From 41e38a5467560a07dde2b4f180950479e3d76c8d Mon Sep 17 00:00:00 2001
From: Jeremy Behreandt <behreandtjeremy@gmail.com>
Date: Mon, 8 Nov 2021 09:22:31 -0600
Subject: [PATCH] Use Luminance In Desaturate Shader

Updated desaturate shader to use perceptual luminance instead of HSV.
---
 src/Shaders/Desaturate.shader | 81 +++++++++++++++++++----------------
 1 file changed, 44 insertions(+), 37 deletions(-)

diff --git a/src/Shaders/Desaturate.shader b/src/Shaders/Desaturate.shader
index 9dc246a74f51..d7f5e0435534 100644
--- a/src/Shaders/Desaturate.shader
+++ b/src/Shaders/Desaturate.shader
@@ -9,54 +9,61 @@ uniform sampler2D selection;
 uniform bool affect_selection;
 uniform bool has_selection;
 
-vec3 rgb2hsb(vec3 c){
-	vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
-	vec4 p = mix(vec4(c.bg, K.wz),
-				vec4(c.gb, K.xy),
-				step(c.b, c.g));
-	vec4 q = mix(vec4(p.xyw, c.r),
-				vec4(c.r, p.yzx),
-				step(p.x, c.r));
-	float d = q.x - min(q.w, q.y);
-	float e = 1.0e-10;
-	return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)),
-				d / (q.x + e),
-				q.x);
+float stolChannel(float x) {
+	return (x < 0.04045) ? (x / 12.92) : pow((x + 0.055) / 1.055, 2.4);	
 }
 
-vec3 hsb2rgb(vec3 c){
-	vec3 rgb = clamp(abs(mod(c.x*6.0+vec3(0.0,4.0,2.0),
-					6.0)-3.0)-1.0,
-					0.0,
-					1.0 );
-	rgb = rgb*rgb*(3.0-2.0*rgb);
-	return c.z * mix(vec3(1.0), rgb, c.y);
+vec3 standardToLinear(vec3 c) {
+	return vec3(
+		stolChannel(c.r),
+		stolChannel(c.g),
+		stolChannel(c.b));
 }
 
+float ltosChannel(float x) {
+	return (x > 0.0031308) ? (pow(x, 1.0 / 2.4) * 1.055 - 0.055) : (x * 12.92);
+}
+
+vec3 linearToStandard(vec3 c) {
+	return vec3(
+		ltosChannel(c.r),
+		ltosChannel(c.g),
+		ltosChannel(c.b));
+}
+
+float luminance(vec3 lin) {
+	return 0.21264935 * lin.r
+	     + 0.71516913 * lin.g
+		 + 0.07218152 * lin.b;
+}
 
 void fragment() {
 	// Get color from the sprite texture at the current pixel we are rendering
 	vec4 original_color = texture(TEXTURE, UV);
 	vec4 selection_color = texture(selection, UV);
 	
-	vec3 col = original_color.rgb;
-	vec3 hsb = rgb2hsb(col);
-	float gray = hsb.z;
-    if (red)
-        col.x = gray;
-    if (green)
-        col.y = gray;
-    if (blue)
-        col.z = gray;
+	// Transform from standard RGB to linear RGB.
+	vec3 std = original_color.rgb;
+	vec3 lin = standardToLinear(std);
+
+	// Find the y component of linear RGB to XYZ transformation.
+	float lum = luminance(lin);
+	vec3 des = vec3(
+		red ? lum : lin.r,
+		green ? lum : lin.g,
+		blue ? lum : lin.b);
+	vec3 stdPrime = linearToStandard(des);
 
 	vec3 output;
-	if(affect_selection && has_selection)
-		output = mix(original_color.rgb, col, selection_color.a);
-	else
-		output = col;
-    if (alpha)
-	    COLOR = vec4(output.rgb, gray);
-    else 
-        COLOR = vec4(output.rgb, original_color.a);
+	if(affect_selection && has_selection) {
+		output = mix(original_color.rgb, stdPrime, selection_color.a);
+	} else {
+		output = stdPrime;
+	}
 
+    if (alpha) {
+	    COLOR = vec4(output.rgb, ltosChannel(lum));
+	} else {
+        COLOR = vec4(output.rgb, original_color.a);
+	}
 }
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