JPEG Quality 80 Is Not a Setting

MP4's boxes are exact about everything: every byte accounted for, every offset written down. JPEG is exact about its structure too, and then hands you one control that has no defined meaning at all. It destroys things. That is the entire point, and it will not tell you how much. Here is one 512x512 PNG, encoded to JPEG four times, on the same machine, with every encoder set to quality 80.…