Render: compound outlines (double, thick-thin, thin-thick, triple)

a:ln/@cmpd was read but every outline was drawn as one band. Each band is
now stroked along the path moved to its place across the width, in the
shares PowerPoint uses (measured on its export of 24 pt lines: double
1/3-1/3-1/3, thin-thick 1/5-1/5-3/5, triple 1/6-1/6-1/3-1/6-1/6), the
first band on the left of travel (the outside of a shape). Line corners
meet at their miter points; curves are offset with kurbo.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/render/src/compound.rs

@@ -0,0 +1,242 @@
+//! Compound outlines (`a:ln/@cmpd`): double, thick-thin, thin-thick and triple lines.
+//!
+//! PowerPoint splits the line's width into bands across the path, measured from its left side
+//! (looking along the path) to its right, as these shares of the width (read from PowerPoint's
+//! export of 24 pt lines of each kind):
+//! - double: 1/3 line, 1/3 gap, 1/3 line;
+//! - thin-thick: 1/5 line, 1/5 gap, 3/5 line; thick-thin the mirror;
+//! - triple: 1/6 line, 1/6 gap, 1/3 line, 1/6 gap, 1/6 line.
+//!
+//! On a shape drawn clockwise (all presets) the left side is the outside.
+
+use deckcraft_model::style::Compound;
+use kurbo::{BezPath, CubicBez, ParamCurve, PathEl, PathSeg, Point, Vec2};
+
+/// The bands of `c` as (from, to) shares of the width, left side first.
+pub fn bands(c: Compound) -> &'static [(f64, f64)] {
+    match c {
+        Compound::Single => &[(0.0, 1.0)],
+        Compound::Double => &[(0.0, 1.0 / 3.0), (2.0 / 3.0, 1.0)],
+        Compound::ThinThick => &[(0.0, 0.2), (0.4, 1.0)],
+        Compound::ThickThin => &[(0.0, 0.6), (0.8, 1.0)],
+        Compound::Triple => &[(0.0, 1.0 / 6.0), (2.0 / 6.0, 4.0 / 6.0), (5.0 / 6.0, 1.0)],
+    }
+}
+
+/// Unit normal pointing to the right of travel (y down), or None for a zero-length tangent.
+fn right(t: Vec2) -> Option<Vec2> {
+    let l = t.hypot();
+    (l > 1e-12 && l.is_finite()).then(|| Vec2::new(-t.y, t.x) / l)
+}
+
+/// `path` moved `d` to the right of travel (negative: to the left). Line corners meet at their
+/// miter point (bevelled when sharper than about 11°); curves are offset by kurbo; other joins are
+/// bridged by a straight line.
+pub fn offset(path: &BezPath, d: f64, tolerance: f64) -> BezPath {
+    let mut out = BezPath::new();
+    if d.abs() < 1e-9 {
+        return path.clone();
+    }
+    // Each subpath as segments, offset one by one.
+    let mut segs: Vec<PathSeg> = Vec::new();
+    let mut closed = false;
+    let flush = |segs: &mut Vec<PathSeg>, closed: bool, out: &mut BezPath| {
+        let pieces: Vec<(PathSeg, BezPath)> = segs.iter().filter_map(|s| offset_seg(*s, d, tolerance).map(|p| (*s, p))).collect();
+        let begin = out.elements().len();
+        for (i, (seg, piece)) in pieces.iter().enumerate() {
+            let Some(PathEl::MoveTo(p)) = piece.elements().first().copied() else { continue };
+            match i.checked_sub(1).and_then(|j| pieces.get(j)) {
+                None => out.move_to(p),
+                Some((prev, _)) => join(out, *prev, *seg, p, d),
+            }
+            for e in piece.elements().iter().skip(1) {
+                out.push(*e);
+            }
+        }
+        if closed && let (Some((last, _)), Some((first, piece))) = (pieces.last(), pieces.first()) {
+            if let Some(PathEl::MoveTo(p)) = piece.elements().first().copied() {
+                // Lines meeting at the start: both ends move to their miter point.
+                match miter(*last, *first, d) {
+                    Some(m) if is_line(*last) && is_line(*first) => {
