[cairo] A better image comparison tool

[Behdad Esfahbod]

Adib Taraben wrote:
> Hello Antoine,
> 
> in the Inkscape team we had a GSOC project that set-up some rendering 
> tests. Currently there is only binary compare of the images (sounds 
> familiar ?)
> There is also a tool for perceptual image difference 
> http://pdiff.sourceforge.net/
> 
> I think/assume that this tool detects only "visible" differences using 
> hard stuff mathematics ;-) You might look at this tool too.
> 
> 1.) perceptualdiff does not handle transparency in images.

pdiff is what we currently use.  It causes more problem than it solves.  (and
it's painfully slow).

behdad

> 2.) Also what I need is to define some exclude and/or include regions.
>      - Some test images have text that I would not like to take into 
> consideration.
>      - Or I would like to specifically point to that line-caps only.
> 
> Those are my 2ct :-)
> 
> HTH,
> 
> Adib.
> ---
> 
> 
> Antoine Azar wrote:
>> Hey all,
>>
>>  
>>
>> this is something we discussed during the Cairo Summit, and I wanted to 
>> take some feedback and ideas on the list before I start working on it.
>>
>>  
>>
>> The idea is to have a better image comparison tool in the Cairo test 
>> suite instead of simply checking for any pixel discrepancy. Carl pointed 
>> out we currently have several reference image sets to compensate for 
>> differences across backends. Such a tool would ideally eliminate these 
>> multiple images as much as possible. Another advantage of such a tool 
>> would be some integration in the build/test bot machines Mozilla is 
>> setting up.  If a given patch changes the results of a render in a (new) 
>> correct way, instead of seeing a broken test suite, we would see e.g. a 
>> 90% similarity score for the given test.
>>
>>  
>>
>> I am considering a couple approaches. The first one I want to try out is 
>> a pretty simple square sum of differences or sum of absolute 
>> differences. There are more fancy algorithms like cross correlation and 
>> mutual information,  which I'll probably try out too. We can then set 
>> custom thresholds for what should appear as "OK", "Warning" and "Error".
>>
>>  
>>
>> Best,
>>
>> Antoine
>>
>>
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