openh264的使用（1）

1.初始化ISVCEncoder

ISVCEncoder* encoder_;
int rv = WelsCreateSVCEncoder(&encoder_);
if (0 != rv || !encoder_) {
        //error
}

  ISVCEncoder提供了一系列的编码有关的接口，如初始化编码器、设置编码参数、编码等；

2. 设置相关参数
  openh264提供了两个结构体来设置编码参数，SEncParamBase与SEncParamExt， SEncParamBase仅通过了最基础的参数设置， 其定义如下:

typedef struct TagEncParamBase {
  EUsageType
  iUsageType;                 ///< application type;1.CAMERA_VIDEO_REAL_TIME:camera video signal; 2.SCREEN_CONTENT_REAL_TIME:screen content signal;

  int       iPicWidth;        ///< width of picture in luminance samples (the maximum of all layers if multiple spatial layers presents)
  int       iPicHeight;       ///< height of picture in luminance samples((the maximum of all layers if multiple spatial layers presents)
  int       iTargetBitrate;   ///< target bitrate desired, in unit of bps
  RC_MODES  iRCMode;          ///< rate control mode
  float     fMaxFrameRate;    ///< maximal input frame rate

} SEncParamBase, *PEncParamBase;

  其中参数iUsageType指明应用的类型，类型包括:

typedef enum {
  CAMERA_VIDEO_REAL_TIME,      ///< camera video for real-time communication
  SCREEN_CONTENT_REAL_TIME,    ///< screen content signal
  CAMERA_VIDEO_NON_REAL_TIME
} EUsageType;

iRCMode指定码率控制的模式, openh264提供的模式如下:

typedef enum {
  RC_QUALITY_MODE = 0,     ///< quality mode
  RC_BITRATE_MODE = 1,     ///< bitrate mode
  RC_BUFFERBASED_MODE = 2, ///< no bitrate control,only using buffer status,adjust the video quality
  RC_TIMESTAMP_MODE = 3, //rate control based timestamp
  RC_BITRATE_MODE_POST_SKIP = 4, ///< this is in-building RC MODE, WILL BE DELETED after algorithm tuning!
  RC_OFF_MODE = -1,         ///< rate control off mode
} RC_MODES;

对编码器的初始化例子如下:

SEncParamBase paramBase;
paramBase.iPicWidth = width_;
paramBase.iPicHeight = height_;
paramBase.fMaxFrameRate = fps_;
paramBase.iTargetBitrate = 10 * width_ * height_;
paramBase.iUsageType = CAMERA_VIDEO_REAL_TIME;
paramBase.iRCMode = RC_BITRATE_MODE;
int ret = encoder_->Initialize(&paramBase);
if (0 != ret) {
        //error
}

3. 编码

  编码需要用到SSourcePicture和SFrameBSInfo两个结构体， SSourcePicture用来保存需要编码的数据信息， 而SFrameBSInfo会保存编码完成后的数据。
SSourcePicture的定义

typedef struct Source_Picture_s {
  int       iColorFormat;          ///< color space type
  int       iStride[4];            ///< stride for each plane pData
  unsigned char*  pData[4];        ///< plane pData
  int       iPicWidth;             ///< luma picture width in x coordinate
  int       iPicHeight;            ///< luma picture height in y coordinate
  long long uiTimeStamp;           ///< timestamp of the source picture, unit: millisecond
} SSourcePicture;

• iColorFormat:颜色空间的类型，如videoFormatI420；
• iStride, 每个plane的stride，对于plane和stride的理解可参考yuv 图像里的stride和plane的解释。
• pData,指向每个plane的指针；

SFrameBSInfo结构体在编码前只需要用memset将结构体中的数据置为0即可。其定义如下:

typedef struct {
  int           iLayerNum;
  SLayerBSInfo  sLayerInfo[MAX_LAYER_NUM_OF_FRAME];

  EVideoFrameType eFrameType;
  int iFrameSizeInBytes;
  long long uiTimeStamp;
} SFrameBSInfo, *PFrameBSInfo;

typedef struct {
  unsigned char uiTemporalId;
  unsigned char uiSpatialId;
  unsigned char uiQualityId;
  EVideoFrameType eFrameType;
  unsigned char uiLayerType;

  /**
   * The sub sequence layers are ordered hierarchically based on their dependency on each other so that any picture in a layer shall not be
   * predicted from any picture on any higher layer.
  */
  int   iSubSeqId;                ///< refer to D.2.11 Sub-sequence information SEI message semantics
  int   iNalCount;              ///< count number of NAL coded already
  int*  pNalLengthInByte;       ///< length of NAL size in byte from 0 to iNalCount-1
  unsigned char*  pBsBuf;       ///< buffer of bitstream contained
} SLayerBSInfo, *PLayerBSInfo;

SFrameBSInfo结构体比较复杂， 具体使用情况下面再解释。
对于i420数据的编码过程:

  SSourcePicture pic;
  memset(&pic, 0, sizeof(pic));
  pic.iPicWidth = width_;
  pic.iPicHeight = height_;
  pic.iColorFormat = videoFormatI420;
  pic.iStride[0] = pic.iPicWidth;
  pic.iStride[1] = pic.iStride[2] = pic.iPicWidth >> 1;
  pic.pData[0] = (unsigned char *) i420Buffer;
  pic.pData[1] = pic.pData[0] + width_ * height_;
  pic.pData[2] = pic.pData[1] + (width_ * height_ >> 2);
  SFrameBSInfo info;
  memset(&info, 0, sizeof(SFrameBSInfo));
  int rv = encoder_->EncodeFrame(&pic, &info);

  int retSize = 0;
  if (rv != cmResultSuccess) {
        //error info
      return retSize;
  }
 if (info.eFrameType != videoFrameTypeSkip) {
        int type = info.eFrameType;
        for (int i = 0; i < info.iLayerNum; ++i) {
            const SLayerBSInfo &layerInfo = info.sLayerInfo[i];
            int layerSize = 0;
            for (int j = 0; j < layerInfo.iNalCount; ++j) {
                layerSize += layerInfo.pNalLengthInByte[j];
            }
            memcpy((char *) (oBuf + retSize), (char *) layerInfo.pBsBuf, layerSize);
            retSize += layerSize;
        }
    }

  其中i420Buffer是指向原始yuv数据的指针，oBuf是指向h264流缓冲区的指针。由上可知，整个过程分为几步：

• 定义SSourcePicture和SFrameBSInfo， 并给SSourcePicture赋值；
• 编码；
• 判断编码是否成功；
• 判断帧类型，如果不是跳帧， 则读取编码后数据；

  SFrameBSInfo的参数iLayerNum表示编码后的NAL数量。编码后的h264数据存放在SFrameBSInfo的sLayerInfo结构数组中，其中每个结构体中的pBsBuf表示编码得到的数据，而长度是结构体pNalLengthInByteint数组加起来的和，数组长度由结构体的iNalCount成员表示。

4. 释放
先调用ISVCEncoder的Uninitialize函数，再调用WelsDestroySVCEncoder即可。