Add mosaic spline fitter; fix stacked holes & relative writer; add test suite

Feature — mosaic spline segment fitter (crates/vtracer/src/mosaic/fit.rs):
open-path cubic fitting for boundary segments, reusing the now-public
visioncortex primitives (PathSimplify::limit_penalties for symmetric,
gap-free staircase removal; open-path SubdivideSmooth::{find_corners,
subdivide_keep_corners,find_splice_points}; fit_points_with_bezier per splice
slice). Matches stacked spline curve quality; endpoints pinned to lattice
nodes so shared boundaries stay seam-free.

Fix — stacked mode punched holes in cluster masks (to_image_with_hole .. true);
stacked must trace solid layers and occlude by paint-order overdraw (false).
Holes left the layer below exposed as hairline seams.

Fix — the relative SVG writer measured a subpath's opening `m` from the last
vertex instead of the subpath start (SVG resets the current point to the start
after Z), misplacing holes / extra subpaths at optimize=1/2.

Tests — new tests/equivalence.rs: stacked-vs-mosaic interior agreement (all
fitters) and a seam guard (a full-coverage image must render fully opaque).
svg round-trip test (absolute vs relative encode identical geometry). mosaic
spline endpoint-pinning test. Regenerated goldens; added disc_mosaic_spline.
resvg added as a dev-dependency (test-only; not compiled for wasm).

Drop unused MosaicOptions placeholder

The strict/seam-stroke mitigations aren't needed — the mosaic geometry is
already gapless and seam-free. Remove the no-op MosaicOptions struct and thread
it out of Compositing::Mosaic and compose_mosaic.

crates/vtracer/Cargo.toml

@@ -16,3 +16,8 @@ path = "src/lib.rs"
 
 [dependencies]
 visioncortex.workspace = true
+
+[dev-dependencies]
+# Rasterize-and-diff equivalence tests (stacked vs mosaic). Test-only; not
+# compiled for wasm targets, so the library stays wasm-safe.
+resvg = "0.45"

crates/vtracer/src/compose/mod.rs

@@ -1,20 +1,20 @@
 //! Compositing: turn a [`Segmentation`] into a [`VectorDoc`].
 //!
-//! Only **stacked** composition is implemented: each layer is traced
-//! independently into closed outlines and stacked in paint order (painter's
-//! algorithm). The **mosaic** compositor — gapless tessellation with shared
-//! boundary geometry — is a separate milestone and not built yet.
+//! * **Stacked** — each layer is traced independently into closed outlines and
+//!   stacked in paint order (painter's algorithm).
+//! * **Mosaic** — a seam-free gapless tessellation with shared boundary
+//!   geometry (see [`crate::mosaic`]).
 
 use crate::fitter::CurveFitter;
 use crate::ir::{Segmentation, Shape, VectorDoc};
-use crate::mosaic::{compose_mosaic, MosaicOptions, SegmentFitter};
+use crate::mosaic::{compose_mosaic, SegmentFitter};
 
 /// Which compositing strategy the pipeline uses. Each variant owns its fitter.
 pub enum Compositing {
     /// Independent per-region closed outlines, stacked bottom-to-top.
     Stacked(Box<dyn CurveFitter>),
     /// Seam-free gapless tessellation via a shared boundary graph.
-    Mosaic(Box<dyn SegmentFitter>, MosaicOptions),
+    Mosaic(Box<dyn SegmentFitter>),
 }
 
 impl Compositing {
@@ -22,7 +22,7 @@ impl Compositing {
     pub fn compose(&self, seg: &Segmentation) -> VectorDoc {
         match self {
             Compositing::Stacked(fitter) => compose_stacked(seg, fitter.as_ref()),
-            Compositing::Mosaic(fitter, opts) => compose_mosaic(seg, fitter.as_ref(), opts),
+            Compositing::Mosaic(fitter) => compose_mosaic(seg, fitter.as_ref()),
         }
     }
 }

crates/vtracer/src/config.rs

@@ -9,7 +9,9 @@ use crate::compose::Compositing;
 use crate::error::Error;
 use crate::fitter::{CurveFitter, FitParams, PixelFitter, PolygonFitter, SplineFitter};
 use crate::frontend::{BinaryFrontend, ColorClusterFrontend, Frontend};
-use crate::mosaic::{MosaicOptions, PixelSegmentFitter, PolygonSegmentFitter, SegmentFitter};
+use crate::mosaic::{
+    PixelSegmentFitter, PolygonSegmentFitter, SegmentFitter, SplineSegmentFitter,
+};
 use crate::optimize::{OptimizerPass, QuantizePass, SimplifyPass};
 use crate::pipeline::Pipeline;
 use crate::svg::SvgWriter;
@@ -165,9 +167,13 @@ impl Config {
         match self.mode {
             FitMode::Pixel => Box::new(PixelSegmentFitter),
             FitMode::Polygon => Box::new(PolygonSegmentFitter::default()),
-            // The spline segment fitter is not implemented yet; mosaic falls
-            // back to the polygon (crack-midline) fitter for now.
-            FitMode::Spline => Box::new(PolygonSegmentFitter::default()),
+            FitMode::Spline => Box::new(SplineSegmentFitter {
+                corner_threshold: deg2rad(self.corner_threshold),
+                length_threshold: self.length_threshold,
+                max_iterations: self.max_iterations,
+                splice_threshold: deg2rad(self.splice_threshold),
+                ..SplineSegmentFitter::default()
+            }),
         }
     }
 
@@ -206,9 +212,7 @@ impl Config {
     pub fn build(&self) -> Result<Pipeline, Error> {
         let compositing = match self.hierarchical {
             Hierarchical::Stacked => Compositing::Stacked(self.fitter()),
-            Hierarchical::Cutout => {
-                Compositing::Mosaic(self.segment_fitter(), MosaicOptions::default())
-            }
+            Hierarchical::Cutout => Compositing::Mosaic(self.segment_fitter()),
         };
 
