Render: rectangle and outline path gradients

Path gradients that follow a rectangle (path="rect") or the shape's own
outline (path="shape") were drawn as circles. They're now rasterised into
a small image of the bounds: rect takes the larger of the horizontal and
vertical shares of the distance from the focus to the edge (X-shaped
contours), shape the share of the way from the focus to the outline along
the same ray (an ellipse gives ellipses, a triangle triangles), as
measured on PowerPoint's export. The two path probe decks now match it to
a mean pixel difference of 1.7 and 0.6 (from 15.6 and 22).

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

crates/render/src/lib.rs

@@ -16,6 +16,8 @@ mod morph_path;
 mod morph_text;
 mod paint;
 mod pattern;
+
+mod pathgrad;
 mod placed;
 mod ramp;
 mod table;

crates/render/src/paint.rs

@@ -37,6 +37,41 @@ pub fn fill_path(
             }
             let stops: Vec<(f64, Rgba)> = g.stops.iter().map(|s| (s.pos, shade(rctx.color(&s.color, ph), mode))).collect();
             ctx.set_paint(gradient(&g.shape, stops, bounds, alpha));
+
+            let mut stops: Vec<(f64, Rgba)> = g.stops.iter().map(|s| (s.pos.clamp(0.0, 1.0), shade(rctx.color(&s.color, ph), mode))).collect();
+            stops.sort_by(|a, b| a.0.total_cmp(&b.0));
+            // Rectangle and outline paths: rasterised (see `pathgrad`).
+            if let GradientShape::Path { path: kind, focus } = &g.shape
+                && matches!(kind.as_str(), "rect" | "shape")
+            {
+                crate::pathgrad::fill(ctx, kind, *focus, &crate::ramp::expand(&stops), bounds, path, m, alpha);
+                return;
+            }
+            // PowerPoint's ramp between stops (linear light, S-shaped), see `ramp`.
+            let stops: Vec<peniko::ColorStop> =
+                crate::ramp::expand(&stops).into_iter().map(|(o, c)| peniko::ColorStop { offset: o as f32, color: color(c, alpha).into() }).collect();
+            let grad = match &g.shape {
+                GradientShape::Linear { angle, .. } => {
+                    // Angle measured clockwise from left→right; span the bounds along that direction.
+                    let a = angle.to_radians();
+                    let (dx, dy) = (a.cos(), a.sin());
+                    let c = bounds.center();
+                    let half = (bounds.width() * dx.abs() + bounds.height() * dy.abs()) / 2.0;
+                    peniko::Gradient::new_linear(Point::new(c.x - dx * half, c.y - dy * half), Point::new(c.x + dx * half, c.y + dy * half))
+                        .with_stops(stops.as_slice())
+                }
+                GradientShape::Path { focus, .. } => {
+                    let fx = bounds.x0 + bounds.width() * (focus[0] + (1.0 - focus[2])) / 2.0;
+                    let fy = bounds.y0 + bounds.height() * (focus[1] + (1.0 - focus[3])) / 2.0;
+                    // Reach the farthest corner, so a gradient from a corner covers the whole box.
+                    let r = [(bounds.x0, bounds.y0), (bounds.x1, bounds.y0), (bounds.x0, bounds.y1), (bounds.x1, bounds.y1)]
+                        .iter()
+                        .map(|(x, y)| ((x - fx).powi(2) + (y - fy).powi(2)).sqrt())
+                        .fold(1e-6, f64::max);
+                    peniko::Gradient::new_radial(Point::new(fx, fy), r as f32).with_stops(stops.as_slice())
+                }
+            };
+            ctx.set_paint(grad);
             ctx.fill_path(path);
         }
         Fill::Picture(pf) => {

