fix: Handle grayscale data properly when encoding AVIFs #278
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hey! thanks so much for the PRs this week. just calling out that we've seen em and will be taking a look after the thanksgiving holidays. |
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AVIF Grayscale Handling
This change fixes an issue I noticed when testing my other fix for #277. The AVIF encoder currently mishandles single-channel (monochrome) input data, accidentally tiling it 3x horizontally in the resulting image.
For instance, this 300x300 source image:
Would transcode to this output image using the transcoder example program (invoked with
example -input grayscale.png -output grayscale.avif -width 300 -height 300 -stretch):(Converted back to a PNG here for previewing compatibility.)
To fix this, this change skips RGB to YUV conversion for single-channel images during the AVIF encoding process, since they aren't RGB anyway. Instead, for grayscale source data, it sets the AVIF output format to YUV400 (i.e. luminance only with no subsambling), and points its Y plane directly at the grayscale data. This requires no conversions, so it has perfect accuracy, requires no memory allocations, and runs quite a bit faster than before in addition to being more correct. Hooray for a win-win!