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by JyrkiAlakuijala
809 days ago
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Moritz, the author of that improvement, implemented the same for jpegli. I believe the standard does not specify what the intermediate progressive renderings should look like. I developed that interpolation mechanism originally for Pik, and Moritz was able to formulate it directly in the DCT space so that we don't need to go into pixels for the smoothing to happen, but he computed it using a few of the low frequency DCT coefficients. |
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This is possibly getting too academic, but IIUC for a progressive JPEG, e.g. encoded by cjpeg to have 10 0xDA Start Of Scan markers, it's actually legitimate to post-process the file, truncating to fewer scans (but re-appending the 0xD9 End Of Image marker). The shorter file is still a valid JPEG, and so still relevant for discussing whether all decoders will render the same pixels.
I might be wrong about validity, though. It's been a while since I've studied the JPEG spec.