使用Android MediaCodec 硬解码延时问题分析
2018年03月29日 09:30:38 珠雨妮儿 阅读数：2492
版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/zhuyunier/article/details/79730872
最近做项目用到Android native层的MediaCodec的接口对H264进行解码，通过在解码前和解码后加打印日志，发现解码耗时200多ms，和IOS的解码耗时10ms相比实在是延时好大。后来研究了两周也没能解决延时问题，很悲惨……不过还是将这过程中分析的思路记录下来，说不定以后万一灵感来了就解决了呢。

 起初在https://software.intel.com/en-us/forums/android-applications-on-intel-architecture/topic/536355和https://software.intel.com/en-us/android/articles/android-hardware-codec-mediacodec这两个网址中找到问题原因和一种解决方法。

  如果编码器类型是H.264，MediaCodec可能会有几帧的延迟（对于IA平台，它是7帧，而另一些则是3-5帧）。如果帧速率为30 FPS，则7帧将具有200毫秒的延迟。什么导致了硬件解码器的延迟？原因可能是主或高配置文件有一个B帧，并且如果当B帧引用后续缓冲区时解码器没有缓冲区，则该播放会短暂停止。英特尔已经考虑了硬件解码器的这种情况，并为主要或高配置文件提供了7个缓冲区，但是由于基线没有B帧，因此将其减少到基线的零缓冲区。其他供应商可能为所有配置文件使用4个缓冲区。
 因此，在基于IA的Android平台上，解决方案是将编码器更改为基线配置文件。如果编码器配置文件无法更改，则可行的解决方法是更改​​解码器端的配置数据。通常，配置文件位于第五个字节中，因此将基准配置文件更改为66会在IA平台上产生最低延迟。也就是说要想低延时，必须：

- 不要设置MediaFormat.KEY_MAX_WIDTH和MediaFormat.KEY_MAX_HEIGHT。

 - 将第一个SPS中的配置文件切换到基线，然后在第一个PPS后以正确的配置文件重新提交。

  先附上解码部分代码：

int OpenMetaCUDAVideoDecoderContext::OnVideoH264(uint8_t * frame, uint32_t frameSize){

DP_MSG("onVideoData=%6d, %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X\n",
       frameSize,
       frame[0],frame[1],frame[2],frame[3],frame[4],frame[5],frame[6],frame[7]
);

int  pos = 0;
if( (frame[4] & 0X1F ) == 0X09 ){
    // this is an iframe
    frame     += 6;
    frameSize -= 6;
}

if( (frame[4] & 0X1F ) == 0X07 ){
    // this is an iframe
    iFrameFound = true;
}
else if(!iFrameFound){
    // iframe not found, do nothing
    return -1;
}

DP_MSG("video264::::%d",frameSize)

uint8_t *data = NULL;
uint8_t *pps = NULL;
uint8_t *sps = NULL;

// I know what my H.264 data source's NALUs look like so I know start code index is always 0.
// if you don't know where it starts, you can use a for loop similar to how i find the 2nd and 3rd start codes
int currentIndex = 0;
long blockLength = 0;

int nalu_type = (frame[currentIndex + 4] & 0x1F);
// if we havent already set up our format description with our SPS PPS parameters, we
// can't process any frames except type 7 that has our parameters
if (nalu_type != 7 && formatDesc == NULL)
{
    return -1;
}

if (nalu_type == 7)
{
    currentIndex = H264_findStartCode(frame,frameSize,currentIndex+4);
    size_t spsSize = currentIndex;

    // find what the second NALU type is
    nalu_type = (frame[currentIndex + 4] & 0x1F);

    DP_MSG("onVideoDataSPs=%6d, %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X\n",
           frameSize,
           frame[0],frame[1],frame[2],frame[3],frame[4],frame[5],frame[6],frame[7]
    );

    // type 8 is the PPS parameter NALU
    if(nalu_type == 8){
        int nextIndex = H264_findStartCode(frame,frameSize,currentIndex+4);
        size_t ppsSize = nextIndex - currentIndex;

