Fix banding artifacts of 3D space view's skybox

crates/re_renderer/shader/generic_skybox.wgsl

@@ -14,11 +14,37 @@ fn skybox_light_srgb(dir: vec3f) -> vec3f {
     return vec3f(0.85) + 0.15 * rgb;
 }
 
+// -----------------------------------------------
+// Adapted from
+// https://www.shadertoy.com/view/llVGzG
+// Originally presented in:
+// Jimenez 2014, "Next Generation Post-Processing in Call of Duty"
+
+fn interleaved_gradient_noise(n: vec2f) -> f32 {
+    let f = 0.06711056 * n.x + 0.00583715 * n.y;
+    return fract(52.9829189 * fract(f));
+}
+
+fn dither_interleaved(rgb: vec3f, levels: f32, frag_coord: vec4<f32>) -> vec3f {
+    var noise = interleaved_gradient_noise(frag_coord.xy);
+    noise = noise - 0.5;
+    return rgb + noise / (levels - 1.0);
+}
+
+// -----------------------------------------------
+
 @fragment
-fn main(in: FragmentInput) -> @location(0) vec4f {
+fn main(in: FragmentInput, @builtin(position) frag_coord: vec4<f32>) -> @location(0) vec4f {
     let camera_dir = camera_ray_direction_from_screenuv(in.texcoord);
     // Messing with direction a bit so it looks like in our old three-d based renderer (for easier comparison)
     let rgb = skybox_dark_srgb(camera_dir); // TODO(andreas): Allow switching to skybox_light
-    return vec4f(linear_from_srgb(rgb), 1.0);
+
+    // Apply dithering in sRGB.
+    // TODO(andreas): Once we switch to HDR outputs, this can be removed.
+    //                As of writing, the render target itself is (s)RGB8, so we need to dither while we still have maximum precision.
+    let rgb_dithered = dither_interleaved(rgb, 256.0, frag_coord);
+
+    return vec4f(linear_from_srgb(rgb_dithered), 1.0);
+    //return vec4f(linear_from_srgb(rgb), 1.0); // Without dithering
     //return vec4f(camera_dir, 1.0);
 }