Luma Key (create alpha mask from image) for iOS

Question

I'm building an app that allows people to upload an image of themselves against a white background and the app will create a silhouette of the person.

I'm having a hard time keying out the background. I am using the GPUImage framework and the GPUImageChromaKeyBlendFilter works great for colors, but if you get to white/black it is really hard to key out one of those colors. If I set the key to white or black it keys both the same.

Any advice?

Answer 1

There is a reason why typically blue or green screens are used in movie production for chroma keying, instead of white. Anything can be white or sufficiently close to white in a photo, especially eyes or highlights or just parts of the skin. Also, it is quite hard to find an uniform white wall without shadows, cast by your subject at least. I would recommend building a histogram, finding the most frequently used color among the brightest ones, then search for the biggest area of that color using some threshold. Then do a flood fill from that area until sufficiently different colors are encountered. All of that can be quite easily done in software, unless you want a realtime video stream.