crates/render/src/pathgrad.rs

@@ -0,0 +1,220 @@
+//! Path gradients (`a:path`) that follow a rectangle or the shape's own outline, which no vello
+//! gradient draws: they're rasterised into a small image of the bounds and painted with it.
+//!
+//! Measured on PowerPoint's export (black to grey to white from the centre):
+//! - `rect`: the colour position is the larger of the horizontal and vertical distances from the
+//!   focus, each as a share of the distance to that side's edge (X-shaped contours);
+//! - `shape`: contours are the outline scaled about the focus (an ellipse gives ellipses, a
+//!   triangle triangles): the share of the way from the focus to the outline along the same ray.
+//!
+//! `circle` stays a radial vello gradient (see `paint::gradient`).
+
+use std::sync::Arc;
+
+use deckcraft_color::Rgba;
+use kurbo::{Affine, BezPath, PathEl, Point, Rect};
+use vello_cpu::{Pixmap, RenderContext, peniko};
+
+/// Longest side of the rasterised gradient, in pixels (bilinear sampling smooths the rest).
+const MAX_SIDE: f64 = 384.0;
+
+/// Colour position (0 at the focus, 1 at the edge) of `p` for path `kind` in `bounds` with the
+/// focus rect `focus` (fractions inset from left, top, right, bottom), `outline` being the
+/// shape's flattened polygon(s) for `shape`.
+pub fn position(kind: &str, bounds: Rect, focus: [f64; 4], outline: &[Vec<Point>], p: Point) -> f64 {
+    let f = Rect::new(
+        bounds.x0 + bounds.width() * focus[0].clamp(0.0, 1.0),
+        bounds.y0 + bounds.height() * focus[1].clamp(0.0, 1.0),
+        bounds.x1 - bounds.width() * focus[2].clamp(0.0, 1.0),
+        bounds.y1 - bounds.height() * focus[3].clamp(0.0, 1.0),
+    );
+    let f = if f.x0 <= f.x1 && f.y0 <= f.y1 { f } else { Rect::from_center_size(f.center(), (0.0, 0.0)) };
+    match kind {
+        "shape" if !outline.is_empty() => shape_position(outline, f.center(), p),
+        _ => {
+            // Distance outside the focus rect along each axis, over the room to that side's edge.
+            let axis = |v: f64, lo: f64, hi: f64, edge_lo: f64, edge_hi: f64| {
+                if v < lo {
+                    (lo - v) / (lo - edge_lo).max(1e-9)
+                } else if v > hi {
+                    (v - hi) / (edge_hi - hi).max(1e-9)
+                } else {
+                    0.0
+                }
+            };
+            axis(p.x, f.x0, f.x1, bounds.x0, bounds.x1).max(axis(p.y, f.y0, f.y1, bounds.y0, bounds.y1)).clamp(0.0, 1.0)
+        }
+    }
+}
+
+/// The share of the way from `c` to the outline along the ray through `p` (1 when the ray meets
+/// no edge).
+fn shape_position(outline: &[Vec<Point>], c: Point, p: Point) -> f64 {
+    let d = p - c;
+    let len = d.hypot();
+    if len < 1e-9 {
+        return 0.0;
+    }
+    // Nearest crossing of the ray c + s·d (s > 0) with an edge.
+    let mut best = f64::INFINITY;
+    for poly in outline {
+        for w in poly.windows(2) {
+            let (a, b) = (w[0], w[1]);
+            let e = b - a;
+            let den = d.cross(e);
+            if den.abs() < 1e-12 {
+                continue;
+            }
+            let ac = a - c;
+            let s = ac.cross(e) / den;
+            let u = ac.cross(d) / den;
+            if s > 1e-9 && (-1e-9..=1.0 + 1e-9).contains(&u) {
+                best = best.min(s);
+            }
+        }
+    }
+    if best.is_finite() { (1.0 / best).clamp(0.0, 1.0) } else { 1.0 }
+}
+
+/// The outline flattened into closed polygons.
+pub fn polygons(path: &BezPath) -> Vec<Vec<Point>> {
+    let mut out: Vec<Vec<Point>> = Vec::new();
+    let mut cur: Vec<Point> = Vec::new();
+    kurbo::flatten(path.iter(), 0.25, |el| match el {
+        PathEl::MoveTo(p) => {
+            if cur.len() > 1 {
+                out.push(std::mem::take(&mut cur));
+            }
+            cur = vec![p];
+        }
+        PathEl::LineTo(p) => cur.push(p),
+        PathEl::ClosePath => {
+            if let Some(f) = cur.first().copied() {
+                cur.push(f);
+            }
+        }
+        _ => {}
+    });
+    if cur.len() > 1 {
+        out.push(cur);
+    }
+    for poly in &mut out {
+        if let (Some(f), Some(l)) = (poly.first().copied(), poly.last().copied())
+            && (f - l).hypot() > 1e-9
+        {
+            poly.push(f);
+        }
+    }
+    out.retain(|p| p.len() > 2 && p.len() < 100_000);
+    out
+}
+
+/// A 256-entry colour ramp for the sorted, already expanded `stops` (linear in sRGB between them).
+pub fn ramp(stops: &[(f64, Rgba)]) -> Vec<Rgba> {
+    (0..256)
+        .map(|i| {
+            let t = i as f64 / 255.0;
+            let after = stops.iter().position(|s| s.0 >= t);
+            match after {
+                Some(0) | None => stops.get(after.unwrap_or(stops.len().saturating_sub(1))).map(|s| s.1).unwrap_or(Rgba::BLACK),
+                Some(j) => {
