CGGraphics memory and optimisation issue

Question

Objective

I am working on a utility for cropping UIImages according to an arbitrary aspect ratio with either aspectFit or aspectFill as a crop option. The aspectFill option will crop the original image in such a way that the final image will be full covered by the original image. aspectFit will make sure that no pixel of the original image is cut, and black stripes will be added to the sides of the original image to make it fit to the aspect ratio. I know that there already are 3rd party libraries which do the same job, but I wanted to make this myself as a learning exercise.

My method

• For aspectFill, I am simply calculating the final image offsets and size and cropping the CGRect from the original UIImage.
• For aspectFit, I calculate the final image size and then make a CGContextRef which is filled using CGContextFillRect. To that context, I draw the original image at the required offsets (keeping the original image in the middle of the final image).

Problem

To test this utility, I am using a 2MB image which is approximately the same size as the iPhone camera photos. The following problems cropped up:

• The utility for aspectFill is working as expected, with each image taking around 0.01ms processing time, which is great. The problem is that if I try to run this utility in a loop for a lot of images (10000+) the memory usage spikes until the app crashes. I added the @autoreleasepool block, but it seems to be making no difference.

• aspectFit utility has the opposite problem. The @autoreleasepool block here works as expected and releases the objects periodically so the app does not crash for any number of images in a loop. But here, each image takes processing time around 130ms, which seems to be a lot. I tried using UIGraphicsBeginImageContext instead of CGContextRef, but that was taking even more time.

Code

Code for aspectFill

//ox, oy are the image crop offsets calculated before and fw, fh are the width and height of final image

@autoreleasepool {
    CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], CGRectMake(ox,oy,fw,fh));
    UIImage *finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    return finalImage;
}     

Code for aspectFit

@autoreleasepool {
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
    CGContextRef context = CGBitmapContextCreate(NULL, fw, fh, CGImageGetBitsPerComponent(image.CGImage), 0, colorSpace, CGImageGetBitmapInfo(image.CGImage));
    CGContextSetFillColorWithColor(context, [UIColor blackColor].CGColor);
    CGContextFillRect(context, CGRectMake(0, 0, fw, fh));
    CGRect rect = CGRectMake(ox, oy, image.size.width, image.size.height);
    CGContextDrawImage(context, rect, image.CGImage);
    CGImageRef newCGImage = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    UIImage *finalImage = [UIImage imageWithCGImage:newCGImage];
    CGImageRelease(newCGImage);
    return finalImage;
}

Can someone point to what I am doing wrong, or give me some optimisations for reducing the processing time for this utility? Thanks in advance!

Answer 1

Solution

Finally, after a lot of messing about with CGGraphics and UIGraphics stuff, I found out that the issue was not with the utility at all. The code above was perfect, but the culprit was the way I was loading the UIImage.

I was using

[UIImage imageNamed:@"twomb.jpg"];

to load the image. The imageNamed method caches the image in memory, all 2MB of it. That was eating up the time. On changing the above line to

[UIImage imageWithContentsOfFile:@"twomb.jpg"];

the time and memory use both reduced drastically.