Dev Audio Lesson

Every audio lesson produces two things: library code and a demo program. The library — common/audio/forge_audio.h — is the primary deliverable. The lessons teach concepts; the library is what remains when the learning is done. It must be correct, efficient, tested, and safe. The demo program visualizes and controls the audio in action, rendered with SDL GPU and controlled through forge UI panels.

When to use this skill:

• You need to teach audio playback, mixing, spatial sound, or DSP effects
• A learner wants to understand PCM audio, streaming, attenuation, or filters
• The concept benefits from a visual 3D scene showing audio source positions
• New functionality needs to be added to common/audio/forge_audio.h

Smart behavior:

• Before creating a lesson, check if an existing audio lesson already covers it
• Library first, demo second. Design, implement, document, and test the library code before writing a single line of the demo program. The demo exercises the library — it does not replace it.
• Audio lessons are interactive — every concept must be audible in the running program and controllable via forge UI
• Focus on why the technique works, not just the code — connect DSP math to what the listener hears
• Use simple geometric shapes for sound source visualization — the audio is the focus
• All scene geometry comes from common/shapes/forge_shapes.h — never write inline geometry generation functions
• Cross-reference math lessons (vectors, dot products) and GPU lessons where relevant

Arguments

The user (or you) can provide:

• Number: two-digit lesson number (e.g. 01, 02)
• Topic name: kebab-case (e.g. audio-basics, sound-effects)
• Description: what this teaches (e.g. "PCM fundamentals, WAV loading, SDL audio streams")

If any are missing, infer from context or ask.

Steps

1. Analyze what's needed

• Check existing audio lessons: Is there already a lesson for this topic?
• Check common/audio/: Does relevant library code already exist?
• Check common/ui/: Does the UI library provide the widgets needed for audio controls? If a required widget is missing, note it.
• Identify the scope: What specific audio concepts does this lesson cover?
• Find cross-references: Which math/GPU/UI lessons relate?
• Check PLAN.md: Where does this lesson fit in the audio track?

2. Create the lesson directory

lessons/audio/NN-topic-name/

With subdirectories:

lessons/audio/NN-topic-name/
  main.c
  CMakeLists.txt
  README.md
  audio.conf           (gitignored — local path to audio files)
  assets/
    screenshot.png     (lesson screenshot — checked in)

Baseline rendering shaders (shadow, scene, grid, sky, UI) are provided by forge_scene.h — no per-lesson copies needed. If the lesson introduces a topic-specific shader (e.g. a waveform visualization pass), add a shaders/ subdirectory for those files only.

3. Design and implement the audio library code

Every audio lesson adds to common/audio/forge_audio.h. This is not optional — it is the primary deliverable. The demo program exists to exercise and visualize the library; the library is what ships.

For the first audio lesson, create the file. For subsequent lessons, extend it. Design the API before writing the demo. The demo calls the library — never the reverse.

Library standards

Correctness:

• Every function must implement a named, well-understood technique. Cite the source in the doc comment — a textbook, paper, or DSP reference.
• Audio processing must not introduce clicks, pops, or DC offset. Transitions (volume changes, crossfades) must be smooth.
• Sample rate conversion and channel mapping must be handled correctly via SDL audio stream properties.
• Buffer sizes and sample counts must be computed correctly — off-by-one errors in audio produce audible artifacts.

Thread safety:

• SDL audio streams deliver data on a separate audio thread. Any shared state between the main thread and audio processing must use SDL atomics or SDL mutexes.
• Volume, panning, and effect parameters should be set atomically so partial updates don't produce glitches.
• Resource lifetimes must be clear — never free audio data while a stream might still reference it.

Performance:

• Audio processing runs in real time — dropped samples cause audible glitches. Keep per-sample operations simple (multiply, add, table lookup).
• Precompute coefficients (filter taps, attenuation curves, panning tables) at init time or when parameters change, not per-sample.
• Use float for all audio processing. Convert to/from integer formats only at the SDL boundary.

Numerical safety:

• Clamp output samples to [-1.0f, 1.0f] before delivery to prevent clipping distortion beyond what the user intends.
• Guard against division by zero in attenuation calculations (zero distance).
• Filter coefficients must be validated to prevent instability (poles inside the unit circle for IIR filters).