+                    let (Some(a), Some(b)) = (stops.get(j - 1), stops.get(j)) else { return Rgba::BLACK };
+                    let k = if b.0 - a.0 > 1e-12 { (t - a.0) / (b.0 - a.0) } else { 1.0 };
+                    a.1.lerp(b.1, k)
+                }
+            }
+        })
+        .collect()
+}
+
+/// Fill `path` with a `rect` or `shape` path gradient of `stops` (sorted and expanded, see
+/// `ramp::expand`) over `bounds`, faded by `alpha`; `m` maps the path to the output.
+#[allow(clippy::too_many_arguments)]
+pub fn fill(ctx: &mut RenderContext, kind: &str, focus: [f64; 4], stops: &[(f64, Rgba)], bounds: Rect, path: &BezPath, m: Affine, alpha: f64) {
+    if bounds.width() <= 0.0 || bounds.height() <= 0.0 || !bounds.is_finite() || stops.is_empty() {
+        return;
+    }
+    let k = crate::scale_of(m);
+    let k = if k.is_finite() && k > 0.0 { k } else { 1.0 };
+    let longest = bounds.width().max(bounds.height()) * k;
+    let s = (MAX_SIDE / longest).min(1.0) * k;
+    let (pw, ph) = (((bounds.width() * s).ceil() as u32).clamp(1, 4096), ((bounds.height() * s).ceil() as u32).clamp(1, 4096));
+    let outline = if kind == "shape" { polygons(path) } else { Vec::new() };
+    let lut = ramp(stops);
+    let a = alpha.clamp(0.0, 1.0);
+    let mut data = Vec::with_capacity((pw * ph) as usize);
+    for j in 0..ph {
+        for i in 0..pw {
+            let p = Point::new(bounds.x0 + (i as f64 + 0.5) / pw as f64 * bounds.width(), bounds.y0 + (j as f64 + 0.5) / ph as f64 * bounds.height());
+            let t = position(kind, bounds, focus, &outline, p);
+            let c = lut.get((t * 255.0).round().clamp(0.0, 255.0) as usize).copied().unwrap_or(Rgba::BLACK);
+            let ca = (c.a as f64 * a).round().clamp(0.0, 255.0);
+            let mul = |v: u8| (v as f64 * ca / 255.0).round().clamp(0.0, 255.0) as u8;
+            data.push(vello_cpu::color::PremulRgba8 { r: mul(c.r), g: mul(c.g), b: mul(c.b), a: ca as u8 });
+        }
+    }
+    let pm = Arc::new(Pixmap::from_parts(data, pw as u16, ph as u16));
+    ctx.set_transform(m);
+    ctx.set_paint(vello_cpu::Image {
+        image: vello_cpu::ImageSource::Pixmap(pm),
+        sampler: peniko::ImageSampler { quality: peniko::ImageQuality::Medium, ..Default::default() },
+    });
+    ctx.set_paint_transform(
+        Affine::translate(bounds.origin().to_vec2()) * Affine::scale_non_uniform(bounds.width() / pw as f64, bounds.height() / ph as f64),
+    );
+    ctx.fill_path(path);
+    ctx.reset_paint_transform();
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use kurbo::Shape;
+
+    const CENTRE: [f64; 4] = [0.5, 0.5, 0.5, 0.5];
+
+    #[test]
+    fn rect_paths_take_the_larger_axis_share() {
+        // PowerPoint, 280x200 box from the centre: 20 px across is 1/7, 20 px down is 1/5.
+        let b = Rect::new(0.0, 0.0, 280.0, 200.0);
+        let at = |x: f64, y: f64| position("rect", b, CENTRE, &[], Point::new(x, y));
+        assert!((at(160.0, 100.0) - 20.0 / 140.0).abs() < 1e-9);
+        assert!((at(140.0, 120.0) - 20.0 / 100.0).abs() < 1e-9);
+        assert!((at(168.0, 120.0) - 0.2).abs() < 1e-9, "the diagonal takes the larger share");
+        assert_eq!(at(0.0, 0.0), 1.0);
+        // From a corner (fillToRect 0,0,100%,100%): the far corner is the end.
+        let corner = [0.0, 0.0, 1.0, 1.0];
+        assert!((position("rect", b, corner, &[], Point::new(140.0, 50.0)) - 0.5).abs() < 1e-9);
+    }
+
+    #[test]
+    fn shape_paths_follow_the_outline() {
+        let b = Rect::new(0.0, 0.0, 280.0, 200.0);
+        let ellipse = polygons(&kurbo::Ellipse::from_rect(b).to_path(0.01));
+        let at = |x: f64, y: f64| position("shape", b, CENTRE, &ellipse, Point::new(x, y));
+        assert!((at(160.0, 100.0) - 20.0 / 140.0).abs() < 0.01);
+        assert!((at(140.0, 120.0) - 0.2).abs() < 0.01);
+        // Half way to the outline along any ray is 0.5.
+        let (dx, dy) = (140.0 * 0.6, 100.0 * 0.8);
+        assert!((at(140.0 + dx * 0.5, 100.0 + dy * 0.5) - 0.5).abs() < 0.01);
+        // A triangle: half way from its centre to its apex is 0.5.
+        let mut tri = BezPath::new();
+        tri.move_to((140.0, 0.0));
+        tri.line_to((280.0, 200.0));
+        tri.line_to((0.0, 200.0));
+        tri.close_path();
+        let polys = polygons(&tri);
+        assert!((position("shape", b, CENTRE, &polys, Point::new(140.0, 50.0)) - 0.5).abs() < 1e-6);
+    }
+
+    #[test]
+    fn the_ramp_runs_through_the_stops() {
+        let stops = [(0.0, Rgba::rgb(0, 0, 0)), (0.5, Rgba::rgb(128, 128, 128)), (1.0, Rgba::rgb(255, 255, 255))];
+        let r = ramp(&stops);
+        assert_eq!(r.len(), 256);
+        assert_eq!(r[0], Rgba::rgb(0, 0, 0));
+        assert_eq!(r[255], Rgba::rgb(255, 255, 255));
+        assert!((r[64].r as i32 - 64).abs() <= 2);
+        assert!(ramp(&[(0.3, Rgba::rgb(9, 9, 9))]).iter().all(|c| *c == Rgba::rgb(9, 9, 9)));
+    }
+}

