在《Android视频编码源码分享》一文中，我们主要学习的是Android视频编码的实现方法，本文要给大家讲述的则是Android中视频的解码过程。

在Android中，解码器的具体实现会被封装在一个OpenMAX组件中，对于视频的编码器，相应的组件为OpenmaxMpeg4AO，该组件被Mpeg4OmxComponentFactory、H263OmxComponentFactory等组件工厂共有，用来实现对H.263和MPEG-4的解码。提供给记录引擎的接口为OpenmaxMpeg4AO::ProcessData()方法，实际的解码工作在 Mpeg4 Decoder_OMX::Mp4DecodeVideo ()方法中实现。下面是该方法的具体实现：

代码：视频的解码过程

MX_BOOL Mpeg4Decoder_OMX::Mp4DecodeVideo(OMX_BUFFERHEADERTYPE* aOutBuffer, OMX_U32*
    aOutputLength,OMX_U8** aInputBuf, OMX_U32* aInBufSize,
    OMX_PARAM_PORTDEFINITIONTYPE* aPortParam, OMX_BOOL aDeBlocking,
    OMX_S32* aFrameCount, OMX_BOOL aMarkerFlag, OMX_BOOL *aResizeFlag)
    {
      OMX_BOOL Status=OMX_TRUE;
      OMX_S32 OldWidth, OldHeight;
      OldWidth=aPortParam->format.video.nFrameWidth;
      OldHeight=aPortParam->format.video.nFrameHeight;
      *aResizeFlag=OMX_FALSE;
      #ifdef _DEBUG
      static OMX_U32 FrameCount=0;
      #endif
      uint UseExtTimestamp=0;
      uint32 TimeStamp;
      OMX_S32 InputSize, InitSize;
      if ((Mpeg4InitCompleteFlag==OMX_FALSE) && (MPEG4_MODE==CodecMode))
      {
        if (!aMarkerFlag)
        {
          InitSize=GetVideoHeader(0, *aInputBuf, *aInBufSize);
        }
        else
        {
          InitSize=*aInBufSize;
        }
      //初始化解码
      if (PV_TRUE!=InitializeVideoDecode(&iDisplay_Width, &iDisplay_Height,aInputBuf, (OMX_S32*)aInBufSize, MPEG4_MODE, aDeBlocking))
      return OMX_FALSE;
      Mpeg4InitCompleteFlag=OMX_TRUE;
      aPortParam->format.video.nFrameWidth=iDisplay_Width;
      aPortParam->format.video.nFrameHeight=iDisplay_Height;
      OMX_U32 min_stride=((aPortParam->format.video.nFrameWidth+15) & (~15));
      OMX_U32 min_sliceheight=((aPortParam->format.video.nFrameHeight+15) & (~15));
      aPortParam->format.video.nStride=min_stride;
      aPortParam->format.video.nSliceHeight=min_sliceheight;
      //解码器组件总是输出YUV420格式
      aPortParam->nBufferSize=(aPortParam->format.video.nSliceHeight*aPortParam->
      format.video.nStride * 3)>>1;
      iFrameSize=(aPortParam->format.video.nSliceHeight * aPortParam->format. video.nStride);
      if ((iDisplay_Width !=OldWidth) || (iDisplay_Height !=OldHeight))
      {
        *aResizeFlag=OMX_TRUE;
      }
      else if (NULL !=aOutBuffer)
      {
        PVSetReferenceYUV(&VideoCtrl, (uint8*)(aOutBuffer->pBuffer));
        BufferCtrlStruct *pBCTRL=(BufferCtrlStruct *)(aOutBuffer-> pOutputPortPrivate);
        pBCTRL->iRefCount++;
        ipRefCtrPreviousReferenceBuffer=&(pBCTRL->iRefCount);
        iReferenceYUVWasSet=OMX_TRUE;
      }
      *aFrameCount=1;
      *aInBufSize -=InitSize;
      return OMX_TRUE;
      }
      if ((*(OMX_S32*)aInBufSize) <=0)
      {
        return OMX_FALSE;
      }
      TimeStamp=0xFFFFFFFF;
      InputSize=*aInBufSize;
      if ((OMX_FALSE==Mpeg4InitCompleteFlag) && (H263_MODE==CodecMode))
      {
      int32 aligned_width, aligned_height;
      int32 display_width, display_height;
      //获取配置信息
      if (iGetM4VConfigInfo(*aInputBuf, *aInBufSize, &aligned_width, &aligned_height,&display_width, &display_height))
