音视频编解码器的ID列表image.png

Codec ID，就是PayloadType类型。
因为每种CodecID必须是只对应视频或者音频的编解码类型。如果PayloadType类型一样，则无法区分是视频还是音频，所以音频和视频不能同时使用102,103,104 其中一个Codec ID 。

设置编解码器ID

image.png

MergeCodecs

h:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\pc\media_session.cc

  1. // Adds all codecs from |reference_codecs| to |offered_codecs| that don't
  2. // already exist in |offered_codecs| and ensure the payload types don't
  3. // collide.
  4. template <class C>
  5. static void MergeCodecs(const std::vector<C>& reference_codecs,
  6.                         std::vector<C>* offered_codecs,
  7.                         UsedPayloadTypes* used_pltypes) {
  8.   // Add all new codecs that are not RTX codecs.
  9.   for (const C& reference_codec : reference_codecs) {
  10.     if (!IsRtxCodec(reference_codec) &&
  11.         !FindMatchingCodec<C>(reference_codecs, *offered_codecs,
  12.                               reference_codec, nullptr)) {
  13.       C codec = reference_codec;
  14.       used_pltypes->FindAndSetIdUsed(&codec);
  15.       offered_codecs->push_back(codec);
  16.     }
  17.   }
  19.   // Add all new RTX codecs.
  20.   for (const C& reference_codec : reference_codecs) {
  21.     if (IsRtxCodec(reference_codec) &&
  22.         !FindMatchingCodec<C>(reference_codecs, *offered_codecs,
  23.                               reference_codec, nullptr)) {
  24.       C rtx_codec = reference_codec;
  25.       const C* associated_codec =
  26.           GetAssociatedCodec(reference_codecs, rtx_codec);
  27.       if (!associated_codec) {
  28.         continue;
  29.       }
  30.       // Find a codec in the offered list that matches the reference codec.
  31.       // Its payload type may be different than the reference codec.
  32.       C matching_codec;
  33.       if (!FindMatchingCodec<C>(reference_codecs, *offered_codecs,
  34.                                 *associated_codec, &matching_codec)) {
  35.         RTC_LOG(LS_WARNING)
  36.             << "Couldn't find matching " << associated_codec->name << " codec.";
  37.         continue;
  38.       }
  40.       rtx_codec.params[kCodecParamAssociatedPayloadType] =
  41.           rtc::ToString(matching_codec.id);
  42.       used_pltypes->FindAndSetIdUsed(&rtx_codec);
  43.       offered_codecs->push_back(rtx_codec);
  44.     }
  45.   }
  46. }

FindAndSetIdUsed

H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\pc\used_ids.h

  1. // Loops through all Id in |ids| and changes its id if it is
  2.   // already in use by another IdStruct. Call this methods with all Id
  3.   // in a session description to make sure no duplicate ids exists.
  4.   // Note that typename Id must be a type of IdStruct.
  5.   template <typename Id>
  6.   void FindAndSetIdUsed(std::vector<Id>* ids) {
  7.     for (const Id& id : *ids) {
  8.       FindAndSetIdUsed(&id);
  9.     }
  10.   }
  12.   // Finds and sets an unused id if the |idstruct| id is already in use.
  13.   void FindAndSetIdUsed(IdStruct* idstruct) {
  14.     const int original_id = idstruct->id;
  15.     int new_id = idstruct->id;
  17.     if (original_id > max_allowed_id_ || original_id < min_allowed_id_) {
  18.       // If the original id is not in range - this is an id that can't be
  19.       // dynamically changed.
  20.       return;
  21.     }
  23.     if (IsIdUsed(original_id)) {
  24.       new_id = FindUnusedId();
  25.       RTC_LOG(LS_WARNING) << "Duplicate id found. Reassigning from "
  26.                           << original_id << " to " << new_id;
  27.       idstruct->id = new_id;
  28.     }
  29.     SetIdUsed(new_id);
  30.   }

编解码器ID范围

image.png

编解码器获取新的ID算法

image.png

结论

image.png
H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\pc\session_description.h
SessionDescription