learn-wgpu/docs/beginner/tutorial3-pipeline/README.md

# The Pipeline

## What's a pipeline?
If you're familiar with OpenGL, you may remember using shader programs. You can think of a pipeline as a more robust version of that. A pipeline describes all the actions the gpu will perform when acting on a set of data. In this section, we will be creating a `RenderPipeline` specifically.

## Wait shaders?
Shaders are mini programs that you send to the gpu to perform operations on your data. There are 3 main types of shader: vertex, fragment, and compute. There are others such as geometry shaders, but they're more of an advanced topic. For now we're just going to use vertex, and fragment shaders.

## Vertex, fragment.. what are those?
A vertex is a point in 3d space (can also be 2d). These vertices are then bundled in groups of 2s to form lines and/or 3s to form triangles.

<img src="./tutorial3-pipeline-vertices.png" />

Most modern rendering uses triangles to make all shapes, from simple shapes (such as cubes), to complex ones (such as people). These triangles are stored as vertices which are the points that make up the corners of the triangles.

<!-- Todo: Find/make an image to put here -->

We use a vertex shader to manipulate the vertices, in order to transform the shape to look the way we want it.

The vertices are then converted into fragments. Every pixel in the result image gets at least one fragment. Each fragment has a color that will be copied to its corresponding pixel. The fragment shader decides what color the fragment will be.

## WGSL

WebGPU supports two shader languages natively: SPIR-V, and WGSL. SPIR-V is actually a binary format developed by Kronos to be a compilation target for other languages such as GLSL and HLSL. It allows for easy porting of code. The only problem is that it's not human readable as it's a binary language. WGSL is meant to fix that. WGSL's development focuses on getting it to easily convert into SPIR-V. WGPU even allows us to supply WGSL for our shaders. 

<div class="note">

If you've gone through this tutorial before you'll likely notice that I've switched from using GLSL to using WGSL. Given that GLSL support is a secondary concern and that WGSL is the first class language of WGPU, I've elected to convert all the tutorials to use WGSL. Some of the showcase examples still use GLSL, but the main tutorial and all examples going forward will be using WGSL.

</div>

<div class="note">

The WGSL spec and it's inclusion in WGPU is still in development. If you run into trouble using it, you may want the folks at [https://app.element.io/#/room/#wgpu:matrix.org](https://app.element.io/#/room/#wgpu:matrix.org) to take a look at your code.

</div>

## Writing the shaders
In the same folder as `main.rs`, create a file `shader.wgsl`. Write the following code in `shader.wgsl`.

```wgsl
// Vertex shader

struct VertexOutput {
    [[builtin(position)]] clip_position: vec4<f32>;
};

[[stage(vertex)]]
fn main(
    [[builtin(vertex_index)]] in_vertex_index: u32,
) -> VertexOutput {
    var out: VertexOutput;
    let x = f32(1 - i32(in_vertex_index)) * 0.5;
    let y = f32(i32(in_vertex_index & 1u) * 2 - 1) * 0.5;
    out.clip_position = vec4<f32>(x, y, 0.0, 1.0);
    return out;
}
```

First we declare `struct` to store the inputs the output of our vertex shader. This consists of only one field currently which is our vertex's `clip_position`. The `[[builtin(position)]]` bit tells WGPU that this is the value we want to use as the vertex's [clip coordinates](https://en.wikipedia.org/wiki/Clip_coordinates). This is analogous to GLSL's `gl_Position` variable.

<div class="note">

Vector types such as `vec4` are generic. Currently you must specify the type of value the vector will contain. Thus a 3D vector using 32bit floats would be `vec3<f32>`.

</div>

The next part of the shader code is the `main` function. We using `[[stage(vertex)]]` to mark this function as a valid entry point for a vertex shader. We expect a `u32` called `in_vertex_index` which gets its value from `[[builtin(vertex_index)]]`.

We then declare a variable called `out` using our `VertexOutput` struct. We create two other variables for the `x`, and `y`, of a triangle.

