Reconsider the tile size / macro block

[technopagan]

Why are tiles square?

Apparently their minimum size does not take chroma subsampling in consideration, for instance if I have a small 256 by 128 pixels JPEG with 4:2:0 or 4:2:2 chroma subsampling AJC will pick an optimaltilesize of 8, and work on tiles that hold less than an entire native Cb or Cr matrix.

I would suggest to set the smallest tile size to the size of an entire macroblock (or MCU in JPEG speak) based on the current JPEG being optimized, in these cases 8 by 16 and 16 by 16.

(submitted by Frédéric Kayser, [email protected])