Port the trail shader

kitty/shaders/trail.slang

@@ -0,0 +1,41 @@
+#language slang 2026
+// Copyright (C) 2026 Kovid Goyal <kovid at kovidgoyal.net>
+// Distributed under terms of the GPLv3 license.
+
+struct VertexOutput
+{
+    float2 frag_pos : TEXCOORD;
+    float4 position : SV_Position;
+};
+
+// Main Vertex Shader Entry Point
+[shader("vertex")]
+VertexOutput vertex_main(uint vertex_id : SV_VertexID, uniform float4 x_coords, uniform float4 y_coords) {
+    VertexOutput output;
+    float2 pos = float2(x_coords[vertex_id], y_coords[vertex_id]);
+    output.position = float4(pos, 1.0, 1.0);
+    output.frag_pos = pos;
+    return output;
+}
+
+
+// Main Fragment Shader Entry Point
+[shader("fragment")]
+float4 fragment_main(
+    float2 frag_pos : TEXCOORD,
+    uniform float2 cursor_edge_x,
+    uniform float2 cursor_edge_y,
+    uniform float3 trail_color,
+    uniform float trail_opacity
+) : SV_Target {
+    float opacity = trail_opacity;
+    // Evaluate if the fragment falls inside the bounding box of the cursor
+    float in_x = step(cursor_edge_x[0], frag_pos.x) * step(frag_pos.x, cursor_edge_x[1]);
+    float in_y = step(cursor_edge_y[1], frag_pos.y) * step(frag_pos.y, cursor_edge_y[0]);
+
+    // Mask out opacity inside the active cursor area
+    opacity *= 1.0f - (in_x * in_y);
+
+    // Output color with premultiplied alpha formatting
+    return float4(trail_color * opacity, opacity);
+}