photocraft/crates/algo/src/render2.rs

//! Procedural renderers of Filter › Render: Flame, Picture Frame and Tree.
//!
//! Each renderer turns its options into a list of anti-aliased vector
//! primitives ([`Prim`]: tapered capsules and shaded ellipses, blended
//! normally or additively). [`composite`] rasterizes the list over a surface
//! tile by tile (in parallel on native targets), in straight sRGB converted to
//! and from the surface's own colour model and depth, mixed by the selection.
//!
//! The shapes are our own designs (L-system-like recursive branching for
//! trees, turbulent flame strands, ornaments walked along the frame edge),
//! built from observation of what the options do, not from any reference
//! implementation.

use photocraft_geom::Rect;
use photocraft_raster::{Surface, from_rgba_into, to_rgba};
use serde::{Deserialize, Serialize};

// ---------------------------------------------------------------------------
// Primitives and compositing
// ---------------------------------------------------------------------------

/// Geometry of a primitive (document pixel coordinates).
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum Shape {
    /// A segment from `a` to `b` whose radius goes from `ra` to `rb`; colour goes `c0` → `c1` along it.
    Capsule { a: (f32, f32), b: (f32, f32), ra: f32, rb: f32 },
    /// An ellipse rotated by `angle` radians; colour goes `c0` (centre) → `c1` (rim).
    Ellipse { c: (f32, f32), rx: f32, ry: f32, angle: f32 },
}

/// One drawable primitive. Colours are straight sRGB RGBA (0–1).
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Prim {
    pub shape: Shape,
    pub c0: [f32; 4],
    pub c1: [f32; 4],
    /// Edge softness in pixels (≥ 1 for anti-aliasing; larger for glows).
    pub soft: f32,
    pub blend: Blend,
}

/// How a primitive combines with what is under it.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
pub enum Blend {
    /// Source over.
    #[default]
    Normal,
    /// Additive light (glows).
    Add,
    /// Per-channel maximum of the premultiplied values (overlapping strands don't build up).
    Lighten,
}

impl Prim {
    pub fn capsule(a: (f32, f32), b: (f32, f32), ra: f32, rb: f32, c0: [f32; 4], c1: [f32; 4]) -> Prim {
        Prim { shape: Shape::Capsule { a, b, ra, rb }, c0, c1, soft: 1.0, blend: Blend::Normal }
    }
    pub fn ellipse(c: (f32, f32), rx: f32, ry: f32, angle: f32, c0: [f32; 4], c1: [f32; 4]) -> Prim {
        Prim { shape: Shape::Ellipse { c, rx, ry, angle }, c0, c1, soft: 1.0, blend: Blend::Normal }
    }
    pub fn disc(c: (f32, f32), r: f32, col: [f32; 4]) -> Prim {
        Prim::ellipse(c, r, r, 0.0, col, col)
    }
    fn with_soft(mut self, s: f32) -> Prim {
        self.soft = s;
        self
    }
    fn additive(mut self) -> Prim {
        self.blend = Blend::Add;
        self
    }
    fn lighten(mut self) -> Prim {
        self.blend = Blend::Lighten;
        self
    }

    /// Pixel bounds this primitive can touch.
    pub fn bounds(&self) -> Rect {
        let s = self.soft.max(1.0) + 1.0;
        let (x0, y0, x1, y1) = match self.shape {
            Shape::Capsule { a, b, ra, rb } => {
                let r = ra.max(rb) + s;
                (a.0.min(b.0) - r, a.1.min(b.1) - r, a.0.max(b.0) + r, a.1.max(b.1) + r)
            }
            Shape::Ellipse { c, rx, ry, angle } => {
                let (sn, cs) = angle.sin_cos();
                let ex = ((rx * cs).powi(2) + (ry * sn).powi(2)).sqrt() + s;
                let ey = ((rx * sn).powi(2) + (ry * cs).powi(2)).sqrt() + s;
                (c.0 - ex, c.1 - ey, c.0 + ex, c.1 + ey)
            }
        };
        if !(x0.is_finite() && y0.is_finite() && x1.is_finite() && y1.is_finite()) {
            return Rect::EMPTY;
        }
        let cl = |v: f32| v.clamp(-1.0e8, 1.0e8) as i32;
        Rect::new(cl(x0.floor()), cl(y0.floor()), cl(x1.ceil()), cl(y1.ceil()))
    }

    /// Coverage (0–1) and colour position (0–1) at a pixel centre.
    #[inline]
    fn eval(&self, x: f32, y: f32) -> (f32, f32) {
        let soft = self.soft.max(1.0);
        match self.shape {
            Shape::Capsule { a, b, ra, rb } => {
                let (dx, dy) = (b.0 - a.0, b.1 - a.1);
                let l2 = dx * dx + dy * dy;
                let t = if l2 > 1e-9 { (((x - a.0) * dx + (y - a.1) * dy) / l2).clamp(0.0, 1.0) } else { 0.0 };
                let (px, py) = (a.0 + dx * t - x, a.1 + dy * t - y);
                let d = (px * px + py * py).sqrt();
                let r = ra + (rb - ra) * t;
                // Hairlines thinner than a pixel fade instead of vanishing.
                let thin = (2.0 * r).clamp(0.0, 1.0);
                (((r.max(0.5) - d) / soft + 0.5).clamp(0.0, 1.0) * thin, t)
            }
            Shape::Ellipse { c, rx, ry, angle } => {
                let (s, co) = angle.sin_cos();
                let (dx, dy) = (x - c.0, y - c.1);
                let (u, v) = (dx * co + dy * s, -dx * s + dy * co);
                let (rx, ry) = (rx.max(0.05), ry.max(0.05));
                let e = ((u / rx).powi(2) + (v / ry).powi(2)).sqrt();
                let edge = (1.0 - e) * rx.min(ry);
                let thin = (2.0 * rx.min(ry)).clamp(0.0, 1.0);
                ((edge / soft + 0.5).clamp(0.0, 1.0) * thin, e.min(1.0))
            }
        }
    }
}

#[inline]
fn lerp4(a: [f32; 4], b: [f32; 4], t: f32) -> [f32; 4] {
    [a[0] + (b[0] - a[0]) * t, a[1] + (b[1] - a[1]) * t, a[2] + (b[2] - a[2]) * t, a[3] + (b[3] - a[3]) * t]
}

/// Rasterizes `prims` (in order) into a premultiplied RGBA buffer covering `t`.
fn raster_tile(prims: &[Prim], idx: &[u32], t: Rect) -> Vec<[f32; 4]> {
    let w = t.width() as usize;
    let mut buf = vec![[0.0f32; 4]; w * t.height() as usize];
    for &k in idx {
        let p = &prims[k as usize];
        let r = p.bounds().intersect(&t);
        if r.is_empty() {
            continue;
        }
        for y in r.y0..r.y1 {
            let row = (y - t.y0) as usize * w;
            for x in r.x0..r.x1 {
                let (cov, ct) = p.eval(x as f32 + 0.5, y as f32 + 0.5);
                if cov <= 0.0 {
                    continue;
                }
                let c = if p.c0 == p.c1 { p.c0 } else { lerp4(p.c0, p.c1, ct) };
                let a = (c[3] * cov).clamp(0.0, 1.0);
                if a <= 0.0 {
                    continue;
                }
                let d = &mut buf[row + (x - t.x0) as usize];
                match p.blend {
                    Blend::Add => {
                        d[0] += c[0] * a;
                        d[1] += c[1] * a;
                        d[2] += c[2] * a;
                        d[3] += a * (1.0 - d[3]);
                    }
                    Blend::Lighten => {
                        d[0] = d[0].max(c[0] * a);
                        d[1] = d[1].max(c[1] * a);
                        d[2] = d[2].max(c[2] * a);
                        d[3] = d[3].max(a);
                    }
                    Blend::Normal => {
                        let k = 1.0 - a;
                        d[0] = c[0] * a + d[0] * k;
                        d[1] = c[1] * a + d[1] * k;
                        d[2] = c[2] * a + d[2] * k;
                        d[3] = a + d[3] * k;
                    }
                }
            }
        }
    }
    buf
}

/// Size of the tiles [`composite`] works in.
const CTILE: i32 = 128;