        DP_MSG("onVideoDataPPs=%6d, %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X %.2X\n",
               frameSize,
               frame[0],frame[1],frame[2],frame[3],frame[4],frame[5],frame[6],frame[7],
               frame[spsSize+1],frame[spsSize+2],frame[spsSize+3],frame[spsSize+4]
        );

        if(decompressionSession == NULL) {
            formatDesc = AMediaFormat_new();
            AMediaFormat_setString(formatDesc, "mime", "video/avc");
            AMediaFormat_setInt32(formatDesc, AMEDIAFORMAT_KEY_WIDTH, 1920); // 视频宽度
            AMediaFormat_setInt32(formatDesc, AMEDIAFORMAT_KEY_HEIGHT, 1088); // 视频高度

            sps = (uint8_t *)malloc(spsSize);
            pps = (uint8_t *)malloc(ppsSize);
            memcpy (sps, &frame[0], spsSize);
            memcpy (pps, &frame[spsSize], ppsSize);

            AMediaFormat_setBuffer(formatDesc, "csd-0", sps, spsSize); // sps
            AMediaFormat_setBuffer(formatDesc, "csd-1", pps, ppsSize); // pps

            DP_MSG("createDecompSession ========= ")
            this->createDecompSession();

            if(sps != NULL)
            {
                free(sps);
                sps = NULL;
            }
            if(pps != NULL)
            {
                free(pps);
                pps = NULL;
            }

        }

        // now lets handle the IDR frame that (should) come after the parameter sets
        // I say "should" because that's how I expect my H264 stream to work, YMMV
        currentIndex = nextIndex;
        nalu_type = (frame[currentIndex + 4] & 0x1F);

        while(currentIndex != -1 && nalu_type != 5 && nalu_type != 1){
            currentIndex = H264_findStartCode(frame,frameSize,currentIndex+4);
            nalu_type = (frame[currentIndex + 4] & 0x1F);
        }
    }
}

int32_t afout = 0;
   nalu_type = 5; // temp
// type 5 is an IDR frame NALU.  The SPS and PPS NALUs should always be followed by an IDR (or IFrame) NALU, as far as I know
if(nalu_type == 5)
{

    DP_MSG("jniPlayTs size=%d and %d \n",AMediaFormat_getInt32(formatDesc,AMEDIAFORMAT_KEY_COLOR_FORMAT,&afout),afout);
    //DP_MSG("theNativeWindow format %p and %d \n",theNativeWindow,ANativeWindow_getFormat(theNativeWindow));
    // find the offset, or where the SPS and PPS NALUs end and the IDR frame NALU begins
    int offset = currentIndex;//
    blockLength = frameSize - offset;

    data = &frame[offset];

    DP_MSG("nalu_type ===5 ")
    //DP_MSG("nalu_type ===5 %s /n ",AMediaFormat_toString(formatDesc))

    if(decompressionSession == nullptr)
    {
        DP_MSG("decompressionSession is null")
    }

    DP_MSG("OpenMetaCUDAVideoDecoder Start decoding: frameIdentifier=%lld",frameIdentifier);
    avx_info("OpenMetaCUDAVideoDecoder Start decoding: ","frameIdentifier=%lld",frameIdentifier);

    static int64_t  llPts = 0;

    ssize_t index = -1;
    index = AMediaCodec_dequeueInputBuffer(decompressionSession, 10000);
    if(index>=0)
    {
        size_t bufsize;
        auto buf = AMediaCodec_getInputBuffer(decompressionSession, index, &bufsize);
        if(buf!= nullptr )
        {
            DP_MSG("buf!= nullptr5======ww")
            memcpy(buf,data,blockLength);

            AMediaCodec_queueInputBuffer(decompressionSession, index, 0, blockLength, (uint64_t)frameIdentifier, 0);

            llPts += 3000;
        }

    }

    AMediaCodecBufferInfo info;
    auto status = AMediaCodec_dequeueOutputBuffer(decompressionSession, &info, 0);
    if (status >= 0) {
        if (info.flags & AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM) {
            DP_MSG("output EOS");
        }

        size_t outsize = 0;
        u_int8_t * outData = AMediaCodec_getOutputBuffer(decompressionSession,status,&outsize);
        auto formatType = AMediaCodec_getOutputFormat(decompressionSession);
        DP_MSG("format formatType to: %s", AMediaFormat_toString(formatType));
        int colorFormat = 0;
        int width = 0;
        int height = 0;
        AMediaFormat_getInt32(formatType,"color-format",&colorFormat);
        AMediaFormat_getInt32(formatType,"width",&width);
        AMediaFormat_getInt32(formatType,"height",&height);
        DP_MSG("format color-format : %d width ：%d height :%d", colorFormat,width,height);

