iOS graphics

http://www.sunsetlakesoftware.com/

https://github.com/BradLarson/GPUImage

https://github.com/BradLarson/GPUImage2

http://code.tutsplus.com/tutorials/build-a-photo-app-with-gpuimage--mobile-12223

https://www.youtube.com/watch?v=6J_0PrmdD38

https://www.raywenderlich.com/77488/ios-8-metal-tutorial-swift-getting-started