[mutter/lamefun/dithering] compositor: improve vignette on background actor

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit bdf8d0f1c31cb87eefdfd3cd956c9f2c309201d8
Author: Nikita Churaev <lamefun x0r gmail com>
Date:   Wed Mar 7 00:14:44 2018 +0300

    compositor: improve vignette on background actor
    
    The shader used for computing a vignette currently has two
    problems:
    
     * The math is wrong such that the vignette isn't stretched
       across the whole actor and so ends abruptly
     * There is noticeable banding in its gradient
    
    This commit corrects both problems by fixing the computing
    and introducing noise dithering.

 src/compositor/meta-background-actor.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/src/compositor/meta-background-actor.c b/src/compositor/meta-background-actor.c
index 2b619eb27..756b29e72 100644
--- a/src/compositor/meta-background-actor.c
+++ b/src/compositor/meta-background-actor.c
@@ -131,15 +131,19 @@ typedef enum {
 #define VIGNETTE_VERTEX_SHADER_CODE                                     \
 "position = cogl_tex_coord0_in.xy * scale + offset;\n"                  \
 
-#define VIGNETTE_FRAGMENT_SHADER_DECLARATIONS                           \
-"uniform float vignette_sharpness;\n"                                   \
-"varying vec2 position;\n"                                              \
+#define VIGNETTE_SQRT_2 "1.4142"
+
+#define VIGNETTE_FRAGMENT_SHADER_DECLARATIONS                                                   \
+"uniform float vignette_sharpness;\n"                                                           \
+"varying vec2 position;\n"                                                                      \
+"float rand(vec2 p) { return fract(sin(dot(p, vec2(12.9898, 78.233))) * 43758.5453123); }\n"    \
 
 #define VIGNETTE_FRAGMENT_SHADER_CODE                                          \
-"float t = 2.0 * length(position);\n"                                          \
+"float t = " VIGNETTE_SQRT_2 " * length(position);\n"                          \
 "t = min(t, 1.0);\n"                                                           \
 "float pixel_brightness = 1.0 - t * vignette_sharpness;\n"                     \
 "cogl_color_out.rgb = cogl_color_out.rgb * pixel_brightness;\n"                \
+"cogl_color_out.rgb += (rand(position) - 0.5) / 255.0;\n"                      \
 
 typedef struct _MetaBackgroundLayer MetaBackgroundLayer;