Pan stereo sounds towards where they are

A sound with a position plays on a 3D channel, but those are mono: a stereo one (a Custom Edition map's gunfire, a bullet's impact) fell back to a stereo channel, which the Xbox never panned, so it was only faded with distance and came from nowhere. update_channels now gives a stereo channel's sound in the world a pan towards it, as the mixer pans a 3D voice; the mixer pans the channels' middle and keeps their difference as wide as the far ear's gain. Unpositioned stereo plays as before, and no channel pools change.

port/linux/src/dsound_sdl.c

@@ -91,6 +91,10 @@ struct sdl_stream
 	/* 2D gains */
 	float volume;             /* SetVolume */
 	float mix_left, mix_right;
+	/* a stereo voice of a sound in the world: panned towards it, -1 left
+	to 1 right (dsound_sdl_stream_set_stereo_pan) */
+	BOOL stereo_positioned;
+	float stereo_pan;
 	float headroom;
 
 	/* 3D */
@@ -399,6 +403,15 @@ static void voice_gains(const struct sdl_stream *stream, float *left, float *rig
 				*room_lowpass = lowpass_coefficient(gain_from_millibels(high_level - level), cosine);
 		}
 	}
+	else if (stream->channels == 2 && stream->stereo_positioned)
+	{
+		/* the equal power pan of a 3D voice, at the gains of a 2D one when
+		centred (the game fades it with distance) */
+		float angle = (stream->stereo_pan + 1.0f) * 0.25f * 3.14159265f;
+
+		*left = cosf(angle) * 1.41421356f * stream->mix_left;
+		*right = sinf(angle) * 1.41421356f * stream->mix_right;
+	}
 	else
 	{
 		*left = stream->mix_left;
@@ -554,6 +567,8 @@ static void mix_voice(struct sdl_stream *stream, float *output, float *send, uns
 	float ramp_direct_lowpass, ramp_room_lowpass;
 	long width;
 	unsigned long frame;
+	/* (a stereo voice panned towards its sound: voice_gains) */
+	BOOL positioned = stream->channels == 2 && stream->stereo_positioned;
 
 	if (stream->paused || !stream->packet_count || !stream->sample_rate)
 		return;
@@ -665,6 +680,18 @@ static void mix_voice(struct sdl_stream *stream, float *output, float *send, uns
 			output[frame * 2] += sample_left * left;
 			output[frame * 2 + 1] += sample_left * right;
 		}
+		else if (positioned)
+		{
+			/* the channels' middle panned, and their difference kept as
+			wide as the far ear's gain: the sound comes from where it is,
+			still stereo */
+			float middle = 0.5f * (sample_left + sample_right);
+			float side = 0.5f * (sample_left - sample_right);
+			float far_gain = left < right ? left : right;
+
+			output[frame * 2] += middle * left + side * far_gain;
+			output[frame * 2 + 1] += middle * right - side * far_gain;
+		}
 		else
 		{
 			output[frame * 2] += sample_left * left;
@@ -1362,10 +1389,26 @@ static HRESULT STDMETHODCALLTYPE stream_flush(IDirectSoundStream *object)
 	}
 	stream->cursor = 0;
 	resampler_reset(stream);
+	/* (the next sound on the channel says where it is) */
+	stream->stereo_positioned = FALSE;
 	pthread_mutex_unlock(&mixer_lock);
 	return S_OK;
 }
 
+/* A stereo voice of a sound in the world (`positioned`) is panned towards
+it, `pan` from -1 (left) to 1 (right); not positioned, it plays as the Xbox
+played every stereo sound, unpanned. The game fades it with distance itself
+(sound_manager.c, update_channels: sound_dsound_xbox.c calls this). */
+void dsound_sdl_stream_set_stereo_pan(IDirectSoundStream *object, BOOL positioned, float pan)
+{
+	struct sdl_stream *stream = stream_from_interface(object);
+
+	pthread_mutex_lock(&mixer_lock);
+	stream->stereo_positioned = positioned && stream->channels == 2;
+	stream->stereo_pan = pan < -1.0f ? -1.0f : (pan > 1.0f ? 1.0f : pan);
+	pthread_mutex_unlock(&mixer_lock);
+}
+
 static IDirectSoundStreamVtbl stream_vtable =
 {
 	stream_add_reference,

source/sound/sound_dsound_xbox.c

@@ -1929,6 +1929,30 @@ static void dsound_virtual_set_location(
 	return;
 }
 
+/* port: a stereo channel's sound in the world is panned towards it, which
+the Xbox's stereo channels were not (sound_manager.c, update_channels;
+port/linux/src/dsound_sdl.c) */
+void dsound_port_set_channel_stereo_pan(
+	short virtual_channel_index,
+	boolean positioned,
+	real pan)
+{
+	extern void dsound_sdl_stream_set_stereo_pan(IDirectSoundStream *stream, BOOL positioned, float pan);
+	short channel_index= dsound_virtual_touch(virtual_channel_index);
+
+	if (channel_index!=NONE)
+	{
+		struct sound_channel *channel= channel_get(channel_index);
+
+		if (channel->stream && TEST_FLAG(channel->type_flags, _sound_channel_stereo_bit))
+		{
+			dsound_sdl_stream_set_stereo_pan(channel->stream, positioned, pan);
+		}
+	}
+
+	return;
+}
+
 static void dsound_virtual_set_properties(
 	short virtual_channel_index,
 	struct platform_sound_channel_properties const *properties,

source/sound/sound_manager.c

@@ -244,6 +244,10 @@ symbols in this file:
 #include <math.h>
 #include <stdio.h>
 
+/* port: a stereo channel's sound in the world panned towards it
+(sound_dsound_xbox.c; update_channels) */
+void dsound_port_set_channel_stereo_pan(short virtual_channel_index, boolean positioned, real pan);
+
 /* ---------- constants */
 
 enum
@@ -3495,6 +3499,11 @@ static void update_channels(
 				switch (sound->source.spatialization_mode)
 				{
 				case _sound_spatialization_mode_none:
+					/* port: (unpanned: dsound_port_set_channel_stereo_pan) */
+					if (TEST_FLAG(channel->type_flags, _sound_channel_stereo_bit))
+					{
+						dsound_port_set_channel_stereo_pan(channel_index, FALSE, 0.f);
+					}
 					break;
 
 				case _sound_spatialization_mode_absolute:
@@ -3530,6 +3539,24 @@ static void update_channels(
 							(maximum_distance - minimum_distance);
 
 						fade *= PIN(attenuation, 0.f, 1.f);
+						/* port: and a stereo sound is panned towards where it
+						is, as the mixer pans a 3D one (port/linux/src/dsound_sdl.c,
+						spatialize: ahead is x, right -y; centred when close),
+						which the Xbox's stereo channels never were: a Custom
+						Edition map's stereo gunfire came from nowhere */
+						if (TEST_FLAG(channel->type_flags, _sound_channel_stereo_bit))
+						{
+							real horizontal = square_root(
+								relative_position.x * relative_position.x +
+								relative_position.y * relative_position.y);
+							real pan = horizontal > 1.0e-4f ? -relative_position.y / horizontal : 0.f;
+
+							if (distance < minimum_distance && minimum_distance > 0.f)
+							{
+								pan *= distance / minimum_distance;
+							}
+							dsound_port_set_channel_stereo_pan(channel_index, TRUE, 0.75f * pan);
+						}
 					}
 					break;