cssvalue: Change color transitions to run premultiplied

Previously, the unpremultiplied values from the GdkRGBA were taken. Now
we premultiply the color values as specified by the CSS specs.
This is only relevant when transitioning with translucent colors.

An example is the halfway transition between transparent (0, 0, 0, 0)
and white (1, 1, 1, 1). Previously, all 4 values where transitioned
separately and the result was semi-transparent gray (0.5, 0.5, 0.5,
0.5).
By depending on the alpha value, the result is now semi-transparent
white (1, 1, 1, 0.5) which is what one would naively expect.

New reftest: color-transition

1 files changed, 9 insertions, 3 deletions

diff --git a/gtk/gtkcssrgbavalue.c b/gtk/gtkcssrgbavalue.c
index 0b66f8a113..7005347e0e 100644
--- a/gtk/gtkcssrgbavalue.c
+++ b/gtk/gtkcssrgbavalue.c
@@ -74,9 +74,15 @@ gtk_css_value_rgba_transition (GtkCssValue *start,
     }
   else
     {
-      result.red = transition (start->rgba.red, end->rgba.red, progress);
-      result.green = transition (start->rgba.green, end->rgba.green, progress);
-      result.blue = transition (start->rgba.blue, end->rgba.blue, progress);
+      result.red = transition (start->rgba.red * start->rgba.alpha,
+                               end->rgba.red * end->rgba.alpha,
+                               progress) / result.alpha;
+      result.green = transition (start->rgba.green * start->rgba.alpha,
+                                 end->rgba.green * end->rgba.alpha,
+                                 progress) / result.alpha;
+      result.blue = transition (start->rgba.blue * start->rgba.alpha,
+                                end->rgba.blue * end->rgba.alpha,
+                                progress) / result.alpha;
     }
 
   return _gtk_css_rgba_value_new_from_rgba (&result);