Always perform color correction and debanding on nonlinear sRGB values.

Fixes #107730

Co-authored-by: LuoZhihao <luo_zhihao@outlook.com>

servers/rendering/renderer_rd/shaders/blit.glsl

@@ -10,7 +10,6 @@ layout(push_constant, std140) uniform Pos {
 
 	float rotation_sin;
 	float rotation_cos;
-	vec2 pad;
 
 	vec2 eye_center;
 	float k1;
@@ -20,6 +19,8 @@ layout(push_constant, std140) uniform Pos {
 	float aspect_ratio;
 	uint layer;
 	bool convert_to_srgb;
+	bool use_debanding;
+	float pad;
 }
 data;
 
@@ -50,7 +51,6 @@ layout(push_constant, std140) uniform Pos {
 
 	float rotation_sin;
 	float rotation_cos;
-	vec2 pad;
 
 	vec2 eye_center;
 	float k1;
@@ -60,6 +60,8 @@ layout(push_constant, std140) uniform Pos {
 	float aspect_ratio;
 	uint layer;
 	bool convert_to_srgb;
+	bool use_debanding;
+	float pad;
 }
 data;
 
@@ -74,12 +76,27 @@ layout(binding = 0) uniform sampler2D src_rt;
 #endif
 
 vec3 linear_to_srgb(vec3 color) {
-	// If going to srgb, clamp from 0 to 1.
-	color = clamp(color, vec3(0.0), vec3(1.0));
 	const vec3 a = vec3(0.055f);
 	return mix((vec3(1.0f) + a) * pow(color.rgb, vec3(1.0f / 2.4f)) - a, 12.92f * color.rgb, lessThan(color.rgb, vec3(0.0031308f)));
 }
 
+// From https://alex.vlachos.com/graphics/Alex_Vlachos_Advanced_VR_Rendering_GDC2015.pdf
+// and https://www.shadertoy.com/view/MslGR8 (5th one starting from the bottom)
+// NOTE: `frag_coord` is in pixels (i.e. not normalized UV).
+// This dithering must be applied after encoding changes (linear/nonlinear) have been applied
+// as the final step before quantization from floating point to integer values.
+vec3 screen_space_dither(vec2 frag_coord) {
+	// Iestyn's RGB dither (7 asm instructions) from Portal 2 X360, slightly modified for VR.
+	// Removed the time component to avoid passing time into this shader.
+	vec3 dither = vec3(dot(vec2(171.0, 231.0), frag_coord));
+	dither.rgb = fract(dither.rgb / vec3(103.0, 71.0, 97.0));
+
+	// Subtract 0.5 to avoid slightly brightening the whole viewport.
+	// Use a dither strength of 100% rather than the 37.5% suggested by the original source.
+	// Divide by 255 to align to 8-bit quantization.
+	return (dither.rgb - 0.5) / 255.0;
+}
+
 void main() {
 #ifdef APPLY_LENS_DISTORTION
 	vec2 coords = uv * 2.0 - 1.0;
@@ -118,5 +135,10 @@ void main() {
 
 	if (data.convert_to_srgb) {
 		color.rgb = linear_to_srgb(color.rgb); // Regular linear -> SRGB conversion.
+		// When convert_to_srgb is true, debanding was skipped in tonemap.glsl.
+		if (data.use_debanding) {
+			color.rgb += screen_space_dither(gl_FragCoord.xy);
+		}
+		color.rgb = clamp(color.rgb, vec3(0.0), vec3(1.0));
 	}
 }