egui_fltk_frontend/

backend.rs

// source: https://github.com/emilk/egui/blob/master/crates/egui-wgpu/src/renderer.rs modified for FLTK

use egui::{epaint::Primitive, PaintCallbackInfo};
use fxhash::FxHashMap;
use std::{borrow::Cow, num::NonZeroU32};
use type_map::concurrent::TypeMap;
use wgpu::util::DeviceExt as _;
use wgpu::{self, TextureViewDescriptor};

const EGUI_WGSL: &str = r#"
// Vertex shader bindings

struct VertexOutput {
    @location(0) tex_coord: vec2<f32>,
    @location(1) color: vec4<f32>,
    @builtin(position) position: vec4<f32>,
};

struct Locals {
    screen_size: vec2<f32>,
    // Uniform buffers need to be at least 16 bytes in WebGL.
    // See https://github.com/gfx-rs/wgpu/issues/2072
    _padding: vec2<u32>,
};
@group(0) @binding(0) var<uniform> r_locals: Locals;

// 0-1 from 0-255
fn linear_from_srgb(srgb: vec3<f32>) -> vec3<f32> {
    let cutoff = srgb < vec3<f32>(10.31475);
    let lower = srgb / vec3<f32>(3294.6);
    let higher = pow((srgb + vec3<f32>(14.025)) / vec3<f32>(269.025), vec3<f32>(2.4));
    return select(higher, lower, cutoff);
}

// [u8; 4] SRGB as u32 -> [r, g, b, a]
fn unpack_color(color: u32) -> vec4<f32> {
    return vec4<f32>(
        f32(color & 255u),
        f32((color >> 8u) & 255u),
        f32((color >> 16u) & 255u),
        f32((color >> 24u) & 255u),
    );
}

fn position_from_screen(screen_pos: vec2<f32>) -> vec4<f32> {
    return vec4<f32>(
        2.0 * screen_pos.x / r_locals.screen_size.x - 1.0,
        1.0 - 2.0 * screen_pos.y / r_locals.screen_size.y,
        0.0,
        1.0,
    );
}

@vertex
fn vs_main(
    @location(0) a_pos: vec2<f32>,
    @location(1) a_tex_coord: vec2<f32>,
    @location(2) a_color: u32,
) -> VertexOutput {
    var out: VertexOutput;
    out.tex_coord = a_tex_coord;
    let color = unpack_color(a_color);
    out.color = vec4<f32>(linear_from_srgb(color.rgb), color.a / 255.0);
    out.position = position_from_screen(a_pos);
    return out;
}

@vertex
fn vs_conv_main(
    @location(0) a_pos: vec2<f32>,
    @location(1) a_tex_coord: vec2<f32>,
    @location(2) a_color: u32,
) -> VertexOutput {
    var out: VertexOutput;
    out.tex_coord = a_tex_coord;
    let color = unpack_color(a_color);
    out.color = vec4<f32>(color.rgba / 255.0);
    out.position = position_from_screen(a_pos);
    return out;
}

// Fragment shader bindings

@group(1) @binding(0) var r_tex_color: texture_2d<f32>;
@group(1) @binding(1) var r_tex_sampler: sampler;

@fragment
fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
    return in.color * textureSample(r_tex_color, r_tex_sampler, in.tex_coord);
}
"#;

const IDX_BUF: &str = "egui_index_buffer";
const VTX_BUF: &str = "egui_vertex_buffer";
const UNI_BUF: &str = "egui_uniform_buffer";
const WG_RPAS: &str = "wgpu render pass";
const GRP_LAB: &str = "egui_pass";

/// A callback function that can be used to compose an [`egui::PaintCallback`] for custom WGPU
/// rendering.
///
/// The callback is composed of two functions: `prepare` and `paint`.
///
/// `prepare` is called every frame before `paint`, and can use the passed-in [`wgpu::Device`] and
/// [`wgpu::Buffer`] to allocate or modify GPU resources such as buffers.
///
/// `paint` is called after `prepare` and is given access to the the [`wgpu::RenderPass`] so that it
/// can issue draw commands.
///
/// The final argument of both the `prepare` and `paint` callbacks is a the
/// [`paint_callback_resources`][crate::renderer::RenderPass::paint_callback_resources].
/// `paint_callback_resources` has the same lifetime as the Egui render pass, so it can be used to
/// store buffers, pipelines, and other information that needs to be accessed during the render
/// pass.
///
/// # Example
///
/// See the [`custom3d_glow`](https://github.com/emilk/egui/blob/master/crates/egui_demo_app/src/apps/custom3d_wgpu.rs) demo source for a detailed usage example.
pub struct CallbackFn {
    prepare: Box<PrepareCallback>,
    paint: Box<PaintCallback>,
}