Header-only implementation:

• static inline for all functions — no separate .c compilation unit
• Guard with #ifndef FORGE_AUDIO_H / #define / #endif
• Include only SDL3, "math/forge_math.h", and standard C headers
• No heap allocation in per-sample paths — allocate buffers at init time
• Deterministic: identical input samples and parameters produce identical output

Documentation (every function, no exceptions):

/* Load a WAV file into an audio buffer.
 *
 * Reads the WAV file at the given path using SDL_LoadWAV and converts
 * it to the specified format. The returned buffer owns its sample data
 * and must be freed with forge_audio_buffer_free().
 *
 * Parameters:
 *   path   — file path to the WAV file (must not be NULL)
 *   spec   — desired output format (sample rate, channels, format)
 *
 * Returns:
 *   ForgeAudioBuffer with sample data, or a zero-initialized struct
 *   on failure (check .data != NULL).
 *
 * Usage:
 *   ForgeAudioBuffer buf = forge_audio_load_wav("/path/to/audio/click.wav", spec);
 *   if (!buf.data) { SDL_Log("Failed to load WAV"); return; }
 *
 * See: Audio Lesson 01 — Audio Basics
 * Ref: SDL3 documentation, SDL_LoadWAV
 */

Every doc comment must include:

• Summary (one sentence — what it does)
• Algorithm or technique being implemented
• Parameters with types, valid ranges, and nullability
• Return value (if any) with format/units
• Usage example
• Cross-reference to the lesson that introduces it
• Reference to the source material

Naming:

• Functions: forge_audio_verb_noun() — e.g. forge_audio_init(), forge_audio_load_wav(), forge_audio_play_oneshot()
• Types: ForgeAudioNoun — e.g. ForgeAudioBuffer, ForgeAudioSource, ForgeAudioMixer, ForgeAudioListener
• Constants: FORGE_AUDIO_UPPER — e.g. FORGE_AUDIO_MAX_SOURCES

Core types to establish in Lesson 01:

#ifndef FORGE_AUDIO_H
#define FORGE_AUDIO_H

#include <SDL3/SDL.h>
#include "math/forge_math.h"

/* --- Audio buffer (decoded sample data) --------------------------------- */

typedef struct ForgeAudioBuffer {
    float *data;          /* interleaved float samples                       */
    int    sample_count;  /* total samples (frames * channels)               */
    int    channels;      /* 1 = mono, 2 = stereo                           */
    int    sample_rate;   /* Hz — typically 44100 or 48000                   */
} ForgeAudioBuffer;

/* --- Audio source (a playing instance) ---------------------------------- */

typedef struct ForgeAudioSource {
    const ForgeAudioBuffer *buffer;  /* sample data (not owned)             */
    int    cursor;          /* current read position in samples              */
    float  volume;          /* linear gain [0..1]                            */
    float  pan;             /* stereo pan [-1..1], 0 = center                */
    bool   looping;         /* true = restart at end of buffer               */
    bool   playing;         /* true = actively producing samples             */
} ForgeAudioSource;

/* ... functions grow lesson by lesson ... */

#endif /* FORGE_AUDIO_H */

Testing the library (MANDATORY)

The audio library is tested independently of the demo program. Tests validate correctness, edge cases, and determinism. Every function added to forge_audio.h must have corresponding tests in tests/audio/test_audio.c.

Test categories (every function must have all applicable categories):

1. Basic correctness — Known inputs produce expected outputs. For audio processing functions, verify output samples against hand-computed values.

2. Edge cases — Empty buffers, zero-length audio, single-sample buffers, zero volume, maximum volume, out-of-range panning values, mismatched sample rates.

3. No artifacts — Volume transitions produce no clicks (verify samples are continuous). Looping produces seamless playback (verify loop-point samples match). Mixing does not overflow [-1, 1] without explicit clipping.

4. Determinism — Two identical sources with identical parameters produce bit-identical output.

5. Thread safety — Parameter changes during playback do not crash. Verify with single-threaded simulation (no need to test actual SDL threads in unit tests).