         Ok(Pipeline {

crates/vtracer/src/frontend/color_cluster.rs

@@ -73,7 +73,12 @@ impl Frontend for ColorClusterFrontend {
         // paint order for the layer stack.
         for &cluster_index in view.clusters_output.iter().rev() {
             let cluster = view.get_cluster(cluster_index);
-            let image = cluster.to_image_with_hole(view.width, true);
+            // Solid cluster masks (no holes punched): stacked mode relies on
+            // paint-order overdraw for occlusion, matching 0.6.x. Punching
+            // holes here would leave the layer below exposed as hairline seams.
+            // The mosaic flatten is unaffected — a higher layer still wins per
+            // pixel — so a solid parent gives the same partition.
+            let image = cluster.to_image_with_hole(view.width, false);
             let mask = RegionMask::new(
                 image,
                 PointI32 {

crates/vtracer/src/mosaic/compose.rs

@@ -12,14 +12,10 @@ use visioncortex::PointF64;
 use super::face::{assemble, Contour, Face};
 use super::fit::{FittedGeom, FittedSegment, SegmentFitter};
 use super::graph::BoundaryGraph;
-use super::{LabelMap, MosaicOptions, Segmentation};
+use super::{LabelMap, Segmentation};
 
 /// Run the full mosaic pipeline: flatten → boundary graph → faces → fit → compose.
-pub fn compose_mosaic(
-    seg: &Segmentation,
-    fitter: &dyn SegmentFitter,
-    _options: &MosaicOptions,
-) -> VectorDoc {
+pub fn compose_mosaic(seg: &Segmentation, fitter: &dyn SegmentFitter) -> VectorDoc {
     let map = LabelMap::from_segmentation(seg);
     let graph = BoundaryGraph::extract(&map);
     let faces = assemble(&graph, &map);

crates/vtracer/src/mosaic/fit.rs

@@ -4,10 +4,13 @@
 //! the same [`FittedSegment`], one traversed reversed. Reversal is exact, so
 //! the shared geometry is bitwise identical and no seam can appear.
 
-use visioncortex::{PointF64, PointI32};
+use visioncortex::{PathI32, PathSimplify, PointF64, PointI32, Spline, SubdivideSmooth};
 
 use super::graph::Segment;
 
+/// Outset ratio for the 4-point subdivision scheme (matches visioncortex).
+const OUTSET_RATIO: f64 = 8.0;
+
 /// Fitted geometry for one boundary segment.
 #[derive(Clone, Debug)]
 pub enum FittedGeom {
@@ -58,116 +61,204 @@ impl SegmentFitter for PixelSegmentFitter {
     }
 }
 
-/// Symmetric open Douglas–Peucker. Endpoints are always kept, so junction
-/// nodes stay pinned. Plain DP (no directional staircase removal) collapses
-/// 1-px staircases to the crack midline — centered between the two regions,
-/// which is what a mosaic wants.
-#[derive(Debug, Clone)]
-pub struct PolygonSegmentFitter {
-    pub tolerance: f64,
-}
+/// Straight-segment fitter. Uses visioncortex's symmetric `limit_penalties`
+/// simplification, which collapses 1px staircases toward the crack midline
+/// (centered, no directional outset) so the boundary stays gapless. Endpoints
+/// are preserved, pinning junction nodes.
+#[derive(Debug, Clone, Default)]
+pub struct PolygonSegmentFitter;
 
-impl Default for PolygonSegmentFitter {
-    fn default() -> Self {
-        Self { tolerance: 0.5 }
+impl PolygonSegmentFitter {
+    fn fit(&self, seg: &Segment) -> FittedSegment {
+        let simplified = PathSimplify::limit_penalties(&PathI32::from_points(seg.points.clone()));
+        FittedSegment {
+            geom: FittedGeom::Polyline(simplified.path.iter().copied().map(pt).collect()),
+        }
     }
 }
 
