From 1252974765068f3f5bdf76d7d18eddaf55d288c8 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Thu, 2 Jul 2026 14:47:36 +0530
Subject: [PATCH] Port the cell fragment shader

---
 kitty/shaders/cell.slang | 96 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)

diff --git a/kitty/shaders/cell.slang b/kitty/shaders/cell.slang
index 45f7992c4..be379b85b 100644
--- a/kitty/shaders/cell.slang
+++ b/kitty/shaders/cell.slang
@@ -7,6 +7,8 @@
 
 import utils;
 import hsluv;
+import alpha_blend;
+import linear2srgb;
 
 #define NUM_COLORS 256
 
@@ -470,4 +472,98 @@ VertexOutput vertex_main(
     return vo;
 }
 
+uniform Sampler2DArray sprites;
+// Scaling factor for the extra text-alpha adjustment for luminance-difference.
+static const float text_gamma_scaling = 0.5;
 
+float clamp_to_unit_float(float x) {
+    // Clamp value to suitable output range
+    return clamp(x, 0.0f, 1.0f);
+}
+
+float4 foreground_contrast_new(float4 over, float3 under, float text_contrast, float text_gamma_adjustment) {
+    float under_luminance = dot(under, Y);
+    float over_lumininace = dot(over.rgb, Y);
+    // Apply additional gamma-adjustment scaled by the luminance difference, the darker the foreground the more adjustment we apply.
+    // A multiplicative contrast is also available to increase saturation.
+    over.a = clamp_to_unit_float(lerp(over.a, pow(over.a, text_gamma_adjustment), (1 - over_lumininace + under_luminance) * text_gamma_scaling) * text_contrast);
+    return over;
+}
+
+float4 foreground_contrast_old(float4 over, float3 under) {
+    // Simulation of gamma-incorrect blending
+    float under_luminance = dot(under, Y);
+    float over_lumininace = dot(over.rgb, Y);
+    // This is the original gamma-incorrect rendering, it is the solution of the following equation:
+    //
+    // linear2srgb(over * overA2 + under * (1 - overA2)) = linear2srgb(over) * over.a + linear2srgb(under) * (1 - over.a)
+    // ^ gamma correct blending with new alpha             ^ gamma incorrect blending with old alpha
+    over.a = clamp_to_unit_float((srgb2linear(linear2srgb(over_lumininace) * over.a + linear2srgb(under_luminance) * (1.0f - over.a)) - under_luminance) / (over_lumininace - under_luminance));
+    return over;
+}
+
+float4 foreground_contrast(float4 over, float3 under, float text_contrast, float text_gamma_adjustment) {
+    if (TEXT_NEW_GAMMA) return foreground_contrast_new(over, under, text_contrast, text_gamma_adjustment);
+    return foreground_contrast_old(over, under);
+}
+
+float4 load_text_foreground_color(float3 sprite_pos, float colored_sprite, float3 cell_foreground) {
+    // For colored sprites use the color from the sprite rather than the text foreground
+    // Return non-premultiplied foreground color
+    float4 text_fg = sprites.Sample(sprite_pos);
+    return float4(lerp(cell_foreground, text_fg.xyz, colored_sprite), text_fg.w);
+}
+
+float4 calculate_premul_foreground_from_sprites(
+    float3 sprite_pos, float3 underline_pos, float3 cursor_pos, float3 strike_pos, uint underline_exclusion_pos,
+    float4 text_fg, float3 decoration_fg, float4 cursor_color_premult,
+    float effective_text_alpha,
+) {
+    // Return premul foreground color from decorations (cursor, underline, strikethrough)
+    int width, height, layer;
+    sprites.GetDimensions(width, height, layer);
+    float3 sz = float3(float(width), float(height), float(layer));
+
+    float underline_alpha = sprites.Sample(underline_pos).w;
+    int3 fetch_coord = int3(int(sprite_pos.x * sz.x), int(underline_exclusion_pos), int(sprite_pos.z));
+    float underline_exclusion = sprites[fetch_coord].w;
+
+    underline_alpha *= 1.0 - underline_exclusion;
+    float strike_alpha = sprites.Sample(strike_pos).w;
+    float cursor_alpha = sprites.Sample(cursor_pos).w;
+
+    float combined_alpha = min(text_fg.w + strike_alpha, 1.0);
+
+    float4 ans = alpha_blend(
+        float4(text_fg.rgb, combined_alpha * effective_text_alpha),
+        float4(decoration_fg, underline_alpha * effective_text_alpha)
+    );
+
+    return lerp(ans, cursor_color_premult, cursor_alpha * cursor_color_premult.w);
+}
+
+float4 adjust_foreground_contrast_with_background(float4 text_fg, float3 bg, float text_contrast, float text_gamma_adjustment) {
+    return foreground_contrast(text_fg, bg, text_contrast, text_gamma_adjustment);
+}
+
+[shader("fragment")]
+float4 fragment_main(
+    VertexOutput vo,
+    uniform float text_contrast,
+    uniform float text_gamma_adjustment,
+) : SV_Target {
+    float4 ans_premul = 0;
+    if (!ONLY_FOREGROUND) ans_premul = vo.effective_background_premul;
+
+    if (!ONLY_BACKGROUND) {
+        // blend in the foreground color
+        float4 text_fg = load_text_foreground_color(vo.sprite_pos, vo.colored_sprite, vo.cell_foreground);
+        text_fg = adjust_foreground_contrast_with_background(text_fg, vo.background, text_contrast, text_gamma_adjustment);
+        float4 text_fg_premul = calculate_premul_foreground_from_sprites(
+            vo.sprite_pos, vo.underline_pos, vo.cursor_pos, vo.strike_pos, vo.underline_exclusion_pos,
+            text_fg, vo.decoration_fg, vo.cursor_color_premult, vo.effective_text_alpha);
+        if (ONLY_FOREGROUND) ans_premul = text_fg_premul;
+        else ans_premul = alpha_blend_premul(text_fg_premul, ans_premul);
+    }
+    return ans_premul;
+}