Test infrastructure:

• Test file: tests/audio/test_audio.c
• Register in root CMakeLists.txt as test_audio
• Use the same ASSERT_NEAR / test macros as tests/math/test_math.c
• Run: cmake --build build --target test_audio && ctest --test-dir build -R audio
• Tests must pass before the demo program is written

4. Create the demo program (main.c)

A focused C program that demonstrates audio concepts with both audible output and visual feedback using SDL GPU and forge UI.

MANDATORY: Use forge_scene.h for the rendering baseline. The scene library provides Blinn-Phong lighting, shadow mapping, grid floor, sky gradient, FPS camera, and UI in a single forge_scene_init() call. This eliminates hundreds of lines of rendering boilerplate and lets the lesson focus on audio. See the forge-scene-renderer skill for the full API.

Every audio lesson MUST include these features:

1. SDL GPU rendered scene via forge_scene.h — A 3D scene providing visual context for the audio. For spatial audio lessons, this means visible sound source positions in the scene. For fundamentals lessons, a simple scene with visual feedback (waveform visualization, VU meters as colored geometry) is sufficient. The rendering baseline (Blinn-Phong, grid, shadow map, depth buffer, sRGB swapchain, sky) is provided by forge_scene.h.

2. First-person camera — Provided by forge_scene.h. The camera position (accessible via forge_scene_cam_pos()) doubles as the audio listener position for spatial audio lessons.

3. Forge UI panel — Provided by forge_scene.h via forge_scene_begin_ui() / forge_scene_end_ui(). Every lesson must have at least:

   • Master volume slider
   • Play/pause button (or status indicator)
   • Lesson-specific controls (per-source volume, panning, effect parameters)

4. Capture support — forge_capture.h integration (when FORGE_CAPTURE defined) via scripts/capture_lesson.py.

Audio requirements:

• SDL audio stream — All audio output goes through SDL_AudioStream. Create the stream at init, feed samples in the iterate callback or via SDL's audio device callback.
• User-supplied audio files — Audio files are not checked into the repository. Each lesson has an audio.conf file (gitignored) that points to a local directory containing the user's audio files. See step 7 for the full config workflow.
• Real-time parameter control — UI sliders and keys should change audio parameters (volume, pan, effect intensity) in real time with audible results.

Simulation controls:

Scene geometry via forge_shapes.h (MANDATORY):

All scene geometry must come from common/shapes/forge_shapes.h. Use spheres or icospheres for sound source positions, cones for directional sources. Never write inline geometry generation functions.

Template structure:

/*
 * Audio Lesson NN — Topic Name
 *
 * Demonstrates: [what this shows]
 *
 * Controls:
 *   WASD / Arrow keys — move camera (listener position)
 *   Mouse             — look around
 *   R                 — reset / replay audio
 *   P                 — pause / resume
 *   1–9               — trigger sound effects
 *   +/-               — adjust master volume
 *   Escape            — release mouse / quit
 *
 * SPDX-License-Identifier: Zlib
 */

#define SDL_MAIN_USE_CALLBACKS 1
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>
#include <stddef.h>

#include "math/forge_math.h"
#include "audio/forge_audio.h"
#include "shapes/forge_shapes.h"

#define FORGE_SCENE_IMPLEMENTATION
#include "scene/forge_scene.h"

/* ── Constants ────────────────────────────────────────────────────── */

#define AUDIO_SAMPLE_RATE 48000

/* ── Types ────────────────────────────────────────────────────────── */

typedef struct app_state {
    ForgeScene scene;  /* rendering, camera, shadow map, grid, sky, UI */

    /* GPU resources — app-owned geometry */
    SDL_GPUBuffer *sphere_vb, *sphere_ib;
    int sphere_index_count;

    /* Audio state — lesson-specific */
    SDL_AudioStream *audio_stream;  /* SDL audio output stream          */
    /* ForgeAudioBuffer buffers[N]; */
    /* ForgeAudioSource sources[N]; */
    float master_volume;            /* linear gain [0..1]               */
    bool  audio_paused;             /* true = audio playback frozen     */
} app_state;

Init pattern:

