fixes iOS alpha blending #565
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Review Status
Actionable comments generated: 2
Configuration used: CodeRabbit UI
Files selected for processing (1)
- ios/graphics/Shader/Metal/RasterShader.metal (1 hunks)
Additional comments: 1
ios/graphics/Shader/Metal/RasterShader.metal (1)
- 65-65: The final color output on line 65 multiplies the
rgbvalues bycolor.a, which is standard for alpha blending. However, it is important to ensure that thergbvalues have not been premultiplied by alpha earlier in the shader. If they have been, this operation would incorrectly apply the alpha twice. Please verify that thergbvalues are not premultiplied by alpha before this line.Verification successful
The search for any lines where
rgbis premultiplied bycolor.ain theRasterShader.metalfile did not yield any results. This suggests that there are no instances ofrgbbeing premultiplied by the alpha component in the shader code prior to the line in question.Based on the information available and the results of the script, it appears that the concern raised in the review comment about
rgbbeing premultiplied by alpha before the final multiplication on line 65 is unfounded.
Scripts Executed
The following scripts were executed for the analysis:
Script:
#!/bin/bash # Search for any premultiplication of rgb values by alpha in the shader file. rg --type metal "rgb \*=.*color.a" ios/graphics/Shader/Metal/RasterShader.metalLength of output: 111
Script:
#!/bin/bash # Search for any premultiplication of rgb values by alpha in the shader file without specifying the file type. rg "rgb \*=.*color.a" ios/graphics/Shader/Metal/RasterShader.metalLength of output: 68
| rgb = pow(rgb, (1.0 / (gamma))); | ||
| rgb = mix(brightnessMin, brightnessMax, min(rgb / color.a, float3(1.0))); | ||
| return float4(rgb * color.a, color.a) * styling[0].opacity; |
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The gamma correction logic on line 63 appears to be correct, as it inverses the gamma to adjust the color's luminance. However, the use of min(rgb / color.a, float3(1.0)) on line 64 is potentially problematic. Dividing rgb by color.a could lead to incorrect results when color.a is not equal to 1. This is because the RGB values are being scaled by an amount inversely proportional to the alpha value, which is not a standard approach in alpha blending and could lead to colors being incorrectly brightened when the alpha is low.
A standard approach would be to premultiply the RGB values by the alpha before applying any blending or gamma correction, ensuring that the transparency affects the color intensity appropriately.
- rgb = mix(brightnessMin, brightnessMax, min(rgb / color.a, float3(1.0)));
+ rgb = mix(brightnessMin, brightnessMax, min(rgb, float3(1.0))); // Assuming rgb is already premultiplied by alphaCommittable suggestion
❗ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation.
| rgb = pow(rgb, (1.0 / (gamma))); | |
| rgb = mix(brightnessMin, brightnessMax, min(rgb / color.a, float3(1.0))); | |
| return float4(rgb * color.a, color.a) * styling[0].opacity; | |
| rgb = pow(rgb, (1.0 / (gamma))); | |
| rgb = mix(brightnessMin, brightnessMax, min(rgb, float3(1.0))); // Assuming rgb is already premultiplied by alpha | |
| return float4(rgb * color.a, color.a) * styling[0].opacity; |
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| rgb = pow(rgb, (1.0 / (gamma))); | ||
| rgb = mix(brightnessMin, brightnessMax, min(rgb / color.a, float3(1.0))); | ||
| return float4(rgb * color.a, color.a) * styling[0].opacity; |
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The final color is multiplied by styling[0].opacity again on line 65, which seems redundant since the alpha component color.a is already scaled by styling[0].opacity at the beginning of the function. This could result in the opacity being applied twice, which is likely not the intended effect. Consider removing the redundant multiplication by opacity.
- return float4(rgb * color.a, color.a) * styling[0].opacity;
+ return float4(rgb * color.a, color.a);Committable suggestion
❗ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation.
| return float4(rgb * color.a, color.a) * styling[0].opacity; | |
| return float4(rgb * color.a, color.a); |
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