From 920b350ac9de335395a1930ed39775902848e87c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Wernst=C3=A5l?= <m4rw3r@gmail.com>
Date: Thu, 27 Apr 2023 18:27:46 +0200
Subject: [PATCH 1/3] feat: more exact sRGB approximation

---
 kitty/cell_fragment.glsl | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/kitty/cell_fragment.glsl b/kitty/cell_fragment.glsl
index 976660290..183b1bc0d 100644
--- a/kitty/cell_fragment.glsl
+++ b/kitty/cell_fragment.glsl
@@ -37,6 +37,22 @@ in float colored_sprite;
 out vec4 final_color;
 
 // Util functions {{{
+float linear2srgb(float x) {
+    // Linear to sRGB conversion.
+    float lower = 12.92 * x;
+    float upper = 1.055 * pow(x, 1.0f / 2.4f) - 0.055f;
+
+    return mix(lower, upper, step(0.0031308f, x));
+}
+
+float srgb2linear(float x) {
+    // sRGB to linear conversion
+    float lower = x / 12.92;
+    float upper = pow((x + 0.055f) / 1.055f, 2.4f);
+
+    return mix(lower, upper, step(0.04045f, x));
+}
+
 vec4 alpha_blend(vec4 over, vec4 under) {
     // Alpha blend two colors returning the resulting color pre-multiplied by its alpha
     // and its alpha.
@@ -103,20 +119,9 @@ vec4 vec4_premul(vec4 rgba) {
 #ifdef NEEDS_FOREGROUND
 // sRGB luminance values
 const vec3 Y = vec3(0.2126, 0.7152, 0.0722);
-const float gamma_factor = 2.2;
 // Scaling factor for the extra text-alpha adjustment for luminance-difference.
 const float text_gamma_scaling = 0.5;
 
-float linear2srgb(float x) {
-    // Approximation of linear-to-sRGB conversion
-    return pow(x, 1.0 / gamma_factor);
-}
-
-float srgb2linear(float x) {
-    // Approximation of sRGB-to-linear conversion
-    return pow(x, gamma_factor);
-}
-
 float clamp_to_unit_float(float x) {
     // Clamp value to suitable output range
     return clamp(x, 0.0f, 1.0f);

From 289957ef1c1480922d954b9c45baa0614068d5a9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Wernst=C3=A5l?= <m4rw3r@gmail.com>
Date: Thu, 27 Apr 2023 18:34:00 +0200
Subject: [PATCH 2/3] style: use ifdef to be consistent with the other cases

---
 kitty/cell_fragment.glsl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kitty/cell_fragment.glsl b/kitty/cell_fragment.glsl
index 183b1bc0d..70519439e 100644
--- a/kitty/cell_fragment.glsl
+++ b/kitty/cell_fragment.glsl
@@ -198,7 +198,7 @@ void main() {
 #endif
 
 #ifdef BACKGROUND
-#if defined(TRANSPARENT)
+#ifdef TRANSPARENT
     final_color = vec4_premul(background, bg_alpha);
 #else
     final_color = vec4(background, draw_bg);

From 9f41183628eac83ad37a7d7dc7bd6626b62079cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Wernst=C3=A5l?= <m4rw3r@gmail.com>
Date: Thu, 27 Apr 2023 18:35:06 +0200
Subject: [PATCH 3/3] fix: account for incorrect gamma-blending performed by
 compositor on transparent windows

Fixes #6209
---
 kitty/cell_fragment.glsl | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kitty/cell_fragment.glsl b/kitty/cell_fragment.glsl
index 70519439e..113b3c8c2 100644
--- a/kitty/cell_fragment.glsl
+++ b/kitty/cell_fragment.glsl
@@ -184,6 +184,12 @@ void main() {
     vec4 fg = calculate_foreground(background);
 #ifdef TRANSPARENT
     final_color = alpha_blend_premul(fg, vec4_premul(background, bg_alpha));
+
+    // Adjust the transparent alpha-channel to account for incorrect
+    // gamma-blending performed by the compositor (true for at least wlroots,
+    // picom, GNOME, MacOS).
+    // This is the last pass:
+    final_color.a = linear2srgb(final_color.a);
 #else
     final_color = alpha_blend_premul(fg, background);
 #endif
@@ -207,6 +213,10 @@ void main() {
 
 #ifdef FOREGROUND
     final_color = calculate_foreground();  // pre-multiplied foreground
+
+    // This is the last pass, adjust alpha to compensate for gamma-incorrect
+    // blending in compositor:
+    final_color.a = linear2srgb(final_color.a);
 #endif
 
 }