Sound: add air absorption

currently hard-coded to 0.1f, for testing
Air absorption is basically a low-pass filter
relative to distance between sound source and listener,
so when in your base or zoomed out, front battles
will be heard as only low frequencies.

rts/System/Sound/ALShared.h

@@ -3,6 +3,16 @@
 
 #include "LogOutput.h"
 
+// Copied from efx.h, as this header only exists under windows
+// so far (February 2010).
+// This will change in future versions of OpenAL.
+#if       !defined(AL_AIR_ABSORPTION_FACTOR)
+#define AL_AIR_ABSORPTION_FACTOR                           0x20007
+#define AL_MIN_AIR_ABSORPTION_FACTOR                       0.0f
+#define AL_MAX_AIR_ABSORPTION_FACTOR                       10.0f
+#define AL_DEFAULT_AIR_ABSORPTION_FACTOR                   0.0f
+#endif // !defined(AL_AIR_ABSORPTION_FACTOR)
+
 extern const CLogSubsystem LOG_SOUND;
 
 bool CheckError(const char* msg);

rts/System/Sound/Sound.cpp

@@ -309,6 +309,8 @@ void CSound::StartThread(int maxSounds)
 				}
 			}
 
+			SoundSource::SetAirAbsorption(0.1f);
+
 			// Generate sound sources
 			for (int i = 0; i < maxSounds; i++)
 			{

rts/System/Sound/SoundSource.cpp

@@ -9,11 +9,13 @@
 #include "OggStream.h"
 #include "ALShared.h"
 #include "float3.h"
+#include "Util.h"
 #include "LogOutput.h"
 
 float SoundSource::globalPitch = 1.0;
 float SoundSource::heightAdjustedRolloffModifier = 1.0;
 float SoundSource::referenceDistance = 200.0f;
+float SoundSource::airAbsorption = 0.0f;
 
 struct SoundSource::StreamControl
 {
@@ -144,6 +146,9 @@ void SoundSource::Play(SoundItem* item, const float3& pos, float3 velocity, floa
 	alSourcei(id, AL_BUFFER, item->buffer->GetId());
 	alSourcef(id, AL_GAIN, item->GetGain() * volume);
 	alSourcef(id, AL_PITCH, item->GetPitch() * globalPitch);
+	if (airAbsorption > 0.0f) {
+		alSourcef(id, AL_AIR_ABSORPTION_FACTOR, airAbsorption);
+	}
 	velocity *= item->dopplerScale;
 	alSource3f(id, AL_VELOCITY, velocity.x, velocity.y, velocity.z);
 	if (item->loopTime > 0)
@@ -273,3 +278,32 @@ void SoundSource::SetVolume(float newVol)
 		alSourcef(id, AL_GAIN, newVol * curStream->current->volume);
 	}
 }
+
+void SoundSource::SetAirAbsorption(float factor)
+{
+	// check if we can use air absorption
+	std::string noAirAbsorpReason = "";
+	if (!alcIsExtensionPresent(NULL, "ALC_EXT_EFX"))
+	{
+		noAirAbsorpReason = "ALC_EXT_EFX not supported";
+	}
+	else if (factor <= AL_MIN_AIR_ABSORPTION_FACTOR)
+	{
+		noAirAbsorpReason = "absorption value (" + FloatToString(factor) + ") <= minimum (" + FloatToString(AL_MIN_AIR_ABSORPTION_FACTOR) + ")";
+	}
+	else if (factor > AL_MAX_AIR_ABSORPTION_FACTOR)
+	{
+		noAirAbsorpReason = "absorption value (" + FloatToString(factor) + ") > maximum (" + FloatToString(AL_MAX_AIR_ABSORPTION_FACTOR) + ")";
+	}
+
+	if (noAirAbsorpReason.empty())
+	{
+		airAbsorption = factor;
+		LogObject(LOG_SOUND) << "air absorption enabled, value: " << airAbsorption;
+	}
+	else
+	{
+		airAbsorption = 0.0f;
+		LogObject(LOG_SOUND) << "air absorption disabled, reason: " << noAirAbsorpReason;
+	}
+}

rts/System/Sound/SoundSource.h

@@ -51,6 +51,14 @@ class SoundSource : boost::noncopyable
 		heightAdjustedRolloffModifier = mod > 0.0f ? mod : 0.0f;
 	};
 
+	/**
+	 * Sets the ammount of air absorption.
+	 * Filters out high-frequency sounds, relative to distance.
+	 * Higher value -> filter out a lot on small distances already.
+	 * @param factor from 0.0f (disabled) till 10.0f
+	 */
+	static void SetAirAbsorption(float factor = 0.0f);
+
 private:
 	/// pitch shared by all sources
 	static float globalPitch;
@@ -63,6 +71,7 @@ class SoundSource : boost::noncopyable
 	unsigned loopStop;
 	static float heightAdjustedRolloffModifier;
 	static float referenceDistance;
+	static float airAbsorption;
 };
 
 #endif