[trak] Keep tracking fractional#404
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The trak tracking value is interpolated per point size and is generally not a whole font unit, but get_h_tracking/get_v_tracking rounded it to an integer before scaling. HarfBuzz keeps it fractional and rounds once, after scaling to output units (get_h_tracking is em_scalef_x(get_tracking())). Return the fractional tracking and scale it with the Scale::scale_x_f /scale_y_f helpers. For SFNSItalic this removes an up-to-half-font-unit per-glyph advance difference from the "ot" shaper at point sizes where the tracking interpolates to a fractional value (e.g. 20.5, 24.5); integer-tracking sizes were already exact. Assisted-by: Claude <noreply@anthropic.com> Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Follow-up to #403, applying the same fix to the AAT
traktable.Problem
The
traktracking value is interpolated by point size and is generally not awhole font unit, but
get_h_tracking/get_v_trackingrounded it to aninteger before scaling. HarfBuzz keeps it fractional and rounds only once,
after scaling to output units (
get_h_trackingisem_scalef_x(get_tracking())).Fix
Return the fractional tracking and scale it with the
Scale::scale_x_f/scale_y_fhelpers (added in #403).Testing
Comparing the
otandharfrustshapers onSFNSItalic.ttf("System italic", size 20, weight 500) across point sizes 14–26 in 0.5 steps:(20.5, 21.5, 22.5, 23.5, 24.5, 25.0, 25.5) every glyph's advance differed
from
otby up to ±0.5 font units (±0.0049px); integer-tracking sizesalready matched.
0.000000pxmatch at every size.cargo test -p harfrust: 6027 passed, 0 failed.