//! Composition viewer maths shared by the frontends and tests: snapping (targets, tolerance and
//! priority), the Show Channel / Exposure display transforms, and the viewer snapshot.
//!
//! Nothing here touches the project; edits go through commands.

use std::sync::Arc;

use effectcraft_color::{ColorSpace, space};
use effectcraft_geom::vec2;
use effectcraft_project::{Layer, LayerId};
use effectcraft_raster::Image;
use effectcraft_render::EvalCtx;
use effectcraft_time::Tick;
use serde::{Deserialize, Serialize};

// ---------------------------------------------------------------- snapping

/// What a snap target is: a point (snaps both axes) or an axis-aligned line.
#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
pub enum SnapKind {
    Point,
    /// A vertical line at `pos[0]` (snaps x only).
    VLine,
    /// A horizontal line at `pos[1]` (snaps y only).
    HLine,
}

/// Where a snap target comes from (for feedback: the highlighted box / cross).
#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
pub enum SnapSource {
    Layer(LayerId),
    Comp,
    Guide,
    Grid,
    /// A mask or shape path vertex of a layer.
    Vertex(LayerId),
}

#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
pub struct SnapTarget {
    pub kind: SnapKind,
    /// Comp pixels.
    pub pos: [f64; 2],
    /// Lower wins when two targets are equally near (vertices and layer features before
    /// guides, guides before the grid).
    pub priority: u8,
    pub source: SnapSource,
}

impl SnapTarget {
    pub fn point(pos: [f64; 2], priority: u8, source: SnapSource) -> SnapTarget {
        SnapTarget { kind: SnapKind::Point, pos, priority, source }
    }
    pub fn vline(x: f64, priority: u8, source: SnapSource) -> SnapTarget {
        SnapTarget { kind: SnapKind::VLine, pos: [x, 0.0], priority, source }
    }
    pub fn hline(y: f64, priority: u8, source: SnapSource) -> SnapTarget {
        SnapTarget { kind: SnapKind::HLine, pos: [0.0, y], priority, source }
    }
}

/// Priorities of the standard targets.
pub const PRI_VERTEX: u8 = 0;
pub const PRI_LAYER: u8 = 1;
pub const PRI_COMP: u8 = 2;
pub const PRI_GUIDE: u8 = 3;
pub const PRI_GRID: u8 = 4;

/// A snap: move the dragged feature by `delta` (comp pixels); `hits` are the targets it lands on.
#[derive(Clone, Debug, PartialEq)]
pub struct Snap {
    pub delta: [f64; 2],
    pub hits: Vec<SnapTarget>,
    /// The snapped feature position.
    pub at: [f64; 2],
}

fn better(a: (f64, u8), b: (f64, u8)) -> bool {
    if (a.0 - b.0).abs() < 1e-6 { a.1 < b.1 } else { a.0 < b.0 }
}