SDL_AppResult SDL_AppInit(void **appstate, int argc, char **argv)
{
    app_state *state = SDL_calloc(1, sizeof(*state));
    if (!state) return SDL_APP_FAILURE;
    *appstate = state;

    ForgeSceneConfig cfg = forge_scene_default_config("Audio NN — Topic");
    cfg.cam_start_pos = vec3_create(0.0f, 4.0f, 12.0f);
    cfg.font_path = "assets/fonts/liberation_mono/LiberationMono-Regular.ttf";

    if (!forge_scene_init(&state->scene, &cfg, argc, argv))
        return SDL_APP_FAILURE;

    state->master_volume = 1.0f;

    /* Read audio directory from audio.conf */
    char audio_dir[512];
    if (!read_audio_conf("audio.conf", audio_dir, sizeof(audio_dir))) {
        SDL_Log("audio.conf not found or not configured.");
        SDL_Log("See the lesson README for setup instructions.");
        return SDL_APP_FAILURE;
    }

    /* Load audio files from user's directory, set up SDL audio stream,
     * upload shapes ... */
    return SDL_APP_CONTINUE;
}

Iterate pattern (camera position as listener):

SDL_AppResult SDL_AppIterate(void *appstate)
{
    app_state *state = appstate;
    ForgeScene *s = &state->scene;

    if (!forge_scene_begin_frame(s)) return SDL_APP_CONTINUE;

    /* Use camera position as audio listener */
    vec3 listener_pos = forge_scene_cam_pos(s);

    /* Shadow pass */
    forge_scene_begin_shadow_pass(s);
    /* forge_scene_draw_shadow_mesh(s, vb, ib, count, model); */
    forge_scene_end_shadow_pass(s);

    /* Main pass */
    forge_scene_begin_main_pass(s);
    /* Draw sound source markers as colored spheres */
    /* forge_scene_draw_mesh(s, vb, ib, count, model, color); */
    forge_scene_draw_grid(s);
    forge_scene_end_main_pass(s);

    /* UI pass — audio controls */
    float mx, my;
    Uint32 buttons = SDL_GetMouseState(&mx, &my);
    forge_scene_begin_ui(s, mx, my, (buttons & SDL_BUTTON_LMASK) != 0);
    /* forge_ui_* slider/button calls for volume, pan, effects ... */
    forge_scene_end_ui(s);

    return forge_scene_end_frame(s);
}

5. Shaders

The baseline shaders (scene, grid, shadow, sky, UI) are compiled into forge_scene.h — no per-lesson copies needed. If the lesson introduces a topic-specific shader (e.g. a waveform or spectrum visualization pass), add those files to shaders/ and compile them:

python scripts/compile_shaders.py audio/NN-topic-name

Most audio lessons need no lesson-specific shaders at all.

6. Create CMakeLists.txt

add_executable(NN-topic-name WIN32 main.c)
target_include_directories(NN-topic-name PRIVATE ${FORGE_COMMON_DIR})
target_link_libraries(NN-topic-name PRIVATE SDL3::SDL3
    $<$<NOT:$<C_COMPILER_ID:MSVC>>:m>)

if(FORGE_CAPTURE)
    target_compile_definitions(NN-topic-name PRIVATE FORGE_CAPTURE)
endif()

# Copy SDL3 DLL next to executable (Windows)
if(TARGET SDL3::SDL3-shared)
    add_custom_command(TARGET NN-topic-name POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy_if_different
            $<TARGET_FILE:SDL3::SDL3-shared>
            $<TARGET_FILE_DIR:NN-topic-name>
    )
endif()

# Copy font asset next to executable
add_custom_command(TARGET NN-topic-name POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E make_directory
        $<TARGET_FILE_DIR:NN-topic-name>/assets/fonts/liberation_mono
    COMMAND ${CMAKE_COMMAND} -E copy_if_different
        ${CMAKE_SOURCE_DIR}/assets/fonts/liberation_mono/LiberationMono-Regular.ttf
        $<TARGET_FILE_DIR:NN-topic-name>/assets/fonts/liberation_mono/
)

# Copy audio.conf next to executable (user-local audio file paths)
if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/audio.conf)
    add_custom_command(TARGET NN-topic-name POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy_if_different
            ${CMAKE_CURRENT_SOURCE_DIR}/audio.conf
            $<TARGET_FILE_DIR:NN-topic-name>/audio.conf
    )
endif()

7. Set up audio file configuration

Audio files are not checked into the repository. Each lesson uses a gitignored audio.conf file that points to a local directory containing the user's own audio files. This avoids distributing copyrighted or large binary assets.

