CSS gradients: add a special case for transparent stops.

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).

1 files changed, 38 insertions, 0 deletions

diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index 5b8a884a1..054632ad7 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -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) {