/// Draws `prims` over `surface` inside `clip`, mixed by the selection coverage
/// (channel 0 of `selection`). Returns the bounds of what was drawn.
pub fn composite(surface: &mut Surface, prims: &[Prim], clip: Rect, selection: Option<&Surface>) -> Rect {
    let mut used = Rect::EMPTY;
    let bounds: Vec<Rect> = prims.iter().map(|p| p.bounds().intersect(&clip)).collect();
    for b in &bounds {
        if !b.is_empty() {
            used = if used.is_empty() { *b } else { used.union(b) };
        }
    }
    if used.is_empty() {
        return used;
    }
    // Bin primitives into tiles (order preserved within a tile).
    let tx0 = used.x0.div_euclid(CTILE);
    let ty0 = used.y0.div_euclid(CTILE);
    let tx1 = (used.x1 - 1).div_euclid(CTILE);
    let ty1 = (used.y1 - 1).div_euclid(CTILE);
    let (nx, ny) = ((tx1 - tx0 + 1) as usize, (ty1 - ty0 + 1) as usize);
    let mut bins: Vec<Vec<u32>> = vec![Vec::new(); nx * ny];
    for (k, b) in bounds.iter().enumerate() {
        if b.is_empty() {
            continue;
        }
        for ty in b.y0.div_euclid(CTILE)..=(b.y1 - 1).div_euclid(CTILE) {
            for tx in b.x0.div_euclid(CTILE)..=(b.x1 - 1).div_euclid(CTILE) {
                bins[(ty - ty0) as usize * nx + (tx - tx0) as usize].push(k as u32);
            }
        }
    }
    let fmt = surface.format();
    let n = fmt.channels();
    let jobs: Vec<(Rect, &Vec<u32>)> = bins
        .iter()
        .enumerate()
        .filter(|(_, b)| !b.is_empty())
        .map(|(i, b)| {
            let (tx, ty) = (tx0 + (i % nx) as i32, ty0 + (i / nx) as i32);
            (Rect::new(tx * CTILE, ty * CTILE, (tx + 1) * CTILE, (ty + 1) * CTILE).intersect(&clip), b)
        })
        .filter(|(r, _)| !r.is_empty())
        .collect();
    let src: &Surface = surface;
    let run = |(t, idx): &(Rect, &Vec<u32>)| -> (Rect, Vec<f32>) {
        let paint = raster_tile(prims, idx, *t);
        let mut data = src.read_region(*t);
        let sel = selection.map(|s| s.read_region(*t));
        let sn = selection.map_or(1, Surface::channels);
        let mut tmp = [0.0f32; 8];
        for (i, px) in data.chunks_exact_mut(n).enumerate() {
            let s = paint[i];
            let k = sel.as_ref().map_or(1.0, |v| v[i * sn].clamp(0.0, 1.0));
            let sa = s[3].clamp(0.0, 1.0) * k;
            if sa <= 0.0 {
                continue;
            }
            let inv = 1.0 / s[3].max(1e-6);
            let sc = [(s[0] * inv).clamp(0.0, 1.0), (s[1] * inv).clamp(0.0, 1.0), (s[2] * inv).clamp(0.0, 1.0)];
            let d = to_rgba(&fmt, px);
            let da = if fmt.alpha { d[3] } else { 1.0 };
            let oa = sa + da * (1.0 - sa);
            let mix = |sv: f32, dv: f32| if oa > 1e-7 { (sv * sa + dv * da * (1.0 - sa)) / oa } else { 0.0 };
            let o = [mix(sc[0], d[0]), mix(sc[1], d[1]), mix(sc[2], d[2]), oa];
            let m = from_rgba_into(&fmt, o, &mut tmp);
            px.copy_from_slice(&tmp[..m.min(n)]);
        }
        (*t, data)
    };
    #[cfg(not(target_arch = "wasm32"))]
    let results: Vec<(Rect, Vec<f32>)> = {
        use rayon::prelude::*;
        jobs.par_iter().map(run).collect()
    };
    #[cfg(target_arch = "wasm32")]
    let results: Vec<(Rect, Vec<f32>)> = jobs.iter().map(run).collect();
    for (t, data) in results {
        surface.write_region(t, &data);
    }
    used
}

// ---------------------------------------------------------------------------
// Randomness
// ---------------------------------------------------------------------------

/// Small deterministic PRNG (SplitMix64).
#[derive(Clone, Debug)]
pub struct Rng(u64);

impl Rng {
    pub fn new(seed: u32, salt: u64) -> Rng {
        Rng(((seed as u64) << 32) ^ salt.wrapping_mul(0x9E37_79B9_7F4A_7C15) ^ 0x2545_F491_4F6C_DD1D)
    }
    pub fn next_u64(&mut self) -> u64 {
        self.0 = self.0.wrapping_add(0x9E37_79B9_7F4A_7C15);
        let mut z = self.0;
        z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
        z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
        z ^ (z >> 31)
    }
    /// Uniform in [0, 1).
    pub fn f(&mut self) -> f32 {
        (self.next_u64() >> 40) as f32 / (1u64 << 24) as f32
    }
    /// Uniform in [a, b).
    pub fn range(&mut self, a: f32, b: f32) -> f32 {
        a + (b - a) * self.f()
    }
    /// Uniform in [-1, 1).
    pub fn sym(&mut self) -> f32 {
        self.f() * 2.0 - 1.0
    }
}

/// Smooth 1-D value noise in [-1, 1].
fn noise1(x: f32, seed: u64) -> f32 {
    let h = |i: i64| -> f32 {
        let mut r = Rng((i as u64).wrapping_mul(0xD6E8_FEB8_6659_FD93) ^ seed);
        r.sym()
    };
    let i = x.floor();
    let f = x - i;
    let i = i as i64;
    let t = f * f * (3.0 - 2.0 * f);
    h(i) + (h(i + 1) - h(i)) * t
}

/// Two octaves of [`noise1`].
fn fbm1(x: f32, seed: u64) -> f32 {
    (noise1(x, seed) + 0.5 * noise1(x * 2.13 + 7.1, seed ^ 0xA5A5)) / 1.5
}

// ---------------------------------------------------------------------------
// Polylines
// ---------------------------------------------------------------------------

/// A polyline parameterized by arc length.
struct Spine {
    pts: Vec<(f32, f32)>,
    cum: Vec<f32>,
}

impl Spine {
    fn new(pts: Vec<(f32, f32)>) -> Spine {
        let mut cum = vec![0.0];
        for w in pts.windows(2) {
            let l = ((w[1].0 - w[0].0).powi(2) + (w[1].1 - w[0].1).powi(2)).sqrt();
            cum.push(cum.last().copied().unwrap_or(0.0) + l);
        }
        Spine { pts, cum }
    }
    fn len(&self) -> f32 {
        self.cum.last().copied().unwrap_or(0.0)
    }
    /// Point and unit tangent at arc length `s`.
    fn at(&self, s: f32) -> ((f32, f32), (f32, f32)) {
        let n = self.pts.len();
        if n == 0 {
            return ((0.0, 0.0), (1.0, 0.0));
        }
        if n == 1 {
            return (self.pts[0], (0.0, -1.0));
        }
        let s = s.clamp(0.0, self.len());
        let k = match self.cum.binary_search_by(|v| v.partial_cmp(&s).unwrap_or(std::cmp::Ordering::Less)) {
            Ok(i) => i.min(n - 2),
            Err(i) => i.saturating_sub(1).min(n - 2),
        };
        let (a, b) = (self.pts[k], self.pts[k + 1]);
        let seg = (self.cum[k + 1] - self.cum[k]).max(1e-6);
        let t = ((s - self.cum[k]) / seg).clamp(0.0, 1.0);
        let (dx, dy) = (b.0 - a.0, b.1 - a.1);
        let l = (dx * dx + dy * dy).sqrt().max(1e-6);
        ((a.0 + dx * t, a.1 + dy * t), (dx / l, dy / l))
    }
}

// ---------------------------------------------------------------------------
// Flame
// ---------------------------------------------------------------------------

/// Flame placement (Photoshop's six flame types).
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, Default)]
#[serde(rename_all = "camelCase")]
pub enum FlameType {
    /// One flame whose spine is the path (path start = flame base).
    #[default]
    OneFlameAlongPath,
    /// Flames at `interval` along the path, rising from its left side.
    MultipleFlamesAlongPath,
    /// Flames at `interval` along the path, pointing along the path's direction.
    MultipleFlamesPathDirections,
    /// Like `multipleFlamesAlongPath` with strongly varying lengths.
    MultipleFlamesVariousLength,
    /// A candle flame at the start of each subpath.
    CandleLight,
    /// Flames at `interval` along the path, all pointing at `angle`.
    MultipleFlamesOneDirection,
}

/// Overall motion of the flame lines.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, Default)]
#[serde(rename_all = "camelCase")]
pub enum FlameStyle {
    #[default]
    Normal,
    Violent,
    Flat,
}

/// How the flame lines converge towards the tip.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, Default)]
#[serde(rename_all = "camelCase")]
pub enum FlameShape {
    #[default]
    Parallel,
    ToCenter,
    Spread,
    Oval,
    Pointed,
}

/// Flame options (Photoshop's Flame dialog, Basic + Advanced).
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase", default)]
pub struct FlameSpec {
    pub flame_type: FlameType,
    /// Flame length in px (1–1000).
    pub length: f32,
    pub randomize_length: bool,
    /// Flame width in px (1–1000).
    pub width: f32,
    /// Direction for `multipleFlamesOneDirection` (degrees, 0 = up, clockwise).
    pub angle: f32,
    /// Spacing between flames along the path in px (1–1000).
    pub interval: f32,
    /// Spread the flames evenly so a closed path loops seamlessly.
    pub adjust_interval_for_loops: bool,
    /// `Some` = custom flame colour; `None` = natural flame colours.
    pub color: Option<[f32; 4]>,
    /// 0–100.
    pub turbulent: f32,
    /// 0–100.
    pub jag: f32,
    /// 0–100 %.
    pub opacity: f32,
    /// Flame lines (complexity) 1–100.
    pub flame_lines: f32,
    /// 0–100: how unevenly the lines start along the flame.
    pub flame_bottom_alignment: f32,
    pub flame_style: FlameStyle,
    pub flame_shape: FlameShape,
    pub randomize_shapes: bool,
    /// 0–100: arrangement seed.
    pub seed: u32,
    /// 0 (draft) … 4 (fine): segments per flame line.
    pub quality: u32,
}

impl Default for FlameSpec {
    fn default() -> Self {
        FlameSpec {
            flame_type: FlameType::OneFlameAlongPath,
            length: 150.0,
            randomize_length: false,
            width: 40.0,
            angle: 0.0,
            interval: 60.0,
            adjust_interval_for_loops: true,
            color: None,
            turbulent: 25.0,
            jag: 25.0,
            opacity: 75.0,
            flame_lines: 20.0,
            flame_bottom_alignment: 20.0,
            flame_style: FlameStyle::Normal,
            flame_shape: FlameShape::Parallel,
            randomize_shapes: false,
            seed: 0,
            quality: 2,
        }
    }
}