type PrepareCallback = dyn Fn(&wgpu::Device, &wgpu::Queue, &mut TypeMap) + Sync + Send;

type PaintCallback =
    dyn for<'a, 'b> Fn(PaintCallbackInfo, &'a mut wgpu::RenderPass<'b>, &'b TypeMap) + Sync + Send;

impl Default for CallbackFn {
    fn default() -> Self {
        CallbackFn {
            prepare: Box::new(|_, _, _| ()),
            paint: Box::new(|_, _, _| ()),
        }
    }
}

#[allow(dead_code)]
impl CallbackFn {
    pub fn new() -> Self {
        Self::default()
    }

    /// Set the prepare callback
    pub fn prepare<F>(mut self, prepare: F) -> Self
    where
        F: Fn(&wgpu::Device, &wgpu::Queue, &mut TypeMap) + Sync + Send + 'static,
    {
        self.prepare = Box::new(prepare) as _;
        self
    }

    /// Set the paint callback
    pub fn paint<F>(mut self, paint: F) -> Self
    where
        F: for<'a, 'b> Fn(PaintCallbackInfo, &'a mut wgpu::RenderPass<'b>, &'b TypeMap)
            + Sync
            + Send
            + 'static,
    {
        self.paint = Box::new(paint) as _;
        self
    }
}

/// Enum for selecting the right buffer type.
#[derive(Debug)]
enum BufferType {
    Uniform,
    Index,
    Vertex,
}

/// Information about the screen used for rendering.
pub struct ScreenDescriptor {
    /// Size of the window in physical pixels.
    pub size_in_pixels: [u32; 2],

    /// HiDPI scale factor (pixels per point).
    pub pixels_per_point: f32,
}

impl ScreenDescriptor {
    /// size in "logical" points
    pub fn screen_size_in_points(&self) -> [f32; 2] {
        [
            self.size_in_pixels[0] as f32 / self.pixels_per_point,
            self.size_in_pixels[1] as f32 / self.pixels_per_point,
        ]
    }
}

/// Uniform buffer used when rendering.
#[derive(Clone, Copy, Debug, bytemuck::Pod, bytemuck::Zeroable)]
#[repr(C)]
struct UniformBuffer {
    screen_size_in_points: [f32; 2],
    // Uniform buffers need to be at least 16 bytes in WebGL.
    // See https://github.com/gfx-rs/wgpu/issues/2072
    _padding: [u32; 2],
}

/// Wraps the buffers and includes additional information.
#[derive(Debug)]
struct SizedBuffer {
    buffer: wgpu::Buffer,
    /// number of bytes
    size: usize,
}

/// Render pass to render a egui based GUI.
pub struct RenderPass<'a> {
    render_pipeline: wgpu::RenderPipeline,
    index_buffers: Vec<SizedBuffer>,
    vertex_buffers: Vec<SizedBuffer>,
    uniform_buffer: SizedBuffer,
    uniform_bind_group: wgpu::BindGroup,
    texture_bind_group_layout: wgpu::BindGroupLayout,
    /// Map of egui texture IDs to textures and their associated bindgroups (texture view +
    /// sampler). The texture may be None if the TextureId is just a handle to a user-provided
    /// sampler.
    textures: FxHashMap<egui::TextureId, (Option<wgpu::Texture>, wgpu::BindGroup)>,
    next_user_texture_id: u64,
    /// Storage for use by [`egui::PaintCallback`]'s that need to store resources such as render
    /// pipelines that must have the lifetime of the renderpass.
    pub paint_callback_resources: TypeMap,
    sampler: wgpu::Sampler,
    texture_size: wgpu::Extent3d,
    pub tex_view_desc: TextureViewDescriptor<'a>,
}

