Don't use Identity matrix coefficients with 4:0:0 #1482
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Thx for the explanation in #1460 (comment) ! |
Chrome doesn't render 4:0:0 with Identity matrix coefficients correctly. Work around this Chrome issue by using matrix coefficients 6 instead with lossless 4:0:0. Fix AOMediaCodec#1481.
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Please review the two commits together rather than separately.
Vincent approved the first commit in this pull request before it was ready for review.
I tested this pull request with a grayscale PNG input and a monochrome y4m input, and added the second commit.
| (input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444)) { | ||
| // User explicitly asked for non YUV444 yuvFormat, while matrixCoefficients was likely | ||
| // set to AVIF_MATRIX_COEFFICIENTS_IDENTITY as a side effect of --lossless, | ||
| // and Identity is only valid with YUV444. Set matrixCoefficients back to the default. |
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Only this change and the new if statement at line 1680 are needed to work around the Chrome issue.
| @@ -1663,8 +1661,7 @@ int main(int argc, char * argv[]) | |||
| } | |||
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| // Check again for y4m input (y4m input ignores input.requestedFormat and retains the format in file). | |||
| if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444) && | |||
| (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV400)) { | |||
| if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) { | |||
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This change satisfies the following requirement in the AV1 spec (on page 119):
If matrix_coefficients is equal to MC_IDENTITY, it is a requirement of bitstream conformance that subsampling_x is equal to 0 and subsampling_y is equal to 0.
In AV1, subsampling_x == 0 && subsampling_y == 0 is equivalent to YUV 4:4:4.
| if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444) && | ||
| (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV400)) { | ||
| // Check again for -y auto or for y4m input (y4m input ignores input.requestedFormat and | ||
| // retains the format in file). |
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The original comment may have been written before -y auto was supported. A y4m input is not the only reason image->yuvFormat may be different from input.requestedFormat now. So I updated this comment.
Chrome doesn't render 4:0:0 with Identity matrix coefficients correctly. Work around this Chrome issue by using matrix coefficients 6 instead with 4:0:0.
Note: On the decoder side, avifImageYUVToRGB() accepts 4:0:0 with Identity matrix coefficients.
Fix #1481.