OSX、VideoToolbox、压缩会话

Question

环境

• OSX 优胜美地
• xcode 6.4
• C++

用例

给定一个由单个关键帧和多个对应的增量帧组成的 h264 GOP，解码 GOP，然后编码单个 4.2Mbit 关键帧

问题描述

我能够解码 GOP 并编码关键帧，但是，生成的关键帧质量低（低比特率），也就是说，尽管我专门将比特率设置为 4.2Mbit （这可以在下面的代码快照中看到），考虑到我只需要一个 单个关键帧，有没有办法设置 VideoToolBox 编码器以输出高比特率关键帧?

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最终质量非常低

Answer 1

还有一个你需要关心的因素，在创建 VTCompressionSession 之后，你需要在 VTCompressionSessionEncodeFrame 中正确设置你的帧时间戳。