Jérôme Bousquié
115 lines
3.5 KiB
Markdown
115 lines
3.5 KiB
Markdown
# Emissive + Exposure
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## Principle
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Two complementary features (Step 22):
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| Feature | Effect | Cost |
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|---------|--------|------|
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| **Exposure** (6.1) | Multiplies luminance before the tone-mapping curve | Zero when HDR is inactive |
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| **Emissive** (6.2) | Adds an emitted color (independent of the lights) | Zero when `emissive = [0,0,0,0]` |
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## Exposure
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### API
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```rust
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// Initialization (optional, default = 1.0)
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let app = AppBuilder::new()
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.with_hdr(ToneMapper::Aces)
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.with_exposure(1.5) // start brighter
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.build().await?;
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// Runtime (in update())
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app.set_exposure(app.exposure() * 1.1); // +1 "stop"
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app.set_exposure(1.0); // reset
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```
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### Behavior
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- Exposure is a **multiplier** applied to the HDR texture before the tone-mapping curve.
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- `exposure = 2.0` → the image is 2× brighter (like opening a camera's aperture).
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- `exposure = 0.5` → the image is 2× darker.
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- Clamped to `[0.01, 10.0]` to avoid degenerate values.
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- **Only has an effect when HDR is active** (`with_hdr(...)`). In LDR the value is ignored.
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### Keyboard (demo)
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| Key | Effect |
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|-----|--------|
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| `+` | ×1.1 (brighter) |
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| `-` | ÷1.1 (darker) |
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| `0` | Reset to 1.0 |
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## Emissive
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### API
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```rust
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use wsg_lib::resources::Material;
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// Create a material with emissivity
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let mut mat = /* ... */;
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mat.emissive = [1.0, 0.3, 0.1, 1.5]; // orange, intensity 1.5 (> 1.0 = HDR glow)
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```
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### Format
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`emissive = [r, g, b, intensity]`:
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- **rgb**: the emission color (same space as the vertex base color)
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- **a (intensity)**: the multiplier. `1.0` = normal color, `> 1.0` = highlight (only visible in HDR)
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### Shader formula
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```
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final_color = lit + base_color * emissive.rgb * emissive.a
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```
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- The emission is **additive**: visible even in total darkness (no light needed).
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- It is **independent of shadows**: an emissive object casts no shadow and is not shadowed.
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- `emissive = [0,0,0,0]` (default) → no change (non-regression guaranteed).
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### Use cases
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| Use | Value |
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|-----|-------|
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| LED / indicator | `[0, 1, 0, 1.0]` (green, normal intensity) |
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| Flame / sun | `[1, 0.8, 0.2, 3.0]` (orange, HDR glow) |
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| Neon | `[0, 0.5, 1, 2.5]` (cyan, glow) |
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| Inactive | `[0, 0, 0, 0]` (default) |
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### Keyboard (demo)
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| Key | Effect |
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|-----|--------|
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| `E` | Toggle orange glow on the sphere/cylinder |
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## Interactions
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| Combination | Result |
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|-------------|--------|
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| Emissive + HDR + ACES | Soft glow, highlights roll off (the nicest) |
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| Emissive + LDR | Clamped to 1.0 (no glow, but the color is visible in the dark) |
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| Emissive + shadows | The emissive object is NOT shadowed (emission bypasses the shadow term) |
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| Exposure + Emissive | Exposure also amplifies the emission (consistent: everything is in the HDR texture) |
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## Non-regression
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- **Emissive**: `[0,0,0,0]` by default → the shader adds `base * 0 * 0 = 0` → no change.
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- **Exposure**: `1.0` by default → `pow(color, 1/1) = color` → no change.
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- Both are **opt-in**: without `with_hdr(...)` and `emissive != 0`, the pipeline is identical
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to the previous state.
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## Limitations (MVP)
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- Emissive is **per material**, not per vertex (no emission gradient within a mesh).
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- Emissive is **static** at material creation (changing `mat.emissive` requires re-registering
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the material via `add_material`).
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- No **bloom** (Step 23): the HDR glow is visible but not "blurred" / spread.
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## Links
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- [User README](../README.md) · [HDR & tone mapping](../effects/hdr.md) · [Examples](../examples.md)
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- [Root README](../../../README.md)
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