 impl SegmentFitter for PolygonSegmentFitter {
     fn fit_open(&self, seg: &Segment) -> FittedSegment {
-        let pts = to_f64(&seg.points);
+        self.fit(seg)
+    }
+    fn fit_ring(&self, seg: &Segment) -> FittedSegment {
+        self.fit(seg)
+    }
+}
+
+/// Smooth (cubic-Bézier) open-path fitter — the mosaic analogue of the stacked
+/// [`crate::fitter::SplineFitter`], but for open segments with pinned
+/// endpoints.
+///
+/// Staircase removal reuses visioncortex's symmetric `limit_penalties`
+/// simplification (the same de-noising stacked mode applies), which collapses
+/// staircases toward the crack midline. Unlike `remove_staircase`, it has no
+/// directional outset, so the boundary stays centered (≤√2/2 px from its
+/// crack) and cannot cross a non-adjacent segment — the tessellation stays
+/// gapless. A distance-based DP can't do this: near the √2/2 threshold it
+/// can't separate staircase noise from real curvature. Smoothing and per-slice
+/// cubic fitting then reuse the same visioncortex machinery stacked mode uses
+/// (open-path variants of the smoothing primitives + `fit_points_with_bezier`),
+/// so the curve character matches stacked.
+#[derive(Debug, Clone)]
+pub struct SplineSegmentFitter {
+    /// Corner angle threshold, radians.
+    pub corner_threshold: f64,
+    /// Subdivide until segments are shorter than this (px).
+    pub length_threshold: f64,
+    pub max_iterations: usize,
+    /// Splice angle threshold, radians.
+    pub splice_threshold: f64,
+}
+
+impl Default for SplineSegmentFitter {
+    fn default() -> Self {
+        Self {
+            corner_threshold: std::f64::consts::PI / 3.0,
+            length_threshold: 4.0,
+            max_iterations: 10,
+            splice_threshold: std::f64::consts::PI / 4.0,
+        }
+    }
+}
+
+fn pt(p: PointI32) -> PointF64 {
+    PointF64 {
+        x: p.x as f64,
+        y: p.y as f64,
+    }
+}
+
+/// A degenerate cubic tracing the straight line `a`→`b`.
+fn straight_cubic(a: PointF64, b: PointF64) -> [PointF64; 4] {
+    let c1 = PointF64 {
+        x: a.x + (b.x - a.x) / 3.0,
+        y: a.y + (b.y - a.y) / 3.0,
+    };
+    let c2 = PointF64 {
+        x: a.x + 2.0 * (b.x - a.x) / 3.0,
+        y: a.y + 2.0 * (b.y - a.y) / 3.0,
+    };
+    [a, c1, c2, b]
+}
+
+/// Error bound for the per-slice cubic fit. Matches the value stacked mode
+/// uses in `Spline::from_path_f64`, so mosaic curves have the same character.
+const FIT_ERROR: f64 = 10.0;
+
+/// Fit one splice slice into a single cubic, exactly as stacked mode does
+/// (`fit_points_with_bezier`: one retract-handled cubic per slice, endpoints
+/// pinned to the slice ends).
+fn fit_slice(slice: &[PointF64], out: &mut Vec<[PointF64; 4]>) {
+    match slice.len() {
+        0 | 1 => {}
+        2 => out.push(straight_cubic(slice[0], slice[1])),
+        _ => out.push(SubdivideSmooth::fit_points_with_bezier(slice, FIT_ERROR)),
+    }
+}
+
+fn spline_to_beziers(spline: &Spline) -> Vec<[PointF64; 4]> {
+    spline
+        .get_control_points()
+        .into_iter()
+        .filter(|w| w.len() == 4)
+        .map(|w| [w[0], w[1], w[2], w[3]])
+        .collect()
+}
+
+impl SegmentFitter for SplineSegmentFitter {
+    fn fit_open(&self, seg: &Segment) -> FittedSegment {
+        if seg.points.len() <= 2 {
+            return FittedSegment {
+                geom: FittedGeom::Polyline(to_f64(&seg.points)),
+            };
+        }
+
+        // 1. Staircase removal via visioncortex's `limit_penalties` — the
+        //    symmetric (area-based, no directional outset) simplifier stacked
+        //    mode runs after remove_staircase. Used alone here it collapses
+        //    staircases toward the crack midline, so the boundary stays
+        //    centered and cannot cross a non-adjacent segment (which would
+        //    open a gap in the tessellation). Endpoints are preserved.
+        let simplified = PathSimplify::limit_penalties(&PathI32::from_points(seg.points.clone()));
+        if simplified.len() <= 2 {
+            return FittedSegment {
+                geom: FittedGeom::Polyline(simplified.path.iter().copied().map(pt).collect()),
+            };
+        }
+
+        // 2. Corner detection (open, endpoints forced as corners).
+        let mut corners = SubdivideSmooth::find_corners(&simplified, self.corner_threshold, false);
+        // 3. Open 4-point subdivision.
+        let mut path = simplified.to_path_f64();
+        for _ in 0..self.max_iterations {
+            let (np, nc, done) = SubdivideSmooth::subdivide_keep_corners(
+                &path,
+                &corners,
+                OUTSET_RATIO,
+                self.length_threshold,
+                false,
+            );
+            path = np;
+            corners = nc;
+            if done {
+                break;
+            }
+        }
+        // 4. Splice points (open, endpoints forced).
+        let splice = SubdivideSmooth::find_splice_points(&path, self.splice_threshold, false);
+        let cuts: Vec<usize> = splice
+            .iter()
+            .enumerate()
+            .filter_map(|(i, &s)| if s { Some(i) } else { None })
+            .collect();
+
+        // 5. Per-slice cubic fit.
+        let mut beziers = Vec::new();
+        for w in cuts.windows(2) {
+            fit_slice(&path.path[w[0]..=w[1]], &mut beziers);
+        }
+
+        if beziers.is_empty() {
+            return FittedSegment {
+                geom: FittedGeom::Polyline(path.path.clone()),
+            };
+        }
+
+        // Pin the segment's endpoints exactly to the lattice nodes so that
+        // segments meeting at a junction share identical coordinates.
+        beziers.first_mut().unwrap()[0] = pt(seg.points[0]);
+        beziers.last_mut().unwrap()[3] = pt(seg.points[seg.points.len() - 1]);
+
         FittedSegment {
-            geom: FittedGeom::Polyline(dp_open(&pts, self.tolerance)),
+            geom: FittedGeom::Beziers(beziers),
         }
     }
 
