AVVideoAverageBitRateKey未生效

Question

我将AVVideoAverageBitRateKey设置为900000,但我录制的视频获得了不同的比特率值.总是不同的价值,有时850k,有时780k,810k,从不900k.为什么？

这是我做的设置:

    NSNumber *compression = [NSNumber numberWithLong:900000];

    AVAsset *videoAsset = [[AVURLAsset alloc] initWithURL:inputURL options:nil];

    AVAssetTrack *videoTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    CGSize videoSize = videoTrack.naturalSize;
    NSDictionary *videoWriterCompressionSettings =  [NSDictionary dictionaryWithObjectsAndKeys:compression, AVVideoAverageBitRateKey, nil];
    NSDictionary *videoWriterSettings = [NSDictionary dictionaryWithObjectsAndKeys:AVVideoCodecH264,
                                         AVVideoCodecKey, videoWriterCompressionSettings,
                                         AVVideoCompressionPropertiesKey, [NSNumber numberWithFloat:videoSize.width],
                                         AVVideoWidthKey, [NSNumber numberWithFloat:videoSize.height],
                                         AVVideoHeightKey, nil];

    AVAssetWriterInput* videoWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoWriterSettings];

    videoWriterInput.expectsMediaDataInRealTime = YES;

    videoWriterInput.transform = videoTrack.preferredTransform;

    AVAssetWriter *videoWriter = [[AVAssetWriter alloc] initWithURL:outputURL fileType:AVFileTypeQuickTimeMovie error:nil];

    [videoWriter addInput:videoWriterInput];

Answer 1

还没有人回答这个问题，但复活了。比特率是一个复杂的值，与视频方程式中的各种其他变量成正比——与帧率、分辨率和其他因素有关。使用的编解码器。恒定比特率还是可变比特率？等等。其中一些因素本身会产生限制影响，这些影响会传递到最终观察到的平均比特率。请注意，您用来指定比特率的键称为AVVideoAverageBitRateKey平均值，而不是固定值。请求比特率就像请求一个大概数字，结果应该接近您所请求的值，但可能不完全相同。