<div class="note">

The `f32()` and `i32()` bits are examples of casts.

</div>

<div class="note">

Variables defined with `var` can be modified, but must specify their type. Variables created with `let` can have their types inferred, but their value cannot be changed during the shader.

</div>

Now we can save our `clip_position` to `out`. We then just return `out` and we're done with the vertex shader!

<div class="note">

We technically didn't need a struct for this example, and could have just done something like the following:

```wgsl
[[stage(vertex)]]
fn main(
    [[builtin(vertex_index)]] in_vertex_index: u32
) -> [[builtin(position)]] vec4<f32> {
    // Vertex shader code...
}
```

We'll be adding more fields to `VertexOutput` later, so we might as well start using it now.

</div>

Next up the fragment shader. Still in `shader.wgsl` add the follow:

```wgsl
// Fragment shader

[[stage(fragment)]]
fn main(in: VertexOutput) -> [[location(0)]] vec4<f32> {
    return vec4<f32>(0.3, 0.2, 0.1, 1.0);
}
```

All this does is set the color of the current fragment to brown color.

<div class="note">

Notice that this function is also called `main`. Because this function is marked as a fragment shader entry point, this is ok. You can change the names around if you like, but I've opted to keep them the same.

</div>

The `[[location(0)]]` bit tells WGPU to store the value the `vec4` returned by this function in the first color target. We'll get into what this is later.

## How do we use the shaders?
This is the part where we finally make the thing in the title: the pipeline. First let's modify `State` to include the following.

```rust
// main.rs
struct State {
    surface: wgpu::Surface,
    device: wgpu::Device,
    queue: wgpu::Queue,
    sc_desc: wgpu::SwapChainDescriptor,
    swap_chain: wgpu::SwapChain,
    size: winit::dpi::PhysicalSize<u32>,
    // NEW!
    render_pipeline: wgpu::RenderPipeline,
}
```

Now let's move to the `new()` method, and start making the pipeline. We'll have to load in those shaders we made earlier, as the `render_pipeline` requires those.

```rust
let shader = device.create_shader_module(&wgpu::ShaderModuleDescriptor {
    label: Some("Shader"),
    flags: wgpu::ShaderFlags::all(),
    source: wgpu::ShaderSource::Wgsl(include_str!("shader.wgsl").into()),
});
```

One more thing, we need to create a `PipelineLayout`. We'll get more into this after we cover `Buffer`s.

```rust
let render_pipeline_layout =
    device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
        label: Some("Render Pipeline Layout"),
        bind_group_layouts: &[],
        push_constant_ranges: &[],
    });
```

Finally we have all we need to create the `render_pipeline`.

```rust
let render_pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
    label: Some("Render Pipeline"),
    layout: Some(&render_pipeline_layout),
    vertex: wgpu::VertexState {
        module: &shader,
        entry_point: "main", // 1.
        buffers: &[], // 2.
    },
    fragment: Some(wgpu::FragmentState { // 3.
        module: &shader,
        entry_point: "main",
        targets: &[wgpu::ColorTargetState { // 4.
            format: sc_desc.format,
            blend: Some(wgpu::BlendState::REPLACE),
            write_mask: wgpu::ColorWrite::ALL,
        }],
    }),
    // continued ...
```

