iOS开发-WebRTC本地直播高分辨率不显示画面问题

在之前使用WebRTC结合ossrs进行推流时候，ossrs的播放端无法看到高分辨率画面问题。根据这个问题，找到了解决方案。

一、WebRTC是什么

WebRTC是什么呢？

WebRTC (Web Real-Time Communications) 是一项实时通讯技术，它允许网络应用或者站点，在不借助中间媒介的情况下，建立浏览器之间点对点（Peer-to-Peer）的连接，实现视频流和（或）音频流或者其他任意数据的传输。

二、ossrs是什么？

ossrs是什么呢？
SRS(Simple Realtime Server)是一个简单高效的实时视频服务器，支持RTMP、WebRTC、HLS、HTTP-FLV、SRT等多种实时流媒体协议。

官网地址：https://ossrs.net/lts/zh-cn/

这里暂时不写iOS Google WebRTC与ossrs实现RTC直播了。暂时值记录一下本地WebRTC直播高分辨率不显示画面问题。

三、高分辨率不显示画面问题解决方案

本地WebRTC直播高分辨率不显示画面问题，这个问题和SDP中的profile-level-id有关。

profile-level-id正好是SPS中的第二至四个字节的base16编码。这三个字节的具体含义是

sps[1] AVCProfileIndication
sps[2] profile_compatibility
sps[3] AVCLevlIndication

http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC#Levels
实际设置时，就是level值乘以10，例如level 1.0，设置值就是0x0A。level 3.0，设置值就是0x1E。比较例外的是level 1b，设置值是0x09

图片来源网络（抱歉忘记地址了，如果引用了您的博客图片，请留言）
在这里插入图片描述

第三个代表level，比如1f值为31，从图中可以看出3.1，分辨率在720480 720576 1280*720

如果需要高分辨率 19201080 25601920 3840*2160 ，需要设置的level 5.1 16进制值为33，这里设置的值是42e033。

下面是一个SDP示例

“code”: 0,
“server”: “vid-415v5lz”,
“sdp”: “v=0\r\no=SRS/4.0.268(Leo) 94003279212192 2 IN IP4 0.0.0.0\r\ns=SRSPublishSession\r\nt=0 0\r\na=ice-lite\r\na=group:BUNDLE 0 1\r\na=msid-semantic: WMS live/livestream\r\nm=audio 9 UDP/TLS/RTP/SAVPF 111\r\nc=IN IP4 0.0.0.0\r\na=ice-ufrag:q01184s8\r\na=ice-pwd:o25158210twbb093o342910094v0wo5k\r\na=fingerprint:sha-256 6A:66:81:7C:68:91:79:18:05:2C:EE:5F:BF:1B:4B:F4:78:C4:01:06:CC:CC:9E:F0:32:5B:72:21:4A:C2:A1:AA\r\na=setup:passive\r\na=mid:0\r\na=recvonly\r\na=rtcp-mux\r\na=rtcp-rsize\r\na=rtpmap:111 opus/48000/2\r\na=fmtp:111 minptime=10;useinbandfec=1\r\na=candidate:0 1 udp 2130706431 10.0.80.128 8000 typ host generation 0\r\na=candidate:1 1 udp 2130706431 112.124.157.141 8000 typ host generation 0\r\nm=video 9 UDP/TLS/RTP/SAVPF 96 127\r\nc=IN IP4 0.0.0.0\r\na=ice-ufrag:q01184s8\r\na=ice-pwd:o25158210twbb093o342910094v0wo5k\r\na=fingerprint:sha-256 6A:66:81:7C:68:91:79:18:05:2C:EE:5F:BF:1B:4B:F4:78:C4:01:06:CC:CC:9E:F0:32:5B:72:21:4A:C2:A1:AA\r\na=setup:passive\r\na=mid:1\r\na=extmap:5 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01\r\na=recvonly\r\na=rtcp-mux\r\na=rtcp-rsize\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 transport-cc\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 nack pli\r\na=fmtp:96 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f\r\na=rtpmap:127 red/90000\r\na=candidate:0 1 udp 2130706431 10.0.80.128 8000 typ host generation 0\r\na=candidate:1 1 udp 2130706431 112.124.157.141 8000 typ host generation 0\r\n”,
“sessionid”: “q01184s8:oPvh”
}

