learn-wgpu/docs/beginner/tutorial7-instancing

Instancing

Our scene right now is very simple: we have one object centered at (0,0,0). What if we wanted more objects? This is were instancing comes in.

Instancing allows us to draw the same object multiple times with different properties (position, orientation, size, color, etc.). There are multiple ways of doing instancing. One way would be to modify the uniform buffer to include these properties and then update it before we draw each instance of our object.

We don't want to use this method for performance reasons. Updating the uniform buffer for each instance would require multiple buffer copies each frame. On top of that, our method to update the uniform buffer currently requires use to create a new buffer to store the updated data. That's a lot of time wasted between draw calls.

If we look at the parameters for the draw_indexed function in the wgpu docs, we can see a solution to our problem.

pub fn draw_indexed(
    &mut self,
    indices: Range<u32>,
    base_vertex: i32,
    instances: Range<u32> // <-- This right here
)

The instances parameter takes a Range<u32>. This parameter tells the GPU how many copies, or instances, of our model we want to draw. Currently we are specifying 0..1, which instructs the GPU to draw our model once, and then stop. If we used 0..5, our code would draw 5 instances.

The fact that instances is a Range<u32> may seem weird as using 1..2 for instances would still draw 1 instance of our object. Seems like it would be simpler to just use a u32 right? The reason it's a range is because sometimes we don't want to draw all of our objects. Sometimes we want to draw a selection of them, because others are not in frame, or we are debugging and want to look at a particular set of instances.

Ok, now we know how to draw multiple instances of an object, how do we tell wgpu what particular instance to draw? We are going to use something known as an instance buffer.

The Instance Buffer

We'll create an instance buffer in a similar way to how we create a uniform buffer. First we'll create a struct called Instance.

// main.rs
// ...

// NEW!
struct Instance {
    position: cgmath::Vector3<f32>,
    rotation: cgmath::Quaternion<f32>,
}

A Quaternion is a mathematical structure often used to represent rotation. The math behind them is beyond me (it involves imaginary numbers and 4D space) so I won't be covering them here. If you really want to dive into them here's a Wolfram Alpha article.

Using these values directly in the shader would be a pain as quaternions don't have a WGSL analog. I don't feel like writing the math in the shader, so we'll convert the Instance data into a matrix and store it into a struct called InstanceRaw.

// NEW!
#[repr(C)]
#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
struct InstanceRaw {
    model: [[f32; 4]; 4],
}

This is the data that will go into the wgpu::Buffer. We keep these separate so that we can update the Instance as much as we want without needing to mess with matrices. We only need to update the raw data before we draw.

Let's create a method on Instance to convert to InstanceRaw.

// NEW!
impl Instance {
    fn to_raw(&self) -> InstanceRaw {
        InstanceRaw {
            model: (cgmath::Matrix4::from_translation(self.position) * cgmath::Matrix4::from(self.rotation)).into(),
        }
    }
}

Now we need to add 2 fields to State: instances, and instance_buffer.

struct State {
    instances: Vec<Instance>,
    instance_buffer: wgpu::Buffer,
}

The cgmath crate uses traits to provide common mathematical methods across its structs such as Vector3, and these traits must be imported before these methods can be called. For convenience, the prelude module within the crate provides the most common of these extension crates when it is imported.

To import this prelude module, put this line near the top of main.rs.

use cgmath::prelude::*;

We'll create the instances in new(). We'll use some constants to simplify things. We'll display our instances in 10 rows of 10, and they'll be spaced evenly apart.

const NUM_INSTANCES_PER_ROW: u32 = 10;
const NUM_INSTANCES: u32 = NUM_INSTANCES_PER_ROW * NUM_INSTANCES_PER_ROW;
const INSTANCE_DISPLACEMENT: cgmath::Vector3<f32> = cgmath::Vector3::new(NUM_INSTANCES_PER_ROW as f32 * 0.5, 0.0, NUM_INSTANCES_PER_ROW as f32 * 0.5);

Now we can create the actual instances.

impl State {
    async fn new(window: &Window) -> Self {
        // ...
        let instances = (0..NUM_INSTANCES_PER_ROW).flat_map(|z| {
            (0..NUM_INSTANCES_PER_ROW).map(move |x| {
                let position = cgmath::Vector3 { x: x as f32, y: 0.0, z: z as f32 } - INSTANCE_DISPLACEMENT;

                let rotation = if position.is_zero() {
                    // this is needed so an object at (0, 0, 0) won't get scaled to zero
                    // as Quaternions can effect scale if they're not created correctly
                    cgmath::Quaternion::from_axis_angle(cgmath::Vector3::unit_z(), cgmath::Deg(0.0))
                } else {
                    cgmath::Quaternion::from_axis_angle(position.normalize(), cgmath::Deg(45.0))
                };

                Instance {
                    position, rotation,
                }
            })
        }).collect::<Vec<_>>();
        // ...
    }
}

