update API doc of fluid_synth_process() and add usage example

doc/Doxyfile

@@ -122,7 +122,7 @@ HTML_OUTPUT = html
 HTML_FILE_EXTENSION = .html
 HTML_HEADER = 
 HTML_FOOTER = 
-HTML_STYLESHEET = 
+HTML_EXTRA_STYLESHEET = ..doc/doxy_formula.css
 HTML_ALIGN_MEMBERS = YES
 HTML_EXTRA_FILES = ../doc/fluidsettings.xml ../doc/fluidsettings.xsl
 GENERATE_HTMLHELP = NO

doc/Doxyfile.cmake

@@ -122,7 +122,7 @@ HTML_OUTPUT = html
 HTML_FILE_EXTENSION = .html
 HTML_HEADER = 
 HTML_FOOTER = 
-HTML_STYLESHEET = 
+HTML_EXTRA_STYLESHEET = @CMAKE_SOURCE_DIR@/doc/doxy_formula.css
 HTML_ALIGN_MEMBERS = YES
 HTML_EXTRA_FILES = @CMAKE_SOURCE_DIR@/doc/fluidsettings.xml @CMAKE_SOURCE_DIR@/doc/fluidsettings.xsl
 GENERATE_HTMLHELP = NO

doc/doxy_formula.css

@@ -0,0 +1,12 @@
+
+code {
+        background-color: #eeeeee;
+	text-shadow: none;
+	color: black;
+	margin-left: 4px;
+	margin-right: 4px;
+	padding-left: 4px;
+	padding-right: 4px;
+	border-radius: 3px;
+	white-space: nowrap;
+}

doc/fluidsynth-v11-devdoc.txt

@@ -892,3 +892,8 @@ Example of how to register audio drivers using fluid_audio_driver_register() (ad
 \example fluidsynth_sfload_mem.c
 Example of how read a soundfont from memory (advanced users only)
 */
+
+/*!
+\example fluidsynth_process.c
+Usage examples of how to render audio using fluid_synth_process() (advanced users only)
+*/

doc/fluidsynth_process.c

@@ -0,0 +1,105 @@
+/*
+ * This is a C99 program that outlines different usage examples for fluid_synth_process()
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include <fluidsynth.h>
+
+int main()
+{
+    // any arbitrary number of audio samples to render during on call of fluid_synth_process()
+    enum { SAMPLES = 512 };
+
+    // ...creation of synth omitted...
+
+    // USECASE1: render all dry audio channels + reverb and chorus to one stereo channel
+    {
+        // planar sample buffers that received synthesized (monophonic) audio
+        float left[SAMPLES], right[SAMPLES];
+
+        // array of buffers used to setup channel mapping
+        float *dry[1*2], *fx[1*2];
+
+        // first make sure to zero out the sample buffers
+        memset(left, 0, sizeof(left1));
+        memset(right, 0, sizeof(right1));
+
+        // setup channel mapping for a single stereo channel to which to render all dry audio to
+        dry[0] = left;
+        dry[1] = right;
+
+        // setup channel mapping for a single effects channel to which to render reverb and chorus to
+        // just using the same sample buffers as for dry audio is fine here, will cause the effects to be mixed with dry output
+        fx[0] = left;
+        fx[1] = right;
+
+        int err = fluid_synth_process(synth, SAMPLES, 2, fx, 2, dry);
+        if(err == FLUID_FAILED)
+            puts(„oops“);
+
+
+        // USECASE2: only render dry audio and discard effects
+        // same as above, but call fluid_synth_process() like:
+        int err = fluid_synth_process(synth, SAMPLES, 0, NULL, 2, dry);
+        if(err == FLUID_FAILED)
+            puts(„oops“);
+    }
+    
+
+    // USECASE3: render audio and discard all samples
+    {
+        int err = fluid_synth_process(synth, SAMPLES, 0, NULL, 0, NULL);
+        if(err == FLUID_FAILED)
+            puts(„oops“);
+    }
+
+    
+    // USECASE4: multi-channel rendering, i.e. render all audio and effects channels to dedicated audio buffers
+    // ofc it‘s not a good idea to allocate all the arrays on the stack
+    {
+        // lookup number of audio and effect (stereo-)channels of the synth
+        // see „synth.audio-channels“ and „synth.effects-channels“ settings respectively
+        int n_aud_chan = fluid_synth_count_audio_channels(synth);
+        int n_fx_chan = fluid_synth_count_effects_channels(synth);
+
+        // allocate one single sample buffer
+        float samp_buf[SAMPLES * (n_aud_chan + n_fx_chan) * 2];
+
+        // array of buffers used to setup channel mapping
+        float *dry[n_aud_chan * 2], *fx[n_fx_chan * 2];
+
+        // setup buffers to mix dry stereo audio to
+        // buffers are alternating left and right for each audio channel, i.e.:
+        // dry[0] = first audio channel left
+        // dry[1] = first audio channel right
+        // dry[2] = second audio channel left
+        // ...
+        // dry[i*2 + 0] = i‘th audio channel left
+        // dry[i*2 + 1] = i‘th audio channel right
+        for(int i=0; i<n_aud_chan*2; i++)
+        {
+            dry[i] = &samp_buf[i * SAMPLES];
+        }
+
+        // setup buffers to mix effects stereo audio to
+        // similar channel layout as above, but currently special as there are only 2 hardcoded effects channels:
+        // fx[0] = global reverb channel left
+        // fx[1] = global reverb channel right
+        // fx[2] = global chorus channel left
+        // fx[3] = global chorus channel right
+        for(int i=0; i<n_fx_chan*2; i++)
+        {
+            fx[i] = &samp_buf[n_aud_chan*2*SAMPLES + i * SAMPLES];
+        }
+
+        // dont forget to zero sample buffer(s) before each rendering
+        memset(samp_buf, 0, sizeof(samp_buf));
+
+        int err = fluid_synth_process(synth, SAMPLES, n_fx_chan*2, fx, n_aud_chan*2, dry);
+        if(err == FLUID_FAILED)
+            puts(„oops“);
+    }
+
+    return 0;
+}

