# Bevy App Compute

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Dispatch and run compute shaders on bevy from App World .

## Getting Started

Add the following line to your `Cargo.toml`

```toml
[dependencies]
bevy_app_compute = "0.10.3"
```

## Usage

### Setup

Declare your shaders in structs implementing `ComputeShader`. The `shader()` fn should point to your shader source code.
You need to derive `TypeUuid` as well and assign a unique Uuid:

```rust
#[derive(TypeUuid)]
#[uuid = "2545ae14-a9bc-4f03-9ea4-4eb43d1075a7"]
struct SimpleShader;

impl ComputeShader for SimpleShader {
    fn shader() -> ShaderRef {
        "shaders/simple.wgsl".into()
    }
}
```

Next, declare a struct implementing `ComputeWorker` to declare the bindings and the logic of your worker:

```rust
#[derive(Resource)]
struct SimpleComputeWorker;

impl ComputeWorker for SimpleComputeWorker {
    fn build(world: &mut World) -> AppComputeWorker<Self> {
        let worker = AppComputeWorkerBuilder::new(world)
            // Add a uniform variable
            .add_uniform("uni", &5.)

            // Add a staging buffer, it will be available from
            // both CPU and GPU land.
            .add_staging("values", &[1., 2., 3., 4.])

            // Create a compute pass from your compute shader
            // and define used variables
            .add_pass::<SimpleShader>([4, 1, 1], &["uni", "values"])
            .build();

        worker
    }
}

```

Don't forget to add a shader file to your `assets/` folder:

```rust

@group(0) @binding(0)
var<uniform> uni: f32;

@group(0) @binding(1)
var<storage, read_write> my_storage: array<f32>;

@compute @workgroup_size(1)
fn main(@builtin(global_invocation_id) invocation_id: vec3<u32>) {
    my_storage[invocation_id.x] = my_storage[invocation_id.x] + uni;
}
```

Add the `AppComputePlugin` plugin to your app, as well as one `AppComputeWorkerPlugin` per struct implementing `ComputeWorker`:

```rust
use bevy::prelude::*;
use bevy_app_compute::AppComputePlugin;

fn main() {
    App::new()
        .add_plugin(AppComputePlugin)
        .add_plugin(AppComputeWorkerPlugin::<SimpleComputeWorker>::default());
}
```

Your compute worker will now run every frame, during the `PostUpdate` stage. To read/write from it, use the `AppComputeWorker<T>` resource!

```rust
fn my_system(
    mut compute_worker: ResMut<AppComputeWorker<SimpleComputeWorker>>
) {
    if !compute_worker.available() {
        return;
    };

    let result: Vec<f32> = compute_worker.read_vec("values");

    compute_worker.write_slice("values", [2., 3., 4., 5.]);

    println!("got {:?}", result)
}
```

(see [simple.rs](https://github.com/kjolnyr/bevy_app_compute/tree/dev/examples/simple.rs))

### Multiple passes

You can have multiple passes without having to copy data back to the CPU in between:

```rust
let worker = AppComputeWorkerBuilder::new(world)
    .add_uniform("value", &3.)
    .add_storage("input", &[1., 2., 3., 4.])
    .add_staging("output", &[0f32; 4])
    // add each item + `value` from `input` to `output`
    .add_pass::<FirstPassShader>([4, 1, 1], &["value", "input", "output"]) 
    // multiply each element of `output` by itself
    .add_pass::<SecondPassShader>([4, 1, 1], &["output"]) 
    .build();

    // the `output` buffer will contain [16.0, 25.0, 36.0, 49.0]

```

(see [multi_pass.rs](https://github.com/kjolnyr/bevy_app_compute/tree/dev/examples/multi_pass.rs))

### One shot computes

You can configure your worker to execute only when requested:

```rust
let worker = AppComputeWorkerBuilder::new(world)
    .add_uniform("uni", &5.)
    .add_staging("values", &[1., 2., 3., 4.])
    .add_pass::<SimpleShader>([4, 1, 1], &["uni", "values"])

    // This `one_shot()` function will configure your worker accordingly
    .one_shot()
    .build();

```

Then, you can call `execute()` on your worker when you are ready to execute it:

```rust
// Execute it only when the left mouse button is pressed.
fn on_click_compute(
    buttons: Res<Input<MouseButton>>,
    mut compute_worker: ResMut<AppComputeWorker<SimpleComputeWorker>>
) {
    if !buttons.just_pressed(MouseButton::Left) { return; }

    compute_worker.execute();
} 
```

It will run at the end of the current frame, and you'll be able to read the data in the next frame.

(see [one_shot.rs](https://github.com/kjolnyr/bevy_app_compute/tree/dev/examples/one_shot.rs))


## Examples

See [examples](https://github.com/kjolnyr/bevy_app_compute/tree/main/examples)


## Features being worked upon

- Ability to read/write between compute passes.
- add more options to the api, like deciding `BufferUsages` or size of buffers.
- Optimization. Right now the code is a complete mess.
- Tests. This badly needs tests.

## Bevy version mapping

|Bevy|bevy_app_compute|
|---|---|
|main|main|
|0.10|0.10.3|