#19 imageblock experiment beginnings

Author: wtholliday

Sources/vger/path.metal

@@ -0,0 +1,52 @@
+// Copyright © 2025 Audulus LLC. All rights reserved.
+
+#include <metal_stdlib>
+using namespace metal;
+
+// New experimental tile-buffer based path rendering.
+// Draw paths by flipping in/out using a triangle fan
+// then flipping using bezier regions.
+
+struct GBufferData {
+
+    /// Inside or outside the path?
+    bool sign [[raster_order_group(0)]];
+
+    /// Approx distance to path curve for AA.
+    float distance [[raster_order_group(0)]];
+};
+
+struct GBufferStore {
+    GBufferData data    [[imageblock_data]];
+};
+
+struct VertexOut {
+    float4 position  [[ position ]];
+    float2 uv;
+};
+
+void flip(thread bool& b) {
+    b = !b;
+}
+
+fragment GBufferStore path_fragment(VertexOut in [[ stage_in ]],
+                                    GBufferStore  fragmentValues [[imageblock_data]]) {
+
+    GBufferStore result;
+    result.data.sign = fragmentValues.data.sign;
+    float u = in.uv.x;
+    float v = in.uv.y;
+
+    if (isnan(u)) {
+        // If we're rendering the triangle fan, flip the
+        // whole triangle.
+        flip(result.data.sign);
+    } else {
+        // Flip the sign only for the interior pixels.
+        if(u*u - v < 0) {
+            flip(result.data.sign);
+        }
+    }
+
+    return result;
+}