[mutter] MetaBackgroundActor: match total dimming if GLSL is not present

[Owen Taylor <otaylor src gnome org>]

commit ef3b0000508d30e15aaee6c7ee9ae58275246062
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Tue Sep 2 18:09:29 2014 -0400

    MetaBackgroundActor: match total dimming if GLSL is not present
    
    Without GLSL, we didn't apply the vignetting, which not only made the
    background uniform in color, it made it much lighter. Adjust for this
    and make the average brightness with the vignette effect the same
    with or without GLSL.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=735637

 src/compositor/meta-background-actor.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/src/compositor/meta-background-actor.c b/src/compositor/meta-background-actor.c
index ad82f20..4423b14 100644
--- a/src/compositor/meta-background-actor.c
+++ b/src/compositor/meta-background-actor.c
@@ -319,7 +319,18 @@ setup_pipeline (MetaBackgroundActor   *self,
     }
 
   if (priv->vignette)
-    color_component = priv->brightness * opacity / 255.;
+    {
+      color_component = priv->brightness * opacity / 255.;
+
+      if (!clutter_feature_available (CLUTTER_FEATURE_SHADERS_GLSL))
+        {
+          /* Darken everything to match the average brightness that would
+           * be there if we were drawing the vignette, which is
+           * (1 - (pi/12.) * vignette_sharpness [exercise for the reader :]
+           */
+          color_component *= (1 - 0.74 * priv->vignette_sharpness);
+        }
+    }
   else
     color_component = opacity / 255.;