Iphone core graphic, mask

I have a problem with the core graphic of iPhone. Below is a drawing which demonstrate this problem:

There is a mask with a human shape, I'm put the mask upon an image, I'm doing "something" (this is the part I need help with), I remove the mas开发者_StackOverflow中文版k, and finally the image became human-shape transparent.

finally I found the solution from the apple sample code called QuartzDemo.