/// Natural flame colour at position `t` along the flame (0 = base, 1 = tip).
fn flame_colour(t: f32, custom: Option<[f32; 4]>) -> [f32; 4] {
    let a = (1.0 - t).powf(0.7) * 0.9 + 0.1 * (1.0 - t);
    match custom {
        Some(c) => {
            // Hot near-white core → the colour → a darker rim.
            let core = [c[0] * 0.4 + 0.6, c[1] * 0.4 + 0.6, c[2] * 0.4 + 0.6];
            let rim = [c[0] * 0.6, c[1] * 0.6, c[2] * 0.6];
            let k = t.clamp(0.0, 1.0);
            let rgb = if k < 0.35 {
                let u = k / 0.35;
                [core[0] + (c[0] - core[0]) * u, core[1] + (c[1] - core[1]) * u, core[2] + (c[2] - core[2]) * u]
            } else {
                let u = (k - 0.35) / 0.65;
                [c[0] + (rim[0] - c[0]) * u, c[1] + (rim[1] - c[1]) * u, c[2] + (rim[2] - c[2]) * u]
            };
            [rgb[0], rgb[1], rgb[2], a * c[3]]
        }
        None => {
            const STOPS: [(f32, [f32; 3]); 5] =
                [(0.0, [1.0, 0.98, 0.85]), (0.2, [1.0, 0.86, 0.35]), (0.45, [1.0, 0.55, 0.08]), (0.75, [0.9, 0.25, 0.02]), (1.0, [0.6, 0.08, 0.0])];
            let k = t.clamp(0.0, 1.0);
            let mut rgb = STOPS[4].1;
            for w in STOPS.windows(2) {
                if k <= w[1].0 {
                    let u = (k - w[0].0) / (w[1].0 - w[0].0);
                    rgb = [w[0].1[0] + (w[1].1[0] - w[0].1[0]) * u, w[0].1[1] + (w[1].1[1] - w[0].1[1]) * u, w[0].1[2] + (w[1].1[2] - w[0].1[2]) * u];
                    break;
                }
            }
            [rgb[0], rgb[1], rgb[2], a]
        }
    }
}

/// One flame: its spine (base → tip), how wide it is, and its random stream.
fn draw_flame(out: &mut Vec<Prim>, spine: &Spine, width: f32, spec: &FlameSpec, rng: &mut Rng) {
    let len = spine.len();
    if len < 1.0 {
        return;
    }
    let lines = (spec.flame_lines.clamp(1.0, 100.0) * 0.6 + 3.0).round() as usize;
    let segs = [6usize, 10, 16, 24, 36][spec.quality.min(4) as usize].max((len / 12.0) as usize).min(160);
    let (mut turb, mut jag) = (spec.turbulent.clamp(0.0, 100.0) / 100.0, spec.jag.clamp(0.0, 100.0) / 100.0);
    match spec.flame_style {
        FlameStyle::Violent => {
            turb = (turb * 1.8 + 0.2).min(2.0);
            jag = (jag * 1.8 + 0.15).min(2.0);
        }
        FlameStyle::Flat => {
            turb *= 0.35;
            jag *= 0.35;
        }
        FlameStyle::Normal => {}
    }
    let shape = if spec.randomize_shapes {
        [FlameShape::Parallel, FlameShape::ToCenter, FlameShape::Spread, FlameShape::Oval, FlameShape::Pointed][(rng.next_u64() % 5) as usize]
    } else {
        spec.flame_shape
    };
    let opacity = spec.opacity.clamp(0.0, 100.0) / 100.0;
    let half = width.max(1.0) / 2.0;
    let seed = rng.next_u64();
    // The whole flame sways; each line adds its own flicker on top.
    let sway = |u: f32| turb * half * 1.6 * fbm1(u * 1.7, seed ^ 0x5A5A) * u.powf(1.4);
    // A soft additive body glow along the spine.
    for k in 0..6 {
        let u = k as f32 / 6.0 * 0.75 + 0.05;
        let ((px, py), (dx, dy)) = spine.at(u * len);
        let o = sway(u);
        let c = flame_colour(u, spec.color);
        let r = half * (1.25 - u) + 2.0;
        out.push(
            Prim::ellipse((px - dy * o, py + dx * o), r, r * 1.2, dy.atan2(dx), [c[0], c[1], c[2], 0.16 * opacity], [c[0], c[1] * 0.6, c[2] * 0.3, 0.0])
                .with_soft(r.max(2.0))
                .additive(),
        );
    }
    let line_r = (half / lines as f32 * 2.2).clamp(0.8, 10.0);
    for li in 0..lines {
        let lane = if lines == 1 { 0.0 } else { li as f32 / (lines - 1) as f32 * 2.0 - 1.0 };
        let lseed = seed ^ (li as u64).wrapping_mul(0x51_7CC1_B727_220A);
        let start = (spec.flame_bottom_alignment.clamp(0.0, 100.0) / 100.0) * rng.f() * 0.4;
        // Outer lines are shorter, giving the flame its tongue shape.
        let reach = (1.0 - 0.55 * lane.abs().powf(1.5)) * rng.range(0.7, 1.0);
        let end = (start + reach * (1.0 - start)).clamp(start + 0.05, 1.0);
        let lr = line_r * rng.range(0.6, 1.3);
        let mut prev: Option<((f32, f32), f32, f32)> = None;
        for k in 0..=segs {
            let u = start + (end - start) * k as f32 / segs as f32;
            let s = u * len;
            let ((px, py), (dx, dy)) = spine.at(s);
            let (nx, ny) = (-dy, dx);
            let spread = match shape {
                FlameShape::Parallel => 1.0 - 0.6 * u,
                FlameShape::ToCenter => 1.0 - u,
                FlameShape::Spread => 1.0 + u,
                FlameShape::Oval => (std::f32::consts::PI * (0.15 + 0.85 * u)).sin() * 1.3,
                FlameShape::Pointed => (1.0 - u).powi(2),
            };
            let flick = turb * half * 0.5 * fbm1(s / (len * 0.2 + 6.0) + li as f32 * 0.37, lseed) * u + jag * half * 0.25 * noise1(s / 5.0, lseed ^ 0x77) * u;
            let off = lane * half * spread + sway(u) + flick;
            let p = (px + nx * off, py + ny * off);
            let r = lr * (1.0 - u * 0.75);
            // Lines fade out towards their own tip.
            let fade_t = ((u - start) / (end - start).max(1e-3)).clamp(0.0, 1.0);
            if let Some((q, rq, uq)) = prev {
                // Fade in over the first tenth (a soft base) and out towards the line's tip.
                let f = |c: [f32; 4], uu: f32, t: f32| [c[0], c[1], c[2], c[3] * opacity * (1.0 - t.powi(3)) * ((uu - start) / 0.08).clamp(0.0, 1.0).sqrt()];
                out.push(
                    Prim::capsule(q, p, rq, r, f(flame_colour(uq, spec.color), uq, fade_t), f(flame_colour(u, spec.color), u, fade_t))
                        .with_soft(1.0 + lr * 0.6)
                        .lighten(),
                );
            }
            prev = Some((p, r, u));
        }
    }
}

/// Builds flame primitives. `paths` are flattened polylines in document pixels.
pub fn flame(spec: &FlameSpec, paths: &[Vec<(f32, f32)>]) -> Vec<Prim> {
    let mut out = Vec::new();
    let mut rng = Rng::new(spec.seed, 0xF1A3);
    let length = spec.length.clamp(1.0, 1000.0);
    let width = spec.width.clamp(1.0, 1000.0);
    let interval = spec.interval.clamp(1.0, 1000.0);
    let pick_len =
        |rng: &mut Rng, var: f32| if spec.randomize_length || var > 0.0 { length * rng.range(1.0 - var.max(0.4), 1.0 + var.max(0.4) * 0.5) } else { length };
    for path in paths.iter().filter(|p| !p.is_empty()) {
        let spine = Spine::new(path.clone());
        let total = spine.len();
        match spec.flame_type {
            FlameType::OneFlameAlongPath if path.len() >= 2 && total >= 1.0 => draw_flame(&mut out, &spine, width, spec, &mut rng),
            FlameType::CandleLight | FlameType::OneFlameAlongPath => {
                // A candle: a short teardrop rising straight up from the start.
                let base = path[0];
                let h = length.min(400.0);
                let candle = Spine::new(vec![base, (base.0, base.1 - h)]);
                let c = FlameSpec { flame_shape: FlameShape::Oval, turbulent: spec.turbulent * 0.3, jag: spec.jag * 0.2, ..spec.clone() };
                draw_flame(&mut out, &candle, (width * 0.6).max(4.0), &c, &mut rng);
                let core = flame_colour(0.05, spec.color);
                out.push(
                    Prim::ellipse(
                        (base.0, base.1 - h * 0.28),
                        (width * 0.12).max(1.5),
                        h * 0.22,
                        0.0,
                        [core[0], core[1], core[2], 0.9 * spec.opacity.clamp(0.0, 100.0) / 100.0],
                        [core[0], core[1], core[2], 0.0],
                    )
                    .with_soft(2.0)
                    .additive(),
                );
            }
            _ => {
                let closed = path.len() > 2 && {
                    let (a, b) = (path[0], path[path.len() - 1]);
                    (a.0 - b.0).abs() < 1.0 && (a.1 - b.1).abs() < 1.0
                };
                let count = if spec.adjust_interval_for_loops && closed {
                    let c = (total / interval).round();
                    if c.is_finite() && c > 1.0 { (c as usize).min(2048) } else { 1 }
                } else {
                    let c = (total / interval).floor();
                    if c.is_finite() && c >= 0.0 { (c as usize).saturating_add(1).min(2048) } else { 1 }
                };
                let step = if spec.adjust_interval_for_loops && closed { total / count as f32 } else { interval };
                for k in 0..count {
                    if out.len() >= 65536 {
                        break;
                    }
                    let s = k as f32 * step;
                    let ((x, y), (dx, dy)) = spine.at(s);
                    let dir = match spec.flame_type {
                        FlameType::MultipleFlamesPathDirections => (dx, dy),
                        FlameType::MultipleFlamesOneDirection => {
                            let a = spec.angle.to_radians();
                            (a.sin(), -a.cos())
                        }
                        // Left of the path direction (up for a left-to-right path).
                        _ => (dy, -dx),
                    };
                    let var = if spec.flame_type == FlameType::MultipleFlamesVariousLength { 0.7 } else { 0.0 };
                    let l = pick_len(&mut rng, var);
                    let fl = Spine::new(vec![(x, y), (x + dir.0 * l, y + dir.1 * l)]);
                    draw_flame(&mut out, &fl, width, spec, &mut rng);
                }
            }
        }
    }
    out
}

