AVAssetImageGenerator有时会从2个连续的帧中返回相同的图像 [英] AVAssetImageGenerator returns sometimes same image from 2 successive frames

问题描述

我目前正在使用AVAssetImageGenerator从视频中提取每一帧，但有时它会连续两次向我返回几乎相同的图像(它们没有相同的帧时间").有趣的是，它总是每5帧发生一次(在我的测试视频中).

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 have the same "frame time"). The funny thing is it always happen (in my test video) each 5 frames.

此处和

Here and here are the two images (open each in new tab then switch the tabs to see the differences).

这是我的代码:

//setting up generator & compositor
self.generator = [AVAssetImageGenerator assetImageGeneratorWithAsset:asset];
generator.appliesPreferredTrackTransform = YES;
self.composition = [AVVideoComposition videoCompositionWithPropertiesOfAsset:asset];

NSTimeInterval duration = CMTimeGetSeconds(asset.duration);
NSTimeInterval frameDuration = CMTimeGetSeconds(composition.frameDuration);
CGFloat totalFrames = round(duration/frameDuration);

NSMutableArray * times = [NSMutableArray array];
for (int i=0; i<totalFrames; i++) {
    NSValue * time = [NSValue valueWithCMTime:CMTimeMakeWithSeconds(i*frameDuration, composition.frameDuration.timescale)];
    [times addObject:time];
}

AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef im, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error){
    // If actualTime is not equal to requestedTime image is ignored
    if(CMTimeCompare(actualTime, requestedTime) == 0) {
        if (result == AVAssetImageGeneratorSucceeded) {
            NSLog(@"%.02f     %.02f", CMTimeGetSeconds(requestedTime), CMTimeGetSeconds(actualTime));
            // Each log have differents actualTimes.
            // frame extraction is here...
        }
    }
};

generator.requestedTimeToleranceBefore = kCMTimeZero;
generator.requestedTimeToleranceAfter = kCMTimeZero;
[generator generateCGImagesAsynchronouslyForTimes:times completionHandler:handler];

有人知道它可能来自哪里吗?

Any idea where it could come from?

推荐答案

请参阅AVAssetImageGenerator的以下属性. 您应该为这两个属性设置kCMTimeZero以获得准确的帧.

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);

在为这两个属性设置kCMTimeZero之前，您会遇到一些针对不同请求时间的相同图像. 只需尝试以下代码即可.

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;

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