Avoid hard-coded magi constant.

OSBindings/Mac/Clock Signal/ScanTarget/CSScanTarget.mm

@@ -129,6 +129,7 @@ struct Uniforms {
 	__fp16 outputAlpha;
 	__fp16 outputGamma;
 	__fp16 outputMultiplier;
+	__fp16 weightedMixAlpha;
 };
 
 // Kernel sizes above and in the shaders themselves assume a maximum filter kernel size.
@@ -273,6 +274,7 @@ using BufferingScanTarget = Outputs::Display::BufferingScanTarget;
 - (void)setAlpha {
 	static constexpr float alpha = 0.64f;
 	self.uniforms->outputAlpha = __fp16(std::sqrt(alpha));
+	self.uniforms->weightedMixAlpha = __fp16(alpha);
 }
 
 - (nonnull instancetype)initWithView:(nonnull MTKView *)view {
@@ -931,6 +933,7 @@ using BufferingScanTarget = Outputs::Display::BufferingScanTarget;
 		[encoder setFragmentTexture:_frameBuffers[_fieldIndex ^ 1] atIndex:0];
 		[encoder setVertexTexture:_frameBuffers[_fieldIndex] atIndex:1];
 		[encoder setFragmentTexture:_frameBuffers[_fieldIndex] atIndex:1];
+		[encoder setFragmentBuffer:_uniformsBuffer offset:0 atIndex:0];
 
 		[encoder drawPrimitives:MTLPrimitiveTypeTriangleStrip vertexStart:0 vertexCount:4];
 		[encoder endEncoding];

OSBindings/Mac/Clock Signal/ScanTarget/Shaders/CopiesAndFills.metal

@@ -69,6 +69,7 @@ fragment half4 interpolateEqualMixFragment(
 
 fragment half4 weightedMixFragment(
 	const CopyInterpolator vert [[stage_in]],
+	const constant Uniforms &uniforms [[buffer(0)]],
 	const metal::texture2d<half> texture1 [[texture(0)]],
 	const metal::texture2d<half> texture2 [[texture(1)]]
 ) {
@@ -76,12 +77,13 @@ fragment half4 weightedMixFragment(
 		metal::mix(
 			texture1.sample(nearestSampler, vert.textureCoordinates),
 			texture2.sample(nearestSampler, vert.textureCoordinates),
-			0.64
+			uniforms.weightedMixAlpha
 		);
 }
 
 fragment half4 interpolateWeightedMixFragment(
 	const CopyInterpolator vert [[stage_in]],
+	const constant Uniforms &uniforms [[buffer(0)]],
 	const metal::texture2d<half> texture1 [[texture(0)]],
 	const metal::texture2d<half> texture2 [[texture(1)]]
 ) {
@@ -89,7 +91,7 @@ fragment half4 interpolateWeightedMixFragment(
 		metal::mix(
 			texture1.sample(linearSampler, vert.textureCoordinates),
 			texture2.sample(linearSampler, vert.textureCoordinates),
-			0.64
+			uniforms.weightedMixAlpha
 		);
 }
 

OSBindings/Mac/Clock Signal/ScanTarget/Shaders/uniforms.hpp

@@ -44,4 +44,7 @@ struct Uniforms {
 
 	// Sets a brightness multiplier for output colours.
 	half outputMultiplier;
+
+	// Sets the opacity used by the weighted-mix copy shaders.
+	half weightedMixAlpha;
 };