From cbebf86fb76109e13ecca6ead65fddef36a9f2d2 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Tue, 30 Jun 2026 22:25:01 +0530
Subject: [PATCH] Port the bgimage shader

---
 kitty/shaders/bgimage.slang | 90 +++++++++++++++++++++++++++++++++++++
 kitty/shaders/slang.py      |  7 ++-
 2 files changed, 95 insertions(+), 2 deletions(-)
 create mode 100644 kitty/shaders/bgimage.slang

diff --git a/kitty/shaders/bgimage.slang b/kitty/shaders/bgimage.slang
new file mode 100644
index 000000000..684f3a4e1
--- /dev/null
+++ b/kitty/shaders/bgimage.slang
@@ -0,0 +1,90 @@
+#language slang 2026
+// Copyright (C) 2026 Kovid Goyal <kovid at kovidgoyal.net>
+// Distributed under terms of the GPLv3 license.
+
+import alpha_blend;
+
+// Constants and Macros
+#define left  0
+#define top  1
+#define right  2
+#define bottom  3
+#define tex_left 0.0
+#define tex_top 0.0
+#define tex_right 1.0
+#define tex_bottom 1.0
+
+#define x_axis 0
+#define y_axis 1
+
+static const float2 tex_map[4] = {
+    float2(tex_left, tex_top),
+    float2(tex_left, tex_bottom),
+    float2(tex_right, tex_bottom),
+    float2(tex_right, tex_top)
+};
+
+
+struct VertexOutput {
+    float2 texcoord : TEXCOORD;
+    float4 position : SV_Position;
+};
+
+// Helper Functions
+float scale_factor(float window_size, float image_size) {
+    return window_size / image_size;
+}
+
+float tiling_factor(int i, float4 sizes, float tiled) {
+    int window = i;
+    int image = i + 2;
+    return tiled * scale_factor(sizes[window], sizes[image]) + (1.0 - tiled);
+}
+
+// Main Vertex Shader Entry Point
+[shader("vertex")]
+VertexOutput vertex_main(
+    uint vertex_id : SV_VertexID,
+    uniform float tiled,
+    uniform float4 sizes,
+    uniform float4 positions,
+) {
+    const float2 pos_map[4] = {
+        float2(positions[left], positions[top]),
+        float2(positions[left], positions[bottom]),
+        float2(positions[right], positions[bottom]),
+        float2(positions[right], positions[top])
+    };
+
+
+    VertexOutput output;
+    // Calculate outputs
+    float2 tex_coords = tex_map[vertex_id];
+    output.texcoord = float2(
+        tex_coords[x_axis] * tiling_factor(x_axis, sizes, tiled),
+        tex_coords[y_axis] * tiling_factor(y_axis, sizes, tiled)
+    );
+    output.position = float4(pos_map[vertex_id], 0.0, 1.0);
+    return output;
+}
+
+// Global Texture Bindings
+uniform Sampler2D image;
+
+// Helper Functions
+float4 alpha_blend(float4 color, float4 bg) {
+    // Performs standard linear alpha blending: src * src.a + dst * (1 - src.a)
+    float3 blended_rgb = color.rgb * color.a + bg.rgb * (1.0 - color.a);
+    float blended_alpha = color.a + bg.a * (1.0 - color.a);
+    return float4(blended_rgb, blended_alpha);
+}
+
+// Main Fragment Shader Entry Point
+[shader("fragment")]
+float4 fragment_main(float2 texcoord: TEXCOORD, uniform float4 background) : SV_Target {
+    // Sample the texture using Slang's intrinsic texture object syntax
+    float4 color = image.Sample(texcoord);
+    // Compute final color with alpha blending
+    float4 premult_color = alpha_blend(color, background);
+    return premult_color;
+}
diff --git a/kitty/shaders/slang.py b/kitty/shaders/slang.py
index 2603122f4..2dba04b17 100644
--- a/kitty/shaders/slang.py
+++ b/kitty/shaders/slang.py
@@ -341,8 +341,11 @@ def fixup_opengl_code(glsl_code: str) -> str:
                     in_uniform_block = True
                     in_uniform_block_contents = False
                     line = '// ' + line
-                elif line.startswith('const ') and '] = {' in line:  # }]
-                    line = line.replace('{', f'{words[1]}[](', 1)  # }])
+                elif '] = {' in line:  # }] this is https://github.com/shader-slang/slang/issues/11802
+                    typename = words[0]
+                    if words[0] == 'const':
+                        typename = words[1]
+                    line = line.replace('{', f'{typename}[](', 1)  # }])
                     line = line.removesuffix('};') + ');'
         lines.append(line)
     ans = '\n'.join(lines)