impl<'a> RenderPass<'a> {
    /// Creates a new render pass to render a egui UI.
    ///
    /// If the format passed is not a *Srgb format, the shader will automatically convert to `sRGB` colors in the shader.
    pub fn new(
        device: &wgpu::Device,
        texture_format: wgpu::TextureFormat,
        msaa_samples: u32,
    ) -> Self {
        let shader = wgpu::ShaderModuleDescriptor {
            label: Some("egui_shader"),
            source: wgpu::ShaderSource::Wgsl(Cow::Borrowed(EGUI_WGSL)),
        };
        let module = device.create_shader_module(shader);

        let uniform_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
            label: Some(UNI_BUF),
            contents: bytemuck::cast_slice(&[UniformBuffer {
                screen_size_in_points: [0.0, 0.0],
                _padding: Default::default(),
            }]),
            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
        });
        let uniform_buffer = SizedBuffer {
            buffer: uniform_buffer,
            size: std::mem::size_of::<UniformBuffer>(),
        };

        let uniform_bind_group_layout =
            device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
                label: Some("egui_uniform_bind_group_layout"),
                entries: &[wgpu::BindGroupLayoutEntry {
                    binding: 0,
                    visibility: wgpu::ShaderStages::VERTEX,
                    ty: wgpu::BindingType::Buffer {
                        has_dynamic_offset: false,
                        min_binding_size: None,
                        ty: wgpu::BufferBindingType::Uniform,
                    },
                    count: None,
                }],
            });

        let uniform_bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
            label: Some("egui_uniform_bind_group"),
            layout: &uniform_bind_group_layout,
            entries: &[wgpu::BindGroupEntry {
                binding: 0,
                resource: wgpu::BindingResource::Buffer(wgpu::BufferBinding {
                    buffer: &uniform_buffer.buffer,
                    offset: 0,
                    size: None,
                }),
            }],
        });

        let texture_bind_group_layout =
            device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
                label: Some("egui_texture_bind_group_layout"),
                entries: &[
                    wgpu::BindGroupLayoutEntry {
                        binding: 0,
                        visibility: wgpu::ShaderStages::FRAGMENT,
                        ty: wgpu::BindingType::Texture {
                            multisampled: false,
                            sample_type: wgpu::TextureSampleType::Float { filterable: true },
                            view_dimension: wgpu::TextureViewDimension::D2,
                        },
                        count: None,
                    },
                    wgpu::BindGroupLayoutEntry {
                        binding: 1,
                        visibility: wgpu::ShaderStages::FRAGMENT,
                        ty: wgpu::BindingType::Sampler(wgpu::SamplerBindingType::Filtering),
                        count: None,
                    },
                ],
            });

        let pipeline_layout = device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
            label: Some("egui_pipeline_layout"),
            bind_group_layouts: &[&uniform_bind_group_layout, &texture_bind_group_layout],
            push_constant_ranges: &[],
        });

        let mut multisample = wgpu::MultisampleState::default();
        multisample.count = msaa_samples;

        let render_pipeline = device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
            label: Some("egui_pipeline"),
            layout: Some(&pipeline_layout),
            vertex: wgpu::VertexState {
                entry_point: if texture_format.describe().srgb {
                    "vs_main"
                } else {
                    "vs_conv_main"
                },
                module: &module,
                buffers: &[wgpu::VertexBufferLayout {
                    array_stride: 5 * 4,
                    step_mode: wgpu::VertexStepMode::Vertex,
                    // 0: vec2 position
                    // 1: vec2 texture coordinates
                    // 2: uint color
                    attributes: &wgpu::vertex_attr_array![0 => Float32x2, 1 => Float32x2, 2 => Uint32],
                }],
            },
            primitive: wgpu::PrimitiveState {
                topology: wgpu::PrimitiveTopology::TriangleList,
                unclipped_depth: false,
                conservative: false,
                cull_mode: None,
                front_face: wgpu::FrontFace::default(),
                polygon_mode: wgpu::PolygonMode::default(),
                strip_index_format: None,
            },
            depth_stencil: None,
            multisample,

            fragment: Some(wgpu::FragmentState {
                module: &module,
                entry_point: "fs_main",
                targets: &[Some(wgpu::ColorTargetState {
                    format: texture_format,
                    blend: Some(wgpu::BlendState {
                        color: wgpu::BlendComponent {
                            src_factor: wgpu::BlendFactor::One,
                            dst_factor: wgpu::BlendFactor::OneMinusSrcAlpha,
                            operation: wgpu::BlendOperation::Add,
                        },
                        alpha: wgpu::BlendComponent {
                            src_factor: wgpu::BlendFactor::OneMinusDstAlpha,
                            dst_factor: wgpu::BlendFactor::One,
                            operation: wgpu::BlendOperation::Add,
                        },
                    }),
                    write_mask: wgpu::ColorWrites::ALL,
                })],
            }),
            multiview: None,
        });

