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CSS gradients: add a special case for transparent stops.

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This causes transparent stops to behave like "true transparent" instead
of "transparent black", even in RGBA space.
i.e.: the gradient will transition to a transparent version of the color
adjacent to the transparent color stop (on either side if not on the edge).
This commit is contained in:
wolfbeast 2018-02-02 20:16:50 +01:00 • committed by Roy Tam
commit fa08bd44f7
1 changed files with 38 additions and 0 deletions
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38
layout/base/nsCSSRendering.cpp
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@ -2670,6 +2670,44 @@ nsCSSRendering::PaintGradient(nsPresContext* aPresContext,
lineLength = rectLen;
}
// Special case for 'transparent'
for (uint32_t i = 0; i < stops.Length(); ++i) {
Color color = stops[i].mColor;
if (color.r == 0 && color.g == 0 && color.b == 0 && color.a == 0) {
// We have (0,0,0,0) as a color stop - this means 'transparent'.
// In this case for the usually intended effect, we change the color
// of the transparent stop to the color of the adjacent stop with
// 0 opacity. If we are not on either edge, we add a stop on both
// sides of the transparent point with the adjacent color value.
// i.e.: c1 -> c1 (alpha 0) | c2 (alpha 0) -> c2
// XXX: We should probably track the use of the transparent keyword
// down from the CSS parsing level to here with a flag in mStops, if
// rgba(0,0,0,0) ever is an intended thing (very much a corner case).
if (i > 0) {
// Change stop color to adjacent-previous (color->T)
color = stops[i - 1].mColor;
color.a = 0;
stops[i].mColor = color;
if (i < stops.Length() - 1) {
// We're in the middle somewhere: insert stop adjacent-next (T->color)
Color color2 = stops[i + 1].mColor;
color2.a = 0;
if (color != color2) {
// Only insert an extra stop if c1 is different than c2 in c1->T->c2
// Note: A transparent stop is never considered an interpolation hint
stops.InsertElementAt(i + 1, ColorStop(stops[i].mPosition, false, color2));
i++;
}
}
} else if (i < stops.Length() - 1) {
// Change stop color to adjacent-next (T->color)
color = stops[i + 1].mColor;
color.a = 0;
stops[i].mColor = color;
}
}
}
// Eliminate negative-position stops if the gradient is radial.
double firstStop = stops[0].mPosition;
if (aGradient->mShape != NS_STYLE_GRADIENT_SHAPE_LINEAR && firstStop < 0.0) {