+                        set_last(out, m);
+                        if let Some(PathEl::MoveTo(_)) = out.elements().get(begin) {
+                            out.elements_mut()[begin] = PathEl::MoveTo(m);
+                        }
+                    }
+                    _ => out.line_to(p),
+                }
+            }
+            out.close_path();
+        }
+        segs.clear();
+    };
+    let mut start = Point::ZERO;
+    let mut cur = Point::ZERO;
+    for el in path.elements() {
+        match *el {
+            PathEl::MoveTo(p) => {
+                flush(&mut segs, closed, &mut out);
+                closed = false;
+                start = p;
+                cur = p;
+            }
+            PathEl::LineTo(p) => {
+                segs.push(PathSeg::Line(kurbo::Line::new(cur, p)));
+                cur = p;
+            }
+            PathEl::QuadTo(a, b) => {
+                segs.push(PathSeg::Quad(kurbo::QuadBez::new(cur, a, b)));
+                cur = b;
+            }
+            PathEl::CurveTo(a, b, c) => {
+                segs.push(PathSeg::Cubic(CubicBez::new(cur, a, b, c)));
+                cur = c;
+            }
+            PathEl::ClosePath => {
+                if (cur - start).hypot() > 1e-9 {
+                    segs.push(PathSeg::Line(kurbo::Line::new(cur, start)));
+                }
+                closed = true;
+                flush(&mut segs, closed, &mut out);
+                closed = false;
+                cur = start;
+            }
+        }
+    }
+    flush(&mut segs, closed, &mut out);
+    out
+}
+
+fn offset_seg(s: PathSeg, d: f64, tolerance: f64) -> Option<BezPath> {
+    // Far-off or non-finite geometry (hostile files) is left out rather than offset.
+    let sane = |p: Point| p.is_finite() && p.x.abs() < 1e7 && p.y.abs() < 1e7;
+    let ok = match s {
+        PathSeg::Line(l) => sane(l.p0) && sane(l.p1),
+        PathSeg::Quad(q) => sane(q.p0) && sane(q.p1) && sane(q.p2),
+        PathSeg::Cubic(c) => sane(c.p0) && sane(c.p1) && sane(c.p2) && sane(c.p3),
+    };
+    if !ok || !d.is_finite() {
+        return None;
+    }
+    match s {
+        PathSeg::Line(l) => {
+            let n = right(l.p1 - l.p0)?;
+            let mut p = BezPath::new();
+            p.move_to(l.p0 + n * d);
+            p.line_to(l.p1 + n * d);
+            Some(p)
+        }
+        PathSeg::Quad(q) => offset_seg(PathSeg::Cubic(q.raise()), d, tolerance),
+        PathSeg::Cubic(c) => {
+            // Nearly straight cubics (collinear control points) are offset as lines.
+            let chord = c.p3 - c.p0;
+            let flat = |p: Point| chord.cross(p - c.p0).abs() <= 1e-9 * chord.hypot2().max(1e-18);
+            if flat(c.p1) && flat(c.p2) {
+                return offset_seg(PathSeg::Line(kurbo::Line::new(c.p0, c.p3)), d, tolerance);
+            }
+            let mut p = BezPath::new();
+            kurbo::offset::offset_cubic(c, d, tolerance, &mut p);
+            Some(p)
+        }
+    }
+}
+
+/// The segment's tangent at its start and at its end (the first non-zero control vector).
+fn tangents(s: PathSeg) -> (Vec2, Vec2) {
+    let first = |v: &[Vec2]| v.iter().copied().find(|v| v.hypot2() > 1e-18).unwrap_or(Vec2::ZERO);
+    match s {
+        PathSeg::Line(l) => (l.p1 - l.p0, l.p1 - l.p0),
+        PathSeg::Quad(q) => (first(&[q.p1 - q.p0, q.p2 - q.p0]), first(&[q.p2 - q.p1, q.p2 - q.p0])),
+        PathSeg::Cubic(c) => (first(&[c.p1 - c.p0, c.p2 - c.p0, c.p3 - c.p0]), first(&[c.p3 - c.p2, c.p3 - c.p1, c.p3 - c.p0])),
+    }
+}
+
+fn is_line(s: PathSeg) -> bool {
+    matches!(s, PathSeg::Line(_))
+}
+