        let sampler = device.create_sampler(&wgpu::SamplerDescriptor {
            label: None,
            mag_filter: wgpu::FilterMode::Linear,
            min_filter: wgpu::FilterMode::Nearest,
            ..Default::default()
        });

        let texture_size = wgpu::Extent3d {
            width: 200,
            height: 200,
            depth_or_array_layers: 1,
        };

        Self {
            render_pipeline,
            vertex_buffers: Vec::with_capacity(64),
            index_buffers: Vec::with_capacity(64),
            uniform_buffer,
            uniform_bind_group,
            texture_bind_group_layout,
            textures: FxHashMap::default(),
            next_user_texture_id: 0,
            paint_callback_resources: TypeMap::default(),
            sampler,
            texture_size,
            tex_view_desc: TextureViewDescriptor::default(),
        }
    }

    /// Executes the egui render pass.
    pub fn execute(
        &self,
        encoder: &mut wgpu::CommandEncoder,
        color_attachment: &wgpu::TextureView,
        paint_jobs: Vec<egui::epaint::ClippedPrimitive>,
        screen_descriptor: &ScreenDescriptor,
        clear_color: Option<wgpu::Color>,
    ) {
        let load_operation = if let Some(color) = clear_color {
            wgpu::LoadOp::Clear(color)
        } else {
            wgpu::LoadOp::Load
        };

        let mut rpass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
            color_attachments: &[Some(wgpu::RenderPassColorAttachment {
                view: color_attachment,
                resolve_target: None,
                ops: wgpu::Operations {
                    load: load_operation,
                    store: true,
                },
            })],
            depth_stencil_attachment: None,
            label: Some(WG_RPAS),
        });
        rpass.push_debug_group(GRP_LAB);

        self.execute_with_renderpass(&mut rpass, paint_jobs, screen_descriptor);

        rpass.pop_debug_group();
    }

    /// Executes the egui render pass onto an existing wgpu renderpass.
    pub fn execute_with_renderpass<'rpass>(
        &'rpass self,
        rpass: &mut wgpu::RenderPass<'rpass>,
        paint_jobs: Vec<egui::epaint::ClippedPrimitive>,
        screen_descriptor: &ScreenDescriptor,
    ) {
        let pixels_per_point = screen_descriptor.pixels_per_point;
        let size_in_pixels = screen_descriptor.size_in_pixels;

        // Whether or not we need to reset the renderpass state because a paint callback has just
        // run.
        let mut needs_reset = true;

        let mut index_buffers = self.index_buffers.iter();
        let mut vertex_buffers = self.vertex_buffers.iter();

        for egui::ClippedPrimitive {
            clip_rect,
            primitive,
        } in paint_jobs
        {
            if needs_reset {
                rpass.set_viewport(
                    0.0,
                    0.0,
                    size_in_pixels[0] as f32,
                    size_in_pixels[1] as f32,
                    0.0,
                    1.0,
                );
                rpass.set_pipeline(&self.render_pipeline);
                rpass.set_bind_group(0, &self.uniform_bind_group, &[]);
                needs_reset = false;
            }

            {
                let rect = ScissorRect::new(&clip_rect, pixels_per_point, size_in_pixels);

                if rect.width == 0 || rect.height == 0 {
                    // Skip rendering with zero-sized clip areas.
                    if let Primitive::Mesh(_) = primitive {
                        // If this is a mesh, we need to advance the index and vertex buffer iterators
                        index_buffers.next().unwrap();
                        vertex_buffers.next().unwrap();
                    }
                    continue;
                }

                rpass.set_scissor_rect(rect.x, rect.y, rect.width, rect.height);
            }

            match primitive {
                Primitive::Mesh(mesh) => {
                    let index_buffer = index_buffers.next().unwrap();
                    let vertex_buffer = vertex_buffers.next().unwrap();