     fn fit_ring(&self, seg: &Segment) -> FittedSegment {
-        // Closed loop: split at the vertex farthest from the start, DP each
-        // half, then rejoin. points[0] == points[last].
-        let pts = to_f64(&seg.points);
-        if pts.len() <= 4 {
+        // Rings are closed loops — this is exactly the stacked closed-spline
+        // pipeline (simplify → smooth → fit).
+        if seg.points.len() <= 4 {
             return FittedSegment {
-                geom: FittedGeom::Polyline(pts),
+                geom: FittedGeom::Polyline(to_f64(&seg.points)),
+            };
+        }
+        let simplified = PathSimplify::limit_penalties(&PathI32::from_points(seg.points.clone()));
+        let smoothed = simplified.smooth(
+            self.corner_threshold,
+            OUTSET_RATIO,
+            self.length_threshold,
+            self.max_iterations,
+        );
+        let spline = Spline::from_path_f64(&smoothed, self.splice_threshold);
+        let beziers = spline_to_beziers(&spline);
+        if beziers.is_empty() {
+            return FittedSegment {
+                geom: FittedGeom::Polyline(to_f64(&seg.points)),
             };
         }
-        let open = &pts[..pts.len() - 1]; // drop duplicate closing point
-        let far = farthest_from(open, 0);
-        let first: Vec<PointF64> = open[0..=far].to_vec();
-        let second: Vec<PointF64> = open[far..]
-            .iter()
-            .chain(std::iter::once(&open[0]))
-            .copied()
-            .collect();
-        let mut a = dp_open(&first, self.tolerance);
-        let b = dp_open(&second, self.tolerance);
-        // `a` ends at `far`, `b` starts at `far` and ends back at start.
-        a.pop(); // drop shared `far`
-        a.extend(b); // ...b includes far..start (closing point == start)
         FittedSegment {
-            geom: FittedGeom::Polyline(a),
+            geom: FittedGeom::Beziers(beziers),
         }
     }
 }
-
-fn farthest_from(pts: &[PointF64], anchor: usize) -> usize {
-    let a = pts[anchor];
-    let mut best = anchor;
-    let mut best_d = -1.0;
-    for (i, p) in pts.iter().enumerate() {
-        let dx = p.x - a.x;
-        let dy = p.y - a.y;
-        let d = dx * dx + dy * dy;
-        if d > best_d {
-            best_d = d;
-            best = i;
-        }
-    }
-    best
-}
-
-/// Douglas–Peucker on an open polyline; first and last points are always kept.
-fn dp_open(pts: &[PointF64], tol: f64) -> Vec<PointF64> {
-    if pts.len() <= 2 {
-        return pts.to_vec();
-    }
-    let mut keep = vec![false; pts.len()];
-    keep[0] = true;
-    keep[pts.len() - 1] = true;
-    dp_recurse(pts, 0, pts.len() - 1, tol, &mut keep);
-    pts.iter()
-        .zip(keep)
-        .filter_map(|(p, k)| if k { Some(*p) } else { None })
-        .collect()
-}
-
-fn dp_recurse(pts: &[PointF64], lo: usize, hi: usize, tol: f64, keep: &mut [bool]) {
-    if hi <= lo + 1 {
-        return;
-    }
-    let mut max_d = -1.0;
-    let mut idx = lo;
-    for i in (lo + 1)..hi {
-        let d = perp_distance(pts[i], pts[lo], pts[hi]);
-        if d > max_d {
-            max_d = d;
-            idx = i;
-        }
-    }
-    if max_d > tol {
-        keep[idx] = true;
-        dp_recurse(pts, lo, idx, tol, keep);
-        dp_recurse(pts, idx, hi, tol, keep);
-    }
-}
-
-/// Perpendicular distance from `p` to the segment `a`–`b`.
-fn perp_distance(p: PointF64, a: PointF64, b: PointF64) -> f64 {
-    let dx = b.x - a.x;
-    let dy = b.y - a.y;
-    let len2 = dx * dx + dy * dy;
-    if len2 == 0.0 {
-        return ((p.x - a.x).powi(2) + (p.y - a.y).powi(2)).sqrt();
-    }
-    let cross = (p.x - a.x) * dy - (p.y - a.y) * dx;
-    cross.abs() / len2.sqrt()
-}

crates/vtracer/src/mosaic/mod.rs

@@ -21,7 +21,7 @@ mod graph;
 
 pub use compose::compose_mosaic;
 pub use fit::{
-    FittedSegment, PixelSegmentFitter, PolygonSegmentFitter, SegmentFitter,
+    FittedSegment, PixelSegmentFitter, PolygonSegmentFitter, SegmentFitter, SplineSegmentFitter,
 };
 pub use graph::{BoundaryGraph, Node, Segment, SegRef};
 
@@ -33,16 +33,6 @@ pub type RegionId = u32;
 /// Sentinel label for pixels outside any region.
 pub const OUTSIDE: RegionId = u32::MAX;
 
-/// Options controlling mosaic fitting and output.
-#[derive(Debug, Clone, Copy, Default)]
-pub struct MosaicOptions {
-    /// Sample fitted segments and fall back to the DP polyline on any that
-    /// exceed the 0.5px deviation budget, restoring a hard no-crossing guarantee.
-    pub strict: bool,
-    /// Stroke each path in its own fill color to hide antialiasing hairlines.
-    pub seam_stroke: bool,
-}
-
 /// A flat partition of the canvas: one region id per pixel, plus the paint for
 /// each region. This is the sole input to the boundary-graph extractor.
 #[derive(Debug, Clone)]
@@ -276,6 +266,44 @@ mod tests {
         assert_pixel_roundtrip(&grid(4, 4, labels));
     }
 
+    #[test]
+    fn spline_segments_pin_endpoints_to_lattice() {
+        use super::fit::{FittedGeom, SegmentFitter, SplineSegmentFitter};
+        // A shape with junctions so there are open (non-ring) segments.
+        let map = grid(4, 4, vec![
+            0, 0, 1, 1,
+            0, 0, 1, 1,
+            2, 2, 1, 1,
+            2, 2, 2, 2,
+        ]);
+        let graph = BoundaryGraph::extract(&map);
+        let fitter = SplineSegmentFitter::default();
+        let mut checked = 0;
+        for seg in &graph.segments {
+            if seg.is_ring() {
+                continue;
+            }
+            let fitted = fitter.fit_open(seg);
+            let start = PointF64 { x: seg.points[0].x as f64, y: seg.points[0].y as f64 };
+            let end = {
+                let p = seg.points[seg.points.len() - 1];
+                PointF64 { x: p.x as f64, y: p.y as f64 }
+            };
+            match fitted.geom {
+                FittedGeom::Beziers(b) => {
+                    assert_eq!(b.first().unwrap()[0], start, "start pinned to node");
+                    assert_eq!(b.last().unwrap()[3], end, "end pinned to node");
+                }
+                FittedGeom::Polyline(p) => {
+                    assert_eq!(*p.first().unwrap(), start);
+                    assert_eq!(*p.last().unwrap(), end);
+                }
+            }
+            checked += 1;
+        }
+        assert!(checked > 0, "expected some open segments");
+    }
+
     #[test]
     fn random_maps_roundtrip() {
         // Deterministic LCG; connectivity not required.

