Don't dither alpha.

For alpha, correct rounding is more important to us than overall
spectral properties, simply because 255 -> 255 is such an important
case.

diff --git a/dither_effect.frag b/dither_effect.frag
index f9c6ad13c443e1919c4506790159c9134106d8a7..6994e8467f0c9645e47d9f74ff96700028eafddf 100644 (file)
--- a/dither_effect.frag
+++ b/dither_effect.frag
@@ -3,10 +3,13 @@ uniform vec2 PREFIX(tc_scale);
 uniform float PREFIX(round_fac), PREFIX(inv_round_fac);
 
 vec4 FUNCNAME(vec2 tc) {
 uniform float PREFIX(round_fac), PREFIX(inv_round_fac);
 
 vec4 FUNCNAME(vec2 tc) {

-       // We also choose to dither alpha, just in case.
-       // Maybe it should in theory have a separate dither,
-       // but I doubt it matters much.
-       vec4 result = INPUT(tc) + texture2D(PREFIX(dither_tex), tc * PREFIX(tc_scale)).xxxx;
+       vec4 result = INPUT(tc);
+
+       // Don't dither alpha; the case of alpha=255 (1.0) is very important to us,
+       // and if there's any inaccuracy earlier in the chain so that it becomes e.g.
+       // 254.8, it's better to just get it rounded off than to dither and have it
+       // possibly get down to 254. This is not the case for the color components.
+       result.rgb += texture2D(PREFIX(dither_tex), tc * PREFIX(tc_scale)).xxx;

 
        // NEED_EXPLICIT_ROUND will be #defined to 1 if the GPU has inaccurate
        // fp32 -> int8 framebuffer rounding, and 0 otherwise.
 
        // NEED_EXPLICIT_ROUND will be #defined to 1 if the GPU has inaccurate
        // fp32 -> int8 framebuffer rounding, and 0 otherwise.