Fix a bug where combined fp16 weights would be horribly wrong.

[movit] / dither_effect.frag

diff --git a/dither_effect.frag b/dither_effect.frag
index f9c6ad13c443e1919c4506790159c9134106d8a7..b12ecd7cedfd267d8a9935a2372303c9f62e3be9 100644 (file)
--- a/dither_effect.frag
+++ b/dither_effect.frag
@@ -1,12 +1,16 @@
-uniform sampler2D PREFIX(dither_tex);
-uniform vec2 PREFIX(tc_scale);
-uniform float PREFIX(round_fac), PREFIX(inv_round_fac);
+// Implicit uniforms:
+// uniform sampler2D PREFIX(dither_tex);
+// uniform vec2 PREFIX(tc_scale);
+// 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 += tex2D(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.