alpha blending diagnostic shaders

src/shaders/diagnose_premult_alpha_detector.frag

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+
+in vec2 v_texCoords;
+uniform sampler2D u_texture;
+out vec4 fragColor;
+
+void main() {
+    vec4 c = texture(u_texture, v_texCoords);
+
+    float maxRGB = max(c.r, max(c.g, c.b));
+
+    // Visualise mismatches:
+    // RED = looks like straight alpha
+    // BLUE = looks like premultiplied alpha
+    // GREEN = impossible / corrupted
+
+    if (maxRGB > c.a + 0.001) {
+        // RGB > A → straight alpha
+        fragColor = vec4(1, 0, 0, 1);
+    }
+    else if (maxRGB < c.a - 0.001) {
+        // RGB < A → premultiplied alpha
+        fragColor = vec4(0, 0, 1, 1);
+    }
+    else {
+        // borderline / linear content
+        fragColor = vec4(0, 1, 0, 1);
+    }
+}

src/shaders/diagnose_premult_alpha_detector_highlight.frag

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+
+in vec2 v_texCoords;
+uniform sampler2D u_texture;
+out vec4 fragColor;
+
+// This version highlights pixels that will cause halos
+// Bright red = guaranteed fringe artifact.
+
+void main() {
+    vec4 c = texture(u_texture, v_texCoords);
+    float maxRGB = max(c.r, max(c.g, c.b));
+
+    float diff = maxRGB - c.a;
+
+    // amplify mismatch
+    float halo = clamp(abs(diff) * 10.0, 0.0, 1.0);
+
+    fragColor = vec4(halo, 0, 0, 1); // red glow where halos will appear
+}
+

src/shaders/diagnose_premult_alpha_simulate_wrong_mode.frag

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+
+in vec2 v_texCoords;
+uniform sampler2D u_texture;
+out vec4 fragColor;
+
+// This shows how the sprite *would* look if alpha mode were wrong
+// If you see garbage edges, your texture is premultiplied.
+
+void main() {
+    vec4 c = texture(u_texture, v_texCoords);
+
+    // simulate wrong blending
+    vec3 wrong = c.rgb / max(c.a, 0.001);
+
+    fragColor = vec4(wrong, 1.0);
+}
+
+