crates/vtracer/src/svg/mod.rs

@@ -131,6 +131,8 @@ struct Emitter {
     precision: Option<u32>,
     out: String,
     cur: PointF64,
+    /// Start of the current subpath; `cur` returns here after `Z`.
+    subpath_start: PointF64,
     started: bool,
     /// Absolute second control point of the previous cubic, for `S` detection.
     prev_cubic_c2: Option<PointF64>,
@@ -144,6 +146,7 @@ impl Emitter {
             precision,
             out: String::new(),
             cur: PointF64::default(),
+            subpath_start: PointF64::default(),
             started: false,
             prev_cubic_c2: None,
         }
@@ -161,6 +164,9 @@ impl Emitter {
                 PathCmd::CubicTo(c1, c2, e) => self.cubic_to(c1, c2, e),
                 PathCmd::Close => {
                     self.out.push('Z');
+                    // SVG resets the current point to the subpath's start after
+                    // Z; a following relative `m`/`l` is measured from there.
+                    self.cur = self.subpath_start;
                     self.prev_cubic_c2 = None;
                 }
             }
@@ -184,6 +190,7 @@ impl Emitter {
             self.out.push_str(&token);
         }
         self.cur = p;
+        self.subpath_start = p;
         self.prev_cubic_c2 = None;
     }
 
@@ -426,4 +433,150 @@ mod tests {
         assert!(!d.contains('c'));
         assert!(d.contains('L'));
     }
+
+    /// A shape with a hole (second subpath). Encoded absolute vs relative must
+    /// describe the *same* geometry — regression for the bug where the current
+    /// point was not reset to the subpath start after `Z`, so the relative `m`
+    /// of the hole was measured from the wrong origin.
+    fn holed_shape() -> Shape {
+        use visioncortex::PointF64;
+        let p = |x, y| PointF64 { x, y };
+        let outer = SubPath {
+            commands: vec![
+                PathCmd::MoveTo(p(0.0, 0.0)),
+                PathCmd::LineTo(p(30.0, 0.0)),
+                PathCmd::LineTo(p(30.0, 30.0)),
+                PathCmd::LineTo(p(0.0, 30.0)),
+                PathCmd::Close,
+            ],
+        };
+        let hole = SubPath {
+            commands: vec![
+                PathCmd::MoveTo(p(10.0, 10.0)),
+                PathCmd::LineTo(p(20.0, 10.0)),
+                PathCmd::LineTo(p(20.0, 20.0)),
+                PathCmd::LineTo(p(10.0, 20.0)),
+                PathCmd::Close,
+            ],
+        };
+        Shape {
+            paint: Paint::Solid(Color::new(0, 0, 0)),
+            path: MultiPath {
+                subpaths: vec![outer, hole],
+            },
+        }
+    }
+
+    /// Parse an SVG `d` (M/m/L/l/H/h/V/v/Z only) into absolute points.
+    fn parse_abs(d: &str) -> Vec<(f64, f64)> {
+        let mut toks = Vec::new();
+        let mut i = 0;
+        let b = d.as_bytes();
+        while i < b.len() {
+            let c = b[i] as char;
+            if c.is_ascii_alphabetic() {
+                toks.push(c.to_string());
+                i += 1;
+            } else if c == '-' || c == '.' || c.is_ascii_digit() {
+                let start = i;
+                i += 1;
+                while i < b.len() && {
+                    let d = b[i] as char;
+                    d.is_ascii_digit() || d == '.'
+                } {
+                    i += 1;
+                }
+                toks.push(d[start..i].to_string());
+            } else {
+                i += 1;
+            }
+        }
+        let mut out = Vec::new();
+        let (mut cx, mut cy, mut sx, mut sy) = (0.0, 0.0, 0.0, 0.0);
+        let mut j = 0;
+        let mut cmd = ' ';
+        let num = |j: &mut usize| -> f64 {
+            let v = toks[*j].parse().unwrap();
+            *j += 1;
+            v
+        };
+        while j < toks.len() {
+            if toks[j].chars().next().unwrap().is_ascii_alphabetic() {
+                cmd = toks[j].chars().next().unwrap();
+                j += 1;
+            }
+            let rel = cmd.is_ascii_lowercase();
+            match cmd.to_ascii_uppercase() {
+                'M' => {
+                    let (mut x, mut y) = (num(&mut j), num(&mut j));
+                    if rel {
+                        x += cx;
+                        y += cy;
+                    }
+                    cx = x;
+                    cy = y;
+                    sx = x;
+                    sy = y;
+                    out.push((cx, cy));
+                    cmd = if rel { 'l' } else { 'L' };
+                }
+                'L' => {
+                    let (mut x, mut y) = (num(&mut j), num(&mut j));
+                    if rel {
+                        x += cx;
+                        y += cy;
+                    }
+                    cx = x;
+                    cy = y;
+                    out.push((cx, cy));
+                }
+                'H' => {
+                    let mut x = num(&mut j);
+                    if rel {
+                        x += cx;
+                    }
+                    cx = x;
+                    out.push((cx, cy));
+                }
+                'V' => {
+                    let mut y = num(&mut j);
+                    if rel {
+                        y += cy;
+                    }
+                    cy = y;
+                    out.push((cx, cy));
+                }
+                'Z' => {
+                    cx = sx;
+                    cy = sy;
+                }
+                _ => unreachable!(),
+            }
+        }
+        out
+    }
+
+    #[test]
+    fn relative_and_absolute_encode_same_geometry() {
+        let shape = holed_shape();
+        let abs = SvgWriter {
+            relative: false,
+            shorthands: false,
+            precision: Some(2),
+        }
+        .encode_path(&shape);
+        for shorthands in [false, true] {
+            let rel = SvgWriter {
+                relative: true,
+                shorthands,
+                precision: Some(2),
+            }
+            .encode_path(&shape);
+            assert_eq!(
+                parse_abs(&abs),
+                parse_abs(&rel),
+                "relative (shorthands={shorthands}) geometry diverges from absolute:\n abs={abs}\n rel={rel}"
+            );
+        }
+    }
 }

