# Emissive + Exposure

## Principle

Two complementary features (Step 22):

| Feature | Effect | Cost |
|---------|--------|------|
| **Exposure** (6.1) | Multiplies luminance before the tone-mapping curve | Zero when HDR is inactive |
| **Emissive** (6.2) | Adds an emitted color (independent of the lights) | Zero when `emissive = [0,0,0,0]` |

## Exposure

### API

```rust
// Initialization (optional, default = 1.0)
let app = AppBuilder::new()
    .with_hdr(ToneMapper::Aces)
    .with_exposure(1.5)  // start brighter
    .build().await?;

// Runtime (in update())
app.set_exposure(app.exposure() * 1.1);  // +1 "stop"
app.set_exposure(1.0);                   // reset
```

### Behavior

- Exposure is a **multiplier** applied to the HDR texture before the tone-mapping curve.
- `exposure = 2.0` → the image is 2× brighter (like opening a camera's aperture).
- `exposure = 0.5` → the image is 2× darker.
- Clamped to `[0.01, 10.0]` to avoid degenerate values.
- **Only has an effect when HDR is active** (`with_hdr(...)`). In LDR the value is ignored.

### Keyboard (demo)

| Key | Effect |
|-----|--------|
| `+` | ×1.1 (brighter) |
| `-` | ÷1.1 (darker) |
| `0` | Reset to 1.0 |

## Emissive

### API

```rust
use wsg_lib::resources::Material;

// Create a material with emissivity
let mut mat = /* ... */;
mat.emissive = [1.0, 0.3, 0.1, 1.5];  // orange, intensity 1.5 (> 1.0 = HDR glow)
```

### Format

`emissive = [r, g, b, intensity]`:

- **rgb**: the emission color (same space as the vertex base color)
- **a (intensity)**: the multiplier. `1.0` = normal color, `> 1.0` = highlight (only visible in HDR)

### Shader formula

```
final_color = lit + base_color * emissive.rgb * emissive.a
```

- The emission is **additive**: visible even in total darkness (no light needed).
- It is **independent of shadows**: an emissive object casts no shadow and is not shadowed.
- `emissive = [0,0,0,0]` (default) → no change (non-regression guaranteed).

### Use cases

| Use | Value |
|-----|-------|
| LED / indicator | `[0, 1, 0, 1.0]` (green, normal intensity) |
| Flame / sun | `[1, 0.8, 0.2, 3.0]` (orange, HDR glow) |
| Neon | `[0, 0.5, 1, 2.5]` (cyan, glow) |
| Inactive | `[0, 0, 0, 0]` (default) |

### Keyboard (demo)

| Key | Effect |
|-----|--------|
| `E` | Toggle orange glow on the sphere/cylinder |

## Interactions

| Combination | Result |
|-------------|--------|
| Emissive + HDR + ACES | Soft glow, highlights roll off (the nicest) |
| Emissive + LDR | Clamped to 1.0 (no glow, but the color is visible in the dark) |
| Emissive + shadows | The emissive object is NOT shadowed (emission bypasses the shadow term) |
| Exposure + Emissive | Exposure also amplifies the emission (consistent: everything is in the HDR texture) |

## Non-regression

- **Emissive**: `[0,0,0,0]` by default → the shader adds `base * 0 * 0 = 0` → no change.
- **Exposure**: `1.0` by default → `pow(color, 1/1) = color` → no change.
- Both are **opt-in**: without `with_hdr(...)` and `emissive != 0`, the pipeline is identical
  to the previous state.

## Limitations (MVP)

- Emissive is **per material**, not per vertex (no emission gradient within a mesh).
- Emissive is **static** at material creation (changing `mat.emissive` requires re-registering
  the material via `add_material`).
- No **bloom** (Step 23): the HDR glow is visible but not "blurred" / spread.

## Links

- [User README](../README.md) · [HDR & tone mapping](../effects/hdr.md) · [Examples](../examples.md)
- [Root README](../../../README.md)