crop

crop/<width>x<height>+<x>+<y>

Cuts a region out of the image. The offset is measured from the top left.

crop/100x100+10+20     100 by 100, starting 10 across and 20 down
crop/100x100           100 by 100 from the centre
crop/50%x50%+0+0       the top left quarter

A region larger than the image is reduced to fit, so a crop that overhangs an edge keeps the part that is on the image.

A region that lies entirely off the image is refused with 400. An offset of exactly the width or the height leaves nothing, so it is refused the same way. The offset one pixel inside the edge keeps a strip one pixel wide.

Some clients escape a + as %20, which arrives as a space. crop converts a space back to a +, so 100x100%20350%200 works.