From d61fde6e94bce9f920e9fcc55a43f94c23c40a01 Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@gmail.com>
Date: Thu, 15 Jun 2017 21:46:53 +0200
Subject: CSS: Stop using premultiplied color space for gradients.

---
 layout/base/nsCSSRendering.cpp | 88 ++----------------------------------------
 1 file changed, 4 insertions(+), 84 deletions(-)

(limited to 'layout/base/nsCSSRendering.cpp')

diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index 1c6d97395..5b8a884a1 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -2268,14 +2268,8 @@ ComputeRadialGradientLine(nsPresContext* aPresContext,
   *aLineEnd = *aLineStart + gfxPoint(radiusX*cos(-angle), radiusY*sin(-angle));
 }
 
-
-static float Interpolate(float aF1, float aF2, float aFrac)
-{
-  return aF1 + aFrac * (aF2 - aF1);
-}
-
 // Returns aFrac*aC2 + (1 - aFrac)*C1. The interpolation is done
-// in unpremultiplied space, which is what SVG gradients and cairo
+// in RGBA color space, which is what SVG gradients and cairo
 // gradients expect.
 static Color
 InterpolateColor(const Color& aC1, const Color& aC2, float aFrac)
@@ -2435,79 +2429,6 @@ static void ResolveMidpoints(nsTArray<ColorStop>& stops)
   }
 }
 
-static Color
-Premultiply(const Color& aColor)
-{
-  gfx::Float a = aColor.a;
-  return Color(aColor.r * a, aColor.g * a, aColor.b * a, a);
-}
-
-static Color
-Unpremultiply(const Color& aColor)
-{
-  gfx::Float a = aColor.a;
-  return (a > 0.f)
-       ? Color(aColor.r / a, aColor.g / a, aColor.b / a, a)
-       : aColor;
-}
-
-static Color
-TransparentColor(Color aColor) {
-  aColor.a = 0;
-  return aColor;
-}
-
-// Adjusts and adds color stops in such a way that drawing the gradient with
-// unpremultiplied interpolation looks nearly the same as if it were drawn with
-// premultiplied interpolation.
-static const float kAlphaIncrementPerGradientStep = 0.1f;
-static void
-ResolvePremultipliedAlpha(nsTArray<ColorStop>& aStops)
-{
-  for (size_t x = 1; x < aStops.Length(); x++) {
-    const ColorStop leftStop = aStops[x - 1];
-    const ColorStop rightStop = aStops[x];
-
-    // if the left and right stop have the same alpha value, we don't need
-    // to do anything
-    if (leftStop.mColor.a == rightStop.mColor.a) {
-      continue;
-    }
-
-    // Is the stop on the left 100% transparent? If so, have it adopt the color
-    // of the right stop
-    if (leftStop.mColor.a == 0) {
-      aStops[x - 1].mColor = TransparentColor(rightStop.mColor);
-      continue;
-    }
-
-    // Is the stop on the right completely transparent?
-    // If so, duplicate it and assign it the color on the left.
-    if (rightStop.mColor.a == 0) {
-      ColorStop newStop = rightStop;
-      newStop.mColor = TransparentColor(leftStop.mColor);
-      aStops.InsertElementAt(x, newStop);
-      x++;
-      continue;
-    }
-
-    // Now handle cases where one or both of the stops are partially transparent.
-    if (leftStop.mColor.a != 1.0f || rightStop.mColor.a != 1.0f) {
-      Color premulLeftColor = Premultiply(leftStop.mColor);
-      Color premulRightColor = Premultiply(rightStop.mColor);
-      // Calculate how many extra steps. We do a step per 10% transparency.
-      size_t stepCount = NSToIntFloor(fabsf(leftStop.mColor.a - rightStop.mColor.a) / kAlphaIncrementPerGradientStep);
-      for (size_t y = 1; y < stepCount; y++) {
-        float frac = static_cast<float>(y) / stepCount;
-        ColorStop newStop(Interpolate(leftStop.mPosition, rightStop.mPosition, frac), false,
-                          Unpremultiply(InterpolateColor(premulLeftColor, premulRightColor, frac)));
-        aStops.InsertElementAt(x, newStop);
-        x++;
-      }
-    }
-  }
-}
-
 static ColorStop
 InterpolateColorStop(const ColorStop& aFirst, const ColorStop& aSecond,
                      double aPosition, const Color& aDefault)
@@ -2522,9 +2443,9 @@ InterpolateColorStop(const ColorStop& aFirst, const ColorStop& aSecond,
   }
 
   return ColorStop(aPosition, false,
-                   Unpremultiply(InterpolateColor(Premultiply(aFirst.mColor),
-                                                  Premultiply(aSecond.mColor),
-                                                  (aPosition - aFirst.mPosition) / delta)));
+                   InterpolateColor(aFirst.mColor,
+                                    aSecond.mColor,
+                                    (aPosition - aFirst.mPosition) / delta));
 }
 
 // Clamp and extend the given ColorStop array in-place to fit exactly into the
@@ -2924,7 +2845,6 @@ nsCSSRendering::PaintGradient(nsPresContext* aPresContext,
   }
 
   ResolveMidpoints(stops);
-  ResolvePremultipliedAlpha(stops);
 
   bool isRepeat = aGradient->mRepeating || forceRepeatToCoverTiles;
 
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