how to i understand the compare -metric

[magick]

The compare program returns a single value, a measure of error between the original and reconstructed image. You can get additional statistics for the original or recontructed image with the identify program:

• identify -verbose img1.bmp

If that is not sufficient, write custom code to compute the values you are interested in.

[anthony]

First use "null:" not just "null" that last saves a miff format file into the filename "null". The former saves NOTHING.

A rough sumary of some image compares are in IM Examples
http://www.cit.gu.edu.au/~anthony/graph ... e/#compare

if you like to revise or contribute more, please do.