General external comparison tool support

[acdha]

We should probably have a simple configurable way to use external comparison tools like the ImageMagick/GraphicsMagick convert utilities. The current support for perceptualdiff could be replaced with this, which would also provide a way to customize arguments.

[jphalip]

I've posted a suggested fix for this in #19. Any thoughts?

[acdha]

At first glance that looks nice, particularly using a class name so we could start pushing more arcane things out into external plugins. I was thinking about adding one using the Image/GraphicsMagick compare utility & wondering what the threshold should be for splitting diff.py into a directory.

[jphalip]

What arcane things are you suggesting to push out?

Also, are you suggesting to split diff.py into a directory like https://github.com/mariocesar/sorl-thumbnail/tree/master/sorl/thumbnail/engines ?

[acdha]

“arcane things” really just meant that if you want to experiment with something new, you can do so in a separate project rather than having to run from a fork. Since e.g. ImageMagick's compare function accepts a ton of different options, there's a certain argument that it might best be developed externally.

What sorl-thumbnail is doing looks exactly like what I had in mind.

[jphalip]

Yes, I totally agree, using the class path allows anyone to plug whatever engine they want.

I'll rearrange diff.py into a directory to make things a bit more organized.

[jphalip]

I've updated the pull request with the new directory structure. What do you think?

[jphalip]

Quick note that unless there's any objection or further feedback I'll merge this PR tomorrow. I'll then try and add more documentation in preparation for a new release soon as well.

[bfirsh]

Looks really great :)

[acdha]

I overlooked your recent changes. Looks great to me - merge away!

[jphalip]

Great, thanks for the feedback!

[jphalip]

Fixed in b308ac07f27e588420a07dfa6028520d3b459d96