7a. Create the audio.conf file

Create lessons/audio/NN-topic-name/audio.conf:

# Audio file configuration for Audio Lesson NN — Topic Name
#
# This file is gitignored. Point audio_dir to a local directory containing
# audio files for this lesson. See README.md for what types of files work
# best.
#
# Royalty-free audio files can be obtained from sites like:
#   freesound.org, pixabay.com/music, opengameart.org, incompetech.com
#
audio_dir = /path/to/your/audio/files

This bare-bones config is created by the developer scaffolding the lesson. It ships with a placeholder path that the user replaces with their own.

7b. Add audio.conf to .gitignore

Ensure the root .gitignore contains:

# Audio lesson config — local paths to user-supplied audio files
lessons/**/audio.conf

Add this entry if it does not already exist.

7c. Document audio file requirements in the README

Every audio lesson README must have an "Audio files" section (see step 8) that explains:

1. What types of audio files the lesson needs — be specific about content, not just format. Examples:

   • "A short percussive sound (click, snap, or drum hit) for one-shot playback"
   • "A looping ambient sound (rain, wind, or drone) for continuous playback"
   • "Two distinct sounds for mixing (e.g. a melody and a rhythm track)"
   • "A mono sound effect for spatial positioning (footstep, ping, or alarm)"

2. Format requirements — WAV preferred, mono or stereo, 16-bit or 32-bit float, 44100 or 48000 Hz. Note any specific requirements (e.g. "must be mono for spatial audio").

3. Where to find royalty-free audio — Point readers to sources:

   • freesound.org (CC0 and CC-BY samples)
   • pixabay.com/sound-effects (royalty-free)
   • opengameart.org (game-oriented audio)
   • incompetech.com (Kevin MacLeod music, CC-BY)

4. How to configure — Tell the reader to edit audio.conf and set audio_dir to the directory containing their files.

7d. Load audio files at runtime via audio.conf

The lesson's main.c reads audio.conf at startup to find the audio directory, then loads files from that directory. Add a config-reading helper to forge_audio.h (or use a local helper if it's lesson-specific):

/* Read the audio_dir path from audio.conf.
 * Returns true on success, false if the file is missing or
 * the path is the unedited placeholder. */
static bool read_audio_conf(const char *conf_path, char *out_dir, int max_len)
{
    SDL_IOStream *io = SDL_IOFromFile(conf_path, "r");
    if (!io) return false;

    char line[1024];
    bool found = false;
    Sint64 size = SDL_GetIOSize(io);
    if (size > 0 && size < (Sint64)sizeof(line)) {
        /* Read entire small file */
        size_t bytes_read = SDL_ReadIO(io, line, (size_t)size);
        if (bytes_read != (size_t)size) { SDL_CloseIO(io); return false; }
        line[bytes_read] = '\0';
        /* Parse audio_dir = <path> */
        const char *key = "audio_dir";
        char *pos = strstr(line, key);
        if (pos) {
            pos += strlen(key);
            while (*pos == ' ' || *pos == '=') pos++;
            /* Trim trailing whitespace / newline */
            char *end = pos + strlen(pos) - 1;
            while (end > pos && (*end == '\n' || *end == '\r' || *end == ' '))
                *end-- = '\0';
            /* Reject placeholder or empty path */
            if (*pos == '\0' || strstr(pos, "/path/to/") || strstr(pos, "\\path\\to\\")) {
                SDL_CloseIO(io);
                return false;
            }
            SDL_snprintf(out_dir, max_len, "%s", pos);
            found = true;
        }
    }
    SDL_CloseIO(io);
    return found;
}

