Resize an image to a KB limit

By encoding the real file again and again. Quality is searched first, by halving the interval between the current setting and 0.40, because quality costs no pixels. Only if 0.40 is still too large — and only if “allow fewer pixels” is on — does the picture come down in 5 % steps, each re-rendered through the same kernel, with quality searched again at each step.

Exactly what moved: for example “quality 0.78, pixels unchanged” or “quality 0.60 and ×0.90 (1728 × 1152)”. The byte count of the saved file is read back and printed beside the KB figure, with its SHA-256.

A ceiling is read as 1000 bytes per KB and a floor as 1024, the strict reading at each end, so the file passes whichever one the receiving system uses.

PNG is lossless: it has no quality setting. Only the pixel count can move, which costs far more detail than JPEG compression would. Write JPEG (or WebP) when a size limit is the goal.

It raises quality toward 1.00 to clear a floor. If the file still falls short — a small, plain picture compresses very well — the page says so in figures rather than padding it with bytes that carry no image.