How to compare 2 images from command line

DA Forsyth iwrtech at iwr.ru.ac.za
Wed Sep 3 08:34:36 UTC 2008


On 2 Sep 2008 , freebsd-questions-request at freebsd.org entreated about
 "freebsd-questions Digest, Vol 231, Issue 5":

> > Yes, there's a huge difference between testing differences in images and 
> > testing differences in files.  What do you mean by "...know if the 
> > display is working correctly."?
> > 
> > Andrew
> 
> I want to determine if Mplayer is working correctly. The best way to be 
> sure is to check if the display on the screen is changing. That's the 
> purpose of the screenshots. The screenshots are taken by Nagios every 5 
> minutes and the new screenshot is compared with the preceding one. But, 
> by now, it doesn't work because every tool we try is seeing a diffence 
> between 2 identical images.

JPG format will always be different because of the 
encoding/compression.  I think you need to save to an uncompressed 
format like BMP.  Maybe PNG has a mode that will work too.  BMP has 
no info headers, just the raw data, so that won't be affected by the 
time of the save (at the least).


--
       DA Fo rsyth            Network Supervisor
Principal Technical Officer -- Institute for Water Research
http://www.ru.ac.za/institutes/iwr/




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