At init time:

char audio_dir[512];
if (!read_audio_conf("audio.conf", audio_dir, sizeof(audio_dir))) {
    SDL_Log("audio.conf not found or not configured.");
    SDL_Log("Create lessons/audio/NN-topic-name/audio.conf with:");
    SDL_Log("  audio_dir = /path/to/your/audio/files");
    SDL_Log("See the lesson README for what audio files are needed.");
    return SDL_APP_FAILURE;
}

/* Build full paths to specific audio files */
char wav_path[512];
SDL_snprintf(wav_path, sizeof(wav_path), "%s/click.wav", audio_dir);
ForgeAudioBuffer click = forge_audio_load_wav(wav_path, spec);

7e. Assist the user with audio file selection

When creating a lesson, if audio.conf already exists with a valid audio_dir, scan that directory and help the user choose appropriate files:

1. List all .wav, .ogg, .mp3, and .flac files in the directory
2. For each file, report: filename, duration, channels, sample rate (use ffprobe or soxi if available, otherwise just list filenames)
3. Based on the lesson's audio requirements, suggest which files would work best. For example:
   • For a one-shot effect lesson: suggest short files (< 2 seconds)
   • For a spatial audio lesson: suggest mono files
   • For a mixing lesson: suggest 2-3 files with different character
4. Present recommendations to the user and let them confirm or choose differently

If audio.conf does not exist or has the placeholder path, ask the user:

This lesson needs audio files. Please provide a path to a directory
containing audio files (WAV, OGG, MP3, or FLAC).

This lesson needs:
- [specific audio requirements from the lesson design]

Royalty-free audio can be found at freesound.org, pixabay.com, or
opengameart.org.

What is the path to your audio directory?

Then create audio.conf with their answer and proceed with file selection.

7f. Reference audio files by name in the code

The lesson code should reference audio files by well-known names that correspond to their purpose, not opaque filenames. Define constants:

/* Audio file names — the user places files with these names in their
 * audio directory, or the lesson maps their chosen files to these names. */
#define AUDIO_FILE_CLICK   "click.wav"
#define AUDIO_FILE_AMBIENT "ambient.wav"
#define AUDIO_FILE_MUSIC   "music.wav"

The README should list these expected filenames and what each one is for, so the user knows which of their files to use (or rename).

8. Create README.md

Structure:

# Audio Lesson NN — Topic Name

[Brief subtitle explaining the audio concept]

## What you'll learn

[Bullet list of audio and implementation concepts covered]

## Result

<!-- TODO: screenshot -->

[Brief description of what the demo shows and sounds like.]

![Screenshot](assets/screenshot.png)

**Controls:**

| Key | Action |
|---|---|
| WASD / Arrows | Move camera (listener position) |
| Mouse | Look around |
| R | Reset / replay audio |
| P | Pause / resume |
| 1–9 | Trigger sound effects |
| +/- | Adjust master volume |
| Escape | Release mouse / quit |

## Audio files

This lesson needs the following audio files. Audio is not included in the
repository — you supply your own files via `audio.conf`.

| Name | Purpose | Requirements |
|---|---|---|
| `click.wav` | One-shot playback demo | Short percussive sound (< 1 s), mono or stereo |
| `ambient.wav` | Looping playback demo | 2–5 s ambient loop, mono or stereo |

**Format:** WAV, 16-bit or 32-bit float, 44100 or 48000 Hz.

**Where to find royalty-free audio:**

- [freesound.org](https://freesound.org) — CC0 and CC-BY samples
- [pixabay.com/sound-effects](https://pixabay.com/sound-effects) — royalty-free
- [opengameart.org](https://opengameart.org) — game-oriented sounds
- [incompetech.com](https://incompetech.com) — Kevin MacLeod music (CC-BY)

**Setup:** Edit `audio.conf` in this lesson directory and set `audio_dir` to
the directory containing your audio files. The program loads files by the
names listed above from that directory.

## The audio

[Main explanation of the audio concepts, with diagrams where helpful.
Use /dev-create-diagram for waveform visualizations, signal flow diagrams,
filter response curves, etc.]

