如何准确计算iOS 8上硬件解码的FPS？

Question

这里有来自iOS8的新的硬编码方法，

我们可以用"VTDecompressionSessionDecodeFrame“从iOS8中解码h264，

我试着编写一个程序来打印硬件解码的fps，

但是这里有一个问题，回调是异步的，

那么，如何准确地计算fp呢？

我找到了一种方法"VTDecompressionSessionWaitForAsynchronousFrames“

是我想要的吗？

解码函数

- (void)render:(CMSampleBufferRef)sampleBuffer
{
    if (_isDecoding == NO) {

        _isDecoding = YES;

        _lastTime = [NSDate date];

    }

    VTDecodeFrameFlags flags = kVTDecodeFrame_EnableAsynchronousDecompression;

    VTDecodeInfoFlags flagOut;

    VTDecompressionSessionDecodeFrame(_decompression, sampleBuffer, flags, NULL, &flagOut);

    VTDecompressionSessionWaitForAsynchronousFrames(_decompression);

    if (_gotFrame == YES) {

        _gotFrame = NO;

        _isDecoding = NO;

    }

    CFRelease(sampleBuffer);
}

解码回调函数

void didDecompress( void *decompressionOutputRefCon, void *sourceFrameRefCon, OSStatus status, VTDecodeInfoFlags infoFlags, CVImageBufferRef imageBuffer, CMTime presentationTimeStamp, CMTime presentationDuration ){

    VideoView* THIS = (__bridge VideoView*)decompressionOutputRefCon;

    THIS->_gotFrame = YES;

    NSDate* currentTime = [NSDate date];

    NSTimeInterval runTime = currentTime.timeIntervalSince1970 - THIS->_lastTime.timeIntervalSince1970;

    THIS->_totalTime += runTime;

    THIS->_counts++;

    THIS->_lastTime = currentTime;

}

Answer 1

我们可以将NSDate设置为sourceRefCon，然后访问回调中的时间戳，以获得确切的解码时间。

void * sourceFrameRefCon in VTDecompressionSessionDecodeFrame

VTDecompressionSessionDecodeFrame(
    VTDecompressionSessionRef       session,
    CMSampleBufferRef               sampleBuffer,
    VTDecodeFrameFlags              decodeFlags, // bit 0 is enableAsynchronousDecompression
    void *                          sourceFrameRefCon,
    VTDecodeInfoFlags               *infoFlagsOut /* may be NULL */ )

译码方法

- (void)render:(CMSampleBufferRef)sampleBuffer
{
    VTDecodeFrameFlags flags = kVTDecodeFrame_EnableAsynchronousDecompression;

    VTDecodeInfoFlags flagOut;

    NSDate* currentTime = [NSDate date];

    VTDecompressionSessionDecodeFrame(_decompression, sampleBuffer, flags, (void*)CFBridgingRetain(currentTime), &flagOut);

    CFRelease(sampleBuffer);
}

解码回调方法

void didDecompress( void *decompressionOutputRefCon, void *sourceFrameRefCon, OSStatus status, VTDecodeInfoFlags infoFlags, CVImageBufferRef imageBuffer, CMTime presentationTimeStamp, CMTime presentationDuration ){

    NSDate* currentTime = (__bridge NSDate *)sourceFrameRefCon;

    if (currentTime != nil) {

        //Do something

    }
}