Port the border shader

kitty/shaders/border.slang

@@ -0,0 +1,91 @@
+#language slang 2026
+// Copyright (C) 2026 Kovid Goyal <kovid at kovidgoyal.net>
+// Distributed under terms of the GPLv3 license.
+
+import utils;
+
+// Macro definitions
+#define DEFAULT_BG 0
+#define ACTIVE_BORDER_COLOR 1
+#define INACTIVE_BORDER_COLOR 2
+#define WINDOW_BACKGROUND_PLACEHOLDER 3
+#define BELL_BORDER_COLOR 4
+#define TAB_BAR_BG_COLOR 5
+#define TAB_BAR_MARGIN_COLOR 6
+#define TAB_BAR_EDGE_LEFT_COLOR 7
+#define TAB_BAR_EDGE_RIGHT_COLOR 8
+
+uniform float gamma_lut[256];
+uniform uint colors[9];
+
+// Vertex attributes matching GLSL input locations
+struct VertexInput {
+    float4 rect; // left, top, right, bottom
+    uint rect_color;
+};
+
+// Vertex shader output structure
+struct VertexOutput
+{
+    float4 color_premul : COLOR_PREMUL;
+    float4 position : SV_Position;
+};
+
+// Indices into the rect vector
+static const int LEFT = 0;
+static const int TOP = 1;
+static const int RIGHT = 2;
+static const int BOTTOM = 3;
+static const uint FF = 0xff;
+
+static const uint2 pos_map[4] = {
+    uint2(RIGHT, TOP),
+    uint2(RIGHT, BOTTOM),
+    uint2(LEFT, BOTTOM),
+    uint2(LEFT, TOP)
+};
+
+float to_color(uint c) {
+    return gamma_lut[c & FF];
+}
+
+float is_integer_value(uint c, int x) {
+    return 1. - step(0.5, abs(float(c) - float(x)));
+}
+
+float3 as_color_vector(uint c, int shift) {
+    return float3(to_color(c >> shift), to_color(c >> (shift - 8)), to_color(c >> (shift - 16)));
+}
+
+[shader("vertex")]
+VertexOutput vertex_main(float4 rect, uint rect_color, uniform float background_opacity, uint vertex_id : SV_VertexID)
+{
+    VertexOutput output;
+
+    uint2 pos = pos_map[vertex_id];
+    output.position = float4(rect[pos.x], rect[pos.y], 0, 1);
+
+    float3 window_bg = as_color_vector(rect_color, 24);
+    uint rc = rect_color & FF;
+    float3 color3 = as_color_vector(colors[rc], 16);
+
+    float is_window_bg = is_integer_value(rc, WINDOW_BACKGROUND_PLACEHOLDER); // used by window padding areas
+    float is_default_bg = is_integer_value(rc, DEFAULT_BG);
+    color3 = if_one_then(is_window_bg, window_bg, color3);
+
+    // Actual border quads and tab bar edge strips must be always drawn opaque
+    float is_not_a_border = zero_or_one(abs(
+        (float(rc) - ACTIVE_BORDER_COLOR) * (float(rc) - INACTIVE_BORDER_COLOR) * (float(rc) - BELL_BORDER_COLOR) *
+        (float(rc) - TAB_BAR_EDGE_LEFT_COLOR) * (float(rc) - TAB_BAR_EDGE_RIGHT_COLOR)
+    ));
+
+    float final_opacity = if_one_then(is_not_a_border, background_opacity, 1.);
+    output.color_premul = vec4_premul(color3, final_opacity);
+
+    return output;
+}
+
+[shader("fragment")]
+float4 fragment_main(float4 color_premul : COLOR_PREMUL) : SV_Target {
+    return color_premul;
+}