shithub: cstory

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Add optional Lanczos filter to the audio mixer

Should be higher-quality than the linear-interpolator, but also much
slower, so it's disabled by default.

--- a/CMakeLists.txt

+++ b/CMakeLists.txt

@@ -20,6 +20,7 @@

 option(FIX_BUGS "Fix various bugs in the game" OFF)

 option(DEBUG_SAVE "Re-enable the ability to drag-and-drop save files onto the window" OFF)

 option(DOCONFIG "Compile a DoConfig clone tool - not useful for console ports" ON)

+option(LANCZOS_RESAMPLER "Use Lanczos filtering for audio resampling instead of linear-interpolation (Lanczos is more performance-intensive, but higher quality)" OFF)

 set(BACKEND_RENDERER "SDLTexture" CACHE STRING "Which renderer the game should use: 'OpenGL3' for an OpenGL 3.2 renderer, 'OpenGLES2' for an OpenGL ES 2.0 renderer, 'SDLTexture' for SDL2's hardware-accelerated Texture API, 'SDLSurface' for SDL2's software-rendered Surface API, 'Wii U' for the Wii U's hardware-accelerated GX2 API, or 'Software' for a handwritten software renderer")

 set(BACKEND_AUDIO "SDL2" CACHE STRING "Which audio backend the game should use: 'SDL2', 'miniaudio', 'WiiU-Hardware', 'WiiU-Software', or 'Null'")

@@ -266,6 +267,10 @@

 if(DEBUG_SAVE)

 	target_compile_definitions(CSE2 PRIVATE DEBUG_SAVE)

+endif()

+

+if(LANCZOS_RESAMPLER)

+	target_compile_definitions(CSE2 PRIVATE LANCZOS_RESAMPLER)

 endif()

 if(PKG_CONFIG_STATIC_LIBS)

--- a/README.md

+++ b/README.md

@@ -95,6 +95,7 @@

 `-DDEBUG_SAVE=ON` | Re-enable the ability to drag-and-drop save files onto the window

 `-DDOCONFIG=OFF` | Disable compiling the DoConfig tool (it is not useful for console ports)

 `-DDOCONFIG_LEGACY_OPENGL=ON` | Make DoConfig use OpenGL 2.1 instead of OpenGL 3.2 (useful for older/limited platforms)

+`-DLANCZOS_RESAMPLER=ON` | Use Lanczos filtering for audio resampling instead of linear-interpolation (Lanczos is more performance-intensive, but higher quality)

 `-DBACKEND_RENDERER=OpenGL3` | Render with OpenGL 3.2 (hardware-accelerated)

 `-DBACKEND_RENDERER=OpenGLES2` | Render with OpenGL ES 2.0 (hardware-accelerated)

 `-DBACKEND_RENDERER=SDLTexture` | (Default) Render with SDL2's Texture API (hardware-accelerated) (note: requires `-DBACKEND_PLATFORM=SDL2`)

--- a/src/Backends/Audio/SoftwareMixer.cpp

+++ b/src/Backends/Audio/SoftwareMixer.cpp

@@ -51,7 +51,11 @@

 	if (sound == NULL)

 		return NULL;

-	sound->samples = (signed char*)malloc(length + 1);

+#ifdef LANCZOS_RESAMPLER

+	sound->samples = (signed char*)malloc(length);

+#else

+	sound->samples = (signed char*)malloc(length + 1);	// +1 for the linear-interpolator

+#endif

 	if (sound->samples == NULL)

 	{

@@ -96,7 +100,9 @@

 	sound->playing = true;

 	sound->looping = looping;

+#ifndef LANCZOS_RESAMPLER

 	sound->samples[sound->frames] = looping ? sound->samples[0] : 0;	// For the linear interpolator

+#endif

 }

 void Mixer_StopSound(Mixer_Sound *sound)

@@ -143,15 +149,43 @@

 			for (size_t frames_done = 0; frames_done < frames_total; ++frames_done)

 			{

+			#ifdef LANCZOS_RESAMPLER

+				// Perform Lanczos resampling

+				const int kernel_radius = 2;

+

+				double accumulator = 0;

+

+				for (int i = -MIN(kernel_radius - 1, sound->position); i <= kernel_radius; ++i)

+				{

+					const signed char input_sample = sound->samples[(sound->position + i) % sound->frames];

+

+					const double kernel_input = ((double)sound->position_subsample / 0x10000) - i;

+

+					if (kernel_input == 0.0)

+					{

+						accumulator += input_sample;

+					}

+					else

+					{

+						const double nx = 3.14159265358979323846 * kernel_input;

+						const double nxa = nx / kernel_radius;

+

+						accumulator += input_sample * (sin(nx) / nx) * (sin(nxa) / nxa);

+					}

+				}

+

+				const short output_sample = (short)(accumulator * 0x100);

+			#else

 				// Perform linear interpolation

-				const unsigned char subsample = sound->position_subsample >> 8;

+				const unsigned char interpolation_scale = sound->position_subsample >> 8;

-				const short interpolated_sample = sound->samples[sound->position] * (0x100 - subsample)

-				                                + sound->samples[sound->position + 1] * subsample;

+				const short output_sample = sound->samples[sound->position] * (0x100 - interpolation_scale)

+				                          + sound->samples[sound->position + 1] * interpolation_scale;

+			#endif

 				// Mix, and apply volume

-				*stream_pointer++ += (interpolated_sample * sound->volume_l) >> 8;

-				*stream_pointer++ += (interpolated_sample * sound->volume_r) >> 8;

+				*stream_pointer++ += (output_sample * sound->volume_l) >> 8;

+				*stream_pointer++ += (output_sample * sound->volume_r) >> 8;

 				// Increment sample

 				const unsigned long next_position_subsample = sound->position_subsample + sound->advance_delta;