Added mixed audio processor (#2929)

* Use RL_QUADS/RL_TRIANGLES for single-pixel drawing

Addresses problem mentioned in
https://github.com/raysan5/raylib/issues/2744#issuecomment-1273568263
(in short: when drawing pixels using DrawPixel{,V} in camera mode,
upscaled pixel becomes a line instead of bigger pixel)

* [rtextures] Fixed scaling down in ImageTextEx

Closes #2755

* Added global audio processor

* Renamed struct member to follow naming conventions

* Added example for AttachAudioMixedProcessor

4 files changed, 190 insertions, 1 deletions

diff --git a/examples/audio/audio_mixed_processor.c b/examples/audio/audio_mixed_processor.c
new file mode 100644
index 00000000..3a008f3e
--- /dev/null
+++ b/examples/audio/audio_mixed_processor.c
@@ -0,0 +1,123 @@
+/*******************************************************************************************
+*
+*   raylib [audio] example - Mixed audio processing
+*
+*   Example originally created with raylib 4.2, last time updated with raylib 4.2
+*
+*   Example contributed by hkc (@hatkidchan) and reviewed by Ramon Santamaria (@raysan5)
+*
+*   Example licensed under an unmodified zlib/libpng license, which is an OSI-certified,
+*   BSD-like license that allows static linking with closed source software
+*
+*   Copyright (c) 2023 hkc (@hatkidchan)
+*
+********************************************************************************************/
+#include "raylib.h"
+#include <math.h>
+
+static float exponent = 1.0f;                 // Audio exponentiation value
+static float averageVolume[400] = { 0.0f };   // Average volume history
+
+//------------------------------------------------------------------------------------
+// Audio processing function
+//------------------------------------------------------------------------------------
+void ProcessAudio(void *buffer, unsigned int frames)
+{
+    float *samples = (float *)buffer;   // Samples internally stored as <float>s
+    float average = 0.0f;               // Temporary average volume
+
+    for (unsigned int frame = 0; frame < frames; frame++)
+    {
+        float *left = &samples[frame * 2 + 0], *right = &samples[frame * 2 + 1];
+
+        *left = powf(fabsf(*left), exponent) * ( (*left < 0.0f)? -1.0f : 1.0f );
+        *right = powf(fabsf(*right), exponent) * ( (*right < 0.0f)? -1.0f : 1.0f );
+
+        average += fabsf(*left) / frames;   // accumulating average volume
+        average += fabsf(*right) / frames;
+    }
+
+    // Moving history to the left
+    for (int i = 0; i < 399; i++) averageVolume[i] = averageVolume[i + 1];
+
+    averageVolume[399] = average;         // Adding last average value
+}
+
+//------------------------------------------------------------------------------------
+// Program main entry point
+//------------------------------------------------------------------------------------
+int main(void)
+{
+    // Initialization
+    //--------------------------------------------------------------------------------------
+    const int screenWidth = 800;
+    const int screenHeight = 450;
+
+    InitWindow(screenWidth, screenHeight, "raylib [audio] example - processing mixed output");
+
+    InitAudioDevice();              // Initialize audio device
+
+    AttachAudioMixedProcessor(ProcessAudio);
+
+    Music music = LoadMusicStream("resources/country.mp3");
+    Sound sound = LoadSound("resources/coin.wav");
+
+    PlayMusicStream(music);
+
+    SetTargetFPS(60);               // Set our game to run at 60 frames-per-second
+    //--------------------------------------------------------------------------------------
+
+    // Main game loop
+    while (!WindowShouldClose())    // Detect window close button or ESC key
+    {
+        // Update
+        //----------------------------------------------------------------------------------
+        UpdateMusicStream(music);   // Update music buffer with new stream data
+
+        // Modify processing variables
+        //----------------------------------------------------------------------------------
+        if (IsKeyPressed(KEY_LEFT)) exponent -= 0.05f;
+        if (IsKeyPressed(KEY_RIGHT)) exponent += 0.05f;
+
+        if (exponent <= 0.5f) exponent = 0.5f;
+        if (exponent >= 3.0f) exponent = 3.0f;
+
+        if (IsKeyPressed(KEY_SPACE)) PlaySound(sound);
+
+        // Draw
+        //----------------------------------------------------------------------------------
+        BeginDrawing();
+
+            ClearBackground(RAYWHITE);
+
+            DrawText("MUSIC SHOULD BE PLAYING!", 255, 150, 20, LIGHTGRAY);
+
+            DrawText(TextFormat("EXPONENT = %.2f", exponent), 215, 180, 20, LIGHTGRAY);
+
+            DrawRectangle(199, 199, 402, 34, LIGHTGRAY);
+            for (int i = 0; i < 400; i++)
+            {
+                DrawLine(201 + i, 232 - averageVolume[i] * 32, 201 + i, 232, MAROON);
+            }
+            DrawRectangleLines(199, 199, 402, 34, GRAY);
+
+            DrawText("PRESS SPACE TO PLAY OTHER SOUND", 200, 250, 20, LIGHTGRAY);
+            DrawText("USE LEFT AND RIGHT ARROWS TO ALTER DISTORTION", 140, 280, 20, LIGHTGRAY);
+
+        EndDrawing();
+        //----------------------------------------------------------------------------------
+    }
+
+    // De-Initialization
+    //--------------------------------------------------------------------------------------
+    UnloadMusicStream(music);   // Unload music stream buffers from RAM
+
+    DetachAudioMixedProcessor(ProcessAudio);  // Disconnect audio processor
+
+    CloseAudioDevice();         // Close audio device (music streaming is automatically stopped)
+
+    CloseWindow();              // Close window and OpenGL context
+    //--------------------------------------------------------------------------------------
+
+    return 0;
+}
diff --git a/examples/audio/audio_mixed_processor.png b/examples/audio/audio_mixed_processor.png
new file mode 100644
index 00000000..8575a836
--- /dev/null
+++ b/examples/audio/audio_mixed_processor.png
diff --git a/src/raudio.c b/src/raudio.c
index 443fb924..5f533215 100644
--- a/src/raudio.c
+++ b/src/raudio.c
@@ -372,6 +372,7 @@ typedef struct AudioData {
         AudioBuffer *last;          // Pointer to last AudioBuffer in the list
         int defaultSize;            // Default audio buffer size for audio streams
     } Buffer;
+    rAudioProcessor *mixedProcessor;
     struct {
         unsigned int poolCounter;                               // AudioBuffer pointers pool counter
         AudioBuffer *pool[MAX_AUDIO_BUFFER_POOL_CHANNELS];      // Multichannel AudioBuffer pointers pool
@@ -388,7 +389,8 @@ static AudioData AUDIO = {          // Global AUDIO context
     // After some math, considering a sampleRate of 48000, a buffer refill rate of 1/60 seconds and a
     // standard double-buffering system, a 4096 samples buffer has been chosen, it should be enough
     // In case of music-stalls, just increase this number
-    .Buffer.defaultSize = 0
+    .Buffer.defaultSize = 0,
+    .mixedProcessor = NULL
 };
 
