iris/core/src/render/atlas.rs

use crate::{
    PatchRect,
    util::{HashMap, Vec2},
};
use image::RgbaImage;
use swash::scale::image::{Content, Image};

/// Side of one page, and so of every layer of `render::page`'s array texture.
pub(crate) const PAGE: u32 = 1024;

/// Transparent margin kept around every glyph, so that sampling one cannot
/// pick up its neighbour along a shared edge.
const PAD: u32 = 1;

#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub struct GlyphKey {
    pub font: u64,
    pub glyph: u32,
    /// Font size in 1/16 px, so sizes that round to the same pixels share a
    /// raster instead of filling the atlas with near-duplicates.
    pub size: u32,
    /// Horizontal subpixel phase, in 1/4 px.
    pub subpixel: u8,
    /// Hash of the variation coordinates; a variable font at two weights is two
    /// different sets of pixels from one glyph id.
    pub coords: u64,
}

#[derive(Clone, Copy)]
pub struct GlyphEntry {
    pub uv_min: Vec2,
    pub uv_max: Vec2,
    /// Offset from the glyph's pen position to the top-left of its pixels.
    pub left: i32,
    pub top: i32,
    pub width: u32,
    pub height: u32,
    pub is_colored: bool,
    /// Which atlas array layer this glyph is on.
    pub layer: u32,
}

impl GlyphEntry {
    const IS_COLORED: u32 = 1;

    pub(crate) fn flags(&self) -> u32 {
        if self.is_colored { Self::IS_COLORED } else { 0 }
    }
}

struct Page {
    image: RgbaImage,
    x: u32,
    y: u32,
    shelf_height: u32,
}

/// A rectangle of one page the renderer has not uploaded yet.
#[derive(Clone, Copy)]
pub struct PageUpload {
    pub layer: u32,
    pub rect: PatchRect,
}

#[derive(Default)]
pub struct GlyphAtlas {
    pages: Vec<Page>,
    /// `None` for a glyph that rasterised to nothing -- a space, say. Cached
    /// too, so it is not re-rasterised on every layout.
    entries: HashMap<GlyphKey, Option<GlyphEntry>>,
    uploads: Vec<PageUpload>,
}

impl GlyphAtlas {
    pub fn get(&self, key: &GlyphKey) -> Option<Option<GlyphEntry>> {
        self.entries.get(key).copied()
    }

    pub fn insert(&mut self, key: GlyphKey, image: &Image) -> Option<GlyphEntry> {
        let w = image.placement.width;
        let h = image.placement.height;
        if w == 0 || h == 0 {
            log::warn!(
                "glyph {} in font {} rasterized at {w}x{h}; skipping it",
                key.glyph,
                key.font,
            );
            self.entries.insert(key, None);
            return None;
        }
        if w > PAGE - PAD * 2 || h > PAGE - PAD * 2 {
            log::warn!(
                "glyph {} in font {} rasterized at {w}x{h}, too large for the {PAGE}x{PAGE} atlas; skipping it",
                key.glyph,
                key.font,
            );
            self.entries.insert(key, None);
            return None;
        }

        let (layer, x, y) = self.allocate(w, h);
        write_glyph(&mut self.pages[layer as usize].image, image, x, y);
        self.uploads.push(PageUpload {
            layer,
            rect: PatchRect {
                x,
                y,
                width: w,
                height: h,
            },
        });

        let scale = 1.0 / PAGE as f32;
        let entry = GlyphEntry {
            uv_min: Vec2::new(x as f32 * scale, y as f32 * scale),
            uv_max: Vec2::new((x + w) as f32 * scale, (y + h) as f32 * scale),
            left: image.placement.left,
            top: image.placement.top,
            width: w,
            height: h,
            is_colored: matches!(image.content, Content::Color),
            layer,
        };
        self.entries.insert(key, Some(entry));
        Some(entry)
    }

    fn allocate(&mut self, w: u32, h: u32) -> (u32, u32, u32) {
        if let Some((i, (x, y))) = self
            .pages
            .iter_mut()
            .enumerate()
            .find_map(|(i, page)| page.allocate(w, h).map(|position| (i, position)))
        {
            return (i as u32, x, y);
        }

        self.pages.push(Page {
            image: RgbaImage::new(PAGE, PAGE),
            x: PAD + w + PAD,
            y: PAD,
            shelf_height: h + PAD,
        });
        (self.pages.len() as u32 - 1, PAD, PAD)
    }

    /// Drains what has been written since the last call, for the renderer to
    /// upload. Nothing else is needed for a new page: wgpu leaves the rest of
    /// a fresh layer transparent, which is what an atlas wants.
    pub fn uploads(&mut self) -> impl Iterator<Item = (PageUpload, &RgbaImage)> {
        let pages = &self.pages;
        self.uploads
            .drain(..)
            .map(|upload| (upload, &pages[upload.layer as usize].image))
    }

    pub fn insert_empty(&mut self, key: GlyphKey) {
        self.entries.insert(key, None);
    }

    pub fn page_count(&self) -> u32 {
        self.pages.len() as u32
    }

    pub fn glyph_count(&self) -> usize {
        self.entries.len()
    }
}

impl Page {
    fn allocate(&mut self, w: u32, h: u32) -> Option<(u32, u32)> {
        let need_w = w + PAD;
        let need_h = h + PAD;
        if self.x + need_w > PAGE {
            if need_w + PAD > PAGE || self.y + self.shelf_height + need_h > PAGE {
                return None;
            }
            self.y += self.shelf_height;
            self.x = PAD;
            self.shelf_height = 0;
        } else if self.y + need_h > PAGE {
            return None;
        }

        let position = (self.x, self.y);
        self.x += need_w;
        self.shelf_height = self.shelf_height.max(need_h);
        Some(position)
    }
}

/// Mask glyphs keep coverage in alpha so their raster can be tinted at draw time.
fn write_glyph(page: &mut RgbaImage, image: &Image, x: u32, y: u32) {
    let width = image.placement.width as usize;
    let height = image.placement.height as usize;
    let page_stride = page.width() as usize * 4;
    let x = x as usize * 4;
    let y = y as usize;
    let page = page.as_mut();

    for row in 0..height {
        let start = (y + row) * page_stride + x;
        let target = &mut page[start..start + width * 4];
        match image.content {
            Content::Color => {
                let start = row * width * 4;
                target.copy_from_slice(&image.data[start..start + width * 4]);
            }
            Content::Mask => {
                let start = row * width;
                for (target, &alpha) in target
                    .as_chunks_mut::<4>()
                    .0
                    .iter_mut()
                    .zip(&image.data[start..start + width])
                {
                    target.copy_from_slice(&[255, 255, 255, alpha]);
                }
            }
            Content::SubpixelMask => {
                let start = row * width * 4;
                for (target, source) in target
                    .as_chunks_mut::<4>()
                    .0
                    .iter_mut()
                    .zip(image.data[start..start + width * 4].as_chunks::<4>().0)
                {
                    target.copy_from_slice(&[255, 255, 255, source[1]]);
                }
            }
        }
    }
}

#[derive(Clone, Copy)]
pub struct PlacedGlyph {
    pub entry: GlyphEntry,
    pub offset: Vec2,
}