Mask a UIImage that has been moved/scaled - ios

Question

I can accomplish masking like this:
CGAffineTransform t = self.imageForEditing.transform;
NSLog(#"x offset: %f, y offset: %f", self.imageForEditing.frame.origin.x, self.imageForEditing.frame.origin.y);
NSLog(#"scale: %f", sqrt(t.a * t.a + t.c * t.c));
UIImage *maskImage = [UIImage imageNamed:#"faceMask"];
UIImage *colorsImage = [self.imageForEditing.image imageRotatedByDegrees:180];
CGImageRef maskRef = maskImage.CGImage;
CGImageRef imageMask = CGImageMaskCreate(CGImageGetWidth(maskRef),
CGImageGetHeight(maskRef),
CGImageGetBitsPerComponent(maskRef),
CGImageGetBitsPerPixel(maskRef),
CGImageGetBytesPerRow(maskRef),
CGImageGetDataProvider(maskRef), NULL, false);
CGImageRef maskedRef = CGImageCreateWithMask(colorsImage.CGImage, imageMask);
UIImage *maskedImage = [UIImage imageWithCGImage:maskedRef];
You can see in the log statements that I can access how much the original imaged has been moved and/or scaled. How can I apply this information so that the new masked image takes it into account? Imagine the mask is just a circular cutout in the middle of the screen. self.imageForEditing (a UIImageView) can be moved around. When the new image is created, it should just be the part that is visible through the cutout. The code above works but doesn't take the moving/scaling of the underlying image into account.
EDIT: I think it might be easier to just create a new image based on the current state of the image view (self.imageForEditing). It lies within a container, self.editContainer. So how can I just create a new image based on the current pixels contained in self.editContainer?

Answer 1

Finally solved this by rendering the container view as an image, then using a UIImage Category I found to trim the transparent pixels. In case it helps someone:
on the container uiview:
UIGraphicsBeginImageContextWithOptions(self.frame.size, NO, 0.0);
[self.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;