wsg/docs/user/gpu-driven.md

# GPU-driven rendering

WSG's scene rendering is **GPU-driven**: the per-entity world matrices and the indirect draw
arguments are computed on the GPU each frame, so the CPU no longer loops over entities to issue
draw calls. This page explains what that means for you and how to opt into **frustum culling**.

## What runs on the GPU

Each frame, before the render passes, two compute passes run over a fixed-capacity slot table
(256 entities, allocated once):

1. **`compute_matrices`** derives each entity's world matrix from its transform
   (translation / rotation / scale). The result feeds the render pipelines as the per-entity
   model matrix.
2. **`cull`** decides per-entity visibility and fills the **indirect draw arguments** (the
   vertex/index count, zeroed when the entity is culled or inactive).

The main and shadow render passes are then **100 % indirect**: each active slot issues one
indirect draw that reads its own count and world matrix. A culled or inactive slot has a zero
count, so its draw is a no-op. The CPU only rewrites the transform slots and the cull uniforms
each frame — it never iterates the entities to issue draws.

You do not need to do anything special to get this: `render_scene` is GPU-driven by default.

## Batching by material

The main render pass batches the draws by material: all entities sharing the same material are
drawn back to back, so the GPU pipeline and the material's texture bind group are switched **once
per distinct material**, not once per entity (the per-draw work — matrix offset, vertex/index
buffers, the indirect draw itself — is unchanged). The grouping is internal: it does not change
the rendered image and there is nothing to configure.

> **Constraint:** the batching reorders the draws, which is safe here because every pipeline in
> the engine is **opaque** (`BlendState::REPLACE`, no alpha blending) — the depth buffer resolves
> the draw order. If transparent materials are ever added, the transparent draws must be isolated
> (sorted back-to-front at the end of the pass) and must not interleave with the grouped opaque
> draws.

## Frustum culling (opt-in)

Culling is **off by default**. The culling pass still runs, but with culling disabled it marks
every active entity visible — so the rendered image is **identical** to a CPU-culled scene.
This protects you from a culling bug (an object that should be visible vanishing) becoming a
silent correctness issue.

To enable culling, build your `App` with `.with_culling(true)`:

```rust
let app = AppBuilder::new()
    .title("My app")
    .with_culling(true)   // skip entities whose bounding sphere leaves the frustum
    .build()
    .await?;
```

Or toggle it at runtime on the renderer:

```rust
app.renderer().set_culling(true);   // enable
app.renderer().set_culling(false);  // disable again
```

## How culling works

When culling is on, each entity's **local-axis-aligned bounding box** (computed once from its
geometry, `Geometry::bbox()`) is treated as a **bounding sphere**:

- **center** = the box center, transformed by the entity's world transform (rotation +
  translation; scale is folded into the radius),
- **radius** = the box's circumradius scaled by the entity's largest scale component.

The sphere is tested against the six camera frustum planes. If it is **fully outside** (beyond
a plane by more than its radius), the entity is culled; otherwise it is drawn.

The sphere is a **conservative** approximation of the box: it can draw an object that is partly
out of view (false negative), but it will **never cull an object that is actually visible**
(false positive). For tight culling you would need per-mesh sphere fitting or per-face tests,
which are out of scope for v1.

## Debugging the GPU path

If something looks wrong — a missing object, a black window — the GPU-side slot tables can be
read back and printed. The `Renderer` ships a debug helper (intentionally **not** part of the
documented API):

```rust
app.renderer().debug_dump(8);   // prints the first 8 GPU slots to stderr
```

It dumps exactly what the GPU sees: the transform slots, the derived world matrices, the
indirect draw arguments, the mesh bounding boxes and the cull uniforms. A slot whose vertex
count reads `0` was zeroed by the cull pass (culled, inactive, or beyond `num_slots`); a full
count means the entity is drawn. In the `demo` example the dump is opt-in via an environment
variable, so the showcase stays silent by default:

```sh
WSG_DEBUG_DUMP=120 cargo run -p wsg-lib --example demo
```

`WSG_DEBUG_DUMP=N` dumps for the first *N* frames. The demo stays **silent** when the variable is
unset; a set-but-non-numeric value (e.g. `WSG_DEBUG_DUMP=on`) gives 3 frames.

**How to verify culling is actually working** (a correct culling pass is invisible — culled
objects were off-screen anyway — so the proof is in the counts, not the image):

1. Launch the demo with `WSG_DEBUG_DUMP=120` (the demo has culling **on** and an orbiting
   camera — drag the mouse to orbit).
2. Note first that orbiting/zooming this camera **cannot cull the entity ring**: the camera
   always looks at the origin, so each entity's angular offset from the view axis is bounded
   by `atan(ring radius / camera distance)` = `atan(1.7/6.1)` ≈ 15.5°, under the ~22° vertical
   half-FOV. The seven demo entities therefore keep their **full** counts (cube `36`, sphere
   `3840`, …) in every orientation — that is the expected and correct behaviour (verified
   2026-09-22: 600-frame camera sweep, GPU cull verdicts matched an independent CPU sphere
   test on all 6000 entity frames, zero flips on the ring).
3. To see the counts actually flip to **`0`**, you need an entity well **off the target axis**
   — e.g. one placed far away so it ends up behind the near plane. Its count then toggles
   `0` ↔ full as the camera orbits, while the on-axis entities stay full. (This off-axis test
   is the one that verified the cull path end-to-end, positive and negative.)
4. Optional A/B: temporarily build with `.with_culling(false)` and repeat — with culling off,
   every entity keeps its full count in **every** orientation (the off-axis one included).

This readback is the reference truth when a shader bug is suspected: it shows both the computed
counts and the raw inputs of the cull pass, independently of what ends up on screen. (It is how
the 2026-09-22 « black window » bug — an inverted WGSL `select` argument order — was diagnosed
and verified fixed, see the D14 note in `docs/tech/ARCHI_CPU_GPU.md`.)

## Limitations

- **Culling is all-or-nothing per entity.** There is no partial (per-triangle) culling.
- **The sphere is a coarse bound** for elongated meshes (a long thin box gets a large sphere).
  If your scene is dominated by such shapes, culling may bring little gain.
- **Capacity is 256 entities per render pass.** Beyond that, extra entities are not drawn.
  This is the largest a single-buffer design can address under WebGPU's two `uniform` rules: a
  single `uniform` binding is capped at 64 KB, *and* a `uniform` offset must be a multiple of 256 B.
  A 64-byte matrix can never be individually addressable by a `uniform` offset, so each matrix
  slot is padded to 256 B — and 256 slots × 256 B = 64 KB is the maximum. It is amply generous
  for a simple scene (the demo has 7).
- **Mesh bounding boxes are recomputed when meshes are added**; a scene whose mesh set changes
  at runtime simply re-uploads the small bbox table (a few bytes per mesh).

Culling is a **performance** feature, not a visual one: with it off you get the same image with
the indirect-draw machinery still active.

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