### [Core concept 1]

[Explanation with equations where relevant. For example:]

Linear attenuation reduces volume based on distance from the listener:

$$
\text{gain} = 1 - \frac{d}{d_{\text{max}}}
$$

### [Core concept 2]

[More explanation]

## The code

### Audio initialization

[Explain SDL audio stream setup and buffer management]

### Processing loop

[Walk through the audio update function with annotated code]

### UI controls

[Explain how forge UI sliders and buttons connect to audio parameters]

### Rendering

[Explain how audio state maps to the visual scene. Brief — point readers
to GPU and UI lessons for rendering details.]

## Key concepts

- **Concept 1** — Brief explanation
- **Concept 2** — Brief explanation

## The audio library

This lesson adds the following to `common/audio/forge_audio.h`:

| Function | Purpose |
|---|---|
| `forge_audio_function_name()` | Brief description |

See: [common/audio/README.md](../../../common/audio/README.md)

## Where it's used

- [Math Lesson NN](../../math/NN-name/) provides the [math concept] used here

## Building

```bash
cmake -B build
cmake --build build --config Debug

# Windows
build\lessons\audio\NN-topic-name\Debug\NN-topic-name.exe

# Linux / macOS
./build/lessons/audio/NN-topic-name/NN-topic-name
```

## Exercises

1. [Exercise extending the audio concept]
2. [Exercise modifying parameters to observe different behavior]
3. [Exercise adding a new audio feature]

## Further reading

- [Relevant math lesson for the underlying math]
- [External resource — The Audio Programming Book, DAFX, Julius O. Smith, etc.]

9. Update project files

• CMakeLists.txt (root): Add add_subdirectory(lessons/audio/NN-topic-name) under an "Audio Lessons" section (create the section if it doesn't exist yet, placed after Physics Lessons)
• README.md (root): Add a row to the audio lessons table in an "Audio Lessons (lessons/audio/)" section — follow the same format as the existing track sections. Create the section if this is the first audio lesson.
• lessons/audio/README.md: Create the track README if this is the first lesson, or add a row to the existing lessons table
• PLAN.md: Check off the audio lesson entry

10. Cross-reference other lessons

• Find related math lessons: Vectors (for spatial audio), signal processing
• Find related GPU lessons: Rendering techniques used (lighting, UI rendering)
• Find related UI lessons: Widgets used in the control panel
• Update those lesson READMEs: Add a note like "See Audio Lesson NN for this concept in action"
• Update audio lesson README: List related lessons in "Where it's used"

11. Build and test

# Compile lesson-specific shaders (skip if lesson has no shaders/ directory)
python scripts/compile_shaders.py audio/NN-topic-name

# Build
cmake -B build
cmake --build build --config Debug

# Run
./build/lessons/audio/NN-topic-name/NN-topic-name

Use a Task agent with model: "haiku" for build commands per project conventions.

Verify:

• The program opens a window with the grid floor and scene visible
• Objects are lit with Blinn-Phong shading
• Camera controls work (WASD + mouse)
• Audio plays through speakers/headphones
• UI panel is visible and interactive
• Volume sliders change audio volume in real time
• R resets/replays the audio
• P pauses/resumes audio

12. Capture screenshots

# Configure with capture support
cmake -B build -DFORGE_CAPTURE=ON
cmake --build build --config Debug --target NN-topic-name

# Static screenshot
python scripts/capture_lesson.py lessons/audio/NN-topic-name

Copy output to lessons/audio/NN-topic-name/assets/.

13. Verify key topics are fully explained

Before finalizing, launch a verification agent using the Task tool (subagent_type: "general-purpose"). Give the agent the paths to the lesson's README.md and main.c and ask it to audit every key topic for completeness.

For each key topic / "What you'll learn" bullet, the agent must check:

1. Explained in the README — Is the concept described clearly enough that a reader encountering it for the first time could understand it?
2. Demonstrated in the program — Does main.c actually exercise this concept with audible and visible behavior?
3. All referenced terms are defined — Read the exact wording of each key topic and identify every technical term. For each term, confirm it is explained somewhere in the lesson.
4. Equations match code — Every formula in the README should have a corresponding implementation in the code, and vice versa.

The lesson is incomplete until every key topic passes all four checks.