include/fluidsynth/audio.h

@@ -41,19 +41,20 @@ extern "C" {
 
 /**
  * Callback function type used with new_fluid_audio_driver2() to allow for
- * custom user audio processing before the audio is sent to the driver.  This
- * function is responsible for rendering the audio to the buffers.
+ * custom user audio processing before the audio is sent to the driver. This
+ * function is responsible for rendering the audio to the buffers. For details
+ * please refer to fluid_synth_process().
  * @param data The user data parameter as passed to new_fluid_audio_driver2().
- * @param len Length of the audio in frames.
- * @param nin Count of buffers in 'in'
- * @param in Not used currently
- * @param nout Count of arrays in 'out' (i.e., channel count)
- * @param out Output buffers, one for each channel
- * @return Should return 0 on success, non-zero if an error occured.
+ * @param len Count of audio frames to synthesize.
+ * @param nfx Count of arrays in \c fx.
+ * @param fx Array of buffers to store effects audio to. Buffers may alias with buffers of \c out.
+ * @param nout Count of arrays in \c out.
+ * @param out Array of buffers to store (dry) audio to. Buffers may alias with buffers of \c fx.
+ * @return Should return #FLUID_OK on success, #FLUID_FAILED if an error occured.
  */
 typedef int (*fluid_audio_func_t)(void* data, int len,
-				 int nin, float** in,
-				 int nout, float** out);
+				 int nfx, float* fx[],
+				 int nout, float* out[]);
 
 FLUIDSYNTH_API fluid_audio_driver_t* new_fluid_audio_driver(fluid_settings_t* settings,
 							 fluid_synth_t* synth);

include/fluidsynth/synth.h

@@ -276,8 +276,8 @@ FLUIDSYNTH_API int fluid_synth_nwrite_float(fluid_synth_t* synth, int len,
 					  float** left, float** right, 
 					  float** fx_left, float** fx_right);
 FLUIDSYNTH_API int fluid_synth_process(fluid_synth_t* synth, int len,
-				     int nin, float** in, 
-				     int nout, float** out);
+				     int nfx, float* fx[], 
+				     int nout, float* out[]);
 
