Fix 5.1 channel mapping on Steam Link

app/streaming/audio/audio.cpp

@@ -145,29 +145,34 @@ int Session::arInit(int /* audioConfiguration */,
 
     SDL_memcpy(&s_ActiveSession->m_AudioConfig, opusConfig, sizeof(*opusConfig));
 
+    s_ActiveSession->m_AudioRenderer = s_ActiveSession->createAudioRenderer(&s_ActiveSession->m_AudioConfig);
+    if (s_ActiveSession->m_AudioRenderer == nullptr) {
+        return -2;
+    }
+
+    // Allow the chosen renderer to remap Opus channels as needed to ensure proper output
+    s_ActiveSession->m_AudioRenderer->remapChannels(&s_ActiveSession->m_AudioConfig);
+
+    // Create the Opus decoder with the renderer's preferred channel mapping
     s_ActiveSession->m_OpusDecoder =
-            opus_multistream_decoder_create(opusConfig->sampleRate,
+            opus_multistream_decoder_create(s_ActiveSession->m_AudioConfig.sampleRate,
-                                            opusConfig->channelCount,
+                                            s_ActiveSession->m_AudioConfig.channelCount,
-                                            opusConfig->streams,
+                                            s_ActiveSession->m_AudioConfig.streams,
-                                            opusConfig->coupledStreams,
+                                            s_ActiveSession->m_AudioConfig.coupledStreams,
-                                            opusConfig->mapping,
+                                            s_ActiveSession->m_AudioConfig.mapping,
                                             &error);
     if (s_ActiveSession->m_OpusDecoder == NULL) {
+        delete s_ActiveSession->m_AudioRenderer;
+        s_ActiveSession->m_AudioRenderer = nullptr;
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                      "Failed to create decoder: %d",
                      error);
         return -1;
     }
 
-    s_ActiveSession->m_AudioRenderer = s_ActiveSession->createAudioRenderer(opusConfig);
-    if (s_ActiveSession->m_AudioRenderer == nullptr) {
-        opus_multistream_decoder_destroy(s_ActiveSession->m_OpusDecoder);
-        return -2;
-    }
-
     SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
                 "Audio stream has %d channels",
-                opusConfig->channelCount);
+                s_ActiveSession->m_AudioConfig.channelCount);
 
     return 0;
 }

app/streaming/audio/renderers/renderer.h

@@ -15,4 +15,14 @@ public:
     virtual bool submitAudio(int bytesWritten) = 0;
 
     virtual int getCapabilities() = 0;
+
+    virtual void remapChannels(POPUS_MULTISTREAM_CONFIGURATION) {
+        // Use default channel mapping:
+        // 0 - Front Left
+        // 1 - Front Right
+        // 2 - Center
+        // 3 - LFE
+        // 4 - Surround Left
+        // 5 - Surround Right
+    }
 };

app/streaming/audio/renderers/slaud.cpp

@@ -43,6 +43,29 @@ bool SLAudioRenderer::prepareForPlayback(const OPUS_MULTISTREAM_CONFIGURATION* o
     return true;
 }
 
+#define SWAP_CHANNEL(i, j) \
+    tmp = opusConfig->mapping[i]; \
+    opusConfig->mapping[i] = opusConfig->mapping[j]; \
+    opusConfig->mapping[j] = tmp
+
+void SLAudioRenderer::remapChannels(POPUS_MULTISTREAM_CONFIGURATION opusConfig) {
+    unsigned char tmp;
+
+    if (opusConfig->channelCount == 6) {
+        // The Moonlight's default channel order is FL,FR,C,LFE,RL,RR
+        // SLAudio expects FL,C,FR,RL,RR,LFE so we swap the channels around to match
+
+        // Swap FR and C - now FL,C,FR,LFE,RL,RR
+        SWAP_CHANNEL(1, 2);
+
+        // Swap LFE and RR - now FL,C,FR,RR,RL,LFE
+        SWAP_CHANNEL(3, 5);
+
+        // Swap RR and RL - now FL,C,FR,RL,RR,LFE
+        SWAP_CHANNEL(4, 5);
+    }
+}
+
 void* SLAudioRenderer::getAudioBuffer(int* size)
 {
     SDL_assert(*size == m_AudioBufferSize);

app/streaming/audio/renderers/slaud.h

@@ -18,6 +18,8 @@ public:
 
     virtual int getCapabilities();
 
+    virtual void remapChannels(POPUS_MULTISTREAM_CONFIGURATION opusConfig);
+
 private:
     static void slLogCallback(void* context, ESLAudioLog logLevel, const char* message);