Regressive JPEGs
Points and comments are a snapshot, not live.
Concatenating JPEG scans can embed multiple frames in one file for progressive playback.
A blog post demonstrates how to create 'regressive' JPEGs by concatenating multiple images and filtering out start-of-image markers. The technique exploits progressive JPEG's multiple scans, each with spectral range headers, to switch between images as the file loads over a slow network. Decoders limit scans (e.g., Chrome stops around 90). Using DC-only scans produces solid 1/16-resolution frames. The author built a video-like image of a black cat walking, noting no timing control exists. Practical applications are limited to trolling, with examples including a pure HTML <dialog> video and an interactive single-page app without CSS or JavaScript.
What commenters are saying
Commenters praised the hack as clever and cursed, with several noting the same technique works for GIF and PNG (via progressive interlacing). One suggested approximating timing by having the server send data in timed chunks (multipart/x-mixed-replace). Another observed that Safari freezes until the image fully downloads. A user pointed out that Motion JPEG (MJPEG) already exists with timing support and is well-supported in browsers. Some discussed the decode-speed penalty of progressive JPEG, while others found the technique useful for steganography or subverting AI analysis.