Macos OSX Lion上AVFoundation屏幕捕获的比特率限制

我正在使用OSX Lion上的AVFoundation进行屏幕捕获。完成如下工作：

    self->screenInput = [[AVCaptureScreenInput alloc] initWithDisplayID:self->screen];
    self->dataOutput = [[AVCaptureVideoDataOutput alloc] init];
    self->session = [[AVCaptureSession alloc] init];
    self->assetWriter = [[AVAssetWriter alloc] initWithURL:url
                                                  fileType:AVFileTypeQuickTimeMovie 
                                                     error:&error];
    self->writerInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo
                                                           outputSettings:nil] retain];
    self->dataOutput.videoSettings=videosettings;

- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
       fromConnection:(AVCaptureConnection *)connection
{

    if(!self->startedWriting)
    {
        [self->assetWriter startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp(sampleBuffer)];
        self->startedWriting = YES;
    }

    if(self->writerInput.readyForMoreMediaData)
    {
        [self->writerInput appendSampleBuffer:sampleBuffer]
    }

}

这导致帧速率约为1 Mbps->3 Mbps。问题在于，在视频设置中，我指定了：

NSMutableDictionary * compressionSettings = [[[NSMutableDictionary alloc] initWithCapacity:1] autorelease];
[compressionSettings setObject:[NSNumber numberWithInt:512000] forKey:AVVideoAverageBitRateKey];
[videosettings setObject:compressionSettings forKey:AVVideoCompressionPropertiesKey];

是用于512K的，比特率较高会导致文件太大（毕竟我们需要上传这些文件）

当我拆下线路时

    self->dataOutput.videoSettings=videosettings;

而是将视频设置应用到writerinput via

self->writerInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo
                                                       outputSettings:videosettings] retain];

我得到的比特率太低（通常为100kbps=>300kbps）。我认为这是因为编码是通过软件而不是硬件进行的（它发生在从

AVCaptureSession

返回数据之后）

我该怎么做才能强制捕获速度从1-3 Mbps下降到512K？如果它能走得更高，我无法想象为什么它不能限制它使用的利率

谢谢

---

-G

来自AVCaptureVideoDataOutput videoSettings属性的文档

Currently, the only supported key is kCVPixelBufferPixelFormatTypeKey. Supported pixel formats are
kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange, kCVPixelFormatType_420YpCbCr8BiPlanarFullRange and kCVPixelFormatType_32BGRA,
except on iPhone 3G, where the supported pixel formats are kCVPixelFormatType_422YpCbCr8 and kCVPixelFormatType_32BGRA.

在这个类上设置压缩设置是没有意义的。这意味着您的AVAssetWriterInput压缩设置为零。因此，您将获得设备的任何默认速率

---

虽然OS-X AVFoundaton实现中肯定存在缺陷，但您收到的比特率可能是正确的。例如，视频中有多少运动？场景有多复杂？还请记住，H264/AVC不是恒定比特率编解码器

“在iPhone 3G上除外”让我相信这是来自iPhone版本AVFoundation的文档。你是对的。即使如此，至少对我来说，将H264编码参数附加到视频输出中也没有多大意义。至少在iOS上，唯一直接进行H264编码的类是AVAssetWriter。