/// The path Flame uses when the document has none: a gentle arc across the lower part of the canvas.
pub fn default_flame_path(canvas: Rect) -> Vec<(f32, f32)> {
    let (w, h) = (canvas.width() as f32, canvas.height() as f32);
    (0..=32)
        .map(|k| {
            let t = k as f32 / 32.0;
            (canvas.x0 as f32 + w * (0.15 + 0.7 * t), canvas.y0 as f32 + h * (0.8 - 0.08 * (std::f32::consts::PI * t).sin()))
        })
        .collect()
}

// ---------------------------------------------------------------------------
// Tree
// ---------------------------------------------------------------------------

/// Leaf form of a tree type.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Leaf {
    None,
    Round,
    Oval,
    Needle,
    Heart,
    Blossom,
    Long,
    Frond,
}

/// Branch architecture of a tree type.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Habit {
    /// Recursive branching (deciduous trees).
    Branching,
    /// A dominant stem with whorls of side branches (conifers).
    Conifer,
    /// A tall trunk crowned with fronds.
    Palm,
    /// Several straight jointed stalks.
    Bamboo,
}

/// One base tree type.
struct TreeKind {
    name: &'static str,
    habit: Habit,
    levels: u32,
    children: u32,
    /// Branch spread (degrees from the parent).
    spread: f32,
    /// Child / parent length ratio.
    ratio: f32,
    /// Trunk share of the total height.
    trunk: f32,
    /// Downward bend per unit length at deep levels (willow > 0, upright < 0).
    droop: f32,
    /// Random bend.
    wiggle: f32,
    /// Trunk radius as a fraction of the height.
    thick: f32,
    leaf: Leaf,
    leaf_col: [f32; 3],
    bark: [f32; 3],
    /// Fruit or blossom accent dots.
    accent: Option<[f32; 3]>,
}

#[allow(clippy::too_many_arguments)]
const fn tk(
    name: &'static str,
    habit: Habit,
    levels: u32,
    children: u32,
    spread: f32,
    ratio: f32,
    trunk: f32,
    droop: f32,
    wiggle: f32,
    thick: f32,
    leaf: Leaf,
    leaf_col: [f32; 3],
    bark: [f32; 3],
    accent: Option<[f32; 3]>,
) -> TreeKind {
    TreeKind { name, habit, levels, children, spread, ratio, trunk, droop, wiggle, thick, leaf, leaf_col, bark, accent }
}

const BROWN: [f32; 3] = [0.36, 0.25, 0.16];
const GREY_BARK: [f32; 3] = [0.45, 0.42, 0.38];
const GREEN: [f32; 3] = [0.24, 0.52, 0.16];
const DARK_GREEN: [f32; 3] = [0.12, 0.36, 0.14];

/// The base tree types (index = `baseTreeType` − 1).
const TREES: [TreeKind; 34] = [
    tk("Oak", Habit::Branching, 5, 3, 38.0, 0.72, 0.35, 0.05, 0.35, 0.035, Leaf::Round, GREEN, BROWN, None),
    tk("Maple", Habit::Branching, 5, 3, 34.0, 0.74, 0.32, 0.0, 0.25, 0.03, Leaf::Heart, [0.3, 0.55, 0.15], BROWN, None),
    tk("Pine", Habit::Conifer, 3, 2, 70.0, 0.5, 0.15, 0.25, 0.1, 0.025, Leaf::Needle, DARK_GREEN, BROWN, None),
    tk("Palm", Habit::Palm, 1, 9, 0.0, 0.0, 0.85, 0.0, 0.15, 0.022, Leaf::Frond, [0.25, 0.5, 0.18], [0.55, 0.45, 0.32], None),
    tk("Birch", Habit::Branching, 5, 2, 28.0, 0.75, 0.4, 0.1, 0.3, 0.02, Leaf::Oval, [0.45, 0.65, 0.2], [0.88, 0.86, 0.82], None),
    tk("Weeping Willow", Habit::Branching, 5, 3, 40.0, 0.78, 0.3, 0.9, 0.2, 0.035, Leaf::Long, [0.45, 0.62, 0.22], GREY_BARK, None),
    tk("Cherry Blossom", Habit::Branching, 5, 3, 42.0, 0.7, 0.3, 0.0, 0.45, 0.03, Leaf::Blossom, [0.98, 0.75, 0.82], [0.3, 0.2, 0.17], None),
    tk("Bamboo", Habit::Bamboo, 1, 5, 6.0, 0.0, 1.0, 0.0, 0.05, 0.012, Leaf::Long, [0.35, 0.6, 0.2], [0.55, 0.65, 0.3], None),
    tk("Cypress", Habit::Branching, 5, 3, 10.0, 0.8, 0.1, -0.1, 0.1, 0.02, Leaf::Needle, DARK_GREEN, BROWN, None),
    tk("Baobab", Habit::Branching, 3, 4, 50.0, 0.55, 0.65, 0.0, 0.3, 0.09, Leaf::Round, [0.35, 0.55, 0.2], [0.55, 0.47, 0.4], None),
    tk("Ginkgo", Habit::Branching, 4, 3, 30.0, 0.72, 0.35, 0.0, 0.2, 0.025, Leaf::Heart, [0.75, 0.75, 0.2], GREY_BARK, None),
    tk("Spruce", Habit::Conifer, 3, 2, 80.0, 0.45, 0.1, 0.45, 0.08, 0.025, Leaf::Needle, [0.1, 0.3, 0.2], BROWN, None),
    tk("Bare Winter Tree", Habit::Branching, 6, 2, 34.0, 0.74, 0.3, 0.05, 0.35, 0.03, Leaf::None, GREEN, [0.3, 0.25, 0.22], None),
    tk("Apple", Habit::Branching, 5, 3, 46.0, 0.68, 0.3, 0.1, 0.4, 0.03, Leaf::Round, GREEN, BROWN, Some([0.85, 0.1, 0.1])),
    tk("Acacia", Habit::Branching, 4, 3, 55.0, 0.75, 0.45, -0.25, 0.25, 0.03, Leaf::Round, [0.35, 0.5, 0.18], BROWN, None),
    tk("Poplar", Habit::Branching, 5, 3, 12.0, 0.8, 0.15, -0.15, 0.1, 0.025, Leaf::Oval, [0.3, 0.55, 0.15], GREY_BARK, None),
    tk("Elm", Habit::Branching, 5, 2, 24.0, 0.8, 0.3, 0.15, 0.25, 0.032, Leaf::Oval, GREEN, BROWN, None),
    tk("Magnolia", Habit::Branching, 4, 3, 40.0, 0.7, 0.3, 0.0, 0.3, 0.03, Leaf::Blossom, [0.98, 0.9, 0.92], [0.35, 0.28, 0.25], Some([0.85, 0.5, 0.65])),
    tk("Banana", Habit::Palm, 1, 7, 0.0, 0.0, 0.6, 0.0, 0.05, 0.04, Leaf::Frond, [0.45, 0.7, 0.2], [0.5, 0.6, 0.3], None),
    tk("Bonsai", Habit::Branching, 4, 3, 60.0, 0.6, 0.45, 0.2, 0.7, 0.05, Leaf::Round, DARK_GREEN, [0.4, 0.3, 0.22], None),
    tk("Jacaranda", Habit::Branching, 5, 3, 44.0, 0.72, 0.3, 0.0, 0.35, 0.03, Leaf::Blossom, [0.6, 0.45, 0.85], BROWN, None),
    tk("Autumn Maple", Habit::Branching, 5, 3, 34.0, 0.74, 0.32, 0.0, 0.25, 0.03, Leaf::Heart, [0.85, 0.3, 0.1], BROWN, None),
    tk("Golden Autumn", Habit::Branching, 5, 3, 36.0, 0.72, 0.33, 0.05, 0.3, 0.03, Leaf::Round, [0.9, 0.7, 0.15], BROWN, None),
    tk("Olive", Habit::Branching, 4, 3, 48.0, 0.68, 0.35, 0.1, 0.6, 0.04, Leaf::Long, [0.5, 0.58, 0.42], GREY_BARK, Some([0.25, 0.28, 0.12])),
    tk("Fir", Habit::Conifer, 3, 2, 75.0, 0.5, 0.12, 0.15, 0.05, 0.025, Leaf::Needle, [0.12, 0.38, 0.22], BROWN, None),
    tk("Cedar", Habit::Conifer, 3, 3, 88.0, 0.6, 0.2, 0.0, 0.15, 0.03, Leaf::Needle, [0.2, 0.4, 0.25], BROWN, None),
    tk("Lemon", Habit::Branching, 4, 3, 45.0, 0.68, 0.3, 0.05, 0.35, 0.03, Leaf::Oval, DARK_GREEN, BROWN, Some([0.98, 0.88, 0.15])),
    tk("Wisteria", Habit::Branching, 5, 3, 45.0, 0.72, 0.3, 0.7, 0.3, 0.03, Leaf::Blossom, [0.7, 0.55, 0.9], [0.35, 0.3, 0.27], None),
    tk("Young Sapling", Habit::Branching, 3, 2, 30.0, 0.7, 0.45, 0.0, 0.2, 0.015, Leaf::Oval, [0.4, 0.7, 0.2], [0.45, 0.35, 0.22], None),
    tk("Dead Tree", Habit::Branching, 5, 2, 45.0, 0.68, 0.4, 0.1, 0.8, 0.035, Leaf::None, GREEN, [0.32, 0.29, 0.26], None),
    tk("Coconut Palm", Habit::Palm, 1, 11, 0.0, 0.0, 0.88, 0.0, 0.35, 0.018, Leaf::Frond, [0.3, 0.55, 0.15], [0.6, 0.5, 0.36], Some([0.4, 0.3, 0.15])),
    tk("Shrub", Habit::Branching, 4, 4, 55.0, 0.7, 0.05, 0.0, 0.35, 0.02, Leaf::Round, GREEN, BROWN, None),
    tk("Plum Blossom", Habit::Branching, 5, 2, 48.0, 0.7, 0.3, 0.1, 0.6, 0.03, Leaf::Blossom, [0.95, 0.6, 0.7], [0.25, 0.18, 0.15], None),
    tk("Cloud Pine", Habit::Branching, 4, 3, 70.0, 0.65, 0.35, 0.2, 0.5, 0.035, Leaf::Needle, [0.15, 0.4, 0.2], [0.35, 0.27, 0.2], None),
];