crates/render/src/tests.rs

@@ -820,3 +820,29 @@ fn compound_lines_draw_their_bands() {
     let tri = runs(Compound::Triple);
     assert!(close(tri.clone(), &[4.0, 8.0, 4.0]), "{tri:?}");
 }
+
+#[test]
+fn rect_path_gradients_draw_x_shaped_contours() {
+    // path="rect" from the centre, black to grey to white (linear): PowerPoint's colour position is
+    // the larger axis share, so 20 pt right of centre in a 280x200 box is 1/7 and 20 pt down 1/5.
+    use deckcraft_model::style::{Fill, Gradient, GradientShape, GradientStop};
+    let stop = |pos, v| GradientStop { pos, color: ColorRef::rgb(deckcraft_color::Rgba::rgb(v, v, v)) };
+    let sh = Shape {
+        id: ShapeId(770),
+        xfrm: Some(Xfrm::new(0.0, 0.0, 280.0, 200.0)),
+        fill: Some(Fill::Gradient(Gradient {
+            stops: vec![stop(0.0, 0), stop(0.5, 128), stop(1.0, 255)],
+            shape: GradientShape::Path { path: "rect".into(), focus: [0.5, 0.5, 0.5, 0.5] },
+            rotate_with_shape: true,
+        })),
+        line: Some(deckcraft_model::style::Line { fill: Some(Fill::None), ..Default::default() }),
+        ..Default::default()
+    };
+    let p = deck_with(vec![sh]);
+    let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+    let v = |x: u32, y: u32| img.pixel(x, y)[0] as f64;
+    assert!((v(160, 100) - 255.0 / 7.0).abs() < 6.0, "{}", v(160, 100));
+    assert!((v(140, 120) - 51.0).abs() < 6.0, "{}", v(140, 120));
+    // Along the diagonal to a corner the colour follows the vertical share.
+    assert!((v(168, 120) - 51.0).abs() < 6.0, "{}", v(168, 120));
+}