Question

UPDATE This piece of code is actually not where the problem is; commenting out all the CoreGraphics lines and returning the first image in the array as the result does not prevent the crashes from happening, so I must look farther upstream.

I am running this on a 75ms NSTimer. It works perfectly with 480×360 images, and will run all day long without crashing.

But when I send it images that are 1024×768, it will crash after about 20 seconds, having given several low memory warnings.

In both cases Instruments shows absolutely normal memory usage: a flat allocations graph, less than one megabyte of live bytes, no leaks the whole time.

So, what’s going on? Is Core Graphics somehow using too much memory without showing it?

Also worth mentioning: there aren’t that many images in (NSMutableArray*)imgs — usually three, sometimes two or four. Crashes regardless. Crashes slightly less soon when there are only two.

- (UIImage*) imagefromImages:(NSMutableArray*)imgs andFilterName:(NSString*)filterName {

    UIImage *tmpResultant = [imgs objectAtIndex:0];

    CGSize s = [tmpResultant size];

    UIGraphicsBeginImageContext(s);

    [tmpResultant drawInRect:CGRectMake(0, 0, s.width, s.height) blendMode:kCGBlendModeNormal alpha:1.0];

    for (int i=1; i<[imgs count]; i++) { [[imgs objectAtIndex:i] drawInRect:CGRectMake(0, 0, s.width, s.height) blendMode:kCGBlendModeMultiply alpha:1.0]; }

    tmpResultant =  UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return tmpResultant;
}

Answer 1

Sounds to me like the problem is outside of the code you have shown. Images that are displayed on screen have a backing store outside of your app’s memory that is width*height*bytes_per_pixel. You also get memory warnings and app termination if you have too many backing stores.

You might need to optimize there, to either create smaller optimized versions of these images for display or to allow for the backing stores to be released. Also turning on rasterization for certain non-changing layers can help here as well as setting layer contents directly to the CGImage as opposed to working with UIImages.

You should make a sample project that demonstrates the issue that has no other code around it and see if you still run out of memory. But as I suspect you’ll find that just with the code you have shown you will not be able to reproduce the isse as it lies elsewhere.