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Rewrite Alpha To Coverage Alpha Clipping Logic
This change rewrites a good deal of the logic within URP's alpha clipping function that's responsible for handling alpha to coverage behavior. In addition to handling the degenerate use case where cutoff is set to zero, this code now also handles the case where alpha and cutoff are equal. It does this by modifying the sharpening function to yield 1.0 rather than 0.5 in the equal case while still maintaining the other parts of the curve.
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‎Packages/com.unity.render-pipelines.universal/ShaderLibrary/ShaderVariablesFunctions.hlsl

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@@ -180,30 +180,40 @@ bool IsAlphaToMaskAvailable()
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return (_AlphaToMaskAvailable != 0.0);
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}
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// Returns a sharpened alpha value for use with alpha to coverage
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// This function behaves correctly in cases where alpha and cutoff are constant values (degenerate usage of alpha clipping)
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half SharpenAlphaStrict(half alpha, half alphaClipTreshold)
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{
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half dAlpha = fwidth(alpha);
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return saturate(((alpha - alphaClipTreshold - (0.5 * dAlpha)) / max(dAlpha, 0.0001)) + 1.0);
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}
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// When AlphaToMask is available: Returns a modified alpha value that should be exported from the shader so it can be combined with the sample mask
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// When AlphaToMask is not available: Terminates the current invocation if the alpha value is below the cutoff and returns the input alpha value otherwise
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half AlphaClip(half alpha, half cutoff)
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{
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// Produce 0.0 if the input value would be clipped by traditional alpha clipping and produce the original input value otherwise.
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// WORKAROUND: The alpha parameter in this ternary expression MUST be converted to a float in order to work around a known HLSL compiler bug.
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// See Fogbugz 934464 for more information
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half clippedAlpha = (alpha >= cutoff) ? float(alpha) : 0.0;
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bool a2c = IsAlphaToMaskAvailable();
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// Calculate a specialized alpha value that should be used when alpha-to-coverage is enabled
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// We explicitly detect cases where the alpha cutoff threshold is zero or below.
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// When this case occurs, we need to modify the alpha to coverage logic to avoid visual artifacts.
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bool zeroCutoff = (cutoff <= 0.0);
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// If the user has specified zero as the cutoff threshold, the expectation is that the shader will function as if alpha-clipping was disabled.
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// Ideally, the user should just turn off the alpha-clipping feature in this case, but in order to make this case work as expected, we force alpha
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// to 1.0 here to ensure that alpha-to-coverage never throws away samples when its active. (This would cause opaque objects to appear transparent)
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half alphaToCoverageAlpha = (cutoff <= 0.0) ? 1.0 : SharpenAlpha(alpha, cutoff);
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half alphaToCoverageAlpha = zeroCutoff ? 1.0 : SharpenAlphaStrict(alpha, cutoff);
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// When the alpha to coverage alpha is used for clipping, we subtract a small value from it to ensure that pixels with zero alpha exit early
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// rather than running the entire shader and then multiplying the sample coverage mask by zero which outputs nothing.
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half clipVal = (a2c && !zeroCutoff) ? (alphaToCoverageAlpha - 0.0001) : (alpha - cutoff);
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// When alpha-to-coverage is available: Use the specialized value which will be exported from the shader and combined with the MSAA coverage mask.
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// When alpha-to-coverage is not available: Use the "clipped" value. A clipped value will always result in thread termination via the clip() logic below.
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alpha = IsAlphaToMaskAvailable() ? alphaToCoverageAlpha : clippedAlpha;
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half outputAlpha = a2c ? alphaToCoverageAlpha : alpha;
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// Terminate any threads that have an alpha value of 0.0 since we know they won't contribute anything to the final image
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clip(alpha - 0.0001);
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clip(clipVal);
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return alpha;
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return outputAlpha;
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}
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#endif
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‎Tests/SRPTests/Projects/UniversalGraphicsTest_Foundation/Assets/Scenes/250_AlphaToCoverage/A0C0.mat

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‎Tests/SRPTests/Projects/UniversalGraphicsTest_Foundation/Assets/Scenes/250_AlphaToCoverage/A0C0.mat.meta

Lines changed: 8 additions & 0 deletions
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‎Tests/SRPTests/Projects/UniversalGraphicsTest_Foundation/Assets/Scenes/250_AlphaToCoverage/A0C1.mat

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‎Tests/SRPTests/Projects/UniversalGraphicsTest_Foundation/Assets/Scenes/250_AlphaToCoverage/A0C1.mat.meta

Lines changed: 8 additions & 0 deletions
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