# Materials & textures

A **`Material`** describes a mesh's appearance: it references a shader (by id) and
optionally a **diffuse texture**. Several materials pointing at the same shader share the
same compiled GPU pipeline (the `PipelineCache` held by the scene).

The engine ships a single shader: **`standard`** — multi-light Phong lighting (see
[Lights](lights.md)), with an **unlit** mode for flat rendering.

## 1. Registering the shader

```rust
app.scene
    .register_shader("standard", wsg_lib::utils::STANDARD_SHADER_PATH)
    .unwrap();
```

> **Note**: `STANDARD_SHADER_PATH` points to an optional file on disk; if it is missing
> (the normal case for the embedded library), loading falls back to the shader **embedded at
> compile time** (`include_str!`, byte-identical). The fallback message you may see is
> therefore **expected and harmless**.

For a custom shader: register your `.wgsl` file path under an id of your choice (it must
expose the same bind groups as `standard` — frame @0, object @1, texture @2, shadow @3 — see
[ARCHI_RENDU](../tech/ARCHI_RENDU.md) and the
[`shaders/standard_shader.wgsl`](../../lib/src/shaders/standard_shader.wgsl) file).

## 2. Creating materials

```rust
// Textureless material: the color comes from per-vertex colors (or white by default).
app.scene.add_material_shader("mat", "standard").unwrap();

// Textured material: the texture must first be registered in the scene (below).
app.scene.add_material_texture("mat_textured", "standard", "my_texture").unwrap();
```

Binding a material to a mesh happens at mesh creation (see [Meshes](meshes.md)):

```rust
app.scene.create_mesh("cube_mesh", cube(1.0), Some("mat_textured")).unwrap();
```

A mesh created with `material = None` is rendered with the scene's **default material**
(`standard`, built once then cached) — that is the behavior of the
[`simple`](../../lib/examples/simple.rs) example.

## 3. Diffuse textures

`Texture` is a GPU image in `Rgba8UnormSrgb` (linear sampler, repeat addressing).
Four constructors:

| Constructor | Usage |
|--------------|-------|
| `Texture::from_rgba8(device, queue, w, h, rgba, label)` | raw RGBA8 bytes (procedural) |
| `Texture::from_bytes(device, queue, label, bytes)` | encoded data (PNG/JPEG… via the `image` crate) |
| `Texture::from_file(device, queue, label, path)` | image file on disk |
| `Texture::white_placeholder(device, queue)` | 1×1 white — used internally when a material has no texture |

You get `device`/`queue` in `setup()` via `app.context()`:

```rust
let (device, queue) = {
    let ctx = app.context();
    (ctx.device.clone(), ctx.queue.clone())
};
let texture = Texture::from_rgba8(&device, &queue, 8, 8, &my_rgba, "checker").unwrap();
app.scene.add_texture("checker_texture", texture).unwrap();
app.scene.add_material_texture("ground_mat", "standard", "checker_texture").unwrap();
```

The exact snippet (8×8 checkerboard + stripes generation) is in
[`demo.rs`](../../lib/examples/demo.rs) and [`cube.rs`](../../lib/examples/cube.rs).

Two conditions for a texture to show up:
1. the material is created via `add_material_texture` (otherwise the 1×1 white placeholder
   is bound — no visual effect, no regression);
2. the `Geometry` carries **UVs** (`.with_uvs(…)`). Without UVs, sampling is constant.
   The procedural primitives (`uv_sphere`, `cube`, …) already provide them.

## 4. Unlit mode (flat / 2D rendering)

"Flat" rendering (vertex colors as-is, no lighting) is a **renderer switch**, not a material:

```rust
app.renderer_mut().set_unlit(true);   // in setup()
```

This is the mode of the `simple` example (2D quad). In this mode the scene's lights are
ignored; per-vertex colors (or white) are rendered directly. 2D is a special case of 3D:
the single `standard` pipeline serves both.

> `clear_lights()` (see [Lights](lights.md)) gives a similar result but keeps the lit
> pipeline: only ambient stays active. Use it when you want to "turn off the lights" without
> switching to unlit.

## Links

- [User README](README.md) · [Meshes](meshes.md) · [Lights](lights.md) · [Examples](examples.md)
- [Root README](../../README.md) · [ARCHI_RENDU](../tech/ARCHI_RENDU.md)