learn-wgpu/docs/beginner/tutorial2-surface

The Surface

First, some housekeeping: State

For convenience, we're going to pack all the fields into a struct and create some methods on that.

// main.rs
use winit::window::Window;

struct State {
    surface: wgpu::Surface,
    device: wgpu::Device,
    queue: wgpu::Queue,
    config: wgpu::SurfaceConfiguration,
    size: winit::dpi::PhysicalSize<u32>,
}

impl State {
    // Creating some of the wgpu types requires async code
    async fn new(window: &Window) -> Self {
        todo!()
    }

    fn resize(&mut self, new_size: winit::dpi::PhysicalSize<u32>) {
        todo!()
    }

    fn input(&mut self, event: &WindowEvent) -> bool {
        todo!()
    }

    fn update(&mut self) {
        todo!()
    }

    fn render(&mut self) -> Result<(), wgpu::SurfaceError> {
        todo!()
    }
}

I'm glossing over States fields, but they'll make more sense as I explain the code behind the methods.

State::new()

The code for this is pretty straight forward, but let's break this down a bit.

impl State {
    // ...
    async fn new(window: &Window) -> Self {
        let size = window.inner_size();

        // The instance is a handle to our GPU
        // Backends::all => Vulkan + Metal + DX12 + Browser WebGPU
        let instance = wgpu::Instance::new(wgpu::Backends::all());
        let surface = unsafe { instance.create_surface(window) };
        let adapter = instance.request_adapter(
            &wgpu::RequestAdapterOptions {
                power_preference: wgpu::PowerPreference::default(),
                compatible_surface: Some(&surface),
                force_fallback_adapter: false,
            },
        ).await.unwrap();

Instance and Adapter

The instance is the first thing you create when using wgpu. Its main purpose is to create Adapters and Surfaces.

The adapter is a handle to our actual graphics card. You can use this to get information about the graphics card such as its name and what backend the adapter uses. We use this to create our Device and Queue later. Let's discuss the fields of RequestAdapterOptions.

• power_preference has two variants: LowPower, and HighPerformance. This means will pick an adapter that favors battery life such as a integrated GPU when using LowPower. HighPerformance as will pick an adapter for more power hungry yet more performant GPU's such as your dedicated graphics card. WGPU will favor LowPower if there is no adapter for the HighPerformance option.
• The compatible_surface field tells wgpu to find an adapter that can present to the supplied surface.
• The force_fallback_adapter forces wgpu to pick an adapter that will work on all harware. This usually means that the rendering backend will use a "software" system, instead of hardware such as a GPU.

The options I've passed to request_adapter aren't guaranteed to work for all devices, but will work for most of them. If wgpu can't find an adapter with the required permissions, request_adapter will return None. If you want to get all adapters for a particular backend you can use enumerate_adapters. This will give you an iterator that you can loop over to check if one of the adapters works for your needs.

let adapter = instance
    .enumerate_adapters(wgpu::Backends::all())
    .filter(|adapter| {
        // Check if this adapter supports our surface
        surface.get_preferred_format(&adapter).is_some()
    })
    .next()
    .unwrap()

Another thing to note is that Adapters are locked to a specific backend. If you are on Windows and have 2 graphics cards you'll have at least 4 adapters available to use, 2 Vulkan and 2 DirectX.

For more fields you can use to refine your search check out the docs.

The Surface

The surface is the part of the window that we draw to. We need it to draw directly to the screen. Our window needs to implement raw-window-handle's HasRawWindowHandle trait to create a surface. Fortunately, winit's Window fits the bill. We also need it to request our adapter.

Device and Queue

Let's use the adapter to create the device and queue.

        let (device, queue) = adapter.request_device(
            &wgpu::DeviceDescriptor {
                features: wgpu::Features::empty(),
                limits: wgpu::Limits::default(),
                label: None,
            },
            None, // Trace path
        ).await.unwrap();

The features field on DeviceDescriptor, allows us to specify what extra features we want. For this simple example, I've decided not to use any extra features.

The graphics card you have limits the features you can use. If you want to use certain features you may need to limit what devices you support, or provide workarounds.

You can get a list of features supported by your device using adapter.features(), or device.features().

You can view a full list of features here.

The limits field describes the limit of certain types of resources that we can create. We'll use the defaults for this tutorial, so we can support most devices. You can view a list of limits here.

        let config = wgpu::SurfaceConfiguration {
            usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
            format: surface.get_preferred_format(&adapter).unwrap(),
            width: size.width,
            height: size.height,
            present_mode: wgpu::PresentMode::Fifo,
        };
        surface.configure(&device, &config);

Here we are defining a config for our surface. This will define how the surface creates its underlying SurfaceTextures. We will talk about SurfaceTexture when we get to the render function. For now lets talk about the config's fields.

