figcraft/crates/render/src/shadow.rs

//! Drop and inner shadows, with spread that offsets the shape's outline.
//!
//! Spread moves a shadow's outline out (or in) by the spread: the shape is filled and its outline
//! stroked twice the spread wide with mitred joins, drawn over it to grow and cut away to shrink.
//! Ellipses stay ellipses, stars keep their points, text keeps its letterforms, and a rectangle's
//! corner radii grow with the spread. Layers whose pixels aren't one outline (groups, frames that
//! don't clip, ...) spread their pixels instead (a square neighbourhood).
//!
//! The canvas, PNG/JPG/WebP exports and the PDF export's shadow images all come from here.

use std::collections::HashMap;
use std::hash::{Hash, Hasher};
use std::sync::{Arc, Mutex, OnceLock};

use figcraft_doc::{Document, Node, NodeKind, SPREAD_MITER_LIMIT, Shadow};
use figcraft_geom::{Affine, BezPath};
use tiny_skia::{FillRule, Pixmap, PixmapPaint, Transform};

use crate::draw::{hash_path, node_outline, stroke_area, stroke_rule, ts_path, ts_transform};
use crate::fx;

/// The largest spread drawn, in device pixels.
const MAX_SPREAD_PX: f64 = 1e6;

/// A shape in local space: an outline and the rule it fills with.
pub type Outline = (BezPath, FillRule);

/// The outlines a node's shadows are cast by when they spread, in local space: its fill outline
/// (when a fill shows) and its stroke area. `None` when the layer's pixels aren't those outlines:
/// groups, frames that don't clip their children or show no fill, text with a stroke.
pub fn silhouette(doc: &Document, n: &Node) -> Option<Vec<Outline>> {
    let has_fill = n.fills.iter().any(|f| f.visible && f.opacity > 0.0);
    let is_bool = matches!(n.kind, NodeKind::BooleanOp { .. });
    let is_text = matches!(n.kind, NodeKind::Text(_));
    let kids = !is_bool && n.children.iter().any(|c| doc.node(*c).is_some_and(|c| c.visible));
    if matches!(n.kind, NodeKind::Group | NodeKind::Page { .. })
        || (kids && !(n.kind.is_frame_like() && n.clip && has_fill))
        || (is_text && n.has_visible_stroke())
    {
        return None;
    }
    let closed = is_bool || is_text || figcraft_doc::geometry::is_closed(n);
    let outline = node_outline(doc, n);
    let mut out = Vec::new();
    if has_fill
        && closed
        && let Some(o) = outline.clone()
    {
        out.push(o);
    }
    if n.has_visible_stroke() {
        let geom = if is_bool { outline.map(|(p, _)| p) } else { figcraft_doc::geometry::stroke_geometry(n) };
        if let Some(g) = geom {
            out.push(((*stroke_area(n, &g, closed)).clone(), stroke_rule(n, closed)));
        }
    }
    (!out.is_empty()).then_some(out)
}

/// Cache of spread bands: (outline, width, direction, resolution) → the stroked band.
fn band_cache() -> &'static Mutex<HashMap<u64, Option<Arc<tiny_skia::Path>>>> {
    static C: OnceLock<Mutex<HashMap<u64, Option<Arc<tiny_skia::Path>>>>> = OnceLock::new();
    C.get_or_init(|| Mutex::new(HashMap::new()))
}

/// The band within `half` of `path`'s outline, mitred at corners, in local space. Shrinking first
/// resolves overlapping non-zero contours (overlapping glyph parts, vector regions), whose inner
/// edges aren't outline and must not be cut away.
fn band(path: &BezPath, rule: FillRule, half: f64, shrink: bool, res: f32) -> Option<Arc<tiny_skia::Path>> {
    let mut h = std::collections::hash_map::DefaultHasher::new();
    hash_path(path, &mut h);
    (rule == FillRule::EvenOdd, half.to_bits(), shrink, res.to_bits()).hash(&mut h);
    let key = h.finish();
    if let Some(b) = band_cache().lock().unwrap_or_else(std::sync::PoisonError::into_inner).get(&key) {
        return b.clone();
    }
    let simplified = (shrink && rule == FillRule::Winding).then(|| figcraft_pathops::simplify(path).ok()).flatten();
    let stroke = tiny_skia::Stroke {
        width: (half * 2.0) as f32,
        line_join: tiny_skia::LineJoin::Miter,
        miter_limit: SPREAD_MITER_LIMIT as f32,
        line_cap: tiny_skia::LineCap::Butt,
        ..Default::default()
    };
    let made = ts_path(simplified.as_ref().unwrap_or(path)).and_then(|p| p.stroke(&stroke, res)).map(Arc::new);
    let mut c = band_cache().lock().unwrap_or_else(std::sync::PoisonError::into_inner);
    if c.len() > 1024 {
        c.clear();
    }
    c.insert(key, made.clone());
    made
}

fn white(blend: tiny_skia::BlendMode) -> tiny_skia::Paint<'static> {
    tiny_skia::Paint { shader: tiny_skia::Shader::SolidColor(tiny_skia::Color::WHITE), anti_alias: true, blend_mode: blend, ..Default::default() }
}