Now that we have our data, we can create the actual instance_buffer.

let instance_data = instances.iter().map(Instance::to_raw).collect::<Vec<_>>();
let instance_buffer = device.create_buffer_init(
    &wgpu::util::BufferInitDescriptor {
        label: Some("Instance Buffer"),
        contents: bytemuck::cast_slice(&instance_data),
        usage: wgpu::BufferUsages::VERTEX,
    }
);

We're going to need to create a new VertexBufferLayout for InstanceRaw.

impl InstanceRaw {
    fn desc<'a>() -> wgpu::VertexBufferLayout<'a> {
        use std::mem;
        wgpu::VertexBufferLayout {
            array_stride: mem::size_of::<InstanceRaw>() as wgpu::BufferAddress,
            // We need to switch from using a step mode of Vertex to Instance
            // This means that our shaders will only change to use the next
            // instance when the shader starts processing a new instance
            step_mode: wgpu::VertexStepMode::Instance,
            attributes: &[
                wgpu::VertexAttribute {
                    offset: 0,
                    // While our vertex shader only uses locations 0, and 1 now, in later tutorials we'll
                    // be using 2, 3, and 4, for Vertex. We'll start at slot 5 not conflict with them later
                    shader_location: 5,
                    format: wgpu::VertexFormat::Float32x4,
                },
                // A mat4 takes up 4 vertex slots as it is technically 4 vec4s. We need to define a slot
                // for each vec4. We'll have to reassemble the mat4 in
                // the shader.
                wgpu::VertexAttribute {
                    offset: mem::size_of::<[f32; 4]>() as wgpu::BufferAddress,
                    shader_location: 6,
                    format: wgpu::VertexFormat::Float32x4,
                },
                wgpu::VertexAttribute {
                    offset: mem::size_of::<[f32; 8]>() as wgpu::BufferAddress,
                    shader_location: 7,
                    format: wgpu::VertexFormat::Float32x4,
                },
                wgpu::VertexAttribute {
                    offset: mem::size_of::<[f32; 12]>() as wgpu::BufferAddress,
                    shader_location: 8,
                    format: wgpu::VertexFormat::Float32x4,
                },
            ],
        }
    }
}

We need to add this descriptor to the render pipeline so that we can use it when we render.

let render_pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
    // ...
    vertex: wgpu::VertexState {
        // ...
        // UPDATED!
        buffers: &[Vertex::desc(), InstanceRaw::desc()],
    },
    // ...
});

Don't forget to return our new variables!

Self {
    // ...
    // NEW!
    instances,
    instance_buffer,
}

The last change we need to make is in the render() method. We need to bind our instance_buffer and we need to change the range we're using in draw_indexed() to include the number of instances.

render_pass.set_pipeline(&self.render_pipeline);
render_pass.set_bind_group(0, &self.diffuse_bind_group, &[]);
render_pass.set_bind_group(1, &self.camera_bind_group, &[]);
render_pass.set_vertex_buffer(0, self.vertex_buffer.slice(..));
// NEW!
render_pass.set_vertex_buffer(1, self.instance_buffer.slice(..));
render_pass.set_index_buffer(self.index_buffer.slice(..), wgpu::IndexFormat::Uint16);

// UPDATED!
render_pass.draw_indexed(0..self.num_indices, 0, 0..self.instances.len() as _);

Make sure if you add new instances to the Vec, that you recreate the instance_buffer and as well as camera_bind_group, otherwise your new instances won't show up correctly.

We need to reference the parts of our new matrix in shader.wgsl so that we can use it for our instances. Add the following to the top of shader.wgsl.

struct InstanceInput {
    [[location(5)]] model_matrix_0: vec4<f32>;
    [[location(6)]] model_matrix_1: vec4<f32>;
    [[location(7)]] model_matrix_2: vec4<f32>;
    [[location(8)]] model_matrix_3: vec4<f32>;
};

We need to reassemble the matrix before we can use it.

[[stage(vertex)]]
fn vs_main(
    model: VertexInput,
    instance: InstanceInput,
) -> VertexOutput {
    let model_matrix = mat4x4<f32>(
        instance.model_matrix_0,
        instance.model_matrix_1,
        instance.model_matrix_2,
        instance.model_matrix_3,
    );
    // Continued...
}

We'll apply the model_matrix before we apply camera_uniform.view_proj. We do this because the camera_uniform.view_proj changes the coordinate system from world space to camera space. Our model_matrix is a world space transformation, so we don't want to be in camera space when using it.

[[stage(vertex)]]
fn vs_main(
    model: VertexInput,
    instance: InstanceInput,
) -> VertexOutput {
    // ...
    var out: VertexOutput;
    out.tex_coords = model.tex_coords;
    out.clip_position = camera.view_proj * model_matrix * vec4<f32>(model.position, 1.0);
    return out;
}

With all that done, we should have a forest of trees!

Challenge

Modify the position and/or rotation of the instances every frame.