Replace all white/nearly white pixels in a UIImage with alpha using CGImage?

Question

I have a UIImage with white background. I would like replace the white background/pixels with alpha-transparent pixels. I've looked at other questions on StackOverflow, along with Quartz documentation, but have yet to find a coherent "start-to-end" for this problem. How is this done?

Answer 1

CGImageCreateWithMaskingColors

A UIImage wraps a CGImage. Take the CGImage, run it through CGImageCreateWithMaskingColors, then either create a new UIImage from the result or assign the result back to the UIImage.

Answer 2

The first step is you need to define some sort of "distance" function to determine how far away a pixel is from being white. Then you need to define a distance threshold below which a pixel is considered white. Then you would need to iterate over the pixels of the image, changing any pixels that were considered white according to your distance and threshold, to being transparent. The main trick, though, is making this efficient... touching pixels through functions will be very slow; your best bet is to touch the pixels directl by gaining access to the memory buffer in which the pixels reside and stepping through them.