The usage field describes how SurfaceTextures will be used. RENDER_ATTACHMENT specifies that the textures will be used to write to the screen (we'll talk about more TextureUsagess later).

The format defines how SurfaceTextures will be stored on the gpu. Different displays prefer different formats. We use surface.get_preferred_format(&adapter) to figure out the best format to use based on the display you're using.

width and height are the width and the height in pixels of a SurfaceTexture. This should usually be the width and the height of the window.

Make sure that the width and height of the `SurfaceTexture` are not 0, as that can cause your app to crash.

present_mode uses wgpu::PresentMode enum which determines how to sync the surface with the display. The option we picked, FIFO, will cap the display rate at the displays framerate. This is essentially VSync. This is also the most optimal mode on mobile. There are other options and you can see all of them in the docs

Now that we've configured our surface properly we can add these new fields at the end of the method.

        Self {
            surface,
            device,
            queue,
            config,
            size,
        }
    }
    // ...
}

We'll want to call this in our main method before we enter the event loop.

// State::new uses async code, so we're going to wait for it to finish
let mut state = pollster::block_on(State::new(&window));

You can use heavier libraries like async_std and tokio to make main async, so you can await futures. I've elected not to use these crates as this tutorial is not about writing an async application, and the futures created by wgpu do not require special executor support. We just need some way to interact with wgpu's async functions, and the pollster crate is enough for that.

resize()

If we want to support resizing in our application, we're going to need to reconfigure the surface everytime the window's size changes. That's the reason we stored the physical size and the config used to configure the surface. With all of these, the resize method is very simple.

// impl State
pub fn resize(&mut self, new_size: winit::dpi::PhysicalSize<u32>) {
    if new_size.width > 0 && new_size.height > 0 {
        self.size = new_size;
        self.config.width = new_size.width;
        self.config.height = new_size.height;
        self.surface.configure(&self.device, &self.config);
    }
}

There's nothing really different here from the initial surface configuration, so I won't get into it.

We call this method in main() in the event loop for the following events.

match event {
    // ...

    } if window_id == window.id() => if !state.input(event) {
        match event {
            // ...

            WindowEvent::Resized(physical_size) => {
                state.resize(*physical_size);
            }
            WindowEvent::ScaleFactorChanged { new_inner_size, .. } => {
                // new_inner_size is &&mut so we have to dereference it twice
                state.resize(**new_inner_size);
            }
            // ...
}

input()

input() returns a bool to indicate whether an event has been fully processed. If the method returns true, the main loop won't process the event any further.

We're just going to return false for now because we don't have any events we want to capture.

// impl State
fn input(&mut self, event: &WindowEvent) -> bool {
    false
}

We need to do a little more work in the event loop. We want State to have priority over main(). Doing that (and previous changes) should have your loop looking like this.

// main()
event_loop.run(move |event, _, control_flow| {
    match event {
        Event::WindowEvent {
            ref event,
            window_id,
        } if window_id == window.id() => if !state.input(event) { // UPDATED!
            match event {
                WindowEvent::CloseRequested
                | WindowEvent::KeyboardInput {
                    input:
                        KeyboardInput {
                            state: ElementState::Pressed,
                            virtual_keycode: Some(VirtualKeyCode::Escape),
                            ..
                        },
                    ..
                } => *control_flow = ControlFlow::Exit,
                WindowEvent::Resized(physical_size) => {
                    state.resize(*physical_size);
                }
                WindowEvent::ScaleFactorChanged { new_inner_size, .. } => {
                    state.resize(**new_inner_size);
                }
                _ => {}
            }
        }
        _ => {}
    }
});

update()

We don't have anything to update yet, so leave the method empty.

fn update(&mut self) {
    // remove `todo!()`
}

We'll add some code here later on to move around objects.

render()

Here's where the magic happens. First we need to get a frame to render to.

// impl State

fn render(&mut self) -> Result<(), wgpu::SurfaceError> {
    let output = self.surface.get_current_texture()?;

The get_current_texture function will wait for the surface to provide a new SurfaceTexture that we will render to. We'll store this in output for later.

    let view = output.texture.create_view(&wgpu::TextureViewDescriptor::default());

This line creates a TextureView with default settings. We need to do this because we want to control how the render code interacts with the texture.