可以看到这里的profile-level-id=42e01f

如果在WebRTC中将本地的SDP的profile-level-id替换成42e033

[weakSelf.webRTCClient offer:^(RTCSessionDescription *sdp) {
        DebugLog(@"changeSDP2Server offer sdp:%@", sdp);
        NSString *offerSDPString = [SDWebRTCSDPUtil setMediaBitrate:sdp.sdp media:@"video" bitrate:(6*1024*1024)];
        DebugLog(@"changeSDP2Server offerSDPString:%@", offerSDPString);
        [weakSelf changeSDP2Server:offerSDPString];
    }];

将调用rtc/v1/publish/接口获得的remoteSDPString中的profile-level-id更改42e033之后再调用

- (void)setRemoteDescription:(RTC_OBJC_TYPE(RTCSessionDescription) *)sdp
           completionHandler:(nullable void (^)(NSError *_Nullable error))completionHandler;

经过测试，切换高分辨率画面可以正常显示。

NSString *resultRemoteSDPString = [SDWebRTCSDPUtil setMediaBitrate:remoteSDPString media:@"video" bitrate:(6*1024*1024)];
               DebugLog(@"changeSDP2Server resultRemoteSDPString:%@", resultRemoteSDPString);

               RTCSessionDescription *remoteSDP = [[RTCSessionDescription alloc] initWithType:RTCSdpTypeAnswer sdp:resultRemoteSDPString];
               [weakSelf.webRTCClient setRemoteSdp:remoteSDP completion:^(NSError * error) {
                   DebugLog(@"changeSDP2Server setRemoteDescription error:%@", error);
               }];

四、通过RTCVideoEncoderFactory解决高分辨率不显示画面问题解决方案

通过将SDP中的profile-level-id进行更改并不是我的最终解决方法，我最后使用的是通过指定RTCVideoEncoder的RTCVideoCodecInfo中constrainedHighParams的profile-level-id值为42e033

设置codecs的constrainedHighInfo和constrainedBaselineInfo元素。

NSDictionary<NSString *, NSString *> *constrainedHighParams = @{
    @"profile-level-id" : kLevelHighConstrainedHigh,
    @"level-asymmetry-allowed" : @"1",
    @"packetization-mode" : @"1",
  };
  RTCVideoCodecInfo *constrainedHighInfo =
      [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name parameters:constrainedHighParams];
  [codecs addObject:constrainedHighInfo];

  NSDictionary<NSString *, NSString *> *constrainedBaselineParams = @{
    @"profile-level-id" : kLevelHighConstrainedBaseline,
    @"level-asymmetry-allowed" : @"1",
    @"packetization-mode" : @"1",
  };
  RTCVideoCodecInfo *constrainedBaselineInfo =
      [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name parameters:constrainedBaselineParams];
  [codecs addObject:constrainedBaselineInfo];

