Add curve filter for applying RGB curves.

[gockxml]

Usage

/filters:curve(<curve of all channels>, <curve of R>, <curve o f G>, <curve of B>)

Format of each curve

[(x1,y1),(x2,y2),...]

Examples of use

Original image

Demo 1

/filters:curve([(0,0),(40,59),(255,255)],[(32,59),(64,80),(92,111),(128,153),(140,169),(164,201),(192,214),(224,215),(240,214),(255,212)],[(34,41),(64,51),(92,76),(128,112),(140,124),(164,147),(192,180),(224,216),(240,236),(255,255)],[(40,46),(64,55),(92,83),(128,127),(140,144),(164,174),(192,197),(224,199),(240,197),(255,198)])/

Demo 2

/filters:curve([(0,0),(255,255)],[(0,50),(16,51),(32,69),(58,85),(92,120),(128,170),(140,186),(167,225),(192,245),(225,255),(244,255),(255,254)],[(0,0),(16,2),(32,18),(64,59),(92,116),(128,182),(167,211),(192,227),(224,240),(244,247),(255,252)],[(0,48),(16,50),(62,77),(92,110),(128,144),(140,153),(167,180),(192,192),(224,217),(244,225),(255,225)])/

[heynemann]

Wow! Very good job! I'll try to have a look at it tomorrow. Maybe @cezarsa can help with reviewing this PR! Thanks a lot for the contribution.

[heynemann]

Do you think you can add some tests for this filter? I'd love to merge it already. Sorry for the long delay.

If you need a sample of filter tests:
https://github.com/thumbor/thumbor/blob/master/vows/rotate_filter_vows.py

[gockxml]

I thought about it, but find it is difficult to do that. The existed filter tests are all about metadata such as rotation, format or file size. Once it is about actual pixels, there seems no good way to test, such as brightness, contrast or watermark. That is the exact situation of this RGB curve filter.

[heynemann]

Sorry @gockxml, I was about to merge your PR, but github says I can't merge until it's rebased. Can you do it? Thanks!

[gockxml]

No problem. Rebased.

[crccheck]

Random guy here, I think there should be a trailing comma here so future changes won't cause a diff on this line.

[gockxml]

My bad. Thank you!

[heynemann]

Sorry but when applying this PR, the build fails due to a test that verifies all the default filters. Can you fix it? The log for the test failure is here:

https://api.travis-ci.org/jobs/46025767/log.txt?deansi=true

And the travis build is here:

https://travis-ci.org/thumbor/thumbor/builds/46025766

Thanks!!!

[gockxml]

@heynemann thank you very much for showing me the complete log. For some weird reason, I couldn't see the whole. I thought the building failure occurred during environment setup. Sorry for my clumsy.

[heynemann]

Thanks a LOT for your contribution.

[heynemann]

Hey @gockxml

Can you please create a wiki page for this plug-in?

Thanks! We'll release a version now!

[gockxml]

Added at https://github.com/thumbor/thumbor/wiki/Filters. Thank you!

[heynemann]

Brilliant!