+/// Move the path's last point to `p`.
+fn set_last(out: &mut BezPath, p: Point) {
+    if let Some(e) = out.elements_mut().last_mut() {
+        match e {
+            PathEl::MoveTo(q) | PathEl::LineTo(q) => *q = p,
+            PathEl::QuadTo(_, q) => *q = p,
+            PathEl::CurveTo(_, _, q) => *q = p,
+            PathEl::ClosePath => {}
+        }
+    }
+}
+
+/// Where the offsets of `prev` and `next` meet at their shared corner (their miter point), or
+/// None for a corner sharper than about 11 degrees or a degenerate tangent.
+fn miter(prev: PathSeg, next: PathSeg, d: f64) -> Option<Point> {
+    let (_, t0) = tangents(prev);
+    let (t1, _) = tangents(next);
+    let (n0, n1) = (right(t0)?, right(t1)?);
+    let denom = 1.0 + n0.dot(n1);
+    (denom > 0.02).then(|| prev.end() + (n0 + n1) * (d / denom)).filter(|m| m.is_finite())
+}
+
+/// Join the offset of `prev` (ending at `out`'s current point) to the offset of `next`, which
+/// starts at `p`: two lines meet at their miter point (both ends moved there, so an inner corner
+/// doesn't loop); anything else is bridged straight.
+fn join(out: &mut BezPath, prev: PathSeg, next: PathSeg, p: Point, d: f64) {
+    match miter(prev, next, d) {
+        Some(m) if is_line(prev) && is_line(next) => set_last(out, m),
+        Some(m) => {
+            out.line_to(m);
+            out.line_to(p);
+        }
+        None => out.line_to(p),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use kurbo::Shape;
+
+    #[test]
+    fn bands_split_the_width_like_powerpoint() {
+        for c in [Compound::Single, Compound::Double, Compound::ThinThick, Compound::ThickThin, Compound::Triple] {
+            let b = bands(c);
+            assert_eq!(b.first().map(|x| x.0), Some(0.0));
+            assert_eq!(b.last().map(|x| x.1), Some(1.0));
+            assert!(b.windows(2).all(|w| w[0].1 < w[1].0), "{c:?}: bands in order with gaps");
+        }
+        // Thin-thick: the thin line on the left, thick-thin its mirror.
+        let (tt, th) = (bands(Compound::ThinThick), bands(Compound::ThickThin));
+        assert!((tt[0].1 - tt[0].0) < (tt[1].1 - tt[1].0));
+        assert!((th[0].1 - th[0].0) > (th[1].1 - th[1].0));
+    }
+
+    #[test]
+    fn offsetting_a_rectangle_grows_or_shrinks_it() {
+        // Clockwise in y-down coordinates: the left of travel is the outside.
+        let r = kurbo::Rect::new(0.0, 0.0, 100.0, 50.0).to_path(0.1);
+        let out = offset(&r, -5.0, 0.01).bounding_box();
+        assert!(
+            (out.x0 + 5.0).abs() < 1e-6 && (out.y0 + 5.0).abs() < 1e-6 && (out.x1 - 105.0).abs() < 1e-6 && (out.y1 - 55.0).abs() < 1e-6,
+            "{out:?}"
+        );
+        let inn = offset(&r, 5.0, 0.01).bounding_box();
+        assert!((inn.x0 - 5.0).abs() < 1e-6 && (inn.y1 - 45.0).abs() < 1e-6, "{inn:?}");
+        // Corners meet at their miter points: the area is the offset rectangle's.
+        assert!((offset(&r, -5.0, 0.01).area().abs() - 110.0 * 60.0).abs() < 1e-6);
+    }
+
+    #[test]
+    fn offsetting_a_circle_changes_its_radius() {
+        let c = kurbo::Circle::new((50.0, 50.0), 20.0).to_path(0.01);
+        let b = offset(&c, 4.0, 0.01).bounding_box();
+        let w = b.width() / 2.0;
+        assert!((w - 16.0).abs() < 0.2 || (w - 24.0).abs() < 0.2, "radius {w}");
+        // A hostile path never panics and yields finite output.
+        let mut p = BezPath::new();
+        p.move_to((0.0, 0.0));
+        p.line_to((0.0, 0.0));
+        p.curve_to((f64::MAX, 0.0), (0.0, f64::MIN_POSITIVE), (1.0, 1.0));
+        p.close_path();
+        let _ = offset(&p, 3.0, 0.01);
+    }
+}

