[rer0tsaz]

Another thing that can produce out-of-gamut colors in JPEG is chroma subsampling. http://www.glennchan.info/articles/technical/chroma/chroma1.... has a good overview.

Clipping is required by JFIF, so you can't implement spilling in the decoder by default, even if you assume the source was 0-255 RGB. So implementing this in the encoder like the article does is best. but I have mixed feelings about it since it's most helpful for images that shouldn't be saved as JPEG at all.

[tedunangst]

There are a lot of photographic jpegs that have black and white text too. Perhaps a caption. Perhaps a road sign in the scene. Reducing block noise seems better than saving such images as bajillion byte pngs.