UIGraphicsImageContext Memory Spike - Reducing Footprint

Question

I have a UIImage Category for iOS Objective-C which tints an image based on a given UIColor value. You can take a look at the method below:

- (UIImage *)tintImageWithTint:(UIColor *)color withIntensity:(float)alpha {
    CGSize size = self.size;

    UIGraphicsBeginImageContextWithOptions(size, NO, [UIScreen mainScreen].scale); // MEMORY SPIKE
    CGContextRef context = UIGraphicsGetCurrentContext();

    [self drawAtPoint:CGPointZero blendMode:kCGBlendModeNormal alpha:1.0]; // MEMORY SPIKE

    CGContextSetFillColorWithColor(context, color.CGColor);
    CGContextSetBlendMode(context, kCGBlendModeOverlay);
    CGContextSetAlpha(context, alpha);

    CGContextFillRect(UIGraphicsGetCurrentContext(), CGRectMake(CGPointZero.x, CGPointZero.y, self.size.width, self.size.height));

    // Get the resized image from the context and a UIImage
    CGImageRef newImageRef = CGBitmapContextCreateImage(context); // MEMORY SPIKE
    UIImage *tintedImage = [UIImage imageWithCGImage:newImageRef];

    CGImageRelease(newImageRef);
    UIGraphicsEndImageContext();

    return tintedImage;
}

I've been testing this method with Xcode's Instruments app and found that it takes up more than 96.0 % of all bytes used during the life cycle of the app. That's crazy! It allocates more than 5 MB very quickly, which causes a level one memory warning and begins to terminate other running processes.

The image that I'm tinting is only a 111 KB, so I can't see how this code could possibly allocate more than 5 MB so quickly.

How can I improve the performance of this code and reduce its memory impact? I'm not very familiar with CoreGraphics, so any help would be appreciated.

Answer 1

your image might have 111 KB compressed as an jpg or a png. But for image processing it needs to be decompressed. It will be loaded into memory with 8 or 16 bit for each color channel. With RGB it would be up to 48 bit per pixel and 8 bit for the alpha cannel in case of png. So 5MB would represent roughly 1.25 million pixel, or something like 1000*1250 pixel.

Answer 2

You could use this approach on iOS 7 devices. I can't see which images you're using, so I don't know if this will be an actual improvement for you, but the new UIView methods are supposedly ultra-performant:

- (UIImage *)tintImageWithTint:(UIColor *)color withIntensity:(float)alpha {
    CGRect frame = CGRectMake(0, 0, self.size.width, self.size.height);
    UIView *view = [[UIView alloc] initWithFrame:frame];

    UIImageView *imageView = [[UIImageView alloc] initWithImage:self];

    UIView *tintView = [[UIView alloc] initWithFrame:frame];
    tintView.backgroundColor = color;
    tintView.alpha = alpha;

    [view addSubview:imageView];
    [view addSubview:tintView];

    UIGraphicsBeginImageContextWithOptions(self.size, NO, [UIScreen mainScreen].scale);

    [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:YES];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}