/// Snap feature points `sources` (comp pixels) to the nearest target within `tol` comp pixels.
/// A point target snaps both axes and wins when it is at least as near as the best lines;
/// otherwise the nearest vertical and horizontal lines snap x and y independently. Ties go to
/// the lower priority value.
pub fn snap(sources: &[[f64; 2]], targets: &[SnapTarget], tol: f64) -> Option<Snap> {
    let mut best_pt: Option<((f64, u8), [f64; 2], SnapTarget)> = None;
    let mut best_x: Option<((f64, u8), [f64; 2], SnapTarget)> = None;
    let mut best_y: Option<((f64, u8), [f64; 2], SnapTarget)> = None;
    for s in sources {
        for t in targets {
            let (d, slot) = match t.kind {
                SnapKind::Point => (((t.pos[0] - s[0]).powi(2) + (t.pos[1] - s[1]).powi(2)).sqrt(), &mut best_pt),
                SnapKind::VLine => ((t.pos[0] - s[0]).abs(), &mut best_x),
                SnapKind::HLine => ((t.pos[1] - s[1]).abs(), &mut best_y),
            };
            if d > tol {
                continue;
            }
            let key = (d, t.priority);
            if slot.as_ref().is_none_or(|(k, _, _)| better(key, *k)) {
                *slot = Some((key, *s, *t));
            }
        }
    }
    let line_best = [best_x.map(|b| b.0.0), best_y.map(|b| b.0.0)].into_iter().flatten().fold(f64::INFINITY, f64::min);
    if let Some((k, s, t)) = best_pt
        && k.0 <= line_best + 1e-9
    {
        return Some(Snap { delta: [t.pos[0] - s[0], t.pos[1] - s[1]], hits: vec![t], at: t.pos });
    }
    if best_x.is_none() && best_y.is_none() {
        return None;
    }
    let mut delta = [0.0; 2];
    let mut hits = vec![];
    let mut at = [f64::NAN; 2];
    if let Some((_, s, t)) = best_x {
        delta[0] = t.pos[0] - s[0];
        at = [t.pos[0], s[1]];
        hits.push(t);
    }
    if let Some((_, s, t)) = best_y {
        delta[1] = t.pos[1] - s[1];
        at = if at[0].is_nan() { [s[0], t.pos[1]] } else { [at[0], t.pos[1]] };
        hits.push(t);
    }
    Some(Snap { delta, hits, at })
}

/// Snap features of a layer in comp space: corners, edge midpoints, centre and anchor point
/// (2D layers; `None` for layers without bounds).
pub fn layer_features(ctx: &EvalCtx, layer: &Layer) -> Vec<[f64; 2]> {
    let mut out = vec![];
    let (m, _) = ctx.layer_to_comp(layer);
    if let Some(b) = effectcraft_render::content_bounds(ctx, layer) {
        let (cx, cy) = ((b[0] + b[2]) / 2.0, (b[1] + b[3]) / 2.0);
        for p in [[b[0], b[1]], [b[2], b[1]], [b[2], b[3]], [b[0], b[3]], [cx, b[1]], [b[2], cy], [cx, b[3]], [b[0], cy], [cx, cy]] {
            let q = m.apply(vec2(p[0], p[1]));
            out.push([q.x, q.y]);
        }
    }
    if let Some(tr) = layer.transform() {
        let a = ctx.v3(layer, tr, "anchor", [0.0; 3]);
        let q = m.apply(vec2(a[0], a[1]));
        out.push([q.x, q.y]);
    }
    out
}

/// Mask and shape-path vertices of a layer in comp space.
pub fn layer_vertices(ctx: &EvalCtx, layer: &Layer) -> Vec<[f64; 2]> {
    let (m, _) = ctx.layer_to_comp(layer);
    let mut out = vec![];
    if let Some(masks) = layer.masks() {
        for g in masks.groups() {
            if let Some(sp) = g.get("path").map(|pr| ctx.value(layer, pr)).and_then(|v| v.as_path().cloned()) {
                out.extend(sp.vertices.iter().map(|v| {
                    let q = m.apply(vec2(v[0], v[1]));
                    [q.x, q.y]
                }));
            }
        }
    }
    for (uid, sp) in shape_paths(ctx, layer) {
        let pm = m * effectcraft_render::shapes::item_matrix(ctx, layer, uid).unwrap_or(effectcraft_geom::Mat3::IDENTITY);
        out.extend(sp.vertices.iter().map(|v| {
            let q = pm.apply(vec2(v[0], v[1]));
            [q.x, q.y]
        }));
    }
    out
}

