Trim transparency of an UIImage

I was wondering what would be the best way to trim the "canvas" of an UIImage (pretty much like any image editor allows out there)

Now, the previous example is not a single UIImage. It's actually 2 UIViews. So clipping the superview against the blue box would do the trick, but I guess I am looking into the best possible way to do this. Given that there could be several blue boxes in the "canvas".

Is there a faster way than going through every pixel?

Thanks!