Regressive JPEGs

6 hours ago

JPEG files can be saved with low frequency components first, allowing partially downloaded images to display at low resolution instead of being cut off.
This is achieved through multiple scans in the file, each with a header; the first scan includes the lowest DC Fourier bin for all three color channels.
Scans define spectral ranges and precision, enabling construction of JPEGs where future scans overwrite already rendered data.
Concatenating multiple images with the same resolution can create a file that switches between images over a slow network, but decoders often limit scans to avoid issues.
To increase frame count, use baseline JPEGs with single scans or DC-only progressive scans, allowing up to 90 frames in some browsers without ghosting issues.
This technique has no practical applications due to lack of timing information, but can be used for unconventional purposes like HTML videos without CSS or JavaScript.

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