/// Shape-layer Path items (uid, evaluated path in the item's group space).
pub fn shape_paths(ctx: &EvalCtx, layer: &Layer) -> Vec<(u64, effectcraft_keyframe::ShapePath)> {
    let mut out = vec![];
    let Some(c) = layer.props.sub("contents") else { return out };
    fn walk(ctx: &EvalCtx, layer: &Layer, g: &effectcraft_project::PropGroup, out: &mut Vec<(u64, effectcraft_keyframe::ShapePath)>) {
        for sub in g.groups() {
            if sub.match_id == "path"
                && let Some(sp) = sub.get("path").map(|pr| ctx.value(layer, pr)).and_then(|v| v.as_path().cloned())
            {
                out.push((sub.uid, sp));
            } else if sub.match_id == "group" || sub.match_id == "contents" {
                walk(ctx, layer, sub, out);
            }
        }
    }
    walk(ctx, layer, c, &mut out);
    out
}

/// Which target families are on.
#[derive(Clone, Copy, Debug, Default)]
pub struct SnapOptions {
    pub guides: bool,
    pub grid: bool,
    pub grid_spacing: f64,
}

/// Snap targets of a comp at the context time: other layers' features and vertices, comp edges
/// and centre, and (when on) guides and grid lines. Layers in `exclude` (the dragged ones) are
/// skipped.
pub fn targets(ctx: &EvalCtx, exclude: &[LayerId], opts: SnapOptions) -> Vec<SnapTarget> {
    let comp = ctx.comp;
    let (w, h) = (comp.width as f64, comp.height as f64);
    let mut out = vec![
        SnapTarget::vline(0.0, PRI_COMP, SnapSource::Comp),
        SnapTarget::vline(w, PRI_COMP, SnapSource::Comp),
        SnapTarget::vline(w / 2.0, PRI_COMP, SnapSource::Comp),
        SnapTarget::hline(0.0, PRI_COMP, SnapSource::Comp),
        SnapTarget::hline(h, PRI_COMP, SnapSource::Comp),
        SnapTarget::hline(h / 2.0, PRI_COMP, SnapSource::Comp),
        SnapTarget::point([w / 2.0, h / 2.0], PRI_COMP, SnapSource::Comp),
    ];
    for l in comp.layers.iter().filter(|l| l.is_active_at(ctx.time) && !exclude.contains(&l.id) && !l.is_camera() && !l.is_light() && !l.is_3d()) {
        let f = layer_features(ctx, l);
        // Edges of unrotated layers are lines too.
        if f.len() >= 9 && (f[0][1] - f[1][1]).abs() < 1e-6 && (f[0][0] - f[3][0]).abs() < 1e-6 {
            for x in [f[0][0], f[1][0]] {
                out.push(SnapTarget { kind: SnapKind::VLine, pos: [x, f[0][1]], priority: PRI_LAYER, source: SnapSource::Layer(l.id) });
            }
            for y in [f[0][1], f[3][1]] {
                out.push(SnapTarget { kind: SnapKind::HLine, pos: [f[0][0], y], priority: PRI_LAYER, source: SnapSource::Layer(l.id) });
            }
        }
        out.extend(f.into_iter().map(|p| SnapTarget::point(p, PRI_LAYER, SnapSource::Layer(l.id))));
        out.extend(layer_vertices(ctx, l).into_iter().map(|p| SnapTarget::point(p, PRI_VERTEX, SnapSource::Vertex(l.id))));
    }
    if opts.guides {
        for g in &comp.guides {
            out.push(if g.vertical {
                SnapTarget::vline(g.position, PRI_GUIDE, SnapSource::Guide)
            } else {
                SnapTarget::hline(g.position, PRI_GUIDE, SnapSource::Guide)
            });
        }
    }
    if opts.grid && opts.grid_spacing >= 1.0 {
        let s = opts.grid_spacing;
        let mut x = 0.0;
        while x <= w + 1e-9 && out.len() < 20_000 {
            out.push(SnapTarget::vline(x, PRI_GRID, SnapSource::Grid));
            x += s;
        }
        let mut y = 0.0;
        while y <= h + 1e-9 && out.len() < 40_000 {
            out.push(SnapTarget::hline(y, PRI_GRID, SnapSource::Grid));
            y += s;
        }
    }
    out
}

