Port rounded_rect shader

kitty/shaders/rounded_rect.slang

@@ -0,0 +1,68 @@
+#language slang 2026
+// Copyright (C) 2026 Kovid Goyal <kovid at kovidgoyal.net>
+// Distributed under terms of the GPLv3 license.
+
+import alpha_blend;
+
+#define left 0
+#define top 1
+#define right 2
+#define bottom 3
+
+static const int2 vertex_pos_map[4] = {
+    int2(right, top),
+    int2(right, bottom),
+    int2(left, bottom),
+    int2(left, top)
+};
+
+static const float4 dest_rect = float4(-1, 1, 1, -1);
+
+[shader("vertex")]
+float4 vertex_main(uint vertex_id : SV_VertexID) : SV_Position {
+    int2 pos = vertex_pos_map[vertex_id];
+    return float4(dest_rect[pos.x], dest_rect[pos.y], 0, 1);
+}
+
+// Signed distance function for a rounded rectangle
+float rounded_rectangle_sdf(float2 p, float2 b, float r) {
+    // signed distance field
+    // first term is used for points outside the rectangle
+    float2 q = abs(p) - b;
+    return length(max(q, 0.0)) + min(max(q.x, q.y), 0.0) - r;
+}
+
+[shader("fragment")]
+float4 fragment_main(
+    uniform float4 rect,
+    uniform float2 params,
+    uniform float4 color,
+    uniform float4 background_color,
+    float4 frag_coord : SV_Position // SV_Position provides gl_FragCoord equivalent in Slang
+) : SV_Target {
+    float2 size = rect.zw; // GLSL .ba maps to .zw or .ba in Slang (using .zw is typical)
+    float2 origin = rect.xy;
+    float thickness = params[0];
+    float corner_radius = params[1];
+
+    // Position must be relative to the center of the rectangle of (size) located at (origin)
+    // frag_coord.xy maps directly to gl_FragCoord.xy
+    float2 position = frag_coord.xy - size / 2.0 - origin;
+
+    // Calculate distance to rounded rectangle
+    float dist = rounded_rectangle_sdf(position, size * 0.5 - corner_radius, corner_radius);
+
+    // The border is outer - inner rects
+    float outer_edge = -dist;
+    float inner_edge = outer_edge - thickness;
+
+    // Smooth borders (anti-alias)
+    static const float step_size = 1.0;
+    float alpha = smoothstep(-step_size, step_size, outer_edge) - smoothstep(-step_size, step_size, inner_edge);
+
+    float4 ans = color;
+    ans.a *= alpha;
+
+    // pre-multiplied output
+    return alpha_blend(ans, background_color);
+}