Fix T49764: Audio strips crackle when animating the volume

- Implemented linear interpolation for volume changes in the software
mixer.
- Using this in the software device.

intern/audaspace/intern/AUD_Mixer.cpp

@@ -95,6 +95,21 @@ void AUD_Mixer::mix(sample_t* buffer, int start, int length, float volume)
 		out[i + start] += buffer[i] * volume;
 }
 
+void AUD_Mixer::mix(sample_t* buffer, int start, int length, float volume_to, float volume_from)
+{
+	sample_t* out = m_buffer.getBuffer();
+
+	length = (std::min(m_length, length + start) - start);
+
+	for(int i = 0; i < length; i++)
+	{
+		float volume = volume_from * (1.0f - i / float(length)) + volume_to * (i / float(length));
+
+		for(int c = 0; c < m_specs.channels; c++)
+			out[(i + start) * m_specs.channels + c] += buffer[i * m_specs.channels + c] * volume;
+	}
+}
+
 void AUD_Mixer::read(data_t* buffer, float volume)
 {
 	sample_t* out = m_buffer.getBuffer();

intern/audaspace/intern/AUD_Mixer.h

@@ -95,6 +95,8 @@ public:
 	 */
 	void mix(sample_t* buffer, int start, int length, float volume);
 
+	void mix(sample_t* buffer, int start, int length, float volume_to, float volume_from);
+
 	/**
 	 * Writes the mixing buffer into an output buffer.
 	 * \param buffer The target buffer for superposing.

intern/audaspace/intern/AUD_SoftwareDevice.cpp

@@ -89,7 +89,7 @@ bool AUD_SoftwareDevice::AUD_SoftwareHandle::pause(bool keep)
 }
 
 AUD_SoftwareDevice::AUD_SoftwareHandle::AUD_SoftwareHandle(AUD_SoftwareDevice* device, boost::shared_ptr<AUD_IReader> reader, boost::shared_ptr<AUD_PitchReader> pitch, boost::shared_ptr<AUD_ResampleReader> resampler, boost::shared_ptr<AUD_ChannelMapperReader> mapper, bool keep) :
-	m_reader(reader), m_pitch(pitch), m_resampler(resampler), m_mapper(mapper), m_keep(keep), m_user_pitch(1.0f), m_user_volume(1.0f), m_user_pan(0.0f), m_volume(1.0f), m_loopcount(0),
+	m_reader(reader), m_pitch(pitch), m_resampler(resampler), m_mapper(mapper), m_keep(keep), m_user_pitch(1.0f), m_user_volume(1.0f), m_user_pan(0.0f), m_volume(1.0f), m_old_volume(1.0f), m_loopcount(0),
 	m_relative(true), m_volume_max(1.0f), m_volume_min(0), m_distance_max(std::numeric_limits<float>::max()),
 	m_distance_reference(1.0f), m_attenuation(1.0f), m_cone_angle_outer(M_PI), m_cone_angle_inner(M_PI), m_cone_volume_outer(0),
 	m_flags(AUD_RENDER_CONE), m_stop(NULL), m_stop_data(NULL), m_status(AUD_STATUS_PLAYING), m_device(device)
@@ -100,6 +100,8 @@ void AUD_SoftwareDevice::AUD_SoftwareHandle::update()
 {
 	int flags = 0;
 
+	m_old_volume = m_volume;
+
 	AUD_Vector3 SL;
 	if(m_relative)
 		SL = -m_location;
@@ -404,7 +406,7 @@ bool AUD_SoftwareDevice::AUD_SoftwareHandle::setVolume(float volume)
 
 	if(volume == 0)
 	{
-		m_volume = volume;
+		m_old_volume = m_volume = volume;
 		m_flags |= AUD_RENDER_VOLUME;
 	}
 	else
@@ -772,7 +774,7 @@ void AUD_SoftwareDevice::mix(data_t* buffer, int length)
 			// in case of looping
 			while(pos + len < length && sound->m_loopcount && eos)
 			{
-				m_mixer->mix(buf, pos, len, sound->m_volume);
+				m_mixer->mix(buf, pos, len, sound->m_volume, sound->m_old_volume);
 
 				pos += len;
 
@@ -789,7 +791,7 @@ void AUD_SoftwareDevice::mix(data_t* buffer, int length)
 					break;
 			}
 
-			m_mixer->mix(buf, pos, len, sound->m_volume);
+			m_mixer->mix(buf, pos, len, sound->m_volume, sound->m_old_volume);
 
 			// in case the end of the sound is reached
 			if(eos && !sound->m_loopcount)

intern/audaspace/intern/AUD_SoftwareDevice.h

@@ -84,6 +84,7 @@ protected:
 
 		/// The calculated final volume of the source.
 		float m_volume;
+		float m_old_volume;
 
 		/// The loop count of the source.
 		int m_loopcount;