libakgl/docs/01-introduction.md

# 01. Introduction

libakgl is a C library for building 2D games on SDL3. This is version **0.7.0**. It ships
156 functions declared across the twenty public headers in `include/akgl/`, plus
`akgl_version()` from the generated `version.h` — 157 exported symbols in
`libakgl.so.0.7`.

It gives you object pools instead of `malloc`, name-based registries instead of pointer
plumbing, a per-frame tick that updates and draws every live actor, JSON asset formats for
sprites and characters, a Tiled map loader, arcade physics, a controller/keyboard input
layer, text, and a three-voice tone synthesizer.

## What it refuses to be

**It is not an engine.** There is no editor, no scene graph, no scripting layer, no asset
pipeline that runs before the compiler, and no runtime that owns your `main`. You write a C
program; libakgl is a library it calls.

The refusals are specific, and each one is a design decision documented in
[Chapter 2](02-design-philosophy.md):

| Not here | Instead |
|---|---|
| Inheritance, RTTI, dynamic dispatch on a type tag | A record of function pointers you populate — `akgl_RenderBackend`, `akgl_PhysicsBackend` |
| Runtime `malloc` | Five statically sized pools with reference counts; `AKGL_ERR_HEAP` when one is full |
| An editor or a project format | Tiled TMJ maps and hand-writable JSON for sprites and characters |
| A scripting layer | C. Behaviour attaches as function pointers on `akgl_Actor` |
| Two worlds at once | Four swappable globals: `akgl_renderer`, `akgl_physics`, `akgl_camera`, `akgl_gamemap` |

The library also does not own your window. `akgl_render_2d_init` creates one for you, and
`akgl_render_2d_bind` is the same job with that half removed, for a host that already has an
`SDL_Renderer` — see [Chapter 8](08-rendering.md).

## What a frame costs

`PERFORMANCE.md` measures a 640x480 game with a full screen of 16-pixel tiles and 64 actors,
software-rasterized. At 60 fps a frame is 16.67 ms:

| Part of the frame | Cost | Share of 16.67 ms |
|---|---:|---:|
| Logic: 64 actor updates + one physics sweep | 0.006 ms | 0.03% |
| All-pairs collision over 64 actors, if you do it | 0.012 ms | 0.07% |
| Clear + present | 0.023 ms | 0.1% |
| 64 actor renders (48x48 blits) | 0.191 ms | 1.1% |
| 1200 tile blits | 16.26 ms | 97.6% |
| Six lines of HUD text | 0.076 ms | 0.5% |

**libakgl's own per-tile overhead is about 0.03 ms per frame, under 0.2%.** That number is
the gap between `akgl_tilemap_draw` at 16.26 ms and a raw `SDL_RenderTexture` loop issuing
the same 1200 blits from the same scattered source tiles at 16.23 ms. It covers the bounds
arithmetic, the tileset scan, the offset-table lookup, the backend indirection and the error
macros.

It does not cover the pixels, and the pixels are the frame. It does not tell you what
libakgl costs on a GPU backend, where the blits get cheap and libakgl's share of a much
shorter frame rises — `PERFORMANCE.md` says so explicitly and is the reason the
per-operation numbers there matter more than these totals. And it is one laptop, one build
type, one afternoon: treat the absolute numbers as that machine's and the ratios as the
library's.

## What is not implemented

Named here rather than discovered later. Each gets a callout in the chapter where you would
hit it, and an entry in `TODO.md`.

| Gap | Behaviour today | Chapter |
|---|---|---|
| Collision response | `akgl_physics_arcade_collide` raises `AKERR_API`, and `akgl_physics_simulate` never calls `collide` at all. An actor walks through a wall | [14](14-physics.md) |
| Terminal velocity | Gravity accumulates into `ey` unbounded. `physics.drag.y` is the only brake | [14](14-physics.md) |
| Friction / deceleration | Releasing a direction zeroes thrust and stops the actor dead. Right for Zelda, wrong for Mario | [14](14-physics.md) |
| Savegames | `akgl_game_save` writes the name tables but not the objects, so a file is not yet enough to restore a session | [07](07-the-game-and-the-frame.md) |
| Mesh drawing | `akgl_render_2d_draw_mesh` raises `AKERR_API`; the hook is reserved for a 3D backend | [08](08-rendering.md) |
| A text cache | Every `akgl_text_rendertextat` rasterizes, uploads, blits and destroys | [16](16-text-and-fonts.md) |