crates/vtracer/tests/equivalence.rs

@@ -0,0 +1,215 @@
+//! Rasterize-and-diff equivalence between stacked and mosaic (cutout) modes.
+//!
+//! Both modes render the *same* flattened partition of the image — stacked by
+//! painting layers top-down, mosaic as a gapless tessellation. So their
+//! rasterizations must agree in every region interior; they may differ only
+//! within a thin band along region boundaries, where the two fitting paths
+//! legitimately place the edge a fraction of a pixel apart. This test asserts
+//! exactly that: any pixel that differs must lie within ~1–2px of a boundary.
+//!
+//! `resvg` is a dev-dependency, so this never enters a wasm build.
+
+use resvg::{tiny_skia, usvg};
+use vtracer::{ColorImage, Config, FitMode, Hierarchical};
+
+/// A few smooth colored discs on a background — curved boundaries, limited
+/// boundary length, no thin (1px) features.
+fn blobs(w: usize, h: usize) -> ColorImage {
+    let discs = [
+        (28.0f64, 30.0, 18.0, (210u8, 60, 60)),
+        (64.0, 40.0, 20.0, (60, 160, 90)),
+        (44.0, 68.0, 16.0, (70, 90, 200)),
+    ];
+    let mut pixels = Vec::with_capacity(w * h * 4);
+    for y in 0..h {
+        for x in 0..w {
+            let mut col = (235u8, 230, 225); // background
+            for &(cx, cy, r, c) in &discs {
+                let dx = x as f64 - cx;
+                let dy = y as f64 - cy;
+                if dx * dx + dy * dy <= r * r {
+                    col = c;
+                }
+            }
+            pixels.extend_from_slice(&[col.0, col.1, col.2, 255]);
+        }
+    }
+    ColorImage {
+        pixels,
+        width: w,
+        height: h,
+    }
+}
+
+fn rasterize(svg: &str, w: u32, h: u32) -> Vec<u8> {
+    let tree = usvg::Tree::from_str(svg, &usvg::Options::default()).expect("parse svg");
+    let mut pixmap = tiny_skia::Pixmap::new(w, h).expect("alloc pixmap");
+    resvg::render(&tree, tiny_skia::Transform::identity(), &mut pixmap.as_mut());
+    pixmap.data().to_vec()
+}
+
+/// Max per-channel difference between two RGBA pixels at index `i`.
+fn pixel_diff(a: &[u8], b: &[u8], i: usize) -> u8 {
+    (0..4)
+        .map(|c| a[i + c].abs_diff(b[i + c]))
+        .max()
+        .unwrap_or(0)
+}
+
+/// Mark pixels within Chebyshev radius `r` of a color edge in either image.
+fn boundary_band(a: &[u8], b: &[u8], w: usize, h: usize, r: i32) -> Vec<bool> {
+    const EDGE: u8 = 24;
+    let idx = |x: usize, y: usize| (y * w + x) * 4;
+    let mut edge = vec![false; w * h];
+    for y in 0..h {
+        for x in 0..w {
+            let i = idx(x, y);
+            // An edge is where either rendering changes color vs its right/down
+            // neighbor.
+            let mut is_edge = false;
+            for img in [a, b] {
+                if x + 1 < w && neighbor_diff(img, i, idx(x + 1, y)) > EDGE {
+                    is_edge = true;
+                }
+                if y + 1 < h && neighbor_diff(img, i, idx(x, y + 1)) > EDGE {
+                    is_edge = true;
+                }
+            }
+            if is_edge {
+                edge[y * w + x] = true;
+            }
+        }
+    }
+    // Dilate the edge set by r.
+    let mut band = vec![false; w * h];
+    for y in 0..h as i32 {
+        for x in 0..w as i32 {
+            let mut near = false;
+            'outer: for dy in -r..=r {
+                for dx in -r..=r {
+                    let (nx, ny) = (x + dx, y + dy);
+                    if nx >= 0 && ny >= 0 && (nx as usize) < w && (ny as usize) < h && edge[ny as usize * w + nx as usize] {
+                        near = true;
+                        break 'outer;
+                    }
+                }
+            }
+            band[y as usize * w + x as usize] = near;
+        }
+    }
+    band
+}
+
+fn neighbor_diff(img: &[u8], i: usize, j: usize) -> u8 {
+    (0..4).map(|c| img[i + c].abs_diff(img[j + c])).max().unwrap_or(0)
+}
+
+fn assert_equivalent(mode: FitMode) {
+    let (w, h) = (96usize, 96usize);
+    let img = blobs(w, h);
+
+    let stacked = Config {
+        mode,
+        hierarchical: Hierarchical::Stacked,
+        ..Config::default()
+    }
+    .build()
+    .unwrap()
+    .to_svg(&img)
+    .unwrap();
+
+    let cutout = Config {
+        mode,
+        hierarchical: Hierarchical::Cutout,
+        ..Config::default()
+    }
+    .build()
+    .unwrap()
+    .to_svg(&img)
+    .unwrap();
+
+    let a = rasterize(&stacked, w as u32, h as u32);
+    let b = rasterize(&cutout, w as u32, h as u32);
+    assert_eq!(a.len(), b.len());
+
+    let band = boundary_band(&a, &b, w, h, 2);
+
+    const DIFF: u8 = 40;
+    let mut interior_mismatches = 0;
+    for p in 0..(w * h) {
+        let i = p * 4;
+        if pixel_diff(&a, &b, i) > DIFF && !band[p] {
+            interior_mismatches += 1;
+        }
+    }
+
+    // Every real difference must live in the boundary band; interiors match.
+    assert_eq!(
+        interior_mismatches, 0,
+        "{mode:?}: {interior_mismatches} interior pixels differ between stacked and cutout \
+         (differences must be confined to the boundary band)"
+    );
+}
+
+#[test]
+fn stacked_and_cutout_agree_in_interiors_spline() {
+    assert_equivalent(FitMode::Spline);
+}
+
+#[test]
+fn stacked_and_cutout_agree_in_interiors_polygon() {
+    assert_equivalent(FitMode::Polygon);
+}
+
+#[test]
+fn stacked_and_cutout_agree_in_interiors_pixel() {
+    assert_equivalent(FitMode::Pixel);
+}
+
+// --- seam / show-through test -------------------------------------------------
+
+fn rasterize_on(svg: &str, w: u32, h: u32, bg: [u8; 4]) -> Vec<u8> {
+    let tree = usvg::Tree::from_str(svg, &usvg::Options::default()).expect("parse svg");
+    let mut pixmap = tiny_skia::Pixmap::new(w, h).expect("alloc pixmap");
+    pixmap.fill(tiny_skia::Color::from_rgba8(bg[0], bg[1], bg[2], 255));
+    resvg::render(&tree, tiny_skia::Transform::identity(), &mut pixmap.as_mut());
+    pixmap.data().to_vec()
+}
+
+/// A full-canvas-coverage image rendered in stacked mode must be fully opaque:
+/// solid layers overdraw with no gaps, so nothing shows through. Show-through
+/// (backdrop-dependent pixels away from the canvas edge) means seams — which is
+/// exactly the hole-punching bug this guards against.
+#[test]
+fn stacked_has_no_seams() {
+    let (w, h) = (96usize, 96usize);
+    let img = blobs(w, h); // background fills the whole canvas
+    let svg = Config {
+        mode: FitMode::Spline,
+        hierarchical: Hierarchical::Stacked,
+        ..Config::default()
+    }
+    .build()
+    .unwrap()
+    .to_svg(&img)
+    .unwrap();
+
+    let white = rasterize_on(&svg, w as u32, h as u32, [255, 255, 255, 255]);
+    let black = rasterize_on(&svg, w as u32, h as u32, [0, 0, 0, 255]);
+
+    // Count backdrop-dependent pixels, ignoring the 1px canvas border (the only
+    // legitimate outer-silhouette antialiasing for a full-coverage image).
+    let mut show_through = 0;
+    for y in 1..h - 1 {
+        for x in 1..w - 1 {
+            let i = (y * w + x) * 4;
+            if (0..3).any(|c| white[i + c].abs_diff(black[i + c]) > 8) {
+                show_through += 1;
+            }
+        }
+    }
+    assert_eq!(
+        show_through, 0,
+        "stacked mode leaked {show_through} backdrop pixels — seams/holes in solid overdraw"
+    );
+}

