OSX，VideoToolbox，压缩会话

OSX, VideoToolbox, Compression Session

本文关键字：会话 压缩 VideoToolbox OSX      更新时间：2023-10-16

环境

  • OSX约塞米蒂
  • xcode 6.4
  • C++

用例

给定由单个关键帧和多个相应的增量帧组成的h264 GOP，对GOP进行解码，然后对单个4.2Mbit关键帧进行编码

问题描述

我能够解码GOP并对关键帧进行编码，然而，生成的关键帧质量较低(低比特率(，也就是说，尽管我特别将比特率设置为4.2Mbit(这可以在下面的代码快照中看到(，但考虑到我只需要一个单个关键帧，有什么方法可以设置VideoToolBox编码器来输出高比特率的关键帧吗？

const CMVideoCodecType  fourcc  = (CMVideoCodecType)CMVideoFormatDescriptionGetCodecType(fmt);
// Else, generate a h264 KeyFrame
CFObjectSmartPtr< CFMutableDictionaryRef >  dictEncSpec;
dictEncSpec.Attach(CFDictionaryCreateMutable(NULL, 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks));
//dictEncSpec.Attach(CFDictionaryCreateMutable(NULL, 0, 0, 0));
// CFDictionarySetNumVal(dictEncSpec, kVTVideoEncoderSpecification_RequireHardwareAcceleratedVideoEncoder, TRUE);
CFDictionarySetValue(dictEncSpec, kVTCompressionPropertyKey_ProfileLevel, kVTProfileLevel_H264_High_3_2);
// CFDictionarySetValue(dictEncSpec, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue);
// CFDictionarySetValue(dictEncSpec, kVTCompressionPropertyKey_H264EntropyMode, kVTH264EntropyMode_CABAC);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_AverageBitRate, 4200000);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_Quality, 1.0f);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_MaxFrameDelayCount, 0);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_MaxKeyFrameIntervalDuration, 1);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_ExpectedDuration, 1);
CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_ExpectedFrameRate, 10);
// CFDictionarySetNumVal(dictEncSpec, kVTCompressionPropertyKey_SourceFrameCount, 1);
CFDictionarySetNumVal(dictEncSpec, kVTVideoEncoderSpecification_EnableHardwareAcceleratedVideoEncoder, TRUE);
CFObjectSmartPtr< VTCompressionSessionRef > sessionCompression;
status = VTCompressionSessionCreate(0, res.width, res.height, fourcc, (CFDictionaryRef)dictEncSpec, 0, 0, CompressionCallback, &ctx, &sessionCompression);
if (noErr != status)
    return HRESULT_FROM_ERRNO(status);
if (noErr != (status = VTCompressionSessionEncodeFrame(sessionCompression, ctx.yuvFrame, timestamp, timestamp, 0, 0, 0)))
    return HRESULT_FROM_ERRNO(status);
if(noErr != (status = VTCompressionSessionCompleteFrames(sessionCompression, timestamp)))
    return HRESULT_FROM_ERRNO(status);
if (0 == ctx.keyFrame) {
    assert(false);
    return E_UNEXPECTED;
}

ctx.keyFrame最终是一个非常低质量的

您需要注意的另一个因素是，在创建VTCompressionSession后，您需要在VTCompressionsSessionEncodeFrame中正确设置帧时间戳。

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