在iPhone 4 / 4S和iPad 2上生成5400万像素的图像

Question

I'm currently working on a project that must generate a collage of a 9000x6000 pixels resolution, generated from 15 photos . The problem that I'm facing is that when I finish drawing I'm getting an empty image (those 15 images are not being drawn in the context).

This problem is only present on devices with 512MB of RAM like iPhone 4/4S or iPad 2 and I think that this is a problem caused by the system because it cannot allocate enough memory for this app. When I run this line: UIGraphicsBeginImageContextWithOptions(outputSize, opaque, 1.0f); the app's memory usage raises by 216MB and the total memory usage gets to ~240MB RAM .

The thing that I cannot understand is why on Earth the images that I'm trying to draw within the for loop are not being rendered always on the currentContext ? I emphasized the word always , because only once in 30 tests the images were rendered (without changing any line of code).

Question nr. 2 : If this is a problem caused by the system because it cannot allocate enough memory, is there any other way to generate this image, like a CGContextRef backed by a file output stream, so that it won't keep the image in the memory?

This is the code :

CGSize outputSize = CGSizeMake(9000, 6000);
BOOL opaque = YES;
UIGraphicsBeginImageContextWithOptions(outputSize, opaque, 1.0f);

CGContextRef currentContext = UIGraphicsGetCurrentContext();

CGContextSetFillColorWithColor(currentContext, [UIColor blackColor].CGColor);
CGContextFillRect(currentContext, CGRectMake(0, 0, outputSize.width, outputSize.height));

for (NSUInteger i = 0; i < strongSelf.images.count; i++)
{
    @autoreleasepool
    {
        AGAutoCollageImageData *imageData = (AGAutoCollageImageData *)strongSelf.layout.images[i];

        CGRect destinationRect = CGRectMake(floorf(imageData.destinationRectangle.origin.x * scaleXRatio),
                                            floorf(imageData.destinationRectangle.origin.y * scaleYRatio),
                                            floorf(imageData.destinationRectangle.size.width * scaleXRatio),
                                            floorf(imageData.destinationRectangle.size.height * scaleYRatio));

        CGRect sourceRect = imageData.sourceRectangle;

        // Draw clipped image
        CGImageRef clippedImageRef = CGImageCreateWithImageInRect(((ALAsset *)strongSelf.images[i]).defaultRepresentation.fullScreenImage, sourceRect);
        CGContextDrawImage(currentContext, drawRect, clippedImageRef);
        CGImageRelease(clippedImageRef);
    }
}

// Pull the image from our context
strongSelf.result = UIGraphicsGetImageFromCurrentImageContext();

// Pop the context
UIGraphicsEndImageContext();

PS: The console doesn't show anything but ' memory warnings ', which are expected to see.

Answer 1

Sound like a cool project.

Tactic : try also releasing imageData at the end of every loop (explicitly, after releasing the clippedImageRef )

Strategic : If you do need to support such "low" RAM requirements with such "high" input, maybe you should consider 2 alternative options:

1. Compress (obviously): even minimal, naked to the eye, JPEG compression can go a long way.
2. Split: never "really" merge the image. Have an arrayed datastructure which represents a BigImage . make utilities for the presentation logic.