/// Names of the base tree types (1-based `baseTreeType` = index + 1).
pub fn tree_type_names() -> Vec<&'static str> {
    TREES.iter().map(|t| t.name).collect()
}

/// Tree options (Photoshop's Tree dialog).
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase", default)]
pub struct TreeSpec {
    /// 1–34.
    pub base_tree_type: u32,
    /// 1 (light from the left) … 3 (above) … 5 (right).
    pub light_direction: u32,
    /// 0–100.
    pub leaves_amount: f32,
    /// 0–200 %.
    pub leaves_size: f32,
    /// 50–300 %: how high the branching starts.
    pub branches_height: f32,
    /// 50–200 %.
    pub branches_thickness: f32,
    /// Use the type's leaf colour; otherwise `leaves_color`.
    pub default_leaves: bool,
    pub leaves_color: [f32; 4],
    /// Use `branches_color` instead of the type's bark.
    pub custom_branch_color: bool,
    pub branches_color: [f32; 4],
    /// Flat shading (no light variation).
    pub flat_shading: bool,
    pub seed: u32,
    /// Base position as a fraction of the canvas.
    pub x: f32,
    pub y: f32,
    /// Height as a fraction of the canvas height.
    pub size: f32,
}

impl Default for TreeSpec {
    fn default() -> Self {
        TreeSpec {
            base_tree_type: 1,
            light_direction: 3,
            leaves_amount: 50.0,
            leaves_size: 100.0,
            branches_height: 100.0,
            branches_thickness: 100.0,
            default_leaves: true,
            leaves_color: [0.24, 0.52, 0.16, 1.0],
            custom_branch_color: false,
            branches_color: [0.36, 0.25, 0.16, 1.0],
            flat_shading: false,
            seed: 0,
            x: 0.5,
            y: 0.95,
            size: 0.8,
        }
    }
}

struct TreeCtx<'a> {
    kind: &'a TreeKind,
    spec: &'a TreeSpec,
    rng: Rng,
    branches: Vec<Prim>,
    leaves: Vec<(f32, Prim)>,
    bark: [f32; 3],
    leaf: [f32; 3],
    light: (f32, f32),
    centre: (f32, f32),
    leaf_px: f32,
}

fn shade(c: [f32; 3], k: f32) -> [f32; 4] {
    [(c[0] * k).clamp(0.0, 1.0), (c[1] * k).clamp(0.0, 1.0), (c[2] * k).clamp(0.0, 1.0), 1.0]
}

impl TreeCtx<'_> {
    /// Brightness of a leaf at `p` (lit side of the crown brighter).
    fn light_at(&mut self, p: (f32, f32)) -> f32 {
        if self.spec.flat_shading {
            return 1.0;
        }
        let (dx, dy) = (p.0 - self.centre.0, p.1 - self.centre.1);
        let l = (dx * dx + dy * dy).sqrt().max(1.0);
        0.78 + 0.32 * ((dx * self.light.0 + dy * self.light.1) / l) + 0.12 * self.rng.sym()
    }

    fn add_leaf(&mut self, p: (f32, f32), dir: f32) {
        let r = &mut self.rng;
        let s = self.leaf_px * r.range(0.7, 1.2);
        let ang = dir + r.sym() * 1.2;
        let k = self.light_at(p);
        let col = self.leaf;
        let prim = match self.kind.leaf {
            Leaf::None => return,
            Leaf::Round => Prim::ellipse(p, s, s * 0.85, ang, shade(col, k * 1.1), shade(col, k * 0.8)),
            Leaf::Oval => Prim::ellipse(p, s * 1.2, s * 0.55, ang, shade(col, k * 1.1), shade(col, k * 0.8)),
            Leaf::Long => Prim::ellipse(p, s * 1.6, s * 0.28, ang, shade(col, k * 1.1), shade(col, k * 0.8)),
            Leaf::Heart => Prim::ellipse(p, s * 0.9, s * 1.1, ang, shade(col, k * 1.1), shade(col, k * 0.75)),
            Leaf::Needle => {
                // A tuft of needles fanning out from the twig.
                let n = 7;
                for j in 0..n {
                    let a = ang + (j as f32 / (n - 1) as f32 - 0.5) * 2.4 + self.rng.sym() * 0.15;
                    let l = s * self.rng.range(1.1, 1.7);
                    let q = (p.0 + a.cos() * l, p.1 + a.sin() * l);
                    let kk = k + 0.1 * self.rng.sym();
                    self.leaves.push((kk, Prim::capsule(p, q, (s * 0.1).max(0.45), (s * 0.05).max(0.3), shade(col, kk * 0.85), shade(col, kk * 1.1))));
                }
                return;
            }
            Leaf::Blossom => Prim::ellipse(p, s * 0.75, s * 0.75, 0.0, shade(col, k * 1.08), shade(col, k * 0.85)),
            Leaf::Frond => Prim::ellipse(p, s * 1.6, s * 0.3, ang, shade(col, k), shade(col, k * 0.8)),
        };
        let order = k + self.rng.f() * 0.05;
        self.leaves.push((order, prim));
        if let Some(acc) = self.kind.accent
            && self.rng.f() < 0.12
        {
            let q = (p.0 + self.rng.sym() * s, p.1 + self.rng.sym() * s);
            let r = (s * 0.45).max(1.0);
            self.leaves.push((k + 1.0, Prim::ellipse(q, r, r, 0.0, shade(acc, k * 1.15), shade(acc, k * 0.75))));
        }
    }

    /// A curved branch from `p` heading `ang` (radians, 0 = right, y down).
    /// Returns its end point and end heading.
    fn limb(&mut self, p: (f32, f32), ang: f32, len: f32, r0: f32, r1: f32, depth01: f32) -> ((f32, f32), f32) {
        let segs = ((len / 6.0) as usize).clamp(2, 10);
        let (mut q, mut a) = (p, ang);
        let droop = self.kind.droop * depth01;
        let col0 = shade(self.bark, 0.9 + 0.2 * depth01);
        for k in 0..segs {
            let t0 = k as f32 / segs as f32;
            let t1 = (k + 1) as f32 / segs as f32;
            a += self.kind.wiggle * self.rng.sym() * 0.25;
            // Gravity pulls the heading towards straight down (π/2).
            let down = std::f32::consts::FRAC_PI_2;
            let diff = (down - a).sin().atan2((down - a).cos());
            a += diff * droop * 0.12;
            let step = len / segs as f32;
            let nq = (q.0 + a.cos() * step, q.1 + a.sin() * step);
            let (ra, rb) = (r0 + (r1 - r0) * t0, r0 + (r1 - r0) * t1);
            self.branches.push(Prim::capsule(q, nq, ra, rb, col0, col0));
            if depth01 > 0.99 && self.kind.leaf != Leaf::None && k > 0 && self.rng.f() < self.spec.leaves_amount.clamp(0.0, 100.0) / 100.0 * 0.9 {
                self.add_leaf(nq, a);
            }
            q = nq;
        }
        (q, a)
    }

    fn branch(&mut self, p: (f32, f32), ang: f32, len: f32, r: f32, level: u32) {
        let levels = self.kind.levels;
        let depth01 = level as f32 / levels.max(1) as f32;
        let r_end = r * 0.62;
        let (end, a) = self.limb(p, ang, len, r, r_end, depth01);
        if level >= levels {
            let amount = self.spec.leaves_amount.clamp(0.0, 100.0) / 100.0;
            if self.kind.leaf != Leaf::None {
                let n = (amount * 9.0).round() as usize;
                for _ in 0..n {
                    let d = len * 0.5 * self.rng.f();
                    let th = self.rng.f() * std::f32::consts::TAU;
                    self.add_leaf((end.0 + th.cos() * d, end.1 + th.sin() * d), a);
                }
            }
            return;
        }
        let spread = self.kind.spread.to_radians();
        let n = self.kind.children;
        for c in 0..n {
            let side = if n == 1 { 0.0 } else { c as f32 / (n - 1) as f32 * 2.0 - 1.0 };
            let ca = a + side * spread + self.rng.sym() * spread * 0.35;
            let cl = len * self.kind.ratio * self.rng.range(0.8, 1.1);
            let cr = r_end * self.rng.range(0.75, 0.95);
            self.branch(end, ca, cl, cr, level + 1);
        }
    }
}

