Color: freeform gradients keep CMYK inks when expanded, meshed or edited

Object > Expand and Object > Create Gradient Mesh mixed a freeform
gradient's points in display RGB and turned the result back into the
points' model with the profile-free formula. CMYK inks changed: points
of C50 M20 Y0 K10 gave mesh points of C45 M24 Y0 K22. RGB points were
stored as display sRGB, so with a working RGB space other than sRGB
the mesh showed other colors. Points colored along CMYK stops, a point
paint.freeform.splitLine added between CMYK points and a point
paint.freeform.addPoint added without a color came out as RGB.

Points that share a color model now mix in it: the C, M, Y and K
values mix as inks, and Gray, RGB and Lab components mix as they are.
Points in different models still mix in display RGB. Object > Expand
and Object > Create Gradient Mesh store the result in the working RGB
space, and in a CMYK document they store it as CMYK through the color
settings, as paint.setFill stores an RGB color. Freeform::recolor and
split_line mix with the new Color::lerp_keep_model, which
vectorcraft_doc::blend::lerp_color now calls.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/cmd/expand.rs

@@ -318,7 +318,7 @@ fn expand_fills(d: &mut Document, l: &Node, to: GradientTo) -> Option<Node> {
 /// fill's opacity, blend mode and effects other than geometry effects ([`fill_effects`]).
 fn gradient_art(d: &mut Document, l: &Node, path: &PathData, bounds: Rect, f: &FillLayer, g: &GradientPaint, to: GradientTo) -> Option<Node> {
     let content = if g.gradient.kind == GradientKind::Freeform {
-        let m = GradientMesh::for_path_with(path, FREEFORM_MESH, FREEFORM_MESH, &*gradient_sampler(g, bounds))?;
+        let m = GradientMesh::for_path_with(path, FREEFORM_MESH, FREEFORM_MESH, &*gradient_sampler(g, bounds, d.color_mode.model()))?;
         vec![Node::new(d.alloc_id(), NodeKind::Mesh(m))]
     } else {
         let frame = Frame::new(g, bounds);
@@ -720,29 +720,24 @@ fn tile_art(d: &mut Document, l: &Node, f: &FillLayer, tiles: &[Node]) -> Option
 
 /// The colour and opacity gradient fill `g` paints at a document point of an object whose box is
 /// `bounds`, as the canvas draws it (the end colours continue past the ends). Colours stay in the
-/// colour model their stops (or freeform points) share; display RGB where models differ.
-pub(crate) fn gradient_sampler(g: &GradientPaint, bounds: Rect) -> Box<dyn Fn(Point) -> (Color, f32) + '_> {
+/// color model their stops share; display RGB where models differ. Freeform points mix in the
+/// model they share ([`vectorcraft_color::Freeform::model_field`]); where their models differ, the
+/// RGB color they mix to is stored in `doc_model` (the document's model) the way `paint.setFill`
+/// stores an RGB color ([`super::paint::stored_color`]).
+pub(crate) fn gradient_sampler(g: &GradientPaint, bounds: Rect, doc_model: Model) -> Box<dyn Fn(Point) -> (Color, f32) + '_> {
     if g.gradient.kind == GradientKind::Freeform {
         let b = bounds.abs();
-        let f = g.freeform_on(b);
-        let model = shared_model(f.points.iter().map(|p| p.color));
-        let field = f.field(spread_scale(b));
+        let field = g.freeform_on(b).model_field(spread_scale(b));
+        let store_in = (field.model().is_none() && doc_model != Model::Rgb).then_some(doc_model);
         return Box::new(move |p| {
-            let ([r, g, b], a) = field.sample(p);
-            let c = Color::rgb(r, g, b);
-            (model.map_or(c, |m| c.in_model(m)), a)
+            let (c, a) = field.sample(p);
+            (super::paint::stored_color(store_in, c), a)
         });
     }
     let geom = g.resolve(bounds);
     Box::new(move |p| g.gradient.sample_with(geom.param_at(g.gradient.kind, p) as f32, lerp_color))
 }
 
-/// The colour model all of `colors` share.
-fn shared_model(mut colors: impl Iterator<Item = Color>) -> Option<Model> {
-    let m = colors.next()?.model();
-    colors.all(|c| c.model() == m).then_some(m)
-}
-
 /// Where the colour changes along a gradient between parameters `t0` and `t1`: both ends, every
 /// stop between them (in its own colour, so coincident stops keep a sharp edge) and every midpoint
 /// off the centre, as (parameter, colour, opacity) in order, with more between them where they

crates/engine/src/cmd/live.rs

@@ -1394,6 +1394,7 @@ fn mesh_create(s: &mut Session, p: &Value) -> Result<Value> {
         let mut out = vec![];
         let mut index = None;
         let mut refused = None;
+        let model = d.color_mode.model();
         for id in &roots {
             let Some(n) = d.node(*id).cloned() else { continue };
             let m = if let NodeKind::Image(im) = &n.kind {
@@ -1413,7 +1414,7 @@ fn mesh_create(s: &mut Session, p: &Value) -> Result<Value> {
                 match n.appearance.fill_paint() {
                     // The points take the colours the gradient paints there.
                     Paint::Gradient(g) => {
-                        path.bounds().and_then(|b| GradientMesh::for_path_with(&path, rows, cols, &*gradient_sampler(&g, b))).map(|mut m| {
+                        path.bounds().and_then(|b| GradientMesh::for_path_with(&path, rows, cols, &*gradient_sampler(&g, b, model))).map(|mut m| {
                             m.highlight(app, highlight);
                             m
                         })

crates/engine/src/cmd/paint.rs

@@ -301,7 +301,7 @@ fn new_color_model(s: &Session, p: &Value) -> Option<vectorcraft_color::cms::Mod
 /// `c` in `model` ([`new_color_model`]) when it is an RGB color, converted through the color
 /// settings (the working spaces and their intent) as Document Color Mode converts it. Other colors
 /// are returned as given.
-fn stored_color(model: Option<vectorcraft_color::cms::Model>, c: Color) -> Color {
+pub(crate) fn stored_color(model: Option<vectorcraft_color::cms::Model>, c: Color) -> Color {
     match (c, model) {
         (Color::Rgb { .. }, Some(m)) => {
             let cms = vectorcraft_color::cms::active();