// ---------------------------------------------------------------- channels and exposure

/// Show Channel and Color Management Settings.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
pub enum Channel {
    #[default]
    Rgb,
    Red,
    Green,
    Blue,
    Alpha,
    RgbStraight,
}

impl Channel {
    pub const ALL: [Channel; 6] = [Channel::Rgb, Channel::Red, Channel::Green, Channel::Blue, Channel::Alpha, Channel::RgbStraight];
    pub fn id(self) -> &'static str {
        match self {
            Channel::Rgb => "rgb",
            Channel::Red => "red",
            Channel::Green => "green",
            Channel::Blue => "blue",
            Channel::Alpha => "alpha",
            Channel::RgbStraight => "rgbStraight",
        }
    }
    pub fn label(self) -> &'static str {
        match self {
            Channel::Rgb => "RGB",
            Channel::Red => "Red",
            Channel::Green => "Green",
            Channel::Blue => "Blue",
            Channel::Alpha => "Alpha",
            Channel::RgbStraight => "RGB Straight",
        }
    }
    pub fn from_name(s: &str) -> Option<Channel> {
        let n = s.to_ascii_lowercase().replace([' ', '_', '-'], "");
        Channel::ALL.into_iter().find(|c| c.id().to_ascii_lowercase() == n || c.label().to_ascii_lowercase().replace(' ', "") == n)
    }
}

fn srgb_to_linear(v: f32) -> f32 {
    if v <= 0.04045 { v / 12.92 } else { ((v + 0.055) / 1.055).powf(2.4) }
}
fn linear_to_srgb(v: f32) -> f32 {
    let v = v.max(0.0);
    if v <= 0.003_130_8 { v * 12.92 } else { 1.055 * v.powf(1.0 / 2.4) - 0.055 }
}

/// 8-bit exposure lookup table: `stops` applied in linear light.
pub fn exposure_lut(stops: f32) -> [u8; 256] {
    let k = 2f32.powf(stops);
    let mut lut = [0u8; 256];
    for (i, o) in lut.iter_mut().enumerate() {
        let lin = srgb_to_linear(i as f32 / 255.0) * k;
        *o = (linear_to_srgb(lin).clamp(0.0, 1.0) * 255.0 + 0.5) as u8;
    }
    lut
}

/// Apply the viewer's channel and exposure to premultiplied 8-bit RGBA pixels in place.
/// RGB shows the image (exposure only); Red/Green/Blue/Alpha show one channel as an opaque
/// grayscale image (or tinted in that channel's colour when `colorized`); RGB Straight shows the
/// unpremultiplied colour, opaque.
pub fn display_transform(px: &mut [[u8; 4]], channel: Channel, colorized: bool, stops: f32) {
    let lut = (stops != 0.0).then(|| exposure_lut(stops));
    let ex = |v: u8| lut.as_ref().map(|l| l[v as usize]).unwrap_or(v);
    for p in px.iter_mut() {
        let [r, g, b, a] = *p;
        *p = match channel {
            Channel::Rgb => {
                if lut.is_none() {
                    continue;
                }
                // Exposure on straight colour, re-premultiplied.
                let un = |c: u8| if a == 0 { 0 } else { ((c as u32 * 255 + a as u32 / 2) / a as u32).min(255) as u8 };
                let pm = |c: u8| ((ex(un(c)) as u32 * a as u32 + 127) / 255) as u8;
                [pm(r), pm(g), pm(b), a]
            }
            Channel::Red | Channel::Green | Channel::Blue => {
                let (i, c) = match channel {
                    Channel::Red => (0, r),
                    Channel::Green => (1, g),
                    _ => (2, b),
                };
                let v = ex(c);
                if colorized {
                    let mut o = [0, 0, 0, 255];
                    o[i] = v;
                    o
                } else {
                    [v, v, v, 255]
                }
            }
            Channel::Alpha => {
                let v = ex(a);
                [v, v, v, 255]
            }
            Channel::RgbStraight => {
                let un = |c: u8| if a == 0 { 0 } else { ((c as u32 * 255 + a as u32 / 2) / a as u32).min(255) as u8 };
                [ex(un(r)), ex(un(g)), ex(un(b)), 255]
            }
        };
    }
}