完整代码如下

SDRTCVideoEncoderFactory.h

#import <Foundation/Foundation.h>
#import <WebRTC/WebRTC.h>

/**
 + (NSArray<RTCVideoCodecInfo *> *)supportedCodecs {
   NSDictionary<NSString *, NSString *> *constrainedHighParams = @{
     @"profile-level-id" : kRTCMaxSupportedH264ProfileLevelConstrainedHigh,
     @"level-asymmetry-allowed" : @"1",
     @"packetization-mode" : @"1",
   };
   RTCVideoCodecInfo *constrainedHighInfo =
       [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name
                                    parameters:constrainedHighParams];
   NSDictionary<NSString *, NSString *> *constrainedBaselineParams = @{
     @"profile-level-id" : kRTCMaxSupportedH264ProfileLevelConstrainedBaseline,
     @"level-asymmetry-allowed" : @"1",
     @"packetization-mode" : @"1",
   };
   RTCVideoCodecInfo *constrainedBaselineInfo =
       [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name
                                    parameters:constrainedBaselineParams];
   RTCVideoCodecInfo *vp8Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp8Name];
 #if defined(RTC_ENABLE_VP9)
   RTCVideoCodecInfo *vp9Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp9Name];
 #endif
   return @[
     constrainedHighInfo,
     constrainedBaselineInfo,
     vp8Info,
 #if defined(RTC_ENABLE_VP9)
     vp9Info,
 #endif
   ];
 }
 */

@interface SDRTCVideoEncoderFactory : NSObject<RTCVideoEncoderFactory>

@end

SDRTCVideoEncoderFactory.m

#import "SDRTCVideoEncoderFactory.h"

static NSString *kLevelHighConstrainedHigh = @"640c33";
static NSString *kLevelHighConstrainedBaseline = @"42e033";

@implementation SDRTCVideoEncoderFactory

- (id<RTCVideoEncoder>)createEncoder:(RTCVideoCodecInfo *)info {
  if ([info.name isEqualToString:kRTCVideoCodecH264Name]) {
    return [[RTCVideoEncoderH264 alloc] initWithCodecInfo:info];
  } else if ([info.name isEqualToString:kRTCVideoCodecVp8Name]) {
    return [RTCVideoEncoderVP8 vp8Encoder];
  } else if ([info.name isEqualToString:kRTCVideoCodecVp9Name]) {
    return [RTCVideoEncoderVP9 vp9Encoder];
  }

  return nil;
}

- (NSArray<RTCVideoCodecInfo *> *)supportedCodecs {
  NSMutableArray<RTCVideoCodecInfo *> *codecs = [NSMutableArray array];

  NSDictionary<NSString *, NSString *> *constrainedHighParams = @{
    @"profile-level-id" : kLevelHighConstrainedHigh,
    @"level-asymmetry-allowed" : @"1",
    @"packetization-mode" : @"1",
  };
  RTCVideoCodecInfo *constrainedHighInfo =
      [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name parameters:constrainedHighParams];
  [codecs addObject:constrainedHighInfo];

  NSDictionary<NSString *, NSString *> *constrainedBaselineParams = @{
    @"profile-level-id" : kLevelHighConstrainedBaseline,
    @"level-asymmetry-allowed" : @"1",
    @"packetization-mode" : @"1",
  };
  RTCVideoCodecInfo *constrainedBaselineInfo =
      [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name parameters:constrainedBaselineParams];
  [codecs addObject:constrainedBaselineInfo];

  RTCVideoCodecInfo *vp8Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp8Name parameters:nil];
  [codecs addObject:vp8Info];

#if defined(RTC_ENABLE_VP9)
    RTCVideoCodecInfo *vp9Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp9Name];
    [codecs addObject:vp9Info];
#endif
  
  return [codecs copy];
}

@end

最后在RTCPeerConnectionFactory初始化的时候做一下设置

#pragma mark - Lazy
- (RTCPeerConnectionFactory *)factory {
    if (!_factory) {
        RTCInitializeSSL();
        
        SDRTCVideoDecoderFactory *decoderFactory = [[SDRTCVideoDecoderFactory alloc] init];
        SDRTCVideoEncoderFactory *encoderFactory = [[SDRTCVideoEncoderFactory alloc] init];
        
//        for (RTCVideoCodecInfo *codec in encoderFactory.supportedCodecs) {
//            DebugLog(@"RTCVideoCodecInfo codec.parameters:%@", codec.parameters);
//        }
        _factory = [[RTCPeerConnectionFactory alloc] initWithEncoderFactory:encoderFactory decoderFactory:decoderFactory];
    }
    return _factory;
}