/// Builds tree primitives inside `canvas`.
pub fn tree(spec: &TreeSpec, canvas: Rect) -> Vec<Prim> {
    let kind = &TREES[(spec.base_tree_type.clamp(1, TREES.len() as u32) - 1) as usize];
    let h = canvas.height() as f32 * spec.size.clamp(0.05, 2.0);
    let base = (canvas.x0 as f32 + canvas.width() as f32 * spec.x, canvas.y0 as f32 + canvas.height() as f32 * spec.y);
    let la = match spec.light_direction.clamp(1, 5) {
        1 => 180.0f32,
        2 => 225.0,
        3 => 270.0,
        4 => 315.0,
        _ => 0.0,
    }
    .to_radians();
    let leaf = if spec.default_leaves { kind.leaf_col } else { [spec.leaves_color[0], spec.leaves_color[1], spec.leaves_color[2]] };
    let bark = if spec.custom_branch_color { [spec.branches_color[0], spec.branches_color[1], spec.branches_color[2]] } else { kind.bark };
    let thick = spec.branches_thickness.clamp(50.0, 200.0) / 100.0;
    let bh = spec.branches_height.clamp(50.0, 300.0) / 100.0;
    let mut t = TreeCtx {
        kind,
        spec,
        rng: Rng::new(spec.seed, 0x7EE + spec.base_tree_type as u64),
        branches: Vec::new(),
        leaves: Vec::new(),
        bark,
        leaf,
        light: (la.cos(), la.sin()),
        centre: (base.0, base.1 - h * 0.65),
        leaf_px: (h * 0.012 * spec.leaves_size.clamp(0.0, 200.0) / 100.0).max(0.6),
    };
    let up = -std::f32::consts::FRAC_PI_2;
    let r0 = (h * kind.thick * thick).max(0.8);
    match kind.habit {
        Habit::Branching => {
            let trunk = (h * kind.trunk * bh).min(h * 0.92);
            let (top, a) = t.limb(base, up, trunk.max(1.0), r0, r0 * 0.75, 0.0);
            // Crown length so the tree reaches about `h`.
            let geo: f32 = (0..kind.levels).map(|k| kind.ratio.powi(k as i32)).sum();
            let first = ((h - trunk).max(h * 0.08) / geo.max(0.5)) * 1.05;
            let n = kind.children.max(2);
            for c in 0..n {
                let side = c as f32 / (n - 1) as f32 * 2.0 - 1.0;
                let sp = kind.spread.to_radians() * 0.8;
                let ca = a + side * sp + t.rng.sym() * 0.15;
                let fl = first * t.rng.range(0.85, 1.05);
                t.branch(top, ca, fl, r0 * 0.7, 1);
            }
        }
        Habit::Conifer => {
            let (top, _) = t.limb(base, up, h, r0, r0 * 0.1, 0.0);
            let start = (h * kind.trunk * bh).min(h * 0.8);
            let whorls = (14.0 * (h / 300.0).sqrt().clamp(0.6, 2.5)) as usize;
            let spread = kind.spread.to_radians();
            for k in 0..whorls {
                let f = k as f32 / whorls as f32;
                let y = base.1 - start - (h - start) * f;
                let x = base.0 + (top.0 - base.0) * ((base.1 - y) / h);
                let span = (h - start) * 0.42 * (1.0 - f).powf(0.9) + h * 0.03;
                for side in [-1.0f32, 1.0] {
                    let ang = up + side * spread + t.rng.sym() * 0.1;
                    t.branch((x, y), ang, span, (r0 * 0.35 * (1.0 - f)).max(0.6), kind.levels);
                    if kind.levels >= 2 {
                        let (s2, c2) = (ang + side * 0.5).sin_cos();
                        let mid = (x + c2 * span * 0.5, y + s2 * span * 0.5);
                        t.branch(mid, ang + side * 0.6, span * 0.4, (r0 * 0.15).max(0.5), kind.levels);
                    }
                }
            }
        }
        Habit::Palm => {
            let trunk = (h * kind.trunk * bh.min(1.15)).max(1.0);
            let lean = t.rng.sym() * kind.wiggle;
            let segs = 12;
            let mut p = base;
            for k in 0..segs {
                let f = k as f32 / segs as f32;
                let a = up + lean * f;
                let q = (p.0 + a.cos() * trunk / segs as f32, p.1 + a.sin() * trunk / segs as f32);
                let r = r0 * (1.0 - 0.35 * f);
                let ring = shade(bark, if k % 2 == 0 { 1.0 } else { 0.85 });
                t.branches.push(Prim::capsule(p, q, r, r * 0.97, ring, ring));
                p = q;
            }
            let fronds = kind.children;
            let flen = h * 0.38 * (spec.leaves_size.clamp(10.0, 200.0) / 100.0).sqrt();
            for k in 0..fronds {
                let a = up + (k as f32 / fronds as f32 - 0.5) * std::f32::consts::PI * 1.7 + t.rng.sym() * 0.2;
                let mut q = p;
                let mut ang = a;
                let segs = 10;
                let leaflets = (spec.leaves_amount.clamp(0.0, 100.0) / 100.0 * 3.0).round() as usize;
                for s in 0..segs {
                    // Fronds arch over and droop towards the ground.
                    let d = std::f32::consts::FRAC_PI_2 - ang;
                    ang += 0.06 * d.sin().atan2(d.cos());
                    let nq = (q.0 + ang.cos() * flen / segs as f32, q.1 + ang.sin() * flen / segs as f32);
                    let k2 = t.light_at(nq);
                    t.leaves
                        .push((k2 - 0.1, Prim::capsule(q, nq, (r0 * 0.15).max(0.6), (r0 * 0.1).max(0.4), shade(t.leaf, k2 * 0.8), shade(t.leaf, k2 * 0.8))));
                    for j in 0..leaflets {
                        let along = j as f32 / leaflets as f32;
                        let at = (q.0 + (nq.0 - q.0) * along, q.1 + (nq.1 - q.1) * along);
                        for side in [-1.0f32, 1.0] {
                            let la = ang + side * 1.1;
                            let ll = flen * 0.22 * (1.0 - s as f32 / segs as f32 * 0.7);
                            let tip = (at.0 + la.cos() * ll, at.1 + la.sin() * ll + ll * 0.25);
                            t.leaves.push((k2, Prim::capsule(at, tip, (ll * 0.08).max(0.5), 0.3, shade(t.leaf, k2), shade(t.leaf, k2 * 0.9))));
                        }
                    }
                    q = nq;
                }
            }
            if let Some(acc) = kind.accent {
                for _ in 0..4 {
                    let c = (p.0 + t.rng.sym() * r0 * 2.0, p.1 + t.rng.f() * r0 * 2.0);
                    t.leaves.push((2.0, Prim::disc(c, r0 * 1.1, shade(acc, 1.0))));
                }
            }
        }
        Habit::Bamboo => {
            let stalks = kind.children;
            for k in 0..stalks {
                let x = base.0 + (k as f32 - (stalks - 1) as f32 / 2.0) * r0 * 5.0 + t.rng.sym() * r0;
                let hh = h * t.rng.range(0.7, 1.0);
                let a = up + t.rng.sym() * kind.spread.to_radians() * 0.5;
                let joints = 8;
                let mut p = (x, base.1);
                for j in 0..joints {
                    let q = (p.0 + a.cos() * hh / joints as f32, p.1 + a.sin() * hh / joints as f32);
                    t.branches.push(Prim::capsule(p, q, r0, r0, shade(bark, 1.0), shade(bark, 0.85)));
                    t.branches.push(Prim::capsule((q.0 - r0 * 1.1, q.1), (q.0 + r0 * 1.1, q.1), r0 * 0.25, r0 * 0.25, shade(bark, 0.7), shade(bark, 0.7)));
                    if j >= joints / 3 {
                        let leaves = (spec.leaves_amount.clamp(0.0, 100.0) / 100.0 * 4.0).round() as usize;
                        for _ in 0..leaves {
                            let side = if t.rng.f() < 0.5 { -1.0 } else { 1.0 };
                            let la = a + side * t.rng.range(0.6, 1.4);
                            let ll = t.leaf_px * 2.2;
                            t.add_leaf((q.0 + la.cos() * ll, q.1 + la.sin() * ll), la);
                        }
                    }
                    p = q;
                }
            }
        }
    }
    // Back leaves first: darker ones underneath, lit ones on top.
    t.leaves.sort_by(|a, b| a.0.partial_cmp(&b.0).unwrap_or(std::cmp::Ordering::Equal));
    let mut out = t.branches;
    out.extend(t.leaves.into_iter().map(|(_, p)| p));
    out
}

// ---------------------------------------------------------------------------
// Picture Frame
// ---------------------------------------------------------------------------

/// Picture frame designs (index = `frame` − 1).
pub const FRAME_STYLES: [&str; 16] = [
    "vineWithFlowers",
    "vineWithLeaves",
    "ivy",
    "roses",
    "daisies",
    "berries",
    "bamboo",
    "waves",
    "zigzag",
    "dots",
    "rope",
    "scallops",
    "doubleLine",
    "snowflakes",
    "stars",
    "hearts",
];