// ---------------------------------------------------------------- display colour management

/// View ▸ Simulate Output: the output device a preview imitates.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum SimProfile {
    #[default]
    None,
    /// HDTV (Rec. 709): ITU-R BT.709 primaries, BT.1886 gamma 2.4.
    Rec709,
    /// UHDTV (Rec. 2020): ITU-R BT.2020 primaries, gamma 2.4.
    Rec2020,
    /// Display P3: SMPTE EG 432-1 primaries, D65, the sRGB curve.
    P3,
    /// Internet Standard RGB (sRGB, IEC 61966-2-1).
    Srgb,
    /// Linear light with Rec. 709 primaries (no transfer curve).
    Linear,
    /// SDTV NTSC: SMPTE 170M primaries, gamma 2.4.
    Ntsc,
    /// SDTV PAL: EBU Tech 3213 primaries, gamma 2.4.
    Pal,
    /// Legacy Macintosh RGB: Rec. 709 primaries, gamma 1.8.
    Mac18,
}

/// A transfer curve of a simulated or display profile.
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum Curve {
    Srgb,
    Gamma(f32),
    Linear,
    /// A colour space's own curve (PQ, HLG…).
    Space(space::Curve),
}

impl Curve {
    fn decode(self, v: f32) -> f32 {
        let a = v.abs();
        let l = match self {
            Curve::Srgb => srgb_to_linear(a),
            Curve::Gamma(g) => a.powf(g),
            Curve::Linear => a,
            Curve::Space(c) => c.decode(a),
        };
        l.copysign(v)
    }
    fn encode(self, v: f32) -> f32 {
        let a = v.abs();
        let e = match self {
            Curve::Srgb => linear_to_srgb(a),
            Curve::Gamma(g) => a.powf(1.0 / g),
            Curve::Linear => a,
            Curve::Space(c) => c.encode(a),
        };
        e.copysign(v)
    }
}

const P709: [[f64; 2]; 3] = [[0.640, 0.330], [0.300, 0.600], [0.150, 0.060]];

impl SimProfile {
    pub const ALL: [SimProfile; 9] = [
        SimProfile::None,
        SimProfile::Rec709,
        SimProfile::Ntsc,
        SimProfile::Pal,
        SimProfile::Mac18,
        SimProfile::Srgb,
        SimProfile::Rec2020,
        SimProfile::P3,
        SimProfile::Linear,
    ];

    pub fn id(self) -> &'static str {
        match self {
            SimProfile::None => "none",
            SimProfile::Rec709 => "rec709",
            SimProfile::Rec2020 => "rec2020",
            SimProfile::P3 => "p3",
            SimProfile::Srgb => "srgb",
            SimProfile::Linear => "linear",
            SimProfile::Ntsc => "ntsc",
            SimProfile::Pal => "pal",
            SimProfile::Mac18 => "mac18",
        }
    }

    pub fn label(self) -> &'static str {
        match self {
            SimProfile::None => "No Output Simulation",
            SimProfile::Rec709 => "HDTV (Rec. 709)",
            SimProfile::Rec2020 => "UHDTV (Rec. 2020)",
            SimProfile::P3 => "Display P3",
            SimProfile::Srgb => "Internet Standard RGB (sRGB)",
            SimProfile::Linear => "Linear (1.0 Gamma)",
            SimProfile::Ntsc => "SDTV NTSC",
            SimProfile::Pal => "SDTV PAL",
            SimProfile::Mac18 => "Legacy Macintosh RGB (Gamma 1.8)",
        }
    }