crates/vtracer/tests/golden.rs

@@ -197,6 +197,15 @@ fn cases() -> Vec<(&'static str, ColorImage, Config)> {
                 ..base()
             },
         ),
+        (
+            "disc_mosaic_spline",
+            disc(),
+            Config {
+                hierarchical: Hierarchical::Cutout,
+                mode: FitMode::Spline,
+                ..base()
+            },
+        ),
     ]
 }
 

crates/vtracer/tests/goldens/bands_palette.svg

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="40">
-<path d="M12,0C23.88,0,35.76,0,48,0c0,13.2,0,26.4,0,40c-11.88,0-23.76,0-36,0c0-13.2,0-26.4,0-40Z" fill="#FFFFFF"/>
+<path d="M0,0C15.84,0,31.68,0,48,0c0,13.2,0,26.4,0,40c-15.84,0-31.68,0-48,0C0,26.8,0,13.6,0,0Z" fill="#FFFFFF"/>
 <path d="M24,0c3.96,0,7.92,0,12,0c0,13.2,0,26.4,0,40c-3.96,0-7.92,0-12,0c0-13.2,0-26.4,0-40Z" fill="#000000"/>
 <path d="M0,0C3.96,0,7.92,0,12,0c0,13.2,0,26.4,0,40c-3.96,0-7.92,0-12,0C0,26.8,0,13.6,0,0Z" fill="#FFFFFF"/>
 </svg>

crates/vtracer/tests/goldens/bands_pixel.svg

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="40">
-<path d="M12,0L36,0L36,40L12,40Z" fill="#28C83C"/>
+<path d="M0,0L48,0L48,40L0,40Z" fill="#28C83C"/>
 <path d="M36,0L48,0L48,40L36,40Z" fill="#E6D228"/>
 <path d="M24,0L36,0L36,40L24,40Z" fill="#323CDC"/>
 <path d="M0,0L12,0L12,40L0,40Z" fill="#DC2828"/>

crates/vtracer/tests/goldens/bands_polygon.svg

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="40">
-<path d="M12,0L36,0l0,40L12,40Z" fill="#28C83C"/>
+<path d="M0,0L48,0l0,40L0,40Z" fill="#28C83C"/>
 <path d="M36,0L48,0l0,40L36,40Z" fill="#E6D228"/>
 <path d="M24,0L36,0l0,40L24,40Z" fill="#323CDC"/>
 <path d="M0,0L12,0l0,40L0,40Z" fill="#DC2828"/>