/// Picture Frame options.
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase", default)]
pub struct FrameSpec {
    /// 1-based design (see [`FRAME_STYLES`]).
    pub frame: u32,
    /// Distance of the frame from the canvas edge, % of the shorter side (0–30).
    pub margin: f32,
    /// Ornament size 1–100.
    pub size: f32,
    /// Ornament spacing / density 1–100 (higher = denser).
    pub arrangement: f32,
    /// Number of parallel lines for line designs (1–5).
    pub lines: u32,
    /// Line / vine thickness 1–100.
    pub thickness: f32,
    /// 0–100 %: fade the ornaments towards the inside.
    pub fade: f32,
    pub vine_color: [f32; 4],
    pub flower_color: [f32; 4],
    pub leaf_color: [f32; 4],
    pub seed: u32,
}

impl Default for FrameSpec {
    fn default() -> Self {
        FrameSpec {
            frame: 1,
            margin: 4.0,
            size: 50.0,
            arrangement: 50.0,
            lines: 1,
            thickness: 30.0,
            fade: 0.0,
            vine_color: [0.25, 0.4, 0.15, 1.0],
            flower_color: [0.9, 0.3, 0.45, 1.0],
            leaf_color: [0.3, 0.6, 0.2, 1.0],
            seed: 0,
        }
    }
}

/// The frame rectangle walked by arc length (clockwise from the top-left).
struct Border {
    x0: f32,
    y0: f32,
    w: f32,
    h: f32,
}

impl Border {
    fn len(&self) -> f32 {
        2.0 * (self.w + self.h)
    }
    /// Point, unit tangent and inward normal at arc length `s`.
    fn at(&self, s: f32) -> ((f32, f32), (f32, f32), (f32, f32)) {
        let s = s.rem_euclid(self.len());
        let (w, h) = (self.w, self.h);
        if s < w {
            ((self.x0 + s, self.y0), (1.0, 0.0), (0.0, 1.0))
        } else if s < w + h {
            ((self.x0 + w, self.y0 + s - w), (0.0, 1.0), (-1.0, 0.0))
        } else if s < 2.0 * w + h {
            ((self.x0 + w - (s - w - h), self.y0 + h), (-1.0, 0.0), (0.0, -1.0))
        } else {
            ((self.x0, self.y0 + h - (s - 2.0 * w - h)), (0.0, -1.0), (1.0, 0.0))
        }
    }
}

fn flower(out: &mut Vec<Prim>, c: (f32, f32), r: f32, petals: usize, petal: [f32; 4], centre: [f32; 4], rot: f32) {
    for k in 0..petals {
        let a = rot + k as f32 / petals as f32 * std::f32::consts::TAU;
        let p = (c.0 + a.cos() * r * 0.55, c.1 + a.sin() * r * 0.55);
        out.push(Prim::ellipse(p, r * 0.5, r * 0.26, a, petal, shade([petal[0], petal[1], petal[2]], 0.8)));
    }
    out.push(Prim::ellipse(c, r * 0.28, r * 0.28, 0.0, centre, shade([centre[0], centre[1], centre[2]], 0.75)));
}

fn leaf(out: &mut Vec<Prim>, p: (f32, f32), ang: f32, len: f32, col: [f32; 4]) {
    let c = (p.0 + ang.cos() * len * 0.5, p.1 + ang.sin() * len * 0.5);
    out.push(Prim::ellipse(c, len * 0.5, len * 0.22, ang, shade([col[0], col[1], col[2]], 1.12), shade([col[0], col[1], col[2]], 0.8)));
}

fn star(out: &mut Vec<Prim>, c: (f32, f32), r: f32, points: usize, col: [f32; 4], rot: f32) {
    for k in 0..points {
        let a = rot + k as f32 / points as f32 * std::f32::consts::TAU;
        out.push(Prim::capsule(c, (c.0 + a.cos() * r, c.1 + a.sin() * r), r * 0.28, 0.4, col, col));
    }
}