## Who owns which documentation

**This manual documents libakgl. It does not re-document its dependencies.** Every project
below is documented by the people who own its code, and a paraphrase here would be wrong the
day one of them changes without anything in this repository noticing. So each chapter
answers two questions — what does libakgl add or constrain here, and what does a libakgl
caller actually write — and links out for the rest.

| Topic | Owned by | What this manual owes you |
|---|---|---|
| `ATTEMPT`/`CLEANUP`/`PROCESS`/`HANDLE`/`FINISH`, `PASS`, `CATCH`, `IGNORE` | libakerror (`deps/libakerror`) | Which statuses libakgl raises and what they mean here — [Chapter 4](04-errors.md) |
| `aksl_strncpy`, `aksl_fclose`, `aksl_fgetc`, `aksl_snprintf` | libakstdlib (`deps/libakstdlib`) | Which ones libakgl requires you to use, and why |
| `SDL_Renderer`, `SDL_Texture`, events, `SDL_PropertiesID` | [SDL3](https://wiki.libsdl.org/SDL3/) | The backend vtable, the frame contract, what libakgl does to the renderer's state |
| Image decoding | [SDL3_image](https://wiki.libsdl.org/SDL3_image/) | Which formats reach a spritesheet, and when they are decoded |
| Audio decoding, `MIX_Audio`, mixers and tracks | [SDL3_mixer](https://wiki.libsdl.org/SDL3_mixer/) | `akgl_load_start_bgm` and the track table — [Chapter 17](17-audio.md) |
| TTF rasterizing and metrics | [SDL3_ttf](https://wiki.libsdl.org/SDL3_ttf/) | The font registry, the teardown ordering trap, the per-call cost — [Chapter 16](16-text-and-fonts.md) |
| `json_t`, `json_decref`, the parser | [jansson](https://jansson.readthedocs.io/) | `akgl_get_json_*` status semantics and the borrowed-reference rule — [Chapter 18](18-utilities.md) |
| The TMJ map format, layers, tilesets, custom properties | [Tiled](https://doc.mapeditor.org/en/stable/reference/json-map-format/) | libakgl's extensions and limits — [Chapter 13](13-tilemaps.md) |

The three-voice synthesizer in [Chapter 17](17-audio.md) is the one audio subsystem that is
libakgl's own, and it is documented here in full. It has nothing to do with SDL3_mixer.

## Where the per-function reference lives

This manual is narrative. It teaches a task and links to the generated Doxygen for
signatures, parameters and per-function `@throws` lists:

```sh norun
doxygen Doxyfile
```

Every header already carries a substantial `@file` block explaining its subsystem's design
rationale, and `Doxyfile` sets `WARN_IF_UNDOCUMENTED = YES` with
`WARN_AS_ERROR = FAIL_ON_WARNINGS`, so an undocumented symbol fails CI. The gap these
chapters fill is navigation and worked examples, not reference text. **They deliberately do
not restate the 156 signatures** — a hand-copied signature table is exactly the artifact
that drifts, and it would compete with a reference that CI already keeps honest.

Where a chapter genuinely needs a declaration or a constant table in front of you, it uses
an `excerpt=` block whose contents are checked against the header on every test run. The
text you read *is* the header.

## Reading order

[Chapter 4](04-errors.md) comes before every subsystem chapter, because all 156 functions
return `akerr_ErrorContext AKERR_NOIGNORE *` and you cannot read a single example until you
can read that return value. After that, [Chapter 3](03-getting-started.md) gets a window on
the screen, and the subsystem chapters can be read in any order.

If you would rather start by building something, the two tutorials —
[Chapter 19](19-tutorial-sidescroller.md) and [Chapter 20](20-tutorial-jrpg.md) — are
complete programs under `examples/`, built by default and smoke-run in CI.