We also need to create a CommandEncoder to create the actual commands to send to the gpu. Most modern graphics frameworks expect commands to be stored in a command buffer before being sent to the gpu. The encoder builds a command buffer that we can then send to the gpu.

    let mut encoder = self.device.create_command_encoder(&wgpu::CommandEncoderDescriptor {
        label: Some("Render Encoder"),
    });

Now we can actually get to clearing the screen (long time coming). We need to use the encoder to create a RenderPass. The RenderPass has all the methods for the actual drawing. The code for creating a RenderPass is a bit nested, so I'll copy it all here before talking about its pieces.

    {
        let _render_pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
            label: Some("Render Pass"),
            color_attachments: &[wgpu::RenderPassColorAttachment {
                view: &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,
        });
    }

    // submit will accept anything that implements IntoIter
    self.queue.submit(std::iter::once(encoder.finish()));
    output.present();

    Ok(())
}

First things first, let's talk about the extra block ({}) around encoder.begin_render_pass(...). begin_render_pass() borrows encoder mutably (aka &mut self). We can't call encoder.finish() until we release that mutable borrow. The block tells rust to drop any variables within it when the code leaves that scope thus releasing the mutable borrow on encoder and allowing us to finish() it. If you don't like the {}, you can also use drop(render_pass) to achieve the same effect.

We can get the same results by removing the {}, and the let _render_pass = line, but we need access to the _render_pass in the next tutorial, so we'll leave it as is.

The last lines of the code tell wgpu to finish the command buffer, and to submit it to the gpu's render queue.

We need to update the event loop again to call this method. We'll also call update before it too.

// main()
event_loop.run(move |event, _, control_flow| {
    match event {
        // ...
        Event::RedrawRequested(window_id) if window_id == window.id() => {
            state.update();
            match state.render() {
                Ok(_) => {}
                // Reconfigure the surface if lost
                Err(wgpu::SurfaceError::Lost) => state.resize(state.size),
                // The system is out of memory, we should probably quit
                Err(wgpu::SurfaceError::OutOfMemory) => *control_flow = ControlFlow::Exit,
                // All other errors (Outdated, Timeout) should be resolved by the next frame
                Err(e) => eprintln!("{:?}", e),
            }
        }
        Event::MainEventsCleared => {
            // RedrawRequested will only trigger once, unless we manually
            // request it.
            window.request_redraw();
        }
        // ...
    }
});

With all that, you should be getting something that looks like this.

Wait, what's going on with RenderPassDescriptor?

Some of you may be able to tell what's going on just by looking at it, but I'd be remiss if I didn't go over it. Let's take a look at the code again.

&wgpu::RenderPassDescriptor {
    label: Some("Render Pass"),
    color_attachments: &[
        // ...
    ],
    depth_stencil_attachment: None,
}

A RenderPassDescriptor only has three fields: label, color_attachments and depth_stencil_attachment. The color_attachements describe where we are going to draw our color to. We use the TextureView we created earlier to make sure that we render to the screen.

We'll use depth_stencil_attachment later, but we'll set it to None for now.

wgpu::RenderPassColorAttachment {
    view: &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,
    },
}

The RenderPassColorAttachment has the view field which informs wgpu what texture to save the colors to. In this case we specify frame.view that we created using surface.get_current_texture(). This means that any colors we draw to this attachment will get drawn to the screen.

The resolve_target is the texture that will receive the resolved output. This will be the same as view unless multisampling is enabled. We don't need to specify this, so we leave it as None.

The ops field takes a wpgu::Operations object. This tells wgpu what to do with the colors on the screen (specified by frame.view). The load field tells wgpu how to handle colors stored from the previous frame. Currently, we are clearing the screen with a bluish color. The store field tells wgpu whether we want to store the rendered results to the Texture behind our TextureView (in this case it's the SurfaceTexture). We use true as we do want to store our render results. There are cases when you wouldn't want to but those

It's not uncommon to not clear the screen if the screen is going to be completely covered up with objects. If your scene doesn't cover the entire screen however you can end up with something like this.

Validation Errors?

If wgpu is using Vulkan on your machine, you may run into validation errors if you are running an older version of the Vulkan SDK. You should be using at least version 1.2.182 as older versions can give out some false positives. If errors persist, you may have encountered a bug in wgpu. You can post an issue at https://github.com/gfx-rs/wgpu

Challenge

Modify the input() method to capture mouse events, and update the clear color using that. Hint: you'll probably need to use WindowEvent::CursorMoved.