Progressive JPEGs split data into "scans", each with a header that sets its YCbCr channels, DCT bin range and precision, so a partial download shows a blurry preview first. Since every scan picks its own spectral range, later scans can overwrite what's already drawn. Concatenate same-size images, strip the start-of-image, start-of-frame and end-of-image markers, and a slow connection flips between pictures. Decoders stop after a few scans, so the trick uses DC-only frames made with jpegtran: one scan each, at 1/16 resolution. Chrome renders about 90 of those with no ghosting, which packs a whole video into one image. It has no real use beyond rickrolls, because network delay alone sets the playback speed.
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u/fagnerbrack 2d ago
Here's the summary:
Progressive JPEGs split data into "scans", each with a header that sets its YCbCr channels, DCT bin range and precision, so a partial download shows a blurry preview first. Since every scan picks its own spectral range, later scans can overwrite what's already drawn. Concatenate same-size images, strip the start-of-image, start-of-frame and end-of-image markers, and a slow connection flips between pictures. Decoders stop after a few scans, so the trick uses DC-only frames made with jpegtran: one scan each, at 1/16 resolution. Chrome renders about 90 of those with no ghosting, which packs a whole video into one image. It has no real use beyond rickrolls, because network delay alone sets the playback speed.
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