 //----------------------------------------------------------------------------------
@@ -2278,6 +2280,60 @@ void DetachAudioStreamProcessor(AudioStream stream, AudioCallback process)
     ma_mutex_unlock(&AUDIO.System.lock);
 }
 
+// Add processor to audio pipeline. Order of processors is important
+// Works the same way as {Attach,Detach}AudioStreamProcessor functions, except
+// these two work on the already mixed output just before sending it to the
+// sound hardware.
+void AttachAudioMixedProcessor(AudioCallback process)
+{
+    ma_mutex_lock(&AUDIO.System.lock);
+
+    rAudioProcessor *processor = (rAudioProcessor *)RL_CALLOC(1, sizeof(rAudioProcessor));
+    processor->process = process;
+
+    rAudioProcessor *last = AUDIO.mixedProcessor;
+
+    while (last && last->next)
+    {
+        last = last->next;
+    }
+    if (last)
+    {
+        processor->prev = last;
+        last->next = processor;
+    }
+    else AUDIO.mixedProcessor = processor;
+
+    ma_mutex_unlock(&AUDIO.System.lock);
+}
+
+void DetachAudioMixedProcessor(AudioCallback process)
+{
+    ma_mutex_lock(&AUDIO.System.lock);
+
+    rAudioProcessor *processor = AUDIO.mixedProcessor;
+
+    while (processor)
+    {
+        rAudioProcessor *next = processor->next;
+        rAudioProcessor *prev = processor->prev;
+
+        if (processor->process == process)
+        {
+            if (AUDIO.mixedProcessor == processor) AUDIO.mixedProcessor = next;
+            if (prev) prev->next = next;
+            if (next) next->prev = prev;
+
+            RL_FREE(processor);
+        }
+
+        processor = next;
+    }
+
+    ma_mutex_unlock(&AUDIO.System.lock);
+}
+
+
 //----------------------------------------------------------------------------------
 // Module specific Functions Definition
 //----------------------------------------------------------------------------------
@@ -2519,6 +2575,13 @@ static void OnSendAudioDataToDevice(ma_device *pDevice, void *pFramesOut, const
         }
     }
 
+    rAudioProcessor *processor = AUDIO.mixedProcessor;
+    while (processor)
+    {
+        processor->process(pFramesOut, frameCount);
+        processor = processor->next;
+    }
+
     ma_mutex_unlock(&AUDIO.System.lock);
 }
 
diff --git a/src/raylib.h b/src/raylib.h
index c1b85abd..73c6d224 100644
--- a/src/raylib.h
+++ b/src/raylib.h
@@ -1580,6 +1580,9 @@ RLAPI void SetAudioStreamCallback(AudioStream stream, AudioCallback callback);
 RLAPI void AttachAudioStreamProcessor(AudioStream stream, AudioCallback processor); // Attach audio stream processor to stream
 RLAPI void DetachAudioStreamProcessor(AudioStream stream, AudioCallback processor); // Detach audio stream processor from stream
 
+RLAPI void AttachAudioMixedProcessor(AudioCallback processor); // Attach audio stream processor to the entire audio pipeline
+RLAPI void DetachAudioMixedProcessor(AudioCallback processor); // Detach audio stream processor from the entire audio pipeline
+
 #if defined(__cplusplus)
 }
 #endif