# Objective
When using instancing, 2 `VertexBufferLayout`s are needed, one for
per-vertex and one for per-instance data. Shader locations of all
attributes must not overlap, so one of the layouts needs to start its
locations at an offset. However,
`VertexBufferLayout::from_vertex_formats` will always start locations at
0, requiring manual adjustment, which is currently pretty verbose.
## Solution
Add `VertexBufferLayout::offset_locations`, which adds an offset to all
attribute locations.
Code using this method looks like this:
```rust
VertexState {
shader: BACKBUFFER_SHADER_HANDLE.typed(),
shader_defs: Vec::new(),
entry_point: "vertex".into(),
buffers: vec![
VertexBufferLayout::from_vertex_formats(
VertexStepMode::Vertex,
[VertexFormat::Float32x2],
),
VertexBufferLayout::from_vertex_formats(
VertexStepMode::Instance,
[VertexFormat::Float32x2, VertexFormat::Float32x3],
)
.offset_locations(1),
],
}
```
Alternative solutions include:
- Pass the starting location to `from_vertex_formats` – this is a bit
simpler than my solution here, but most calls don't need an offset, so
they'd always pass 0 there.
- Do nothing and make the user hand-write this.
---
## Changelog
- Add `VertexBufferLayout::offset_locations` to simplify buffer layout
construction when using instancing.
---------
Co-authored-by: Nicola Papale <nicopap@users.noreply.github.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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