    pub fn parse(s: &str) -> Option<SimProfile> {
        let k: String = s.chars().filter(|c| c.is_ascii_alphanumeric()).collect::<String>().to_ascii_lowercase();
        SimProfile::ALL
            .into_iter()
            .find(|p| p.id() == k || p.label().chars().filter(|c| c.is_ascii_alphanumeric()).collect::<String>().to_ascii_lowercase() == k)
            .or(match k.as_str() {
                "off" => Some(SimProfile::None),
                "hdtv" | "bt709" => Some(SimProfile::Rec709),
                "bt2020" | "uhdtv" => Some(SimProfile::Rec2020),
                "displayp3" => Some(SimProfile::P3),
                "lin" | "linearlight" => Some(SimProfile::Linear),
                _ => None,
            })
    }

    /// (primaries, curve); `None` for No Output Simulation.
    pub fn space(self) -> Option<([[f64; 2]; 3], Curve)> {
        Some(match self {
            SimProfile::None => return None,
            SimProfile::Rec709 => (P709, Curve::Gamma(2.4)),
            SimProfile::Rec2020 => (ColorSpace::Rec2020.primaries(), Curve::Gamma(2.4)),
            SimProfile::P3 => (ColorSpace::DisplayP3.primaries(), Curve::Srgb),
            SimProfile::Srgb => (P709, Curve::Srgb),
            SimProfile::Linear => (P709, Curve::Linear),
            SimProfile::Ntsc => ([[0.630, 0.340], [0.310, 0.595], [0.155, 0.070]], Curve::Gamma(2.4)),
            SimProfile::Pal => ([[0.640, 0.330], [0.290, 0.600], [0.150, 0.060]], Curve::Gamma(2.4)),
            SimProfile::Mac18 => (P709, Curve::Gamma(1.8)),
        })
    }
}

/// View ▸ Simulate Output setting: the profile, and Preserve RGB (the output numbers are sent
/// to the display unconverted, as on a device that doesn't colour-manage).
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
#[serde(default)]
pub struct Simulation {
    pub profile: SimProfile,
    pub preserve_rgb: bool,
}

/// An RGB space for viewer conversions: primaries (D65) and curve.
#[derive(Clone, Copy, Debug, PartialEq)]
struct Prof {
    /// Linear RGB → XYZ D65.
    xyz: [[f64; 3]; 3],
    curve: Curve,
}

impl Prof {
    fn new(prims: [[f64; 2]; 3], curve: Curve) -> Prof {
        Prof { xyz: space::rgb_to_xyz(prims, D65), curve }
    }
    /// A colour space (linear when `linear`).
    fn of(cs: ColorSpace, linear: bool) -> Prof {
        let curve = if linear { Curve::Linear } else { Curve::Space(cs.curve()) };
        Prof { xyz: cs.to_xyz(), curve }
    }
}

/// A linear-RGB matrix plus curves between two profiles.
#[derive(Clone, Copy, Debug)]
struct Conv {
    from: Curve,
    m: Option<[[f32; 3]; 3]>,
    to: Curve,
}

impl Conv {
    fn new(a: Prof, b: Prof) -> Conv {
        let m = (a.xyz != b.xyz).then(|| {
            let m = space::mul(&space::invert(&b.xyz), &a.xyz);
            m.map(|r| r.map(|v| v as f32))
        });
        Conv { from: a.curve, m, to: b.curve }
    }
    fn apply(&self, c: [f32; 3]) -> [f32; 3] {
        let l = c.map(|v| self.from.decode(v));
        let l = match &self.m {
            Some(m) => [0, 1, 2].map(|i| m[i][0] * l[0] + m[i][1] * l[1] + m[i][2] * l[2]),
            None => l,
        };
        l.map(|v| self.to.encode(v))
    }
}

const D65: [f64; 2] = [0.3127, 0.3290];

