-record adjustments and anchor coordinates -- were computed with ItemVariationStore::compute_delta, which rounds the delta to whole font units, and then scaled to output units. That double rounding discards up to half a font unit of the advance, placement, or attachment adjustment. HarfBuzz instead keeps the delta fractional and rounds only once, after scaling: Device::get_x_delta is em_scalef_x(get_delta()), i.e. roundf(delta * scale / upem), used for both value records and Anchor::get_anchor. For variable fonts this made the "harfrust" shaper disagree with "ot" by sub-font-unit amounts on non-default instances (#402, e.g. SFNSItalic at weights other than the default). Compute the deltas with compute_float_delta and scale them with new Scale::scale_x_f/scale_y_f helpers that mirror HarfBuzz's em_multf (roundf(v * scale / upem)), keeping f32 arithmetic to match it. resolve_anchor now returns fractional font units, and its cursive and mark callers round once when scaling. The static value-record path stays on the integer scale, matching HarfBuzz (static -> em_scale, device -> em_scalef). Bump the read-fonts dependency to 0.41.0, which makes the FloatItemDelta::to_f64 accessor public (googlefonts/fontations#1954); compute_float_delta's result is read through it. Advances now match the "ot" shaper exactly (0.000000px) for SFNSItalic across weights 100-900; the harfrust and HarfBuzz shape suites pass with no regressions. Fixes: #402 Assisted-by: Claude <noreply@anthropic.com> Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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GPOS Device/VariationIndex deltas -- both value-record adjustments and anchor coordinates -- were computed with ItemVariationStore::compute_delta, which rounds the delta to whole font units, and then scaled to output units. That double rounding discards up to half a font unit of the advance, placement, or attachment adjustment. HarfBuzz instead keeps the delta fractional and rounds only once, after scaling: Device::get_x_delta is em_scalef_x(get_delta()), i.e. roundf(delta * scale / upem), used for both value records and Anchor::get_anchor. For variable fonts this made the "harfrust" shaper disagree with "ot" by sub-font-unit amounts on non-default instances (#402, e.g. SFNSItalic at weights other than the default). Compute the deltas with compute_float_delta and scale them with new Scale::scale_x_f/scale_y_f helpers that mirror HarfBuzz's em_multf (roundf(v * scale / upem)), keeping f32 arithmetic to match it. resolve_anchor now returns fractional font units, and its cursive and mark callers round once when scaling. The static value-record path stays on the integer scale, matching HarfBuzz (static -> em_scale, device -> em_scalef). Bump the read-fonts dependency to 0.41.0, which makes the FloatItemDelta::to_f64 accessor public (googlefonts/fontations#1954); compute_float_delta's result is read through it. Advances now match the "ot" shaper exactly (0.000000px) for SFNSItalic across weights 100-900; the harfrust and HarfBuzz shape suites pass with no regressions. Fixes: #402 Assisted-by: Claude <noreply@anthropic.com> Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
behdad
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GPOS Device/VariationIndex deltas -- both value-record adjustments and anchor coordinates -- were computed with ItemVariationStore::compute_delta, which rounds the delta to whole font units, and then scaled to output units. That double rounding discards up to half a font unit of the advance, placement, or attachment adjustment. HarfBuzz instead keeps the delta fractional and rounds only once, after scaling: Device::get_x_delta is em_scalef_x(get_delta()), i.e. roundf(delta * scale / upem), used for both value records and Anchor::get_anchor. For variable fonts this made the "harfrust" shaper disagree with "ot" by sub-font-unit amounts on non-default instances (#402, e.g. SFNSItalic at weights other than the default). Compute the deltas with compute_float_delta and scale them with new Scale::scale_x_f/scale_y_f helpers that mirror HarfBuzz's em_multf (roundf(v * scale / upem)), keeping f32 arithmetic to match it. resolve_anchor now returns fractional font units, and its cursive and mark callers round once when scaling. The static value-record path stays on the integer scale, matching HarfBuzz (static -> em_scale, device -> em_scalef). Bump the read-fonts dependency to 0.41.0, which makes the FloatItemDelta::to_f64 accessor public (googlefonts/fontations#1954); compute_float_delta's result is read through it. Advances now match the "ot" shaper exactly (0.000000px) for SFNSItalic across weights 100-900; the harfrust and HarfBuzz shape suites pass with no regressions. Fixes: #402 Assisted-by: Claude <noreply@anthropic.com> Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
behdad
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Jul 3, 2026
GPOS Device/VariationIndex deltas -- both value-record adjustments and anchor coordinates -- were computed with ItemVariationStore::compute_delta, which rounds the delta to whole font units, and then scaled to output units. That double rounding discards up to half a font unit of the advance, placement, or attachment adjustment. HarfBuzz instead keeps the delta fractional and rounds only once, after scaling: Device::get_x_delta is em_scalef_x(get_delta()), i.e. roundf(delta * scale / upem), used for both value records and Anchor::get_anchor. For variable fonts this made the "harfrust" shaper disagree with "ot" by sub-font-unit amounts on non-default instances (#402, e.g. SFNSItalic at weights other than the default). Compute the deltas with compute_float_delta and scale them with new Scale::scale_x_f/scale_y_f helpers that mirror HarfBuzz's em_multf (roundf(v * scale / upem)), keeping f32 arithmetic to match it. resolve_anchor now returns fractional font units, and its cursive and mark callers round once when scaling. The static value-record path stays on the integer scale, matching HarfBuzz (static -> em_scale, device -> em_scalef). Bump the read-fonts dependency to 0.41.0, which makes the FloatItemDelta::to_f64 accessor public (googlefonts/fontations#1954); compute_float_delta's result is read through it. Advances now match the "ot" shaper exactly (0.000000px) for SFNSItalic across weights 100-900; the harfrust and HarfBuzz shape suites pass with no regressions. Fixes: #402 Assisted-by: Claude <noreply@anthropic.com> Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Expose FloatItemDelta::to_f32 (new) and to_f64 (was pub(crate)) so downstream crates can read the unrounded item-variation delta.
Previously the only reachable accessor was the integer-rounded ItemVariationStore::compute_delta. Callers that need HarfBuzz-compatible positioning must keep the delta fractional and round only once, after scaling to output units; that requires the raw float value from compute_float_delta.
Assisted by Claude Opus 4.8 (1M context)