                    if let Some((_texture, bind_group)) = self.textures.get(&mesh.texture_id) {
                        rpass.set_bind_group(1, bind_group, &[]);
                        rpass.set_index_buffer(
                            index_buffer.buffer.slice(..),
                            wgpu::IndexFormat::Uint32,
                        );
                        rpass.set_vertex_buffer(0, vertex_buffer.buffer.slice(..));
                        rpass.draw_indexed(0..mesh.indices.len() as u32, 0, 0..1);
                    } else {
                        eprintln!("Missing texture: {:?}", mesh.texture_id);
                    }
                }
                Primitive::Callback(callback) => {
                    let cbfn = if let Some(c) = callback.callback.downcast_ref::<CallbackFn>() {
                        c
                    } else {
                        // We already warned in the `prepare` callback
                        continue;
                    };

                    if callback.rect.is_positive() {
                        needs_reset = true;

                        {
                            // Set the viewport rect
                            // Transform callback rect to physical pixels:
                            let rect_min_x = pixels_per_point * callback.rect.min.x;
                            let rect_min_y = pixels_per_point * callback.rect.min.y;
                            let rect_max_x = pixels_per_point * callback.rect.max.x;
                            let rect_max_y = pixels_per_point * callback.rect.max.y;

                            let rect_min_x = rect_min_x.round();
                            let rect_min_y = rect_min_y.round();
                            let rect_max_x = rect_max_x.round();
                            let rect_max_y = rect_max_y.round();

                            rpass.set_viewport(
                                rect_min_x,
                                rect_min_y,
                                rect_max_x - rect_min_x,
                                rect_max_y - rect_min_y,
                                0.0,
                                1.0,
                            );
                        }

                        (cbfn.paint)(
                            PaintCallbackInfo {
                                viewport: callback.rect,
                                clip_rect,
                                pixels_per_point,
                                screen_size_px: size_in_pixels,
                            },
                            rpass,
                            &self.paint_callback_resources,
                        );
                    }
                }
            }
        }

        rpass.set_scissor_rect(0, 0, size_in_pixels[0], size_in_pixels[1]);
    }

    /// Should be called before `execute()`.
    pub fn update_texture(
        &mut self,
        device: &wgpu::Device,
        queue: &wgpu::Queue,
        id: egui::TextureId,
        image_delta: &egui::epaint::ImageDelta,
    ) {
        let width = image_delta.image.width() as u32;
        let height = image_delta.image.height() as u32;

        self.texture_size.width = width;
        self.texture_size.height = height;

        let data_color32 = match &image_delta.image {
            egui::ImageData::Color(image) => {
                assert_eq!(
                    width as usize * height as usize,
                    image.pixels.len(),
                    "Mismatch between texture size and texel count"
                );
                Cow::Borrowed(&image.pixels)
            }
            egui::ImageData::Font(image) => {
                assert_eq!(
                    width as usize * height as usize,
                    image.pixels.len(),
                    "Mismatch between texture size and texel count"
                );
                Cow::Owned(image.srgba_pixels(1.0).collect::<Vec<_>>())
            }
        };

        let queue_write_data_to_texture = |texture, origin| {
            queue.write_texture(
                wgpu::ImageCopyTexture {
                    texture,
                    mip_level: 0,
                    origin,
                    aspect: wgpu::TextureAspect::All,
                },
                bytemuck::cast_slice(data_color32.as_slice()),
                wgpu::ImageDataLayout {
                    offset: 0,
                    bytes_per_row: NonZeroU32::new(4 * width),
                    rows_per_image: NonZeroU32::new(height),
                },
                self.texture_size,
            );
        };

        if let Some(pos) = image_delta.pos {
            // update the existing texture
            let (texture, _bind_group) = self
                .textures
                .get(&id)
                .expect("Tried to update a texture that has not been allocated yet.");
            let origin = wgpu::Origin3d {
                x: pos[0] as u32,
                y: pos[1] as u32,
                z: 0,
            };
            queue_write_data_to_texture(
                texture.as_ref().expect("Tried to update user texture."),
                origin,
            );
        } else {
            // allocate a new texture
            let texture = device.create_texture(&wgpu::TextureDescriptor {
                label: None,
                size: self.texture_size,
                mip_level_count: 1,
                sample_count: 1,
                dimension: wgpu::TextureDimension::D2,
                format: wgpu::TextureFormat::Rgba8UnormSrgb,
                usage: wgpu::TextureUsages::TEXTURE_BINDING | wgpu::TextureUsages::COPY_DST,
            });

