I wanted to easily blend a UIImage on top of another background image, so wrote a category method for UIImage, adapted from http://stackoverflow.com/questions/1309757/blend-two-uiimages :

- (UIImage *) blendedImageOn:(UIImage *) backgroundImage  {
 NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
 UIGraphicsBeginImageContext(backgroundImage.size);

 CGRect rect = CGRectMake(0, 0, backgroundImage.size.width, backgroundImage.size.height);
 [backgroundImage drawInRect:rect];
 [self drawInRect:rect];
 UIImage* blendedImage = UIGraphicsGetImageFromCurrentImageContext();

 UIGraphicsEndImageContext();
 [pool release];

 return blendedImage;
}

Unfortunately my app that uses the above method to load around 20 images and blend them with background and gloss images (so probably around 40 calls), is being jettisoned on the device.

An Instruments session revealed that calls to malloc stemming from the calls to drawInRect: are responsible for the bulk of the memory usage. I tried replacing the drawInRect: messages with equivalent function calls to the function CGContextDrawImage but it didn't help. The AutoReleasePool was added after I found the memory usage problem; it also didn't make a difference.

I'm thinking this is probably because I'm not using graphics contexts appropriately. Would calling the above method in a loop be a bad idea because of the number of contexts I create? Or did I simply miss something?