		AMediaFormat_delete(formatType);
        DP_MSG("OpenMetaCUDAVideoDecoder End   decoding: frameIdentifier=%lld,%d,%d,%p",info.presentationTimeUs,info.size,outsize,outData)
		if(this->kController){
            //OpenMetaPixelBuffer _kPixelBuffer(pixelBuffer,sizeof(pixelBuffer));
            OpenMetaPixelBuffer _kPixelBuffer(outData,sizeof(outData));
            _kPixelBuffer.kLine[0]  = outData;
            _kPixelBuffer.kLine[1]  = outData + width * height;
            _kPixelBuffer.kSizeX    = width;
            _kPixelBuffer.kSizeY    = height;
            _kPixelBuffer.kDataType = 1;
            _kPixelBuffer.kTimeStamp = info.presentationTimeUs;

            if(this!=NULL && this->kController!=NULL)
            {
                this->kController->OnVideoImage(&_kPixelBuffer);
            }

        }
        avx_info("OpenMetaCUDAVideoDecoder End   decoding:","%lld,%d,%d,%p",info.presentationTimeUs,info.size,outsize,outData);
        AMediaCodec_releaseOutputBuffer(decompressionSession, status, info.size != 0);

    } else if (status == AMEDIACODEC_INFO_OUTPUT_BUFFERS_CHANGED) {
        DP_MSG("output buffers changed");
    } else if (status == AMEDIACODEC_INFO_OUTPUT_FORMAT_CHANGED) {
        auto format = AMediaCodec_getOutputFormat(decompressionSession);
        DP_MSG("format changed to: %s", AMediaFormat_toString(format));
        AMediaFormat_delete(format);
    } else if (status == AMEDIACODEC_INFO_TRY_AGAIN_LATER) {
        DP_MSG("no output buffer right now");
    } else {
        DP_MSG("unexpected info code: %zd", status);
    }
  
}

return 0;

}

  针对查到的解决办法，对以上代码进行了修改，分别是：

//将第一个sps的改为基线配置
sps = (uint8_t *)malloc(spsSize);
sps[0+5] = 66;
memcpy (sps, &frame[0], spsSize);

pps = (uint8_t *)malloc(ppsSize);
memcpy (pps, &frame[spsSize], ppsSize);

AMediaFormat_setBuffer(formatDesc, “csd-0”, sps, spsSize); // sps
AMediaFormat_setBuffer(formatDesc, “csd-1”, pps, ppsSize); // pps

//拷贝两份sps，将第一个sps的改为基线配置
sps = (uint8_t *)malloc(spsSize * 2);
memcpy (sps, &frame[0], spsSize);
sps[0+5] = 66;
memcpy (sps + spsSize, &frame[0], spsSize);

pps = (uint8_t *)malloc(ppsSize);
memcpy (pps, &frame[spsSize], ppsSize);

AMediaFormat_setBuffer(formatDesc, “csd-0”, sps, spsSize * 2); // sps
AMediaFormat_setBuffer(formatDesc, “csd-1”, pps, ppsSize); // pps
//拷贝两份sps，将第二份的第一个sps的改为基线配置
sps = (uint8_t *)malloc(spsSize * 2);
memcpy (sps, &frame[0], spsSize);
memcpy (sps + spsSize, &frame[0], spsSize);
sps[spsSize+5] = 66;

pps = (uint8_t *)malloc(ppsSize);
memcpy (pps, &frame[spsSize], ppsSize);

AMediaFormat_setBuffer(formatDesc, “csd-0”, sps, spsSize * 2); // sps
AMediaFormat_setBuffer(formatDesc, “csd-1”, pps, ppsSize); // pps
通过查看打印的开始解码与结束解码的时间发现，几种方法的时间差距并不是很大，依旧是200-250ms