SVG: write translucent paints as color plus opacity instead of rgba() - #386
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SVG 1.1 does not allow a function like rgba() where a <color> is expected, so renderers implementing SVG 1.1 rather than CSS Color 4 discard the value and fall back to the initial fill, black. Split the alpha off into a separate fill-opacity/stroke-opacity/stop-opacity property and write the color as plain hex, which both SVG 1.1 renderers and browsers understand. The alpha is never written twice: splitAlpha un-premultiplies the color and returns an opaque one, so the value in the *-opacity property is the only place the alpha appears. This was the hazard that made 7d6baa9 remove the previous fix for this (tdewolff#263). Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Fixes #385.
The SVG renderer wrote translucent paints as
fill:rgba(58,130,246,.69803922). SVG 1.1 does not allow a function where a<color>is expected, so renderers implementing SVG 1.1 rather than CSS Color 4 discard the declaration and fall back to the initialfill— black. Browsers accept it, so it only shows up outside a browser (Inkscape, librsvg, most print/CAD toolchains).Fix
splitAlphaun-premultiplies a color into an opaque#rrggbbplus an opacity in[0,1].writePaintreturns both, and each call site writes the opacity to the matching property —fill-opacity,stroke-opacity, orstop-opacity— only when it is not1.0. Output for the issue's reproducer:Covered: path fill and stroke in both attribute and
styleform, the explicitly-drawn stroke path (strokeUnsupported), text fill inRenderTextand all three branches ofwriteFontStyle, and linear/radial gradient stops (stop-colorhad the same problem).Not writing the alpha twice
#263 fixed this in Dec 2023 and 7d6baa9 removed it in Jul 2024 as a drive-by while working on #307. I could not find a recorded failure case, but the obvious hazard is emitting the alpha in both
rgba()andfill-opacity, which darkens the result. That cannot happen here:splitAlphareturns a color withA = 255, so the alpha exists in exactly one place.TestSplitAlphapins that, and every case inTestPaintOpacityadditionally assertsrgba(never appears in the output, so this cannot regress a third time.The SVG parser in
svg.goalready handlesfill-opacity,stroke-opacityandstop-opacity, so round-tripping is unaffected.Verification
inkscape out.svg -o out.pngon the reproducer from the issue now samplessrgba(57,130,248,0.698039)inside the square, instead of solid black.go test ./renderers/svg/ ./renderers/pdf/ .passes.One leftover:
examples/go-chart/output.svgis a committed render that still contains onergba(...)from before this change. I left it alone rather than regenerate the example's four output files in this PR — happy to add that if you'd prefer.🤖 Generated with Claude Code