1、编解码的格式

std::unique_ptr<VideoEncoder> InternalEncoderFactory::CreateVideoEncoder(
    const SdpVideoFormat& format) {
  if (absl::EqualsIgnoreCase(format.name, cricket::kVp8CodecName))
    return VP8Encoder::Create();
  if (absl::EqualsIgnoreCase(format.name, cricket::kVp9CodecName))
    return VP9Encoder::Create(cricket::VideoCodec(format));
  if (absl::EqualsIgnoreCase(format.name, cricket::kH264CodecName))
    return H264Encoder::Create(cricket::VideoCodec(format));
  RTC_LOG(LS_ERROR) << "Trying to created encoder of unsupported format "
                    << format.name;
  return nullptr;
}

　从上到下的优先级的形式设置