Should consider IMCU size

[jepler]

Should consider IMCU size

[za3k]

en.wikipedia.org/wiki/JPEG. It says "After subsampling, each channel must be split into 8×8 blocks. Depending on chroma subsampling, this yields (Minimum Coded Unit) MCU blocks of size 8×8 (4:4:4 – no subsampling), 16×8 (4:2:2), or most commonly 16×16 (4:2:0)." (via http://www.imagemagick.org/discourse-server/viewtopic.php?t=23162)

I'm having a bit of a time figuring out whether "iMCU" and "MCU" are different, but I can't get ahold of a copy of the JPEG standard, which would make this a lot easier.

Here's how you check that in imagemagick: http://www.imagemagick.org/discourse-server/viewtopic.php?t=17145
I don't know how with your libraries.

[xiota]

Is there any real need for this? jpegtran already considers iMCU when it does the actual cropping. Perhaps this issue can be closed?

[onno74]

True, jpegtran considers iMCU. However, when iMCU size is 16, this can give a result that differs by 8 pixels from what I see in cropgui.
When I want to cut out an image with sharp borders (such as a scan of a photograph, where the scanned region is a little larger than the photograph) 8 pixels of the border might remain in the image. The result is very ugly.
Also, when cropping photos of text on signs, small differences between preview in cropgui and final result by jpegtran can be annoying.