crates/vtracer/tests/goldens/bands_spline.svg

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="40">
-<path d="M12,0c7.92,0,15.84,0,24,0c0,13.2,0,26.4,0,40c-7.92,0-15.84,0-24,0c0-13.2,0-26.4,0-40Z" fill="#28C83C"/>
+<path d="M0,0C15.84,0,31.68,0,48,0c0,13.2,0,26.4,0,40c-15.84,0-31.68,0-48,0C0,26.8,0,13.6,0,0Z" fill="#28C83C"/>
 <path d="M36,0c3.96,0,7.92,0,12,0c0,13.2,0,26.4,0,40c-3.96,0-7.92,0-12,0c0-13.2,0-26.4,0-40Z" fill="#E6D228"/>
 <path d="M24,0c3.96,0,7.92,0,12,0c0,13.2,0,26.4,0,40c-3.96,0-7.92,0-12,0c0-13.2,0-26.4,0-40Z" fill="#323CDC"/>
 <path d="M0,0C3.96,0,7.92,0,12,0c0,13.2,0,26.4,0,40c-3.96,0-7.92,0-12,0C0,26.8,0,13.6,0,0Z" fill="#DC2828"/>

crates/vtracer/tests/goldens/checker_spline.svg

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="48">
-<path d="M0,0C15.84,0,31.68,0,48,0c0,13.2,0,26.4,0,40c-2.64,0-5.28,0-8,0c0,2.64,0,5.28,0,8c-13.2,0-26.4,0-40,0C0,32.16,0,16.32,0,0Z" fill="#EBEBEB"/>
+<path d="M0,0C15.84,0,31.68,0,48,0c0,15.84,0,31.68,0,48c-15.84,0-31.68,0-48,0C0,32.16,0,16.32,0,0Z" fill="#EBEBEB"/>
 <path d="M40,40c2.64,0,5.28,0,8,0c0,2.64,0,5.28,0,8c-2.64,0-5.28,0-8,0c0-2.64,0-5.28,0-8Z" fill="#141414"/>
 <path d="M32,40c2.64,0,5.28,0,8,0c0,2.64,0,5.28,0,8c-2.64,0-5.28,0-8,0c0-2.64,0-5.28,0-8Z" fill="#EBEBEB"/>
 <path d="M24,40c2.64,0,5.28,0,8,0c0,2.64,0,5.28,0,8c-2.64,0-5.28,0-8,0c0-2.64,0-5.28,0-8Z" fill="#141414"/>

crates/vtracer/tests/goldens/disc_mosaic_spline.svg

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="48">
+<path d="M0,0C0,16.32,0,32.16,0,48c16.32,0,32.16,0,48,0c0-16.32,0-32.16,0-48C31.68,0,15.84,0,0,0ZM31.56,9.75c5.48,2.81,7.28,7.78,9.13,13.31c.37,2.34,.06,3.7-.69,5.94c-.25,.83-.49,1.65-.75,2.5c-1.98,3.96-4.86,5.85-8.81,7.69C24,41.33,24,41.33,20,40c-.82-.25-1.65-.49-2.5-.75c-3.96-1.98-5.85-4.86-7.69-8.81C7.67,24,7.67,24,9,20c.25-.82,.49-1.65,.75-2.5C14,8.99,23.31,7.33,31.56,9.75Z" fill="#F0F0F0"/>
+<path d="M31.56,9.75C23.31,7.33,14,8.99,9.75,17.5c-.26,.85-.5,1.68-.75,2.5c-1.33,4-1.33,4,.81,10.44c1.84,3.95,3.73,6.83,7.69,8.81c.85,.26,1.68,.5,2.5,.75c4,1.33,4,1.33,10.44-.81c3.95-1.84,6.83-3.73,8.81-7.69c.26-.85,.5-1.67,.75-2.5c.75-2.24,1.06-3.6,.69-5.94c-1.85-5.53-3.65-10.5-9.13-13.31Z" fill="#C83C3C"/>
+</svg>

crates/vtracer/tests/goldens/disc_opt0.svg

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="48">
-<path d="M0,0C15.84,0,31.68,0,48,0C48,15.84,48,31.68,48,48C32.16,48,16.32,48,0,48C0,32.16,0,16.32,0,0ZM12.06,13.69C9.23,18.23,7.55,21.93,8.34,27.34C9.88,32.82,12.06,35.92,17,39C21.43,41.03,24.98,41.31,29.69,40C34.72,38.01,37.57,35.12,39.85,30.15C41.43,25.38,40.94,21.68,39.11,17.05C36.65,12.5,32.98,10.09,28.19,8.5C21.83,7.23,16.73,9.42,12.06,13.69Z" fill="#F0F0F0"/>
+<path d="M0,0C15.84,0,31.68,0,48,0C48,15.84,48,31.68,48,48C32.16,48,16.32,48,0,48C0,32.16,0,16.32,0,0Z" fill="#F0F0F0"/>
 <path d="M35.31,12.06C39.1,16.2,41.16,20.44,40.91,26.15C39.91,31.12,37.77,34.55,34,38C29.69,40.33,25.67,41.47,20.81,40.5C16.02,38.91,12.35,36.5,9.89,31.95C8.06,27.32,7.57,23.62,9.15,18.85C11.43,13.88,14.28,10.99,19.31,9C25.64,7.23,29.86,8.66,35.31,12.06Z" fill="#C83C3C"/>
 </svg>

crates/vtracer/tests/goldens/disc_opt2.svg

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Generator: visioncortex VTracer 1.0.0-alpha.1 -->
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="48" height="48">
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+<path d="M0,0C15.84,0,31.68,0,48,0c0,15.84,0,31.68,0,48c-15.84,0-31.68,0-48,0C0,32.16,0,16.32,0,0Z" fill="#F0F0F0"/>
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crates/vtracer/tests/goldens/disc_spline.svg

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crates/vtracer/tests/goldens/ring_spline.svg

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crates/vtracer/tests/goldens/swatches_color.svg

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crates/vtracer/tests/goldens/swatches_quant4.svg

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