WEBRTC 支持H264编解码

WEBRTC视频编解码支持H264 VP8 VP9 但是默认是VP8 ，根据SDP描述协商

WEBRTC H264编码采用OPENH264 解码采用FFMPEG

一 让WEBRTC支持H264编码

1. 修改配置支持H264编码

 webrtc/build/common.gypi  rtc_use_h264=1(只要有都设为1)，这样OPENH264就会生成

 然后需要重新编译 Python webrtc/build/gyp_webrtc ninja -C out/Debug

2 修改SDP生成次序

webrtcvideoengine2.cc DefaultVideoCodecList 可以修改SDP中 视频编码的次序把

放在最前面，让SDP支持H264

 if (CodecIsInternallySupported(kH264CodecName)) {
   VideoCodec codec = MakeVideoCodecWithDefaultFeedbackParams(
       kDefaultH264PlType, kH264CodecName);
   // TODO(hta): Move all parameter generation for SDP into the codec
   // implementation, for all codecs and parameters.
   // TODO(hta): Move selection of profile-level-id to H.264 codec
   // implementation.
   // TODO(hta): Set FMTP parameters for all codecs of type H264.
   codec.SetParam(kH264FmtpProfileLevelId,
                  kH264ProfileLevelConstrainedBaseline);
   codec.SetParam(kH264FmtpLevelAsymmetryAllowed, "1");
   codec.SetParam(kH264FmtpPacketizationMode, "1");
   codecs.push_back(codec);
   codecs.push_back(
       VideoCodec::CreateRtxCodec(kDefaultRtxH264PlType, kDefaultH264PlType));
 }

二 让WEBRTC支持H264解码
thirdpart 下有ffmpeg源码，不用下载ffmpeg编译
1.修改gyp配置 支持ffmpeg编译 2.默认ffmpeg 不会把h264.c等文件编译进ffmpeg 就会导致 H264DecoderImpl::InitDecode 调用 avcodec_find_decoder 失败，
找不到解码器需要修改 ffmpeg_generated.gypi 默认只支持chrome chromeOS中编译H264，找到h264.c 
然后 在Linux X64 中去掉chrome

 ['(OS == "linux" and target_arch == "arm" and arm_neon == 1 and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "arm" and arm_neon == 1 and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "arm" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "arm" and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "arm64" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "arm64" and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "ia32" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "ia32" and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "mipsel" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "mipsel" and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "x64" and ffmpeg_branding == "Chrome") or(OS == "linux" and target_arch == "x64")or (OS == "mac" and target_arch == "x64" and ffmpeg_branding == "Chrome") or (OS == "win" and target_arch == "ia32" and ffmpeg_branding == "Chrome") or (OS == "win" and target_arch == "x64" and ffmpeg_branding == "Chrome")', {

再找到

 'libavcodec/x86/h264_qpel.c',
 'libavcodec/x86/h264chroma_init.c',
 'libavcodec/x86/h264dsp_init.c',

对应得

 ['(OS == "linux" and target_arch == "ia32" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "ia32" and ffmpeg_branding == "ChromeOS") or (OS == "linux" and target_arch == "x64" and ffmpeg_branding == "Chrome") or (OS == "linux" and target_arch == "x64") or (OS == "mac" and target_arch == "x64" and ffmpeg_branding == "Chrome") or (OS == "win" and target_arch == "ia32" and ffmpeg_branding == "Chrome") or (OS == "win" and target_arch == "x64" and ffmpeg_branding == "Chrome")', {

在LINUX X64 中去掉chrome

3.重新生成ninja python webrtc/build/gyp_webrtc ninja -C out/Debug

可在video_send_stream.cc 函数，中调试 encoded_image._buffer 看码流或写入文件

 int32_t VideoSendStream::Encoded(const EncodedImage& encoded_image,
                                  const CodecSpecificInfo* codec_specific_info,
                                  const RTPFragmentationHeader* fragmentation)

 class="cpp" name="code" snippet_file_name="blog_20160630_8_8873287" code_snippet_id="1736351">