Image Trace: Gradients

Gradients (params `gradients`, a checkbox in the panel for Grayscale and Color;
off unless asked for): an area whose colour changes evenly along one direction
is traced as one path filled with a linear gradient, not as bands of flat colour.

Gradients are found before the palette is made, so no colour is spent on their
bands: a connected run of smoothly changing pixels is a gradient when a plane
fits each colour channel with all three sloping the same way, the colours along
that way are at most six stops with ramps between them, and painting them back
gives the area's own pixels. Flat colour, soft edges between flat colours and
shading that goes two ways are left as they were.

The harness gains two linear-gradient inputs with known lines and stops, and
measures whether each is found, the angle of its line and the colours of its ramp.

crates/engine/src/cmd/buildcmds.rs

@@ -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\"|\"logo\", threshold: 0-255, palette: \"limited\"|\"fullTone\"|\"automatic\"|\"documentLibrary\" (color mode; limited: the `colors` best colours, fullTone: as many as the tones need, more with colorDetail, automatic: flat art with its own colours, else fullTone, documentLibrary: at most `colors` of the library's colours, as they are), colors: 2-256, colorDetail: 0-100 (fullTone, automatic), library: \"document\" (the document's swatches) or a swatch library id or name (swatch.library.list; documentLibrary), paths: 0-100, corners: 0-100, noise: px, method: \"abutting\"|\"overlapping\", ignoreWhite (leave out white areas), ignoreColor?: \"#rrggbb\" (with ignoreWhite: the colour left out instead of white), snapCurvesToLines, shapes: false (filled outlines that are a circle, an ellipse, a rectangle or a regular polygon come out as that live shape), autoGrouping: false (each path that has others inside it is grouped with them), fills: true, strokes: false (Create: lines up to strokeWidth wide become stroked centre lines; without fills, wider areas become stroked outlines), strokeWidth: px (10), logoColors?: [\"#rrggbb\"…] (mode logo: flat-colour logos only; colours found automatically or named here, white is always its own colour, at most ten; noise = areas under noise/10 px ignored; edges to a fraction of a pixel, sharp corners restored, lines and circles snapped; enclosed transparent areas are painted white unless ignoreWhite makes them holes; fills/strokes are ignored; gradients and photos are refused)}, view?: as imageTrace.setView (default: the traced object's own, else tracingResult), ids?: [id…] (instead of id)} → {id, paths, anchors, colors}",
+            "{id?: image or Image Trace group (default: selection), preset?: name (imageTrace.presets), params?: {mode: \"blackAndWhite\"|\"grayscale\"|\"color\"|\"logo\", threshold: 0-255, palette: \"limited\"|\"fullTone\"|\"automatic\"|\"documentLibrary\" (color mode; limited: the `colors` best colours, fullTone: as many as the tones need, more with colorDetail, automatic: flat art with its own colours, else fullTone, documentLibrary: at most `colors` of the library's colours, as they are), colors: 2-256, colorDetail: 0-100 (fullTone, automatic), library: \"document\" (the document's swatches) or a swatch library id or name (swatch.library.list; documentLibrary), paths: 0-100, corners: 0-100, noise: px, method: \"abutting\"|\"overlapping\", ignoreWhite (leave out white areas), ignoreColor?: \"#rrggbb\" (with ignoreWhite: the colour left out instead of white), snapCurvesToLines, shapes: false (filled outlines that are a circle, an ellipse, a rectangle or a regular polygon come out as that live shape), autoGrouping: false (each path that has others inside it is grouped with them), gradients: false (grayscale and color: an area whose colour changes evenly along one direction is one path with a linear gradient fill), fills: true, strokes: false (Create: lines up to strokeWidth wide become stroked centre lines; without fills, wider areas become stroked outlines), strokeWidth: px (10), logoColors?: [\"#rrggbb\"…] (mode logo: flat-colour logos only; colours found automatically or named here, white is always its own colour, at most ten; noise = areas under noise/10 px ignored; edges to a fraction of a pixel, sharp corners restored, lines and circles snapped; enclosed transparent areas are painted white unless ignoreWhite makes them holes; fills/strokes are ignored; gradients and photos are refused)}, view?: as imageTrace.setView (default: the traced object's own, else tracingResult), ids?: [id…] (instead of id)} → {id, paths, anchors, colors}",
             has_selection_or_ids,
             |s, p| trace_make(s, p, false)
         ),
@@ -727,6 +727,16 @@ fn traced_shape(shape: tr::TracedShape, xf: vectorcraft_geom::Affine) -> LiveSha
     live
 }
 
+/// The gradient fill for a linear gradient the tracer found (in the image's pixels), placed in the
+/// document by `xf`.
+fn traced_gradient(g: &tr::TracedGradient, xf: vectorcraft_geom::Affine) -> Paint {
+    use vectorcraft_color::{Gradient, GradientGeom, GradientKind, GradientPaint, GradientStop};
+    let stops = g.stops.iter().map(|&(at, c)| GradientStop::new(at as f32, Color::rgb8(c[0], c[1], c[2]))).collect();
+    let mut paint = GradientPaint::new(Gradient { kind: GradientKind::Linear, stops });
+    paint.geom = Some(GradientGeom { start: xf * g.start, end: xf * g.end, aspect: 1.0, focal: None });
+    Paint::Gradient(Box::new(paint))
+}
+
 /// Auto Grouping: `nodes` (one per traced path, bottom-most first) with each path that has others
 /// inside it (`parents`, as the tracer gives them) in a group with them: the path, then what is
 /// inside it. Without parents, the nodes as they are.
@@ -835,7 +845,8 @@ fn trace_make(s: &mut Session, p: &Value, expand: bool) -> Result<Value> {
             }
             // A library colour fills with the swatch's own colour (its model kept).
             let color = library.iter().find(|(rgb, _)| *rgb == tp.color).map_or_else(|| Color::rgb8(tp.color[0], tp.color[1], tp.color[2]), |l| l.1);
-            let paint = Paint::solid(color);
+            // Gradients: a path the tracer found to be a linear gradient is filled with it.
+            let paint = tp.gradient.as_ref().map_or_else(|| Paint::solid(color), |g| traced_gradient(g, xf));
             n.appearance = match tp.stroke {
                 // Traced lines: round ends and corners, as drawn lines have.
                 Some(width) => Appearance {