objective-c - cropping UIImage

Question

I have 4 points on an image (not perfectly a rectangle, just a 4 point polygon). For example:

I want to extract the image bounded by the 4 points, resulting image:

Using Quartz, I can only specify a rectangle to do this. However, i might not have a perfect rectangle all the time, so what's the best way to do this? Thanks

Answer 1

What about to use OpenGL "Render to texture" technique? Use those 4 points as input texture coordinates and render straight rectangle to the output texture.