      {
        return OMX_FALSE;
      }
      Mpeg4InitCompleteFlag=OMX_TRUE;
      iDisplay_Width=display_width;
      iDisplay_Height=display_height;
      aPortParam->format.video.nFrameWidth=iDisplay_Width; // use non 16byte aligned values (display_width) for H263
      aPortParam->format.video.nFrameHeight=iDisplay_Height; // like in the case of M4V (PVGetVideoDimensions also returns display_width/height)
      OMX_U32 min_stride=((aPortParam->format.video.nFrameWidth+15) & (~15));
      OMX_U32 min_sliceheight=((aPortParam->format.video.nFrameHeight+15) & (~15));
      aPortParam->format.video.nStride=min_stride;
      aPortParam->format.video.nSliceHeight=min_sliceheight;
      aPortParam->nBufferSize=(aPortParam->format.video.nSliceHeight * aPortParam->format.video.nStride * 3) >> 1;
      iFrameSize=(aPortParam->format.video.nSliceHeight * aPortParam->format.video.nStride);
      if ((iDisplay_Width !=OldWidth)||(iDisplay_Height !=OldHeight))
      {
        *aResizeFlag=OMX_TRUE;
      }
      else if (NULL !=aOutBuffer)
      {
        PVSetReferenceYUV(&VideoCtrl, (uint8*)(aOutBuffer->pBuffer));
        BufferCtrlStruct *pBCTRL=(BufferCtrlStruct *)(aOutBuffer->pOutputPortPrivate);
        pBCTRL->iRefCount++;
        ipRefCtrPreviousReferenceBuffer=&(pBCTRL->iRefCount);
        iReferenceYUVWasSet=OMX_TRUE;
      }
      *aFrameCount=1;
      return OMX_TRUE;
      }
      if (iReferenceYUVWasSet==OMX_FALSE)
      {
        PVSetReferenceYUV(&VideoCtrl, (uint8*)(aOutBuffer->pBuffer));
        BufferCtrlStruct *pBCTRL=(BufferCtrlStruct *)(aOutBuffer-> pOutputPortPrivate);
        pBCTRL->iRefCount++;
        ipRefCtrPreviousReferenceBuffer=&(pBCTRL->iRefCount);
        iReferenceYUVWasSet=OMX_TRUE;
        return OMX_TRUE;
      }
      #if PROFILING_ON
      OMX_U32 StartTime=OsclTickCount::TickCount();
      #endif
      //针对一帧进行解码
      Status=(OMX_BOOL) PVDecodeVideoFrame(&VideoCtrl, aInputBuf,&TimeStamp,(int32*)aInBufSize,&UseExtTimestamp,(OMX_U8*)(aOutBuffer->pBuffer));
#if PROFILING_ON
      OMX_U32 EndTime=OsclTickCount::TickCount();
      iTotalTicks +=(EndTime-StartTime);
      #endif
      if (Status==PV_TRUE)
      {
        #ifdef _DEBUG
        #endif
        *aInputBuf +=(InputSize-*aInBufSize);
        (*ipRefCtrPreviousReferenceBuffer)--;
        if ((*ipRefCtrPreviousReferenceBuffer)==0)
        {
          ipOMXComponent->iNumAvailableOutputBuffers++;
        }
        BufferCtrlStruct *pBCTRL=(BufferCtrlStruct *)(aOutBuffer-> pOutputPortPrivate);
        pBCTRL->iRefCount++;
        ipRefCtrPreviousReferenceBuffer=&(pBCTRL->iRefCount);
        *aOutputLength=(iFrameSize * 3)>>1;
        (*aFrameCount)++;
      }
      else
      {
        *aInBufSize=InputSize;
        *aOutputLength=0;
      }
      return Status;
    }

在进行解码时，会首先判断编码格式是MPEG4_MODE还是H263_MODE，如果是MPEG4_MODE，则首先通过GetVideoHeader()方法读取视频的头信息，然后通过InitializeVideoDecode()方法初始化视频解码器，终将数据解码为YUV420格式的元数据。如果编码格式为H263_MODE，则首先通过iGetM4VConfigInfo()方法获取M4V文件的配置信息，终将数据解码为YUV420格式的元数据。