I spent some time between projects today exploring the idea of progressive
image decoding using the VP9 video codec -- sort of a mashup of progressive
JPEG and WebP.
Like a progressive JPEG, each resolution step (a separate frame of the
"video") encodes only the differences from the previous resolution step.
Like WebP, it's more space-efficient than the ancient JPEG codec.
This sort of technique might be useful for lazy-loading images in our
modern internet, where screen densities keep going up and network speeds
can vary by a factor of thousands. On a slow network the user sees
immediate feedback during load, and on a fast network they can reach full
resolution quickly, still in less bandwidth than a JPEG. And since JS would
have control of loading, we can halt cleanly if the image scrolls
offscreen, or pick a maximum resolution based on actual network speed.
Detail notes on my blog:
Sample page if you just want to look at some decoded images at various
resolutions (loading not optimized for slow networks yet!):
It looks plausible, and should be able to use native VP9 decoding in
Firefox, Chrome, and eventually MS Edge in some configurations with a
frames into a single .webm, but to avoid loading unneeded high-resolution
frames they should eventually be in separate files.