ngs2: mix ports without matrix levels with the default matrix

Co-authored-by: Phr3d13 <phr3d13@gmail.com>

src/libs/ngs2.cpp

@@ -1803,6 +1803,18 @@ static void Ngs2ConsumeSamples(Ngs2VoiceInternal& voice, uint32_t grain) {
 	voice.has_samples = output != 0;
 }
 
+// Mono and stereo convert at -3 dB, mono feeds the surround front center, others map 1:1.
+static float Ngs2DefaultMatrixLevel(uint32_t src_channels, uint32_t src, uint32_t dst_channels,
+                                    size_t dst) {
+	if ((src_channels == 1 && dst_channels == 2) || (src_channels == 2 && dst_channels == 1)) {
+		return std::numbers::sqrt2_v<float> / 2;
+	}
+	if (src_channels == 1 && dst_channels > 2) {
+		return dst == 2 ? 1.0f : 0.0f;
+	}
+	return src == dst ? 1.0f : 0.0f;
+}
+
 static void Ngs2RenderVoice(Ngs2VoiceInternal& voice, const std::vector<Ngs2VoiceInternal*>& voices,
                             uint32_t grain) {
 	if (voice.rendered) {
@@ -1851,7 +1863,11 @@ static void Ngs2RenderVoice(Ngs2VoiceInternal& voice, const std::vector<Ngs2Voic
 				EXIT_NOT_IMPLEMENTED(port.input >= voice.rack->NumBuffers());
 				const auto* input  = source->Buffer(source->rack->PortBufferId(p), grain);
 				auto*       output = voice.Buffer(port.input, grain);
+				// A port without a matrix, or whose matrix has no levels, uses the default mix.
 				const auto* matrix = port.matrix < 0 ? nullptr : &source->matrices.at(port.matrix);
+				if (matrix != nullptr && matrix->empty()) {
+					matrix = nullptr;
+				}
 				const size_t output_channels =
 				    matrix == nullptr
 				        ? voice.channels
@@ -1860,7 +1876,8 @@ static void Ngs2RenderVoice(Ngs2VoiceInternal& voice, const std::vector<Ngs2Voic
 					for (uint32_t src = 0; src < source->channels; ++src) {
 						const float level =
 						    port.volume * (matrix == nullptr
-						                       ? (src == dst ? 1.0f : 0.0f)
+						                       ? Ngs2DefaultMatrixLevel(source->channels, src,
+						                                                voice.channels, dst)
 						                       : (*matrix)[dst * source->channels + src]);
 						for (uint32_t i = 0; i < grain; ++i) {
 							output[dst * grain + i] += input[src * grain + i] * level;

tests/Ngs2SamplerTests.cpp

@@ -1096,6 +1096,35 @@ void TestMonoRateAndRouting() {
 	      "22.05 kHz effect duration was incorrect");
 }
 
+void TestDefaultMatrix() {
+	struct Case {
+		uint32_t           src_channels;
+		int32_t            matrix;
+		uint32_t           dst_channels;
+		std::vector<float> expected;
+	};
+	constexpr float minus_3db = std::numbers::sqrt2_v<float> / 2;
+	const int16_t   pcm[]     = {8192, -16384};
+	for (const auto& c: {Case {1, -1, 2, {0.25f * minus_3db, 0.25f * minus_3db}},
+	                     Case {1, 0, 2, {0.25f * minus_3db, 0.25f * minus_3db}},
+	                     Case {2, -1, 1, {0.25f * minus_3db + -0.5f * minus_3db}},
+	                     Case {1, -1, 6, {0, 0, 0.25f, 0, 0, 0}}}) {
+		Fixture          f(48000, c.src_channels);
+		Ngs2RackInternal rack {};
+		rack.ngs  = &f.system;
+		rack.type = Ngs2RackType::Mastering;
+		Ngs2VoiceInternal dest;
+		dest.rack     = &rack;
+		dest.channels = c.dst_channels;
+		dest.state    = Ngs2VoicePlayState::Playing;
+		f.voice.matrices.resize(1);
+		f.voice.ports.push_back({&dest, 0, 1.0f, c.matrix});
+		f.Queue(pcm, 1, 0);
+		Ngs2RenderVoice(dest, {&f.voice, &dest}, 1);
+		Check(dest.samples == c.expected, "default port mix produced the wrong channel levels");
+	}
+}
+
 void TestMatrixOutputChannels() {
 	Fixture f(48000, 2);
 	Ngs2RackInternal mastering_rack {};
@@ -1933,6 +1962,7 @@ int main() {
 	TestPcmSkipAcrossBlocks();
 	TestPcmBlocksEndWithTheirData();
 	TestMonoRateAndRouting();
+	TestDefaultMatrix();
 	TestMatrixOutputChannels();
 	TestCustomPcmStillPlays();
 	TestSetupQuery();