/// The coverage of `outlines` grown (`spread > 0`) or shrunk (`spread < 0`) by `spread` local
/// units, drawn through `xf` (local → pixels) into a `w` × `h` pixmap (white, premultiplied).
pub fn spread_alpha(outlines: &[Outline], spread: f64, xf: Affine, w: u32, h: u32) -> Option<Pixmap> {
    let mut p = Pixmap::new(w, h)?;
    let t = ts_transform(xf);
    let fill = white(tiny_skia::BlendMode::SourceOver);
    for (path, rule) in outlines {
        if let Some(tp) = ts_path(path) {
            p.fill_path(&tp, &fill, *rule, t, None);
        }
    }
    // Past a million pixels every spread looks the same, and the rasteriser needs sane coordinates.
    let px = xf.determinant().abs().sqrt();
    let half = if px.is_finite() && px > 0.0 { spread.abs().min(MAX_SPREAD_PX / px) } else { 0.0 };
    if !(half.is_finite() && half > 1e-6) {
        return Some(p);
    }
    // Curves flatten for the pixel size, bucketed to powers of two so zooming reuses bands.
    let res = tiny_skia::PathStroker::compute_resolution_scale(&t);
    let res = if res.is_finite() && res > 0.0 { 2f32.powi(res.log2().ceil().clamp(-8.0, 8.0) as i32) } else { 1.0 };
    let paint = white(if spread > 0.0 { tiny_skia::BlendMode::SourceOver } else { tiny_skia::BlendMode::Clear });
    for (path, rule) in outlines {
        if let Some(b) = band(path, *rule, half, spread < 0.0, res) {
            p.fill_path(&b, &paint, FillRule::Winding, t, None);
        }
    }
    Some(p)
}

/// A device distance as a whole pixel offset (non-finite → 0, capped).
pub(crate) fn whole_px(v: f64) -> i32 {
    if v.is_finite() { v.round().clamp(-100_000.0, 100_000.0) as i32 } else { 0 }
}

fn color4(c: figcraft_color::Color) -> [f32; 4] {
    let c = c.sanitized();
    [c.r, c.g, c.b, c.a]
}

/// What casts a node's shadows.
pub struct Caster<'a> {
    /// The node's pixels (content and inner shadows, with the effects drawn on top of them).
    pub layer: &'a Pixmap,
    /// Local → layer pixels.
    pub xf: Affine,
    /// Device pixels per document unit.
    pub scale: f64,
    /// See [`silhouette`]; needed only for spread.
    pub silhouette: Option<&'a [Outline]>,
}

/// A drop shadow in the layer's pixels: offset, spread, blurred, tinted and (unless it shows
/// behind the layer) hidden behind the layer's own pixels.
pub fn drop_shadow(c: &Caster, s: &Shadow) -> Option<Pixmap> {
    let (w, h) = (c.layer.width(), c.layer.height());
    let (dx, dy) = (s.offset.0 * c.scale, s.offset.1 * c.scale);
    let spread = if s.spread.is_finite() { s.spread } else { 0.0 };
    let mut sh = match c.silhouette {
        Some(sil) if spread != 0.0 && dx.is_finite() && dy.is_finite() => spread_alpha(sil, spread, Affine::translate((dx, dy)) * c.xf, w, h)?,
        _ => {
            let mut base = c.layer.clone();
            fx::morph(&mut base, whole_px(spread * c.scale));
            let mut out = Pixmap::new(w, h)?;
            out.draw_pixmap(whole_px(dx), whole_px(dy), base.as_ref(), &PixmapPaint::default(), Transform::identity(), None);
            out
        }
    };
    fx::blur(&mut sh, s.radius / 2.0 * c.scale);
    fx::tint(&mut sh, color4(s.color));
    if !s.show_behind {
        fx::mask_by(&mut sh, c.layer, true);
    }
    Some(sh)
}

/// An inner shadow in a `w` × `h` layer whose `shape` maps through `xf`: everything outside the
/// shape (shrunk by the spread) at the offset, blurred, tinted and clipped to the shape.
pub fn inner_shadow(shape: &Outline, w: u32, h: u32, xf: Affine, scale: f64, s: &Shadow) -> Option<Pixmap> {
    let (dx, dy) = (s.offset.0 * scale, s.offset.1 * scale);
    let spread = if s.spread.is_finite() { s.spread } else { 0.0 };
    if !(dx.is_finite() && dy.is_finite()) {
        return None;
    }
    // A buffer padded by the shadow's reach, with the shape drawn at its offset: everything outside
    // the shape casts shadow inwards, from every edge. The shape itself (not the layer's box)
    // decides what is outside: the layer can be cropped (to a render band or the view) where the
    // shape carries on.
    let reach = s.radius.abs() / 2.0 * scale * 3.0 + spread.abs() * scale * SPREAD_MITER_LIMIT + dx.abs().max(dy.abs());
    let pad = if reach.is_finite() { reach.ceil().clamp(0.0, 2048.0) as u32 + 2 } else { 2 };
    let at = Affine::translate((f64::from(pad) + dx, f64::from(pad) + dy)) * xf;
    let mut big = spread_alpha(std::slice::from_ref(shape), -spread, at, w + pad * 2, h + pad * 2)?;
    fx::invert_alpha(&mut big);
    fx::blur(&mut big, s.radius / 2.0 * scale);
    let mut out = Pixmap::new(w, h)?;
    let pad = i32::try_from(pad).ok()?;
    out.draw_pixmap(
        -pad,
        -pad,
        big.as_ref(),
        &PixmapPaint { blend_mode: tiny_skia::BlendMode::Source, ..Default::default() },
        Transform::identity(),
        None,
    );
    let sa = spread_alpha(std::slice::from_ref(shape), 0.0, xf, w, h)?;
    fx::mask_by(&mut out, &sa, false);
    fx::tint(&mut out, color4(s.color));
    Some(out)
}