CGImage/UIImage lazily loading on UI thread causes stutter

Question

My program displays a horizontal scrolling surface tiled with UIImageViews from left to right. Code runs on the UI thread to ensure that newly-visible UIImageViews have a freshly loaded UIImage assigned to them. The loading happens on a background thread.

Everything works almost fine, except there is a stutter as each image becomes visible. At first I thought my background worker was locking something in the UI thread. I spent a lot of time looking at it and eventually realized that the UIImage is doing some extra lazy processing on the UI thread when it first becomes visible. This puzzles me, since my worker thread has explicit code for decompressing JPEG data.

Anyway, on a hunch I wrote some code to render into a temporary graphics context on the background thread and - sure enough, the stutter went away. The UIImage is now being pre-loaded on my worker thread. So far so good.

The issue is that my new "force lazy load of image" method is unreliable. It causes intermittent EXC_BAD_ACCESS. I have no idea what UIImage is actually doing behind the scenes. Perhaps it is decompressing the JPEG data. Anyway, the method is:

+ (void)forceLazyLoadOfImage: (UIImage*)image
{
 CGImageRef imgRef = image.CGImage;

 CGFloat currentWidth = CGImageGetWidth(imgRef);
 CGFloat currentHeight = CGImageGetHeight(imgRef);

    CGRect bounds = CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);

 CGAffineTransform transform = CGAffineTransformIdentity;
 CGFloat scaleRatioX = bounds.size.width / currentWidth;
 CGFloat scaleRatioY = bounds.size.height / currentHeight;

 UIGraphicsBeginImageContext(bounds.size);

 CGContextRef context = UIGraphicsGetCurrentContext();
 CGContextScaleCTM(context, scaleRatioX, -scaleRatioY);
 CGContextTranslateCTM(context, 0, -currentHeight);
 CGContextConcatCTM(context, transform);
 CGContextDrawImage(context, CGRectMake(0, 0, currentWidth, currentHeight), imgRef);

 UIGraphicsEndImageContext();
}

And the EXC_BAD_ACCESS happens on the CGContextDrawImage line. QUESTION 1: Am I allowed to do this on a thread other than the UI thread? QUESTION 2: What is the UIImage actually "pre-loading"? QUESTION 3: What is the official way to solve this problem?

Thanks for reading all that, any advice would be greatly appreciated!

Answer 1

I've had the same stuttering problem, with some help I figured out the proper solution here: Non-lazy image loading in iOS

Two important things to mention:

• Don't use UIKit methods in a worker-thread. Use CoreGraphics instead.
• Even if you have a background thread for loading and decompressing images, you'll still have a little stutter if you use the wrong bitmask for your CGBitmapContext. This are the options you have to choose (it's still a bit unclear to me why):

-

CGBitmapContextCreate(imageBuffer, width, height, 8, width*4, colourSpace,
                          kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);

I've posted a sample project here: SwapTest, it has about the same performace as Apples' Photos app for loading/displaying images.

Answer 2

I used @jasamer's SwapTest UIImage category to force load my large UIImage (about 3000x2100 px) in a worker thread (with NSOperationQueue). This reduces the stutter time when setting the image into the UIImageView to an acceptable value (about 0.5 sec on iPad1).

Here is SwapTest UIImage category... thanks again @jasamer :)

UIImage+ImmediateLoading.h file

@interface UIImage (UIImage_ImmediateLoading)

- (UIImage*)initImmediateLoadWithContentsOfFile:(NSString*)path;
+ (UIImage*)imageImmediateLoadWithContentsOfFile:(NSString*)path;

@end

UIImage+ImmediateLoading.m file

#import "UIImage+ImmediateLoading.h"

@implementation UIImage (UIImage_ImmediateLoading)

+ (UIImage*)imageImmediateLoadWithContentsOfFile:(NSString*)path {
    return [[[UIImage alloc] initImmediateLoadWithContentsOfFile: path] autorelease];
}

- (UIImage*)initImmediateLoadWithContentsOfFile:(NSString*)path {
    UIImage *image = [[UIImage alloc] initWithContentsOfFile:path];
    CGImageRef imageRef = [image CGImage];
    CGRect rect = CGRectMake(0.f, 0.f, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
                                                       rect.size.width,
                                                       rect.size.height,
                                                       CGImageGetBitsPerComponent(imageRef),
                                                       CGImageGetBytesPerRow(imageRef),
                                                       CGImageGetColorSpace(imageRef),
                                                       kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little
                                                       );
    //kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little are the bit flags required so that the main thread doesn't have any conversions to do.

    CGContextDrawImage(bitmapContext, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(bitmapContext);
    UIImage* decompressedImage = [[UIImage alloc] initWithCGImage: decompressedImageRef];
    CGImageRelease(decompressedImageRef);
    CGContextRelease(bitmapContext);
    [image release];

    return decompressedImage;
}

@end

And this is how I create NSOpeationQueue and set the image on main thread...

// Loads low-res UIImage at a given index and start loading a hi-res one in background.
// After finish loading, set the hi-res image into UIImageView. Remember, we need to 
// update UI "on main thread" otherwise its result will be unpredictable.
-(void)loadPageAtIndex:(int)index {
    prevPage = index;

    //load low-res
    imageViewForZoom.image = [images objectAtIndex:index];

    //load hi-res on another thread
    [operationQueue cancelAllOperations];  
    NSInvocationOperation *operation = [NSInvocationOperation alloc];
    filePath = [imagesHD objectAtIndex:index];
    operation = [operation initWithTarget:self selector:@selector(loadHiResImage:) object:[imagesHD objectAtIndex:index]];
    [operationQueue addOperation:operation];
    [operation release];
    operation = nil;
}

// background thread
-(void)loadHiResImage:(NSString*)file {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSLog(@"loading");

    // This doesn't load the image.
    //UIImage *hiRes = [UIImage imageNamed:file];

    // Loads UIImage. There is no UI updating so it should be thread-safe.
    UIImage *hiRes = [[UIImage alloc] initImmediateLoadWithContentsOfFile:[[NSBundle mainBundle] pathForResource:file ofType: nil]];

    [imageViewForZoom performSelectorOnMainThread:@selector(setImage:) withObject:hiRes waitUntilDone:NO];

    [hiRes release];
    NSLog(@"loaded");
    [pool release];
}