[magick-users] Identify-ing a JPEG image

[Wolfgang Hugemann]

> I've always read that as:
> 
> For every 2x2 set of Y samples, there's a 1x1 set of Cr and Cb sample(s).
> In other words, just what you'd expected - Cb and Cr are sampled at a rate
> 1/4 (in 2d) of the Y samples.

Having tested various settings of the command line option 
"-sampling-factor", I've found this explanation to be correct:

"2x2, 1x1, 1x1" is IM's standard subsampling method and corresponds to 
4:2:0, see http://en.wikipedia.org/wiki/Chroma_subsampling

BTW: The option
-sampling-factor 4:2:0
does not work correctly in IM; it produces the same output as 4:2:2.

This seems to be a bug in IM.

Wolfgang Hugemann