# Render Context & GPU Setup

Stage 2 reference for `oxide_engine::render` — how the engine acquires the
GPU and drives a window surface. For the event loop that calls into this each
frame, see [windowing.md](windowing.md).

## Overview

Stage 2 rendering is deliberately minimal: acquire the GPU, configure the
window surface, and clear it to a configurable color every frame. Meshes,
materials, and passes arrive in Stage 4+. The module still establishes the
two long-lived types every later stage builds on:

- **`Gpu`** — instance, adapter, and the device/queue pair. Everything that
  touches the GPU goes through these four objects.
- **`RenderContext`** — a `Gpu` plus a window's surface and its
  configuration; owns the per-frame acquire → clear → present cycle.

Both are created for you by [`run()`](windowing.md); applications normally
reach them through `AppCtx::render()`.

## `Gpu`

```rust
use oxide_engine::prelude::*;

let gpu = Gpu::headless()?;            // offscreen / tests
let device: &wgpu::Device = gpu.device();
let queue:  &wgpu::Queue  = gpu.queue();
# Ok::<(), oxide_engine::render::RenderError>(())
```

Acquisition asks for a high-performance adapter (compatible with the window
surface in the windowed path) and a default-limits device. The chosen adapter
and backend are logged at `info` level on startup.

`Gpu::headless()` skips the surface entirely — used by offscreen rendering
and the automated Stage 2 integration test. Backend selection and debug flags
remain overridable through wgpu's standard `WGPU_*` environment variables
(e.g. `WGPU_BACKEND=vulkan`).

## `RenderContext`

Owns the surface lifecycle:

- **Creation** — builds the wgpu instance (the window doubles as the display
  handle), creates the surface, acquires the `Gpu`, and configures the
  surface with `get_default_config` (the platform's preferred format and
  present mode).
- **`resize(width, height)`** — reconfigures the surface. Zero dimensions
  (minimized windows) are clamped to 1 so the surface stays valid. Called
  automatically by the event loop on `Resized`.
- **`set_clear_color(color)` / `clear_color()`** — the color the next frame
  is cleared to. The engine's `Color` is linear f32 RGBA, matching what the
  surface expects (conversion to `wgpu::Color` is `render::to_wgpu_color`).
- **`render_frame()`** — one frame: acquire the next surface texture, record
  a clear pass, submit, present.
- **`size()`, `gpu()`** — current surface size (physical pixels) and the
  underlying `Gpu`.

### Frame acquisition and transient failures

`get_current_texture` can fail for reasons that are *normal* during resizes
and window-manager activity. `render_frame()` maps them as follows:

| Surface state | Behavior |
|---------------|----------|
| `Success` / `Suboptimal` | Clear and present (a suboptimal frame is still presentable; the next resize reconfigures anyway) |
| `Lost` / `Outdated` | Reconfigure the surface, skip the frame |
| `Timeout` / `Occluded` | Skip the frame |
| `Validation` | Returned as `RenderError::SurfaceValidation` — a real bug, not transient |

Skipped frames are invisible in practice: the next `RedrawRequested` arrives
within milliseconds.

## `clear_view`

The single render operation Stage 2 owns:

```rust
oxide_engine::render::clear_view(device, queue, &texture_view, Color::RED);
```

Records and submits a render pass whose only work is a load-op clear. Both
the windowed path (`render_frame`) and offscreen targets go through it, which
is what makes the GPU path automatically testable: the integration test
`stage2::headless_clear_fills_texture_with_clear_color` clears an offscreen
texture headless, reads the pixels back, and asserts the exact clear color —
no window or human needed. (It self-skips on machines with no GPU adapter.)

## Errors

`RenderError` (a `thiserror` enum) distinguishes the failure modes callers
might handle: `NoAdapter`, `Device`, `CreateSurface`, `UnsupportedSurface`,
and `SurfaceValidation`. Binaries typically just propagate it via `anyhow`
out of `run()`.

## Design notes

- The window is held as `Arc<winit::window::Window>` so the surface, which
  borrows the window, can be `'static` — winit hands windows out from inside
  its event loop, and wgpu surfaces must outlive every frame.
- `Gpu` and `RenderContext` are separate types on purpose: later stages (and
  tests today) need the device/queue without any window, and composability is
  a core project principle.