/// Builds picture-frame primitives inside `canvas`.
pub fn picture_frame(spec: &FrameSpec, canvas: Rect) -> Vec<Prim> {
    let mut out = Vec::new();
    let short = (canvas.width().min(canvas.height()) as f32).max(8.0);
    let margin = short * spec.margin.clamp(0.0, 30.0) / 100.0;
    let unit = short * (0.008 + spec.size.clamp(1.0, 100.0) / 100.0 * 0.05);
    let thick = (unit * 0.04 + short * 0.00012 * spec.thickness.clamp(1.0, 100.0)).max(0.6);
    let spacing = unit * (4.0 - 3.2 * spec.arrangement.clamp(1.0, 100.0) / 100.0);
    let inset = margin + unit;
    let b = Border {
        x0: canvas.x0 as f32 + inset,
        y0: canvas.y0 as f32 + inset,
        w: (canvas.width() as f32 - 2.0 * inset).max(1.0),
        h: (canvas.height() as f32 - 2.0 * inset).max(1.0),
    };
    let total = b.len();
    let count = (total / spacing).round().max(4.0) as usize;
    let step = total / count as f32;
    let mut rng = Rng::new(spec.seed, 0xF8A3E + spec.frame as u64);
    let (vine, fl, lf) = (spec.vine_color, spec.flower_color, spec.leaf_color);
    let style = FRAME_STYLES[(spec.frame.clamp(1, FRAME_STYLES.len() as u32) - 1) as usize];
    let lines = spec.lines.clamp(1, 5) as usize;
    let white = [1.0, 1.0, 1.0, 1.0];
    let yellow = [0.98, 0.82, 0.2, 1.0];
    // A wavy vine along the border.
    let vine_path = |out: &mut Vec<Prim>, amp: f32, wl: f32, col: [f32; 4], r: f32| {
        let n = (total / 3.0).ceil().max(8.0) as usize;
        let mut prev: Option<(f32, f32)> = None;
        for k in 0..=n {
            let s = k as f32 / n as f32 * total;
            let (p, _, nn) = b.at(s);
            let o = amp * (s / wl * std::f32::consts::TAU).sin();
            let q = (p.0 + nn.0 * o, p.1 + nn.1 * o);
            if let Some(pp) = prev {
                out.push(Prim::capsule(pp, q, r, r, col, col));
            }
            prev = Some(q);
        }
    };
    let wave_off = |s: f32, amp: f32, wl: f32| amp * (s / wl * std::f32::consts::TAU).sin();
    match style {
        "vineWithFlowers" | "vineWithLeaves" | "ivy" | "roses" | "daisies" | "berries" => {
            let amp = unit * 0.5;
            let wl = step * 2.0;
            vine_path(&mut out, amp, wl, vine, thick);
            for k in 0..count {
                let s = k as f32 * step + step * 0.5;
                let (p, t, nn) = b.at(s);
                let o = wave_off(s, amp, wl);
                let q = (p.0 + nn.0 * o, p.1 + nn.1 * o);
                let side = if k % 2 == 0 { 1.0 } else { -1.0 };
                let ta = t.1.atan2(t.0);
                let la = ta + side * 0.9 + rng.sym() * 0.3;
                let ll = unit * rng.range(0.9, 1.2);
                match style {
                    "vineWithFlowers" => {
                        leaf(&mut out, q, la, ll, lf);
                        if k % 2 == 0 {
                            let c = (q.0 + nn.0 * unit * 0.7 * side, q.1 + nn.1 * unit * 0.7 * side);
                            flower(&mut out, c, unit * 0.65, 5, fl, yellow, rng.f() * 6.3);
                        }
                    }
                    "vineWithLeaves" => {
                        leaf(&mut out, q, la, ll, lf);
                        leaf(&mut out, q, ta - side * 0.9, ll * 0.8, lf);
                    }
                    "ivy" => {
                        for j in 0..3 {
                            let a = la + (j as f32 - 1.0) * 0.7;
                            leaf(&mut out, q, a, ll * 0.75, shade([lf[0], lf[1], lf[2]], 0.75 + 0.15 * j as f32));
                        }
                    }
                    "roses" => {
                        leaf(&mut out, q, la, ll, lf);
                        if k % 3 == 0 {
                            let r = unit * 0.7;
                            for (j, f) in [1.0f32, 0.75, 0.5, 0.28].iter().enumerate() {
                                let c = shade([fl[0], fl[1], fl[2]], 0.75 + 0.1 * j as f32);
                                out.push(Prim::ellipse(q, r * f, r * f * 0.9, rng.f() * 3.0, c, shade([fl[0], fl[1], fl[2]], 0.55)));
                            }
                        }
                    }
                    "daisies" => {
                        leaf(&mut out, q, la, ll * 0.8, lf);
                        if k % 2 == 1 {
                            flower(&mut out, q, unit * 0.6, 12, white, yellow, rng.f());
                        }
                    }
                    _ => {
                        leaf(&mut out, q, la, ll, lf);
                        for _ in 0..3 {
                            let c = (q.0 + rng.sym() * unit * 0.4, q.1 + rng.sym() * unit * 0.4);
                            out.push(Prim::ellipse(c, unit * 0.2, unit * 0.2, 0.0, shade([fl[0], fl[1], fl[2]], 1.2), shade([fl[0], fl[1], fl[2]], 0.6)));
                        }
                    }
                }
            }
        }
        "bamboo" => {
            for edge in 0..4 {
                let (s0, l) = match edge {
                    0 => (0.0, b.w),
                    1 => (b.w, b.h),
                    2 => (b.w + b.h, b.w),
                    _ => (2.0 * b.w + b.h, b.h),
                };
                let (a, _, _) = b.at(s0 + 0.01);
                let (z, _, _) = b.at(s0 + l - 0.01);
                let joints = (l / (spacing * 2.0)).round().max(1.0) as usize;
                for j in 0..joints {
                    let f0 = j as f32 / joints as f32;
                    let f1 = (j + 1) as f32 / joints as f32;
                    let p0 = (a.0 + (z.0 - a.0) * f0, a.1 + (z.1 - a.1) * f0);
                    let p1 = (a.0 + (z.0 - a.0) * f1, a.1 + (z.1 - a.1) * f1);
                    out.push(Prim::capsule(p0, p1, unit * 0.35, unit * 0.35, vine, shade([vine[0], vine[1], vine[2]], 0.8)));
                    out.push(Prim::disc(p1, unit * 0.4, shade([vine[0], vine[1], vine[2]], 0.7)));
                }
                // A few leaves per edge.
                for _ in 0..(joints / 2).max(1) {
                    let f = rng.f();
                    let p = (a.0 + (z.0 - a.0) * f, a.1 + (z.1 - a.1) * f);
                    leaf(&mut out, p, rng.f() * 6.3, unit * 1.4, lf);
                }
            }
        }
        "waves" | "zigzag" | "doubleLine" | "rope" => {
            for li in 0..lines {
                let off = (li as f32 - (lines - 1) as f32 / 2.0) * thick * 4.0;
                let n = (total / 2.0).ceil().max(8.0) as usize;
                let mut prev: Option<(f32, f32)> = None;
                for k in 0..=n {
                    let s = k as f32 / n as f32 * total;
                    let (p, _, nn) = b.at(s);
                    let o = off
                        + match style {
                            "waves" => wave_off(s + li as f32 * step * 0.25, unit * 0.4, step),
                            "zigzag" => {
                                let ph = (s / step).fract();
                                unit * 0.4 * (4.0 * (ph - 0.5).abs() - 1.0)
                            }
                            _ => 0.0,
                        };
                    let q = (p.0 + nn.0 * o, p.1 + nn.1 * o);
                    if style == "rope" {
                        if k % 2 == 0 {
                            let (_, t, _) = b.at(s);
                            out.push(Prim::ellipse(
                                q,
                                unit * 0.45,
                                unit * 0.2,
                                t.1.atan2(t.0) + 0.7,
                                shade([vine[0], vine[1], vine[2]], 1.1),
                                shade([vine[0], vine[1], vine[2]], 0.65),
                            ));
                        }
                    } else if let Some(pp) = prev {
                        out.push(Prim::capsule(pp, q, thick, thick, vine, vine));
                    }
                    prev = Some(q);
                }
                if style == "doubleLine" {
                    // A second rule inside each line and corner squares.
                    let inner = Border {
                        x0: b.x0 + unit * 0.6 + off,
                        y0: b.y0 + unit * 0.6 + off,
                        w: b.w - 2.0 * (unit * 0.6 + off),
                        h: b.h - 2.0 * (unit * 0.6 + off),
                    };
                    for e in 0..4 {
                        let (p, _, _) = inner.at([0.0, inner.w, inner.w + inner.h, 2.0 * inner.w + inner.h][e]);
                        let (q, _, _) = inner.at([inner.w, inner.w + inner.h, 2.0 * inner.w + inner.h, inner.len() - 0.001][e]);
                        out.push(Prim::capsule(p, q, thick * 0.6, thick * 0.6, vine, vine));
                    }
                }
            }
            if style == "doubleLine" {
                for e in 0..4 {
                    let (p, _, _) = b.at([0.0, b.w, b.w + b.h, 2.0 * b.w + b.h][e]);
                    out.push(Prim::ellipse(p, unit * 0.45, unit * 0.45, std::f32::consts::FRAC_PI_4, fl, fl));
                }
            }
        }
        "dots" | "scallops" | "snowflakes" | "stars" | "hearts" => {
            for k in 0..count {
                let s = k as f32 * step;
                let (p, t, nn) = b.at(s);
                let ta = t.1.atan2(t.0);
                match style {
                    "dots" => {
                        let r = unit * if k % 2 == 0 { 0.4 } else { 0.22 };
                        out.push(Prim::ellipse(p, r, r, 0.0, shade([fl[0], fl[1], fl[2]], 1.15), shade([fl[0], fl[1], fl[2]], 0.75)));
                    }
                    "scallops" => {
                        let (c0, _, _) = b.at(s - step * 0.5);
                        let mid = ((c0.0 + p.0) / 2.0, (c0.1 + p.1) / 2.0);
                        let n = 8;
                        for j in 0..n {
                            let a0 = std::f32::consts::PI * j as f32 / n as f32;
                            let a1 = std::f32::consts::PI * (j + 1) as f32 / n as f32;
                            let r = step * 0.5;
                            let pt = |a: f32| (mid.0 - t.0 * r * a.cos() - nn.0 * r * a.sin(), mid.1 - t.1 * r * a.cos() - nn.1 * r * a.sin());
                            out.push(Prim::capsule(pt(a0), pt(a1), thick, thick, vine, vine));
                        }
                    }
                    "snowflakes" => {
                        let r = unit * 0.6;
                        let c = [0.85, 0.92, 1.0, 1.0];
                        for j in 0..6 {
                            let a = ta + j as f32 * std::f32::consts::FRAC_PI_3;
                            let e = (p.0 + a.cos() * r, p.1 + a.sin() * r);
                            out.push(Prim::capsule(p, e, (thick * 0.6).max(0.6), (thick * 0.4).max(0.5), c, c));
                            let m = (p.0 + a.cos() * r * 0.6, p.1 + a.sin() * r * 0.6);
                            for sd in [-0.7f32, 0.7] {
                                let b2 = (m.0 + (a + sd).cos() * r * 0.3, m.1 + (a + sd).sin() * r * 0.3);
                                out.push(Prim::capsule(m, b2, (thick * 0.4).max(0.5), 0.4, c, c));
                            }
                        }
                    }
                    "stars" => star(&mut out, p, unit * 0.55, 5, if k % 2 == 0 { yellow } else { fl }, ta - std::f32::consts::FRAC_PI_2),
                    _ => {
                        // Heart: two lobes and a point towards the inside.
                        let r = unit * 0.3;
                        let (tx, ty) = t;
                        let l = (p.0 - tx * r * 0.8 - nn.0 * r * 0.3, p.1 - ty * r * 0.8 - nn.1 * r * 0.3);
                        let rr = (p.0 + tx * r * 0.8 - nn.0 * r * 0.3, p.1 + ty * r * 0.8 - nn.1 * r * 0.3);
                        let tip = (p.0 + nn.0 * r * 1.6, p.1 + nn.1 * r * 1.6);
                        out.push(Prim::capsule(l, tip, r, 0.6, fl, fl));
                        out.push(Prim::capsule(rr, tip, r, 0.6, fl, fl));
                    }
                }
            }
        }
        _ => {}
    }
    // Fade: lower the opacity of ornaments that reach further inside.
    let fade = spec.fade.clamp(0.0, 100.0) / 100.0;
    if fade > 0.0 {
        let cx = canvas.x0 as f32 + canvas.width() as f32 / 2.0;
        let cy = canvas.y0 as f32 + canvas.height() as f32 / 2.0;
        for p in &mut out {
            let c = match p.shape {
                Shape::Capsule { a, .. } => a,
                Shape::Ellipse { c, .. } => c,
            };
            let edge = ((c.0 - cx).abs() / (b.w / 2.0)).max((c.1 - cy).abs() / (b.h / 2.0)).clamp(0.0, 1.0);
            let k = 1.0 - fade * (1.0 - edge) * 8.0;
            p.c0[3] *= k.clamp(0.0, 1.0);
            p.c1[3] *= k.clamp(0.0, 1.0);
        }
    }
    out
}

#[cfg(test)]
mod tests {
    use super::*;
    use photocraft_color::{ColorMode, PixelFormat, SampleType};

    #[test]
    fn composite_draws_a_disc_and_respects_the_clip() {
        let mut s = Surface::new(PixelFormat::new(ColorMode::Rgb, SampleType::U8, true));
        let p = [Prim::disc((20.0, 20.0), 8.0, [1.0, 0.0, 0.0, 1.0])];
        let used = composite(&mut s, &p, Rect::new(0, 0, 20, 40), None);
        assert_eq!(used.x1, 20);
        let c = s.pixel(18, 20);
        assert!(c[0] > 0.99 && c[3] > 0.99);
        assert_eq!(s.pixel(22, 20)[3], 0.0, "clipped");
        assert_eq!(s.pixel(2, 2)[3], 0.0);
    }

    #[test]
    fn renderers_are_deterministic_and_seeded() {
        let canvas = Rect::new(0, 0, 400, 300);
        let a = tree(&TreeSpec::default(), canvas);
        assert_eq!(a, tree(&TreeSpec::default(), canvas));
        assert_ne!(a, tree(&TreeSpec { seed: 7, ..Default::default() }, canvas));
        for k in 1..=34 {
            assert!(!tree(&TreeSpec { base_tree_type: k, ..Default::default() }, canvas).is_empty(), "tree {k}");
        }
        for k in 1..=FRAME_STYLES.len() as u32 {
            assert!(!picture_frame(&FrameSpec { frame: k, ..Default::default() }, canvas).is_empty(), "frame {k}");
        }
        let path = default_flame_path(canvas);
        for t in [
            FlameType::OneFlameAlongPath,
            FlameType::MultipleFlamesAlongPath,
            FlameType::MultipleFlamesPathDirections,
            FlameType::MultipleFlamesVariousLength,
            FlameType::CandleLight,
            FlameType::MultipleFlamesOneDirection,
        ] {
            assert!(!flame(&FlameSpec { flame_type: t, ..Default::default() }, std::slice::from_ref(&path)).is_empty(), "{t:?}");
        }
    }
}