How to cut an image at an angle from the center?

Like this:

I basically want to do some sort of radial cut / trim on a Texture2D. I couldn't find much about it because I don't know the name of the process, if there is a name for that. My knowledge of image processing is close to none, but I'm interested on the algorithm for that. I know this is not strictly Unity related but I didn't know where else to post it.

So what I want to do exactly is: manipulate the pixels of an image, so that I can cut the image at an arbitrary angle from the center, setting the alpha of every pixel between that angle and 180 degrees to zero.