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fix(font): bound a color glyph by the spec's per-format rules - #553
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📝 Walkthrough
Merge Risk: 🔵 Low · up to Some invalid COLR glyphs can pass the new boundedness gate and render instead of being rejected. Correct the atop rules before merging; the affected behavior is limited to these composite cases. Security Architecture Review
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely identifies the main change: applying the specification's per-format boundedness rules to color glyphs. |
| Description check | |
The description directly explains the boundedness analysis, composite handling, rendering behavior, and verification results described by the changeset. |
| Docstring Coverage | |
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 2 files. (1 skipped: 1… |
| Linked Issues check | |
Check skipped because no linked issues were found for this pull request. |
| Out of Scope Changes check | |
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- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @crates/neomacs-font-materializer/src/color_glyph/colr.rs:
- Around line 1146-1155: Update composite_is_bounded to handle SourceAtop as
bounded by the backdrop and DestinationAtop as bounded by the source, rather
than letting these modes use the fallback requiring both operands to be bounded.
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🔵 Needs a closer look
It is a subtle rendering-correctness change resting on non-obvious invariants (the adjacent-pop composite heuristic, measure/paint consistency) and I found one concrete format-32 gap (SourceAtop/DestinationAtop misclassification), so it warrants human review.
1 open finding
What changed in this PR
This PR refines COLRv1 color-glyph rendering (follow-up to #544) so that boundedness is decided for the whole paint graph rather than per individual fill. The engine now analyzes the recorded step list before painting: unbounded fills that escape to the root scope drop the glyph entirely (per the OpenType spec), while the format-32 composite table can bound a sub-graph whose own fills are unbounded — and in that permitted case, the previously-skipped unbounded fill now paints the whole surface in its recorded space, so a source-in over a bare solid keeps its ink instead of rendering nothing.
Changes:
- Add
graph_is_bounded(whole-graph scope folding overPushLayer/PopLayerwith adjacent-pop composite detection) andcomposite_is_bounded(format-32 table) incolr.rs, gating the glyph before paint. - Rework the fill replay so an unbounded fill allowed through a composite paints a surface-covering rect (computed by inverse-mapping the pixmap corners into paint space), with no clip applied.
- Update synthetic tests: the bare-layer-fill case now drops the whole glyph, plus new
source-in-over-unbounded-backdrop andclear-over-unbounded cases.
| File | Description |
|---|---|
| crates/neomacs-font-materializer/src/color_glyph/colr.rs | Adds whole-graph boundedness gate and format-32 composite table; reworks unbounded-fill replay to paint the full surface and restructures clip selection. |
| crates/neomacs-font-materializer/src/color_glyph/tests/synthetic_colr_test.rs | Updates the bare-fill test to expect the whole glyph to drop; adds source-in and clear composite boundedness cases. |
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| CompositeMode::Source | CompositeMode::SourceOut => source, | ||
| CompositeMode::Destination | CompositeMode::DestinationOut => backdrop, | ||
| CompositeMode::SourceIn | CompositeMode::DestinationIn => source || backdrop, | ||
| _ => source && backdrop, |
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Inline comments:
Review comments at @crates/neomacs-font-materializer/src/color_glyph/colr.rs:
- Around line 1156-1157: Update the boundedness handling for
CompositeMode::SourceAtop and CompositeMode::DestinationAtop so neither mode is
accepted when only one operand is bounded; remove these special-case rows and
let invalid root graphs reach the existing rejection path.
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crates/neomacs-font-materializer/src/color_glyph/colr.rscrates/neomacs-font-materializer/src/color_glyph/tests/synthetic_colr_test.rslisp/ldefs-boot.el
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| CompositeMode::SourceAtop => backdrop, | ||
| CompositeMode::DestinationAtop => source, |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
Keep both atop modes under the format-32 boundedness rule.
If only one operand is bounded, these rows accept the composite. The OpenType COLR table instead requires both operands to be bounded for SourceAtop and DestinationAtop. Remove these rows so an invalid root graph falls through to the existing rejection path. The output-alpha calculation does not change the specification's boundedness rule. (learn.microsoft.com)
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Review comment at @crates/neomacs-font-materializer/src/color_glyph/colr.rs
around lines 1156 - 1157:
Update the boundedness handling for CompositeMode::SourceAtop and
CompositeMode::DestinationAtop so neither mode is accepted when only one operand
is bounded; remove these special-case rows and let invalid root graphs reach the
existing rejection path.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
Boundedness was decided per fill: a fill with no shape and no clip was skipped, and the rest of the graph still rendered. The spec decides it for the whole graph — a bare solid makes the definition unbounded and "must not render" — except inside a composite, where the format-32 table can bound a sub-graph whose own fills are unbounded (source-in needs only one bounded operand, destination-out only a bounded backdrop). The recorder's step list is now analyzed before painting: layer scopes come from the composite driver's backdrop/source pushes with their adjacent pops, each composite applies the table, and anything unbounded that reaches the root drops the glyph. A fill the table lets through paints the whole surface in the space its paint was recorded in, which is what an unbounded fill means once some bounded content has given the surface its size. Verified: materializer 71/71 — the layer-list case now drops the whole glyph, a source-in over a bare solid keeps its ink, and clear over unbounded content measures empty.
Review of the format-32 table: source-atop and destination-atop were falling through to the both-operands rule. Porter-Duff says their output alpha is the backdrop's and the source's respectively, so a graph with one bounded operand is bounded and must render — rejecting it would have dropped valid color glyphs to the monochrome fallback. Verified: materializer 85/85, the two new tests failing before the rows existed.
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Follow-up to #544. Boundedness was decided per fill: a fill with no shape and no clip was skipped, and the rest of the graph still rendered. The spec decides it for the whole graph — a bare solid makes the definition unbounded, and "applications must confirm that the color glyph definition is bounded, and must not render the color glyph if the defining graph is not bounded" — except inside a composite, where the format-32 table can bound a sub-graph whose own fills are unbounded (
source-inneeds only one bounded operand;destination-outonly a bounded backdrop;clearis always bounded).Implementation. The recorded step list is analyzed before painting:
composite_is_bounded).graph_is_bounded).source-inover a bare solid rendered nothing).Verified. materializer 71/71, including new cases: the layer-list-with-a-bare-fill now drops the whole glyph;
source-inover a bare solid keeps its ink at the source colour;clearover unbounded content measures empty; and the existing source-over/composite tests still hold. GUIcolrv1_emojigreen on a fresh release build + pdump, and the golden emoji sheet from #552 also passes against this binary — the rule moved no real pixels.