Jérôme Bousquié
78 lines
2.9 KiB
Markdown
78 lines
2.9 KiB
Markdown
# HDR & Tone Mapping
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> **Étape 20** — Opt-in HDR rendering with tone mapping.
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## What it does
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By default, the WSG renderer draws directly to the window's sRGB surface. Color values
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above 1.0 are **clipped** (saturated to white) — you lose all information in bright areas.
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When HDR is enabled, the pipeline becomes:
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```
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Main pass → offscreen Rgba16Float texture (unbounded float)
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TM pass → fullscreen triangle samples HDR texture, applies curve, writes to sRGB surface
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```
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The tone mapping **compresses** the [0, ∞) range to [0, 1] with a perceptual curve,
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so bright areas are smoothly rolled off instead of clipping.
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## Enabling HDR
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```rust
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use wsg_lib::core::ToneMapper;
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use wsg_lib::app::AppBuilder;
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let app = AppBuilder::new()
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.title("My HDR App")
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.with_hdr(ToneMapper::Aces) // ← enables HDR
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.build()
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.await?;
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```
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Without `.with_hdr(...)`, the renderer operates in LDR mode (direct to surface, zero overhead).
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## Tone mapping curves
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| Variant | Curve | Use case |
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|---------|-------|----------|
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| `ToneMapper::Aces` | ACES Filmic (Narkowicz 2015) | Cinematic look, soft highlight rolloff, good contrast |
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| `ToneMapper::Reinhard` | `x / (1 + x)` | Simple, flat; less contrast but computationally trivial |
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The curve is **compiled into the pipeline** at construction time (one WGSL entry point
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per variant) — there is no runtime branching cost.
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## Cost
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| HDR state | Extra per-frame cost |
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|-----------|---------------------|
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| Disabled (default) | **Zero** — no texture, no pass, no pipeline |
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| Enabled | +1 fullscreen render pass (triangle, 3 verts) + 1 offscreen texture (same size as window) |
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The extra pass is negligible on any GPU (a few hundred microseconds). The offscreen
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texture costs ~12 bytes/pixel of VRAM (RGBA16F = 8 bytes/px + the surface's own buffer).
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## How it works (technical)
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- **Offscreen texture**: `Rgba16Float`, same size as the window. Created in `Renderer::new`,
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recreated on resize.
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- **Main pass**: the color attachment targets the HDR texture instead of the surface.
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The `standard_shader.wgsl` fragment output (linear float, unbounded) is stored as-is.
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- **TM pass**: a fullscreen triangle (3 vertices, no vertex buffer) samples the HDR texture,
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multiplies by exposure (currently fixed at 1.0), applies the tone curve, and writes to
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the sRGB surface. The hardware performs the linear→sRGB gamma conversion automatically
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(the surface format is `Rgba8UnormSrgb`).
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- **No double gamma**: the shader outputs linear [0,1]; the sRGB surface encoding is
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handled by the rasterizer.
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## Exposure
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Currently fixed at 1.0 (no user control yet). A future step will expose an
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`exposure` field in a `HdrConfig` struct for live adjustment.
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## See also
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- [Shadows](shadows.md) — the other opt-in visual feature
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- [GPU-driven rendering](gpu-driven.md) — the compute pipeline that feeds the main pass
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- [Examples](examples.md) — the `demo` example enables HDR by default
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