Background colour/tint of overview in GNOME 3

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I happened to be in the same place as several of the core GNOME developers today, so I asked one of them about this, and he said that this specific behavior (the lightbox effect in the overview) was handled differently depending on which gnome-shell release you're running. I happened to be running 3.12 (from this copr repo, so he explained how it works in 3.12 and 3.14.

The lightbox effect in those releases is handled by the mutter library, specifically in src/compositor/meta-background.c. This is the code that does the magic:

 #define VIGNETTE_CODE                                                          \
 "vec2 position = cogl_tex_coord_in[0].xy * texture_scale - offset;\n"          \`
 "float t = length(2.0 * (position / actor_size));\n"                           \
 "t = clamp(t, 0.0, 1.0);\n"                                                    \
 "float pixel_brightness = mix(1.0, 1.0 - vignette_sharpness, t);\n"            \
 "cogl_color_out.rgb = cogl_color_out.rgb * pixel_brightness * brightness;\n"

Specifically, it is darkening the background colors through multiplying the brightness values to achieve the lightbox effect. This results in the "black wash", but it's not calculated on a specific color. The quick hack is to change the code to mix in a specific color as part of the process, the diff looks like this:

diff -up mutter-3.12.2/src/compositor/meta-background.c.colorchange mutter-3.12.2/src/compositor/meta-background.c
--- mutter-3.12.2/src/compositor/meta-background.c.colorchange  2014-08-09 13:49:27.603141789 +0200
+++ mutter-3.12.2/src/compositor/meta-background.c  2014-08-09 13:55:46.205277383 +0200
@@ -45,12 +45,15 @@
 "uniform float brightness;\n"                                                  \
 "uniform float vignette_sharpness;\n"                                          \

+/* 1.0, 1.0, 0.0 is yellow */
+
 #define VIGNETTE_CODE                                                          \
 "vec2 position = cogl_tex_coord_in[0].xy * texture_scale - offset;\n"          \
 "float t = length(2.0 * (position / actor_size));\n"                           \
 "t = clamp(t, 0.0, 1.0);\n"                                                    \
 "float pixel_brightness = mix(1.0, 1.0 - vignette_sharpness, t);\n"            \
-"cogl_color_out.rgb = cogl_color_out.rgb * pixel_brightness * brightness;\n"
+"vec3 constant_color = vec3(1.0, 1.0, 0.0);\n"                                 \
+"cogl_color_out.rgb = mix(cogl_color_out.rgb, constant_color, pixel_brightness * brightness);\n"

 /* We allow creating multiple MetaBackgrounds for the same monitor to
  * allow different rendering options to be set for different copies.

This results in this effect:

I asked about the reprocussions of changing this behavior in mutter, and he was reasonably confident that nothing else besides the shell overview mode was using this code in mutter. He also said that there were plans in the near term to rewrite how this code works entirely, but that he thought a patch to add a CSS option to enable the color choice for the overview lightbox would be accepted.

The same fix may apply to 3.10 mutter, but I do not have an instance of Fedora running 3.10 handy at the moment, and he seemed to think it might not work exactly the same there.

Here are some files that will help if you want to use my hack:

• patch against mutter 3.12.2
• patched srpm for mutter from GNOME 3.12 copr
• mutter-3.12.2-1.spot.fc20 x86_64 RPM with patch applied
• mutter-devel-3.12.2-1.fc20 x86_64 ...