How can I release memory of UIImages no longer used - ios

Question

I am attempting to merge a number of smaller images into a larger one. The app crashes because it runs out of memory, but I cannot figure out how to release the memory after it is used, so it keeps building up til the app crashes.
The addImageToImage and resizeImage routines appears to be causing the crash since I cannot free up their memory after it is no longer needed. I am using Automatic Reference Counting in this project. I have tried setting the image to nil but that does not stop the crashing.
testImages is in one class that is called from the main ViewController, while addImageToImage and resizeImage are in another class called ImageUtils.
Can someone look at this code and explain to me how to properly release the memory allocated by these two routines. I cannot call release on the images since the project uses ARC and setting them to nil has no effect.
+ (void)testImages
{
const int IMAGE_WIDTH = 394;
const int IMAGE_HEIGHT = 150;
const int PAGE_WIDTH = 1275;
const int PAGE_HEIGHT = 1650;
const int COLUMN_WIDTH = 30;
const int ROW_OFFSET = 75;
CGSize imageSize = CGSizeMake(PAGE_WIDTH, PAGE_HEIGHT);
UIGraphicsBeginImageContextWithOptions(imageSize, YES, 0);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextFillRect(context, CGRectMake(0, 0, imageSize.width, imageSize.height));
UIImage *psheet = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
CGSize collageSize = CGSizeMake(IMAGE_WIDTH, IMAGE_HEIGHT);
UIGraphicsBeginImageContextWithOptions(collageSize, YES, 0);
CGContextRef pcontext = UIGraphicsGetCurrentContext();
CGContextFillRect(pcontext, CGRectMake(0, 0, collageSize.width, collageSize.height));
UIImage *collage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
float row = 1;
float column = 1;
int index = 1;
int group = 1;
for (int i = 0; i < 64; i++)
{
NSLog(#"processing group %i - file %i ", group, index++);
psheet = [ImageUtils addImageToImage:psheet withImage2:collage andRect:CGRectMake((IMAGE_WIDTH*(column-1)) + (COLUMN_WIDTH * column), (IMAGE_HEIGHT * (row-1)) + ROW_OFFSET, IMAGE_WIDTH, IMAGE_HEIGHT) withImageWidth:PAGE_WIDTH withImageHeight:PAGE_HEIGHT];
column++;
if (column > 3) {
column = 1;
row++;
}
if (index == 15)
{
group++;
index = 1;
row = 1;
column = 1;
UIImage *editedImage = [ImageUtils resizeImage:psheet withWidth:PAGE_WIDTH * 2 withHeight:PAGE_HEIGHT * 2];
editedImage = nil;
}
}
}
ImageUtils methods
+(UIImage *) addImageToImage:(UIImage *)sheet withImage2:(UIImage *)label andRect:(CGRect)cropRect withImageWidth:(int) width withImageHeight:(int) height
{
CGSize size = CGSizeMake(width,height);
UIGraphicsBeginImageContext(size);
CGPoint pointImg1 = CGPointMake(0,0);
[sheet drawAtPoint:pointImg1];
CGPoint pointImg2 = cropRect.origin;
[label drawAtPoint: pointImg2];
UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return result;
}
+ (UIImage*)resizeImage:(UIImage*)image withWidth:(CGFloat)width withHeight:(CGFloat)height
{
CGSize newSize = CGSizeMake(width, height);
CGFloat widthRatio = newSize.width/image.size.width;
CGFloat heightRatio = newSize.height/image.size.height;
if(widthRatio > heightRatio)
{
newSize=CGSizeMake(image.size.width*heightRatio,image.size.height*heightRatio);
}
else
{
newSize=CGSizeMake(image.size.width*widthRatio,image.size.height*widthRatio);
}
UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0);
[image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}

Answer 1

Maybe your images are not deallocated but moved to autorelease pool.
Many programs create temporary objects that are autoreleased. These
objects add to the program’s memory footprint until the end of the
block. In many situations, allowing temporary objects to accumulate
until the end of the current event-loop iteration does not result in
excessive overhead; in some situations, however, you may create a
large number of temporary objects that add substantially to memory
footprint and that you want to dispose of more quickly. In these
latter cases, you can create your own autorelease pool block. At the
end of the block, the temporary objects are released, which typically
results in their deallocation thereby reducing the program’s memory
footprint
Try to wrap code inside the loop with #autoreleasepool {} :
for (int i = 0; i < 64; i++)
{
#autoreleasepool {
NSLog(#"processing group %i - file %i ", group, index++);
psheet = [ImageUtils addImageToImage:psheet withImage2:collage andRect:CGRectMake((IMAGE_WIDTH*(column-1)) + (COLUMN_WIDTH * column), (IMAGE_HEIGHT * (row-1)) + ROW_OFFSET, IMAGE_WIDTH, IMAGE_HEIGHT) withImageWidth:PAGE_WIDTH withImageHeight:PAGE_HEIGHT];
column++;
if (column > 3) {
column = 1;
row++;
}
if (index == 15)
{
group++;
index = 1;
row = 1;
column = 1;
UIImage *editedImage = [ImageUtils resizeImage:psheet withWidth:PAGE_WIDTH * 2 withHeight:PAGE_HEIGHT * 2];
editedImage = nil;
}
}
}