AVAssetImageGenerator returns sometimes same image from 2 successive frames

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北海茫月
北海茫月 2021-02-06 07:26

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

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  •  旧巷少年郎
    2021-02-06 08:01

    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...
    }
    

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