            let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
                label: None,
                layout: &self.texture_bind_group_layout,
                entries: &[
                    wgpu::BindGroupEntry {
                        binding: 0,
                        resource: wgpu::BindingResource::TextureView(
                            &texture.create_view(&self.tex_view_desc),
                        ),
                    },
                    wgpu::BindGroupEntry {
                        binding: 1,
                        resource: wgpu::BindingResource::Sampler(&self.sampler),
                    },
                ],
            });

            let origin = wgpu::Origin3d::ZERO;
            queue_write_data_to_texture(&texture, origin);
            self.textures.insert(id, (Some(texture), bind_group));
        };
    }

    pub fn free_texture(&mut self, id: &egui::TextureId) {
        self.textures.remove(id);
    }

    /// Get the WGPU texture and bind group associated to a texture that has been allocated by egui.
    ///
    /// This could be used by custom paint hooks to render images that have been added through with
    /// [`egui_extras::RetainedImage`](https://docs.rs/egui_extras/latest/egui_extras/image/struct.RetainedImage.html)
    /// or [`egui::Context::load_texture`].
    pub fn texture(
        &self,
        id: &egui::TextureId,
    ) -> Option<&(Option<wgpu::Texture>, wgpu::BindGroup)> {
        self.textures.get(id)
    }

    /// Registers a `wgpu::Texture` with a `egui::TextureId`.
    ///
    /// This enables the application to reference the texture inside an image ui element.
    /// This effectively enables off-screen rendering inside the egui UI. Texture must have
    /// the texture format `TextureFormat::Rgba8UnormSrgb` and
    /// Texture usage `TextureUsage::SAMPLED`.
    pub fn register_native_texture(
        &mut self,
        device: &wgpu::Device,
        texture: &wgpu::TextureView,
        texture_filter: wgpu::FilterMode,
    ) -> egui::TextureId {
        self.register_native_texture_with_sampler_options(
            device,
            texture,
            wgpu::SamplerDescriptor {
                label: Some(
                    format!(
                        "egui_user_image_{}_texture_sampler",
                        self.next_user_texture_id
                    )
                    .as_str(),
                ),
                mag_filter: texture_filter,
                min_filter: texture_filter,
                ..Default::default()
            },
        )
    }

    /// Registers a `wgpu::Texture` with a `egui::TextureId` while also accepting custom
    /// `wgpu::SamplerDescriptor` options.
    ///
    /// This allows applications to specify individual minification/magnification filters as well as
    /// custom mipmap and tiling options.
    ///
    /// The `Texture` must have the format `TextureFormat::Rgba8UnormSrgb` and usage
    /// `TextureUsage::SAMPLED`. Any compare function supplied in the `SamplerDescriptor` will be
    /// ignored.
    #[allow(clippy::needless_pass_by_value)] // false positive
    pub fn register_native_texture_with_sampler_options(
        &mut self,
        device: &wgpu::Device,
        texture: &wgpu::TextureView,
        sampler_descriptor: wgpu::SamplerDescriptor<'_>,
    ) -> egui::TextureId {
        let sampler = device.create_sampler(&wgpu::SamplerDescriptor {
            compare: None,
            ..sampler_descriptor
        });

        let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
            label: Some(
                format!(
                    "egui_user_image_{}_texture_bind_group",
                    self.next_user_texture_id
                )
                .as_str(),
            ),
            layout: &self.texture_bind_group_layout,
            entries: &[
                wgpu::BindGroupEntry {
                    binding: 0,
                    resource: wgpu::BindingResource::TextureView(texture),
                },
                wgpu::BindGroupEntry {
                    binding: 1,
                    resource: wgpu::BindingResource::Sampler(&sampler),
                },
            ],
        });

        let id = egui::TextureId::User(self.next_user_texture_id);
        self.textures.insert(id, (None, bind_group));
        self.next_user_texture_id += 1;

        id
    }

    /// Uploads the uniform, vertex and index data used by the render pass.
    /// Should be called before `execute()`.
    pub fn update_buffers(
        &mut self,
        device: &wgpu::Device,
        queue: &wgpu::Queue,
        paint_jobs: &[egui::epaint::ClippedPrimitive],
        screen_descriptor: &ScreenDescriptor,
    ) {
        let screen_size_in_points = screen_descriptor.screen_size_in_points();

