Part of #641: Image Trace: Create Strokes

Advanced › Create: Fills and/or Strokes, with Stroke (the widest line, px).
With Strokes, each colour's 8-connected parts no wider than the stroke width
(chamfer distance transform) trace as open centre lines instead of filled
outlines: the line is thinned to one pixel (Zhang-Suen with each removal
re-checked, so two-pixel lines survive; staircase corners dropped), its
skeleton becomes runs between ends and junctions (junction pixels merged,
spurs pruned, runs through two-way junctions joined, rings closed), and each
run is simplified and fitted with curves. The stroke width is the line's mean
width (pixels over length). Traced lines get round caps and joins in the
document. Without Fills, wider areas become 1 px stroked outlines.
`imageTrace.make` takes fills, strokes and strokeWidth, and refuses neither
fills nor strokes and a non-positive width.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/buildcmds.rs

@@ -14,7 +14,7 @@ use std::sync::Arc;
 
 use serde_json::{Value, json};
 use vectorcraft_color::{Color, Paint};
-use vectorcraft_doc::appearance::{AppearanceItem, FillLayer, StrokeLayer};
+use vectorcraft_doc::appearance::{AppearanceItem, FillLayer, LineCap, LineJoin, StrokeLayer};
 use vectorcraft_doc::{Appearance, Document, Node, NodeId, NodeKind, Selection};
 use vectorcraft_geom::{FillRule, PathData, Point, Rect, SubPath};
 use vectorcraft_pathops as po;
@@ -120,7 +120,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Make",
             ["Object", "Image Trace"],
             None,
-            "{id?: image or Image Trace group (default: selection), preset?: name (imageTrace.presets), params?: {mode: \"blackAndWhite\"|\"grayscale\"|\"color\", threshold: 0-255, colors: 2-256, paths: 0-100, corners: 0-100, noise: px, method: \"abutting\"|\"overlapping\", ignoreWhite, snapCurvesToLines}} → {id, paths, anchors, colors}",
+            "{id?: image or Image Trace group (default: selection), preset?: name (imageTrace.presets), params?: {mode: \"blackAndWhite\"|\"grayscale\"|\"color\", threshold: 0-255, colors: 2-256, paths: 0-100, corners: 0-100, noise: px, method: \"abutting\"|\"overlapping\", ignoreWhite, snapCurvesToLines, fills: true, strokes: false (Create: lines up to strokeWidth wide become stroked centre lines; without fills, wider areas become stroked outlines), strokeWidth: px (10)}} → {id, paths, anchors, colors}",
             has_selection_or_ids,
             |s, p| trace_make(s, p, false)
         ),
@@ -648,7 +648,14 @@ fn trace_params(p: &Value) -> Result<tr::TraceParams> {
     for (k, val) in src {
         o.insert(k.clone(), val.clone());
     }
-    serde_json::from_value(v).map_err(|e| bad("imageTrace.make", e.to_string()))
+    let params: tr::TraceParams = serde_json::from_value(v).map_err(|e| bad("imageTrace.make", e.to_string()))?;
+    if !params.fills && !params.strokes {
+        return Err(bad("imageTrace.make", "Create needs fills, strokes or both"));
+    }
+    if !(params.stroke_width.is_finite() && params.stroke_width > 0.0) {
+        return Err(bad("imageTrace.make", "strokeWidth must be a positive number of pixels"));
+    }
+    Ok(params)
 }
 
 /// The most anchor points an Image Trace may make: a photo traced at high fidelity can make
@@ -692,6 +699,8 @@ fn trace_make(s: &mut Session, p: &Value, expand: bool) -> Result<Value> {
     let sx = img.width.max(1) as f64 / raster.width.max(1) as f64;
     let sy = img.height.max(1) as f64 / raster.height.max(1) as f64;
     let xf = img.xf * vectorcraft_geom::Affine::scale_non_uniform(sx, sy);
+    // Document units per raster pixel (stroke widths).
+    let unit = xf.determinant().abs().sqrt();
     let (npaths, anchors, colors) = (res.paths.len(), res.anchor_count(), res.palette.len());
     let label = if expand { "Image Trace (Make and Expand)" } else { "Image Trace" };
     let id = s.edit(label, |d, sel| {
@@ -714,7 +723,19 @@ fn trace_make(s: &mut Session, p: &Value, expand: bool) -> Result<Value> {
         for tp in res.paths {
             let path = tp.path.transformed(xf);
             let mut n = shape_node(d, path, None);
-            n.appearance = fill_only(Paint::solid(Color::rgb8(tp.color[0], tp.color[1], tp.color[2])));
+            let paint = Paint::solid(Color::rgb8(tp.color[0], tp.color[1], tp.color[2]));
+            n.appearance = match tp.stroke {
+                // Traced lines: round ends and corners, as drawn lines have.
+                Some(width) => Appearance {
+                    items: vec![AppearanceItem::Stroke(StrokeLayer {
+                        cap: LineCap::Round,
+                        join: LineJoin::Round,
+                        ..StrokeLayer::new(paint, width * unit)
+                    })],
+                    ..Default::default()
+                },
+                None => fill_only(paint),
+            };
             children.push(Arc::new(n));
         }
         let gid = d.alloc_id();