Load each frame in separate threads about olimageview

[koudelka]

I toyed around with the implementation below, using a dispatch group, it worked really well up until I hit the problem of collecting the frames in-order. I tried using NSMutableArray's insertObject:atIndex:, but it has some really weird behavior when you insert out of order (which I think would be necessary for multi-threaded operation).

+ (id)imageWithData:(NSData *)data
{
    CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)(data), NULL);
    if (!imageSource) {
        return nil;
    }
    NSUInteger numberOfFrames = CGImageSourceGetCount(imageSource);
    if (!UTTypeConformsTo(CGImageSourceGetType(imageSource), kUTTypeGIF) || numberOfFrames == 1) {
        CFRelease(imageSource);
        return [UIImage imageWithData:data];
    }

    NSDictionary *imageProperties = CFBridgingRelease(CGImageSourceCopyProperties(imageSource, NULL));
    NSDictionary *GIFProperties = [imageProperties objectForKey:(NSString *)kCGImagePropertyGIFDictionary];

    OLImage *animatedImage = [[OLImage  alloc] init];
    animatedImage.images = [NSMutableArray arrayWithCapacity:numberOfFrames];
    animatedImage.frameDurations = (NSTimeInterval *) malloc(numberOfFrames  * sizeof(NSTimeInterval));
    animatedImage.totalDuration = 0.0f;
    animatedImage.loopCount = [GIFProperties[(NSString *)kCGImagePropertyGIFLoopCount] unsignedIntegerValue];

    dispatch_group_t dispatch_group = dispatch_group_create();

    for (NSUInteger i = 0; i < numberOfFrames; ++i)
        dispatch_group_async(dispatch_group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //Implement as Browsers do, to ensure UX.
            //See: http://nullsleep.tumblr.com/post/16524517190/animated-gif-minimum-frame-delay-browser-compatibility
            //See also: http://blogs.msdn.com/b/ieinternals/archive/2010/06/08/animated-gifs-slow-down-to-under-20-frames-per-second.aspx
            double proposedFrameDuration = CGImageSourceGetGifFrameDelay(imageSource, i);
#ifndef OLExactGIFRepresentation
            proposedFrameDuration = (proposedFrameDuration >= 0.02) ? proposedFrameDuration : 0.10f;
#endif
            animatedImage.frameDurations[i] = proposedFrameDuration;
            CGImageRef theImage = CGImageSourceCreateImageAtIndex(imageSource, i, NULL);
            [animatedImage.images insertObject:[UIImage imageWithCGImage:theImage] atIndex:i];
            CFRelease(theImage);
            animatedImage.totalDuration += animatedImage.frameDurations[i];
      });

    dispatch_group_wait(dispatch_group, DISPATCH_TIME_FOREVER);

    CFRelease(imageSource);
    return animatedImage;
}

Using just addObject: (and not caring about frame ordering), I was seeing about a 10% speedup, but it varied quite a bit. I suspect that the amount of work per dispatch is low enough that the overhead eats into the gains. A better solution might be to dispatch a certain percentage of frames instead (but that still leaves the ordering problem).

[koudelka]

So I've fixed the weirdness with insertObject:atIndex: by sticking a null in its place and removing it later. But this solution is slower that the single-threaded one. :(

+ (id)imageWithData:(NSData *)data
{
    CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)(data), NULL);
    if (!imageSource) {
        return nil;
    }
    NSUInteger numberOfFrames = CGImageSourceGetCount(imageSource);
    if (!UTTypeConformsTo(CGImageSourceGetType(imageSource), kUTTypeGIF) || numberOfFrames == 1) {
        CFRelease(imageSource);
        return [UIImage imageWithData:data];
    }

    NSDictionary *imageProperties = CFBridgingRelease(CGImageSourceCopyProperties(imageSource, NULL));
    NSDictionary *GIFProperties = [imageProperties objectForKey:(NSString *)kCGImagePropertyGIFDictionary];

    OLImage *animatedImage = [[OLImage  alloc] init];
    animatedImage.images = [NSMutableArray arrayWithCapacity:numberOfFrames];
    animatedImage.frameDurations = (NSTimeInterval *) malloc(numberOfFrames  * sizeof(NSTimeInterval));
    animatedImage.totalDuration = 0.0f;
    animatedImage.loopCount = [GIFProperties[(NSString *)kCGImagePropertyGIFLoopCount] unsignedIntegerValue];

    dispatch_group_t dispatch_group = dispatch_group_create();

    for (NSUInteger i = 0; i < numberOfFrames; ++i)
        [animatedImage.images addObject:[NSNull null]];

    for (NSUInteger i = 0; i < numberOfFrames; ++i)
        dispatch_group_async(dispatch_group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //Implement as Browsers do, to ensure UX.
            //See: http://nullsleep.tumblr.com/post/16524517190/animated-gif-minimum-frame-delay-browser-compatibility
            //See also: http://blogs.msdn.com/b/ieinternals/archive/2010/06/08/animated-gifs-slow-down-to-under-20-frames-per-second.aspx
            double proposedFrameDuration = CGImageSourceGetGifFrameDelay(imageSource, i);
#ifndef OLExactGIFRepresentation
            proposedFrameDuration = (proposedFrameDuration >= 0.02) ? proposedFrameDuration : 0.10f;
#endif
            animatedImage.frameDurations[i] = proposedFrameDuration;
            CGImageRef theImage = CGImageSourceCreateImageAtIndex(imageSource, i, NULL);
            [animatedImage.images replaceObjectAtIndex:i withObject:[UIImage imageWithCGImage:theImage]];
            CFRelease(theImage);
            animatedImage.totalDuration += animatedImage.frameDurations[i];
      });

    dispatch_group_wait(dispatch_group, DISPATCH_TIME_FOREVER);

    CFRelease(imageSource);
    return animatedImage;
}

[dtorres]

Hey Michael,
Awesome that you took a look at this but is a hard for me to identify your improvements if you post your changes here.

Can you create a fork a commit the changes there so it's easier for me to read it and help you on it
And also it will be easier to merge the changes after 👍

[koudelka]

Hey Diego,

You can check out the changes on my fork, in the "multithreaded" branch: https://github.com/koudelka/OLImageView/commits/multithreaded

I didn't submit a pull request as it looks like my solution is actually slower than the singlethread solution. The dispatch overhead seems to be greater than the savings of having multiple threads decode the frames. :(

[dtorres]

Since multithreading seems slower than the current solution, i'm closing this for now.
Would be nice to reevaluate in the future