 
 /* Synthesizer's interface to handle SoundFont loaders */

src/synth/fluid_synth.c

@@ -2845,7 +2845,7 @@ fluid_synth_program_reset(fluid_synth_t* synth)
  * @note Should only be called from synthesis thread.
  * 
  * Usage example:
- * @code
+ * @code{.cpp}
     const int FramesToRender = 64;
     int channels;
     // retrieve number of stereo audio channels
@@ -3006,25 +3006,64 @@ fluid_synth_nwrite_float(fluid_synth_t* synth, int len,
 }
 
 /**
- * Synthesize floating point audio to audio buffers.
- * @param synth FluidSynth instance
- * @param len Count of audio frames to synthesize
- * @param nin Ignored
- * @param in Ignored
- * @param nout Count of arrays in 'out'
- * @param out Array of arrays to store audio to
- * @return #FLUID_OK on success, #FLUID_FAILED otherwise
+ * @brief Synthesize floating point audio to planar audio buffers.
  *
- * This function implements the default interface defined in fluidsynth/audio.h.
- * 
+ * Synthesize and <strong>mix</strong> audio to a given number of stereo audio channels.
+ * Therefore pass <code>nout = i*2</code> float buffers to \p out in order to render
+ * the synthesized audio to \p i stereo channels. Each float buffer must be
+ * able to hold \p len elements.
+ *
+ * \p out contains an array of planar buffers for normal, dry, stereo
+ * audio (alternating left and right). Like:
+@code{.cpp}
+out[0]  = left_buffer_channel_1
+out[1]  = right_buffer_channel_1
+out[2]  = left_buffer_channel_2
+out[3]  = right_buffer_channel_2
+...
+out[ (i-1) * 2 + 0 ]  = left_buffer_channel_i
+out[ (i-1) * 2 + 1 ]  = right_buffer_channel_i
+@endcode
+ *
+ * for one-based channel index \p i.
+ * Same buffer layout is used for \p fx for storing effects
+ * like reverb and chorus audio.
+ * This function implements the default interface #fluid_audio_func_t.
+ *
+ * @param synth FluidSynth instance
+ * @param len Count of audio frames to synthesize.
+ * @param nfx Count of arrays in \c fx. Must be a multiple of 2 (because of stereo)
+ * and in the range <code>0 <= nfx/2 <= fluid_synth_count_effects_channels()</code>.
+ * @param fx Array of buffers to store effects audio to. Buffers may
+alias with buffers of \c out.
+ * @param nout Count of arrays in \c out. Must be a multiple of 2
+(because of stereo) and in the range <code>0 <= nout/2 <= fluid_synth_count_audio_channels()</code>.
+ * @param out Array of buffers to store (dry) audio to. Buffers may
+alias with buffers of \c fx.
+ * @return #FLUID_OK on success, #FLUID_FAILED otherwise.
+ *
+ * @parblock
+ * @note Make sure to zero out the sample buffers before calling this
+ * function as any synthesized audio is mixed (i.e. added) to the buffers.
+ * @endparblock
+ *
+ * @parblock
+ * @note No matter how many buffers you pass in, fluid_synth_process()
+ * will always render all fluid_synth_count_audio_channels() to the
+ * buffers in \c out and all fluid_synth_count_effects_channels() to the
+ * buffers in \c fx, provided that <code>nout > 0</code> and <code>nfx > 0</code> respectively. If
+ * <code>nout/2 < fluid_synth_count_audio_channels()</code> it will wrap around. Same
+ * is true for effects audio if <code>nfx/2 < fluid_synth_count_effects_channels()</code>.
+ * See usage examples below.
+ * @endparblock
+ *
+ * @parblock
  * @note Should only be called from synthesis thread.
- */
-/*
- * FIXME: Currently if nout != 2 memory allocation will occur!
+ * @endparblock
  */
 int
-fluid_synth_process(fluid_synth_t* synth, int len, int nin, float** in,
-                    int nout, float** out)
+fluid_synth_process(fluid_synth_t* synth, int len, int nfx, float* fx[],
+                    int nout, float* out[])
 {
   if (nout==2) {
     return fluid_synth_write_float(synth, len, out[0], 0, 1, out[1], 0, 1);