AVAssetImageGenerator returns sometimes same image from 2 successive frames

Question

I\'m currently extracting every frame from a video with AVAssetImageGenerator, but sometimes it returns me successively 2 times almost the same image (they do not h

Answer 1

Please see the following properties of AVAssetImageGenerator. You should set kCMTimeZero for both properties to get the exact frames.

/* The actual time of the generated images will be within the range [requestedTime-toleranceBefore, requestedTime+toleranceAfter] and may differ from the requested time for efficiency.
   Pass kCMTimeZero for both toleranceBefore and toleranceAfter to request frame-accurate image generation; this may incur additional decoding delay.
   Default is kCMTimePositiveInfinity. */
@property (nonatomic) CMTime requestedTimeToleranceBefore NS_AVAILABLE(10_7, 5_0);
@property (nonatomic) CMTime requestedTimeToleranceAfter NS_AVAILABLE(10_7, 5_0);

Before I set kCMTimeZero for both properties, I got some same images for different request time as you experienced. Just try the following code.

self.imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:myAsset];
self.imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;
self.imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;

Answer 2

I was having the same issue of you, but much more evident, the duplication was happening when the interval between the two frames was under 1.0 second and I realised it was depending on the timescale I was using for generating CMTime values.

Before

CMTime requestTime = CMTimeMakeWithSeconds(imageTime, 1);

After

CMTime requestTime = CMTimeMakeWithSeconds(imageTime, playerItem.asset.duration.timescale);

... and Boom, no more duplication :)

So maybe you can try to increase, double perhaps, the timescale, using your code:

NSValue * time = [NSValue valueWithCMTime:CMTimeMakeWithSeconds(i*frameDuration, composition.frameDuration.timescale*2)]; // *2 at the end

For future references here is my code:

    playerItem = [AVPlayerItem playerItemWithURL:item.movieUrl];
    imageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:playerItem.asset];
    imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
    imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;

    float duration = item.duration;
    float interval = item.interval;

    NSLog(@"\nItem info:\n%f \n%f", duration,interval);

    NSString *srcPath = nil;
    NSString *zipPath = nil;

    srcPath = [item.path stringByAppendingPathComponent:@"info.json"];
    zipPath = [NSString stringWithFormat:@"/%@/info.json",galleryID];

    [zip addFileToZip:srcPath newname:zipPath level:0];

    NSTimeInterval frameNum = item.duration / item.interval;
    for (int i=0; i<=frameNum; i++)
    {
        NSArray* cachePathArray = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
        NSString* cachePath = [cachePathArray lastObject];

        srcPath = [cachePath stringByAppendingPathComponent:@"export-tmp.jpg"];
        zipPath = [NSString stringWithFormat:@"/%@/%d.jpg",galleryID,i];

        float imageTime = ( i * interval );

        NSError *error = nil;
        CMTime requestTime = CMTimeMakeWithSeconds(imageTime, playerItem.asset.duration.timescale);
        CMTime actualTime;

        CGImageRef imageRef = [imageGenerator copyCGImageAtTime:requestTime actualTime:&actualTime error:&error];

        if (error == nil) {
            float req = ((float)requestTime.value/requestTime.timescale);
            float real = ((float)actualTime.value/actualTime.timescale);
            float diff = fabsf(req-real);

            NSLog(@"copyCGImageAtTime: %.2f, %.2f, %f",req,real,diff);
        }
        else
        {
            NSLog(@"copyCGImageAtTime: error: %@",error.localizedDescription);
        }



        // consider using CGImageDestination -> http://stackoverflow.com/questions/1320988/saving-cgimageref-to-a-png-file
        UIImage *img = [UIImage imageWithCGImage:imageRef];
        CGImageRelease(imageRef);  // CGImageRef won't be released by ARC



        [UIImageJPEGRepresentation(img, 100) writeToFile:srcPath atomically:YES];

        if (srcPath != nil && zipPath!= nil)
        {
            [zip addFileToZip:srcPath newname:zipPath level:0]; // 0 = no compression. everything is a jpg image
            unlink([srcPath UTF8String]);
        }

Answer 3

I was using a slightly different way for calculating the CMTime request, and it seemed to work. Here is the code (assuming iOS) :

-(void)extractImagesFromMovie {

// set the asset
    NSString* path = [[NSBundle mainBundle] pathForResource:@"myMovie" ofType:@"MOV"];
    NSURL* movURL = [NSURL fileURLWithPath:path];

NSMutableDictionary* myDict = [NSMutableDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES] , 
                                 AVURLAssetPreferPreciseDurationAndTimingKey , 
                                [NSNumber numberWithInt:0],
                                AVURLAssetReferenceRestrictionsKey, nil];

AVURLAsset* movie = [[AVURLAsset alloc] initWithURL:movURL options:myDict];


// set the generator
AVAssetImageGenerator* generator = [[AVAssetImageGenerator assetImageGeneratorWithAsset:movie] retain];
generator.requestedTimeToleranceBefore = kCMTimeZero;
generator.requestedTimeToleranceAfter = kCMTimeZero;

// look for the video track
AVAssetTrack* videoTrack;
bool foundTrack = NO;

for (AVAssetTrack* track in movie.tracks) {

    if ([track.mediaType isEqualToString:@"vide"]) {
        if (foundTrack) {NSLog (@"Error - - - more than one video tracks"); return(-1);}
        else {
            videoTrack = track;
            foundTrack = YES;
        }
    }
}
if (foundTrack == NO) {NSLog (@"Error - - No Video Tracks at all"); return(-1);}

// set the number of frames in the movie
int frameRate = videoTrack.nominalFrameRate;
float value = movie.duration.value;
float timeScale = movie.duration.timescale;
float totalSeconds = value / timeScale;
int totalFrames = totalSeconds * frameRate;

NSLog (@"total frames %d", totalFrames);

int timeValuePerFrame = movie.duration.timescale / frameRate;

NSMutableArray* allFrames = [[NSMutableArray new] retain];

// get each frame
for (int k=0; k< totalFrames; k++) {

    int timeValue = timeValuePerFrame * k;
    CMTime frameTime;
    frameTime.value = timeValue;
    frameTime.timescale = movie.duration.timescale;
    frameTime.flags = movie.duration.flags;
    frameTime.epoch = movie.duration.epoch;

    CMTime gotTime;

    CGImageRef myRef = [generator copyCGImageAtTime:frameTime actualTime:&gotTime error:nil];
    [allFrames addObject:[UIImage imageWithCGImage:myRef]];

    if (gotTime.value != frameTime.value) NSLog (@"requested %lld got %lld for k %d", frameTime.value, gotTime.value, k)

}

NSLog (@"got %d images in the array", [allFrames count]);
// do something with images here...
}