14. Run markdown linting

Use the /dev-markdown-lint skill to check all markdown files:

npx markdownlint-cli2 "**/*.md"

Audio Lesson Conventions

Scope

Audio lessons cover sound concepts with real-time playback and visual feedback:

• Fundamentals — PCM audio, sample rates, bit depth, channels, WAV loading
• Playback — One-shot and looping sounds, volume control, fading
• Mixing — Combining multiple sources, panning, master volume, clipping
• Spatial audio — Distance attenuation, stereo panning, Doppler effect
• Streaming — Large file playback, crossfading, looping with intro sections
• DSP effects — Filters, reverb, echo, chorus, per-source and master bus

Audio lessons do not cover:

• Advanced rendering techniques in depth (point to GPU lessons instead)
• UI widget implementation details (point to UI lessons instead)
• Mathematical derivations from scratch (point to math lessons instead)
• Codec implementation (WAV only — SDL handles the format)

Rendering and UI baseline

Every audio lesson includes the same rendering and UI foundation so the focus stays on the audio. This baseline is not optional.

Use forge_scene.h — one forge_scene_init() call provides all of the following. See the forge-scene-renderer skill for the full API.

Audio-specific visual elements

Sound sources and audio state should have visual representation:

• Sound source markers — Spheres or icons at the 3D position of each audio source, color-coded by state (playing = green, paused = yellow, stopped = gray)
• Volume indicators — Sphere scale or color intensity proportional to volume
• Distance rings — Optional: circles on the ground showing attenuation distance thresholds (for spatial audio lessons)
• Waveform display — UI panel widget showing the current waveform or spectrum (for fundamentals and DSP lessons)
• VU meters — UI panel widget showing output level per channel

Tone

Audio lessons should be practical and grounded. Digital audio has deep signal-processing roots — present the essential math clearly while keeping the focus on audible results.

• Name the techniques — "Constant-power panning", "inverse-distance attenuation", "biquad filter" — named techniques carry weight
• Hear it, then understand it — Let the listener experience the effect before explaining the math
• Explain artifacts — Why clipping sounds harsh, why discontinuities cause clicks, why aliasing produces unexpected frequencies
• Encourage experimentation — Audio is best learned by tweaking parameters and listening. Exercises should modify volume, distance, filter cutoffs, and other values.

Code style

Follow the same conventions as all forge-gpu code:

• C99, matching SDL's style
• ForgeAudio prefix for public types, forge_audio_ for functions
• PascalCase for typedefs, lowercase_snake_case for locals
• UPPER_SNAKE_CASE for #define constants
• No magic numbers — #define or enum everything
• Extensive comments explaining why and purpose

Diagrams

Use the /dev-create-diagram skill for:

• Signal flow diagrams (source → mixer → master → speakers)
• Waveform visualizations (sine, square, noise)
• Filter frequency response curves
• Attenuation curves (linear, inverse, exponential)
• Stereo panning diagrams
• Audio buffer memory layout

KaTeX math

Audio lessons use formulas for DSP concepts. Use inline $...$ and display $$...$$ math notation. Display math blocks must be split across three lines (CI enforces this).

When NOT to Create an Audio Lesson

• The topic is covered by an existing audio lesson
• The concept is pure math with no audio aspect (belongs in a math lesson)
• The concept is purely about rendering (belongs in a GPU lesson)
• The concept is about UI widget implementation (belongs in a UI lesson)
• The topic is too narrow for a full lesson (add to an existing lesson instead)

In these cases, update existing documentation or plan for later.

Tips

• Start with the audio — Get the sound working with placeholder rendering first, then polish the visuals. Correct audio with basic visuals is better than a beautiful scene with broken sound.
• Use headphones — Spatial audio and stereo effects are best evaluated with headphones. Note this in the README.
• Audio files are user-supplied — The repo does not distribute audio. Each lesson's audio.conf (gitignored) points to a local directory. The README describes what types of files work best and where to find royalty-free audio.
• Show state — Use forge UI to display volume levels, cursor positions, buffer states, and other audio internals. Audio is easier to learn when you can see the numbers alongside hearing the results.
• ASCII-only console output — Use only ASCII characters in printf output for cross-platform compatibility.