        self.update_buffer(
            device,
            queue,
            &BufferType::Uniform,
            0,
            bytemuck::cast_slice(&[UniformBuffer {
                screen_size_in_points,
                _padding: Default::default(),
            }]),
        );

        let mut mesh_idx = 0;
        for egui::ClippedPrimitive { primitive, .. } in paint_jobs.iter() {
            match primitive {
                Primitive::Mesh(mesh) => {
                    {
                        let data: &[u8] = bytemuck::cast_slice(&mesh.indices);
                        if mesh_idx < self.index_buffers.len() {
                            self.update_buffer(device, queue, &BufferType::Index, mesh_idx, data);
                        } else {
                            let buffer =
                                device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                                    label: Some(IDX_BUF),
                                    contents: data,
                                    usage: wgpu::BufferUsages::INDEX | wgpu::BufferUsages::COPY_DST,
                                });
                            self.index_buffers.push(SizedBuffer {
                                buffer,
                                size: data.len(),
                            });
                        }
                    }

                    let data: &[u8] = bytemuck::cast_slice(&mesh.vertices);
                    if mesh_idx < self.vertex_buffers.len() {
                        self.update_buffer(device, queue, &BufferType::Vertex, mesh_idx, data);
                    } else {
                        let buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                            label: Some(VTX_BUF),
                            contents: data,
                            usage: wgpu::BufferUsages::VERTEX | wgpu::BufferUsages::COPY_DST,
                        });

                        self.vertex_buffers.push(SizedBuffer {
                            buffer,
                            size: data.len(),
                        });
                    }

                    mesh_idx += 1;
                }
                Primitive::Callback(callback) => {
                    let cbfn = if let Some(c) = callback.callback.downcast_ref::<CallbackFn>() {
                        c
                    } else {
                        eprintln!("Unknown paint callback: expected `egui_gpu::CallbackFn`");
                        continue;
                    };

                    (cbfn.prepare)(device, queue, &mut self.paint_callback_resources);
                }
            }
        }
    }

    /// Updates the buffers used by egui. Will properly re-size the buffers if needed.
    fn update_buffer(
        &mut self,
        device: &wgpu::Device,
        queue: &wgpu::Queue,
        buffer_type: &BufferType,
        index: usize,
        data: &[u8],
    ) {
        let (buffer, storage, label) = match buffer_type {
            BufferType::Index => (
                &mut self.index_buffers[index],
                wgpu::BufferUsages::INDEX,
                IDX_BUF,
            ),
            BufferType::Vertex => (
                &mut self.vertex_buffers[index],
                wgpu::BufferUsages::VERTEX,
                VTX_BUF,
            ),
            BufferType::Uniform => (
                &mut self.uniform_buffer,
                wgpu::BufferUsages::UNIFORM,
                UNI_BUF,
            ),
        };

        if data.len() > buffer.size {
            buffer.size = data.len();
            buffer.buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some(label),
                contents: bytemuck::cast_slice(data),
                usage: storage | wgpu::BufferUsages::COPY_DST,
            });
        } else {
            queue.write_buffer(&buffer.buffer, 0, data);
        }
    }
}

/// A Rect in physical pixel space, used for setting cliipping rectangles.
struct ScissorRect {
    x: u32,
    y: u32,
    width: u32,
    height: u32,
}

impl ScissorRect {
    fn new(clip_rect: &egui::Rect, pixels_per_point: f32, target_size: [u32; 2]) -> Self {
        // Transform clip rect to physical pixels:
        let clip_min_x = (pixels_per_point * clip_rect.min.x).round() as u32;
        let clip_min_y = (pixels_per_point * clip_rect.min.y).round() as u32;
        let clip_max_x = (pixels_per_point * clip_rect.max.x).round() as u32;
        let clip_max_y = (pixels_per_point * clip_rect.max.y).round() as u32;

        // Clamp:
        let clip_min_x = clip_min_x.clamp(0, target_size[0]);
        let clip_min_y = clip_min_y.clamp(0, target_size[1]);
        let clip_max_x = clip_max_x.clamp(clip_min_x, target_size[0]);
        let clip_max_y = clip_max_y.clamp(clip_min_y, target_size[1]);

        ScissorRect {
            x: clip_min_x,
            y: clip_min_y,
            width: clip_max_x - clip_min_x,
            height: clip_max_y - clip_min_y,
        }
    }
}