/// The viewer's colour conversion from the rendered frame (sRGB-encoded display pixels) to
/// what the monitor shows (View ▸ Use Display Color Management, View ▸ Simulate Output and
/// the display profile in Settings ▸ Previews). `None` when nothing changes.
#[derive(Clone, Copy, Debug)]
pub struct DisplayColor {
    sim: Option<(Conv, Option<Conv>)>,
    display: Option<Conv>,
}

impl DisplayColor {
    /// `working`: the project working space (None = unmanaged: nothing is converted, as in
    /// After Effects); `linear`: the working space is linear; `source`: the space rendered
    /// frames are in (the project's output space, sRGB by default); `dcm`: Use Display Color
    /// Management; `display`: the monitor's space.
    pub fn new(working: Option<ColorSpace>, linear: bool, source: ColorSpace, dcm: bool, display: ColorSpace, sim: Simulation) -> Option<DisplayColor> {
        let ws = working?;
        let src = Prof::of(source, false);
        if !dcm {
            // Without display colour management the working space's numbers go to the screen.
            let raw = Conv::new(src, Prof::of(ws, linear));
            return Some(DisplayColor { sim: None, display: Some(raw) });
        }
        let sim = sim.profile.space().map(|(prims, curve)| {
            let out = Prof::new(prims, curve);
            (
                Conv::new(src, out),
                Some(if sim.preserve_rgb {
                    Conv::new(Prof::new(P709, Curve::Srgb), Prof::of(display, false))
                } else {
                    Conv::new(out, Prof::of(display, false))
                }),
            )
        });
        let display = (display != source).then(|| Conv::new(src, Prof::of(display, false)));
        (sim.is_some() || display.is_some()).then_some(DisplayColor { sim, display })
    }

    /// The conversion a session's viewer uses.
    pub fn of(s: &crate::Session) -> Option<DisplayColor> {
        let st = &s.project.settings;
        let display = ColorSpace::parse(&s.prefs.previews.display_profile).unwrap_or(ColorSpace::Srgb);
        let v = &s.state.viewer;
        let linear = st.working_space.is_some_and(|w| st.linearize || w.is_linear());
        DisplayColor::new(st.working_space, linear, st.output_space.unwrap_or(ColorSpace::Srgb), v.display_color_management, display, v.simulation)
    }

    /// One straight colour (0..1 floats).
    pub fn apply_straight(&self, c: [f32; 3]) -> [f32; 3] {
        let c = match &self.sim {
            // Through the output's 8-bit encoding (clipped and quantised like the real output),
            // then shown on the display: converted from the output profile, or — Preserve RGB —
            // the output numbers taken as if they were sRGB.
            Some((to, show)) => {
                let o = to.apply(c).map(|v| (v.clamp(0.0, 1.0) * 255.0).round() / 255.0);
                show.map_or(o, |b| b.apply(o))
            }
            None => self.display.map_or(c, |d| d.apply(c)),
        };
        c.map(|v| v.clamp(0.0, 1.0))
    }

    /// Premultiplied 8-bit RGBA pixels in place.
    pub fn apply(&self, px: &mut [[u8; 4]]) {
        use rayon::prelude::*;
        px.par_iter_mut().for_each(|p| {
            let a = p[3];
            if a == 0 {
                return;
            }
            let af = a as f32 / 255.0;
            let c = [0, 1, 2].map(|i| (p[i] as f32 / 255.0 / af).min(1.0));
            let o = self.apply_straight(c);
            for i in 0..3 {
                p[i] = (o[i] * af * 255.0 + 0.5) as u8;
            }
        });
    }
}

// ---------------------------------------------------------------- snapshot

/// Take Snapshot (Shift+F5): a rendered frame kept for comparison (Show Snapshot, F5).
#[derive(Clone, Debug)]
pub struct Snapshot {
    pub comp: effectcraft_project::ItemId,
    pub time: Tick,
    /// Render scale the image was taken at.
    pub scale: f64,
    pub image: Arc<Image>,
}