crates/render/src/lib.rs

@@ -9,6 +9,8 @@
 
 mod chart;
 mod colr;
+
+mod compound;
 mod images;
 mod morph_path;
 mod morph_text;
@@ -941,6 +943,19 @@ fn stroke(ctx: &mut RenderContext, rctx: &Ctx, line: &Line, ph: Option<Rgba>, pa
         }
         _ => ctx.set_paint(color(Rgba::BLACK, alpha)),
     }
+    // Compound lines: each band stroked along the path moved to its place across the width.
+    let compound = line.compound.unwrap_or_default();
+    if compound != deckcraft_model::style::Compound::Single && lw * scale_of(m) >= 1.5 {
+        let tolerance = 0.1 / scale_of(m).max(1e-6);
+        for &(a, b) in compound::bands(compound) {
+            let d = (-0.5 + (a + b) / 2.0) * lw;
+            let mut band = sk.clone();
+            band.width = (b - a) * lw;
+            ctx.set_stroke(band);
+            ctx.stroke_path(&compound::offset(path, d, tolerance));
+        }
+        return;
+    }
     ctx.set_stroke(sk);
     ctx.stroke_path(path);
 }

crates/render/src/tests.rs

@@ -776,3 +776,47 @@ fn pattern_fills_draw_powerpoints_tiles() {
         assert_eq!(dark(x, y), dark(x + 1, y + 1), "pct50 repeats diagonally at {x},{y}");
     }
 }
+
+#[test]
+fn compound_lines_draw_their_bands() {
+    // A 24 pt outline on a rectangle, as PowerPoint's export draws each kind (top edge, outside
+    // first): double 8/8/8, thin-thick 4.8/4.8/14.4, triple 4/4/8/4/4.
+    use deckcraft_model::style::{Compound, Fill, Line};
+    let runs = |c: Compound| {
+        let sh = Shape {
+            id: ShapeId(760),
+            xfrm: Some(Xfrm::new(100.0, 100.0, 300.0, 200.0)),
+            fill: Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(255, 255, 255)))),
+            line: Some(Line {
+                fill: Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(0, 0, 0)))),
+                width: Some(24.0),
+                compound: Some(c),
+                ..Default::default()
+            }),
+            ..Default::default()
+        };
+        let p = deck_with(vec![sh]);
+        let img = render_slide(&p, 0, &RenderOpts { scale: 2.0, ..Default::default() });
+        // Down the middle of the top edge (y 100 pt), from 80 pt to 120 pt: lengths of dark and
+        // light runs in points.
+        let mut out: Vec<(bool, f64)> = vec![];
+        for y in 160..240 {
+            let dark = img.pixel(500, y)[0] < 128;
+            match out.last_mut() {
+                Some((d, n)) if *d == dark => *n += 0.5,
+                _ => out.push((dark, 0.5)),
+            }
+        }
+        out.into_iter().filter(|(d, _)| *d).map(|(_, n)| n).collect::<Vec<f64>>()
+    };
+    let close = |got: Vec<f64>, want: &[f64]| got.len() == want.len() && got.iter().zip(want).all(|(g, w)| (g - w).abs() <= 1.0);
+    assert!(close(runs(Compound::Single), &[24.0]));
+    let d = runs(Compound::Double);
+    assert!(close(d.clone(), &[8.0, 8.0]), "{d:?}");
+    let t = runs(Compound::ThinThick);
+    assert!(close(t.clone(), &[4.8, 14.4]), "{t:?}");
+    let k = runs(Compound::ThickThin);
+    assert!(close(k.clone(), &[14.4, 4.8]), "{k:?}");
+    let tri = runs(Compound::Triple);
+    assert!(close(tri.clone(), &[4.0, 8.0, 4.0]), "{tri:?}");
+}