mask image via another image

Alright what I am trying to do is:

• given an image where there 开发者_开发百科is a circle within that image that is "blank". I want to take an existing image from user library and then mask it so that only a certain part of that image is shown on the "blank" image..

I have tried a few masking code but they all seem to work the other way around ... any tips on how to tackle this?

Unfortunately you can't use CoreAnimation to do this (which would make it rather easy). Looking at Apple's CoreAnimation documentation:

iOS Note: As a performance consideration, iOS does not support the mask property.

Therefore the next best way to do this is to use Quartz 2D (as answered here):

CGContextRef mainViewContentContext;
CGColorSpaceRef colorSpace;

colorSpace = CGColorSpaceCreateDeviceRGB();

// create a bitmap graphics context the size of the image
mainViewContentContext = CGBitmapContextCreate (NULL, targetSize.width, targetSize.height, 8, 0, colorSpace, kCGImageAlphaPremultipliedLast);

// free the rgb colorspace
CGColorSpaceRelease(colorSpace);    

if (mainViewContentContext==NULL)
    return NULL;

CGImageRef maskImage = [[UIImage imageNamed:@"mask.png"] CGImage];
CGContextClipToMask(mainViewContentContext, CGRectMake(0, 0, targetSize.width, targetSize.height), maskImage);
CGContextDrawImage(mainViewContentContext, CGRectMake(thumbnailPoint.x, thumbnailPoint.y, scaledWidth, scaledHeight), self.CGImage);


// Create CGImageRef of the main view bitmap content, and then
// release that bitmap context
CGImageRef mainViewContentBitmapContext = CGBitmapContextCreateImage(mainViewContentContext);
CGContextRelease(mainViewContentContext);

// convert the finished resized image to a UIImage 
UIImage *theImage = [UIImage imageWithCGImage:mainViewContentBitmapContext];
// image is retained by the property setting above, so we can 
// release the original
CGImageRelease(mainViewContentBitmapContext);

// return the image
return theImage;