Merge branch 'imbris/srgb-dither' into 'master'

Dither in nonlinear srgb space and dithering related experimental shader options

See merge request veloren/veloren!3160

assets/voxygen/shaders/include/random.glsl

@@ -109,4 +109,14 @@ vec3 smooth_rand(vec3 pos, float lerp_axis) {
     vec3 r1 = rand_perm_3(vec3(pos.x, pos.y, pos.z) + floor(lerp_axis + 1.0));
     return r0 + (r1 - r0) * fract(lerp_axis);
 }
+
+// Transform normal distribution to triangle distribution.
+float norm2tri(float n) {
+   // TODO: compare perf with adding two normal noise distributions
+   bool flip = n > 0.5;
+   n = flip ? 1.0 - n : n;
+   n = sqrt(n / 2.0);
+   n = flip ? 1.0 - n : n;
+   return n;
+}
 #endif

assets/voxygen/shaders/postprocess-frag.glsl

@@ -282,9 +282,26 @@ void main() {
     }
 #endif
 
+#ifndef EXPERIMENTAL_NODITHER
     // Add a small amount of very cheap dithering noise to remove banding from gradients
-    // TODO: Instead of 256, detect the colour resolution of the display
-    final_color.rgb = max(vec3(0), final_color.rgb - hash_two(uvec2(uv * screen_res.xy)) / 256.0);
+    // TODO: Consider dithering each color channel independently.
+    // TODO: Consider varying dither over time.
+    // TODO: Instead of 255, detect the colour resolution of the color attachment
+    float noise = hash_two(uvec2(uv * screen_res.xy));
+    #ifndef EXPERIMENTAL_NONSRGBDITHER
+        #ifndef EXPERIMENTAL_TRIANGLENOISEDITHER
+            noise = noise - 0.5;
+        #else
+            // TODO: there is something special we have to do to remove bias
+            // on the bounds when using triangle distribution
+            noise = 2.0 * norm2tri(noise) - 1.0;
+        #endif
+        final_color.rgb = srgb_to_linear(linear_to_srgb(final_color.rgb) + noise / 255.0);
+    #else
+        // NOTE: GPU will clamp value
+        final_color.rgb = final_color.rgb - noise / 255.0;
+    #endif
+#endif
 
     tgt_color = vec4(final_color.rgb, 1);
 }

voxygen/src/render/mod.rs

@@ -447,4 +447,10 @@ pub enum ExperimentalShader {
     Underwarper,
     /// Remove caustics from underwater terrain when shiny water is enabled.
     NoCaustics,
+    /// Don't dither color in post-processing.
+    NoDither,
+    /// Don't use the nonlinear srgb space for dithering color.
+    NonSrgbDither,
+    /// Use triangle PDF noise for dithering instead of uniform noise.
+    TriangleNoiseDither,
 }