使用 AVAssetWriter/AVAssetReader 设置视频帧速率问题

Here is an idea to select frames. Suppose the fps of source video is F and target fps is TF. rate = TF/F

Initiate a variable n equal to -rate and add rate each time, when the integer part of n changed, select the frame.

e.g. rate = 0.3
n: -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1
^              ^           ^
frame index:      0  1   2   3   4   5   6   7
select 0 4 7

float rate = 0.39999f; // TF/F
float n =  -rate; // to make sure first frame will be selected
for (int i = 0; i < 100; ++i, n += rate) { // i stands for frame index, take a video with 100 frames as an example
int m = floor(n);
int tmp = n+rate;
// if rate > 1.0 repeat i
// if rate < 1.0 some of the frames will be dropped
for (int j = 0; m+j < tmp; ++j) {
// Use this frame
printf("%d ", i);
}
}

    NSMutableDictionary *writerInputParams = [[NSMutableDictionary alloc] init];
[writerInputParams setObject:AVVideoCodecTypeH264 forKey:AVVideoCodecKey];
[writerInputParams setObject:[NSNumber numberWithInt:width] forKey:AVVideoWidthKey];
[writerInputParams setObject:[NSNumber numberWithInt:height] forKey:AVVideoHeightKey];
[writerInputParams setObject:AVVideoScalingModeResizeAspectFill forKey:AVVideoScalingModeKey];
NSMutableDictionary * compressionProperties = [[NSMutableDictionary alloc] init];
[compressionProperties setObject:[NSNumber numberWithInt: 20] forKey:AVVideoExpectedSourceFrameRateKey];
[compressionProperties setObject:[NSNumber numberWithInt: 20] forKey:AVVideoAverageNonDroppableFrameRateKey];
[compressionProperties setObject:[NSNumber numberWithInt: 0.0] forKey:AVVideoMaxKeyFrameIntervalDurationKey];
[compressionProperties setObject:[NSNumber numberWithInt: 1] forKey:AVVideoMaxKeyFrameIntervalKey];
[compressionProperties setObject:[NSNumber numberWithBool:YES] forKey:AVVideoAllowFrameReorderingKey];
[compressionProperties setObject:AVVideoProfileLevelH264BaselineAutoLevel forKey:AVVideoProfileLevelKey];
[writerInputParams setObject:compressionProperties forKey:AVVideoCompressionPropertiesKey];


self.assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:writerInputParams];
self.assetWriterInput.expectsMediaDataInRealTime = YES;

It has been verified that SCNView refreshes 60 frames per second, but using AVAssetWriter only wants to save 20 frames per second, what should to do?

Neither AVVideoExpectedSourceFrameRateKey nor AVVideoAverageNonDroppableFrameRateKey above will not affect fps, config fps will not work !!! // Set this to make sure that a functional movie is produced, even if the recording is cut off mid-stream. Only the last second should be lost in that case. self.videoWriter.movieFragmentInterval = CMTimeMakeWithSeconds(1.0, 1000); self.videoWriter.shouldOptimizeForNetworkUse = YES; self.videoWriter.movieTimeScale = 20; The above configuration will not affect fps either.

self.assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:writerInputParams];
self.assetWriterInput.expectsMediaDataInRealTime = YES;
/// this config will change video frame presenttime to fit fps, but it will be change video duration.

// self.assetWriterInput.mediaTimeScale = 20; self.assetWriterInput.mediaTimeScale will affect the fps, but will cause the video duration to be stretched by 3 times, because BOOL isSUc = [self.writerAdaptor appendPixelBuffer:cvBuffer withPresentationTime:presentationTime]; The time of the filled frame will be re-modified, so the self.assetWriterInput.mediaTimeScale value is configured, which is seriously inconsistent with expectations, and the video duration should not be stretched.

So if you want to control the fps of the video that AVAssetWriter finally saves, you must pass the control, and must make sure call 20 per second.

CMTime presentationTime = CMTimeMake(_writeCount * (1.0/20.0) * 1000, 1000);
BOOL isSUc = [self.writerAdaptor appendPixelBuffer:cvBuffer withPresentationTime:presentationTime];
_writeCount += 1;