# llimphi-surface

External surfaces inside the Elm loop.

An `ExternalSurface` is an RGBA8 texture living on the GPU, painted over a rect
of the Llimphi frame every time the app exposes it through
`View::gpu_paint_with`. The byte source runs outside the Elm loop: a video
decoder, a camera capture, a PDF raster, a raw texture produced by another engine
— any producer generating RGBA can push frames with `ExternalSurface::upload` and
see the result on the next raster pass.

The crate provides:

- `ExternalSurface`: owner of the texture + render pipeline + bind group.
  `upload(rgba, w, h)` uploads bytes and recreates the texture if `w`/`h` changed.
- `ExternalSurface::view`: a helper that builds a `View` with `gpu_paint_with`
  already wired. The app only chooses the node's `Style` (which portion of the
  layout it occupies).

## Design

The pipeline is a classic textured quad: two triangles cover the destination
rect, and the fragment shader samples the external texture with a bilinear
sampler. The quad's NDC coordinates are computed on the GPU from
`(rect, viewport)` travelling through a uniform — which is why the callback needs
the `viewport` that `llimphi-ui` began propagating in `GpuPaintFn`.

The intermediate texture Llimphi paints vello into is `Rgba8Unorm` (see
`llimphi-hal::INTERMEDIATE_FORMAT`). The pipeline emits `Rgba8Unorm` too — the
render pass target is that same intermediate with `LoadOp::Load`, so the vello
background is preserved.

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Part of **llimphi** — see [llimphi](../README.md).