Two things to note here:
1. Here you can specify which function inside of the shader should be called, which is known as the `entry_point`. These are the functions we marked with `[[stage(vertex)]]` and `[[stage(fragment)]]`
2. The `buffers` field tells `wgpu` what type of vertices we want to pass to the vertex shader. We're specifying the vertices in the vertex shader itself so we'll leave this empty. We'll put something there in the next tutorial.
3. The `fragment` is technically optional, so you have to wrap it in `Some()`. We need it if we want to store color data to the `swap_chain`.
4. The `targets` field tells `wgpu` what color outputs it should set up.Currently we only need one for the `swap_chain`. We use the `swap_chain`'s format so that copying to it is easy, and we specify that the blending should just replace old pixel data with new data. We also tell `wgpu` to write to all colors: red, blue, green, and alpha. *We'll talk more about*`color_state` *when we talk about textures.*

```rust
    primitive: wgpu::PrimitiveState {
        topology: wgpu::PrimitiveTopology::TriangleList, // 1.
        strip_index_format: None,
        front_face: wgpu::FrontFace::Ccw, // 2.
        cull_mode: Some(wgpu::Face::Back),
        // Setting this to anything other than Fill requires Features::NON_FILL_POLYGON_MODE
        polygon_mode: wgpu::PolygonMode::Fill,
    },
    // continued ...
```

The `primitive` field describes how to interpret our vertices when converting them into triangles.

1. Using `PrimitiveTopology::TriangleList` means that each three vertices will correspond to one triangle.
2. The `front_face` and `cull_mode` fields tell `wgpu` how to determine whether a given triangle is facing forward or not. `FrontFace::Ccw` means that a triangle is facing forward if the vertices are arranged in a counter clockwise direction. Triangles that are not considered facing forward are culled (not included in the render) as specified by `CullMode::Back`. We'll cover culling a bit more when we cover `Buffer`s.

```rust
    depth_stencil: None, // 1.
    multisample: wgpu::MultisampleState {
        count: 1, // 2.
        mask: !0, // 3.
        alpha_to_coverage_enabled: false, // 4.
    },
});
```

The rest of the method is pretty simple:
1. We're not using a depth/stencil buffer currently, so we leave `depth_stencil` as `None`. *This will change later*.
2. This determines how many samples this pipeline will use. Multisampling is a complex topic, so we won't get into it here.
3. `sample_mask` specifies which samples should be active. In this case we are using all of them.
4. `alpha_to_coverage_enabled` has to do with anti-aliasing. We're not covering anti-aliasing here, so we'll leave this as false now.

<!-- https://gamedev.stackexchange.com/questions/22507/what-is-the-alphatocoverage-blend-state-useful-for -->

Now all we have to do is save the `render_pipeline` to `State` and then we can use it!

```rust
// new()
Self {
    surface,
    device,
    queue,
    sc_desc,
    swap_chain,
    size,
    // NEW!
    render_pipeline,
}
```
## Using a pipeline

If you run your program now, it'll take a little longer to start, but it will still show the blue screen we got in the last section. That's because while we created the `render_pipeline`, we need to modify the code in `render()` to actually use it.

```rust
// render()

// ...
{
    // 1.
    let mut render_pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
        label: Some("Render Pass"),
        color_attachments: &[
            // This is what [[location(0)]] in the fragment shader targets
            wgpu::RenderPassColorAttachment {
                view: &frame.view,
                resolve_target: None,
                ops: wgpu::Operations {
                    load: wgpu::LoadOp::Clear(
                        wgpu::Color {
                            r: 0.1,
                            g: 0.2,
                            b: 0.3,
                            a: 1.0,
                        }
                    ),
                    store: true,
                }
            }
        ],
        depth_stencil_attachment: None,
    });

    // NEW!
    render_pass.set_pipeline(&self.render_pipeline); // 2.
    render_pass.draw(0..3, 0..1); // 3.
}
// ...
```

We didn't change much, but let's talk about what we did change.
1. We renamed `_render_pass` to `render_pass` and made it mutable.
2. We set the pipeline on the `render_pass` using the one we just created.
3. We tell `wgpu` to draw *something* with 3 vertices, and 1 instance. This is where `[[builtin(vertex_index)]]` comes from.

With all that you should be seeing a lovely brown triangle.

![Said lovely brown triangle](./tutorial3-pipeline-triangle.png)


## Challenge
Create a second pipeline that uses the triangle's position data to create a color that it then sends to the fragment shader. Have the app swap between these when you press the spacebar. *Hint: you'll need to modify* `VertexOutput`

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