PDF export: a Perceptual gradient that mixes spot and process inks keeps its inks

A Perceptual linear or radial gradient that mixes spot colors with each
other or with CMYK or gray stops was written as an image in process
colors. It is now written as a DeviceN shading of its inks, with the
stops on its Perceptual curve: at each one, the inks of the two stops on
either side are mixed at the weight where the two colors mixed in
display RGB have the OKLab lightness of the color on the curve.

- Gradient::ink_mixes gives the curve's stops as weights between stops,
  under the same 4,096-stop limit as ink_tints.
- Perceptual gradients that mix a spot color with RGB or Lab stops,
  dithered fills and freeform gradients stay images, and the warning
  now names those cases.

crates/engine/src/cmd/gradient.rs

@@ -23,7 +23,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Gradient",
             ["Window", "Gradient"],
             None,
-            "{stroke?: bool (default: the targeted item's kind, else the active proxy), kind?: linear|radial|freeform (freeform places points on each object, coloured along the stops), mode?: points|lines (freeform: how the Gradient tool adds points), method?: classic|perceptual (how the colours between the stops mix: in display RGB, or perceptually (OKLab) so the steps look even, which SVG and PDF exports draw as an image, except that PDF writes one made of tints of one spot color in that ink; freeform gradients mix their own way), stops?: [{offset 0..1, color? (needed without swatch), opacity? 0..1 (or 0..100), midpoint? 0.13..0.87, swatch?: colour swatch name (a global or spot colour or tint swatch links the stop, so swatch edits recolour it and a spot stop prints on its plate; a process colour just gives its colour), tint?: 0..100 (% of the linked swatch; default 100, or a tint swatch's own)}] (at least 2; a freeform gradient's points are recoloured along them), keepModel?: bool (as paint.setFill; a stop color the gradient already has keeps its model), angle?: deg, aspect?: %, reverse?: bool, dither?: bool (fills only: a linear or radial gradient fill is drawn with fine noise that hides banding, and as pixels in SVG and PDF exports), item?: fill/stroke item index|null (omitted: the Appearance panel's active item when it is of the edited kind), ids?|id?, strokeMode?: within|along|across (strokes only: the gradient lies on the page and shows through the stroke, runs from the start of each subpath to its end, or runs from the stroke's left edge to its right all along it; with nothing selected, for the next object drawn; type characters' own strokes always paint within), target?: \"object\"|\"contents\"} edit the gradient in place (keeps its placement); solid/none paints become the default gradient",
+            "{stroke?: bool (default: the targeted item's kind, else the active proxy), kind?: linear|radial|freeform (freeform places points on each object, coloured along the stops), mode?: points|lines (freeform: how the Gradient tool adds points), method?: classic|perceptual (how the colours between the stops mix: in display RGB, or perceptually (OKLab) so the steps look even, which SVG and PDF exports draw as an image, except that PDF writes one with spot color stops in its inks when every other stop is paper white, CMYK or gray; freeform gradients mix their own way), stops?: [{offset 0..1, color? (needed without swatch), opacity? 0..1 (or 0..100), midpoint? 0.13..0.87, swatch?: colour swatch name (a global or spot colour or tint swatch links the stop, so swatch edits recolour it and a spot stop prints on its plate; a process colour just gives its colour), tint?: 0..100 (% of the linked swatch; default 100, or a tint swatch's own)}] (at least 2; a freeform gradient's points are recoloured along them), keepModel?: bool (as paint.setFill; a stop color the gradient already has keeps its model), angle?: deg, aspect?: %, reverse?: bool, dither?: bool (fills only: a linear or radial gradient fill is drawn with fine noise that hides banding, and as pixels in SVG and PDF exports), item?: fill/stroke item index|null (omitted: the Appearance panel's active item when it is of the edited kind), ids?|id?, strokeMode?: within|along|across (strokes only: the gradient lies on the page and shows through the stroke, runs from the start of each subpath to its end, or runs from the stroke's left edge to its right all along it; with nothing selected, for the next object drawn; type characters' own strokes always paint within), target?: \"object\"|\"contents\"} edit the gradient in place (keeps its placement); solid/none paints become the default gradient",
             has_doc,
             edit_gradient
         ),

crates/engine/src/tests/linked_stops.rs

@@ -236,11 +236,39 @@ fn perceptual_spot_gradients_export_as_separation_shadings() {
     assert!(pdf.contains("/C0[1]") && pdf.contains("/C1[0.4"), "white ink at 100% to 40%: {}", &pdf[at..(at + 200).min(pdf.len())]);
 }
 
+#[test]
+fn perceptual_spot_gradients_mixed_with_other_inks_export_as_device_n_shadings() {
+    let (mut s, _) = setup();
+    run(&mut s, "swatch.new", json!({"name": "Leaf", "color": {"c": 1, "m": 0, "y": 1, "k": 0}, "spot": true}));
+    let ink_to_cyan = json!({"stops": [{"offset": 0, "swatch": "Ink"}, {"offset": 1, "color": {"c": 1, "m": 0, "y": 0, "k": 0}}]});
+    let ink_to_leaf = json!({"stops": [{"offset": 0, "swatch": "Ink", "midpoint": 30}, {"offset": 1, "swatch": "Leaf", "tint": 50}]});
+    for (what, edit, space) in [
+        ("Ink → cyan", with(ink_to_cyan.clone(), json!({"method": "perceptual"})), "/DeviceN[/Cyan/Ink]/DeviceCMYK"),
+        ("Ink → Leaf 50 %", with(ink_to_leaf, json!({"method": "perceptual"})), "/DeviceN[/Ink/Leaf]/DeviceCMYK"),
+    ] {
+        run(&mut s, "paint.editGradient", edit);
+        let (pdf, warnings) = pdf_of(&s);
+        assert!(!pdf.contains("/Subtype/Image"), "{what}: no image");
+        assert!(pdf.contains("/ShadingType 2") && pdf.contains(space), "{what}: a DeviceN shading");
+        // The curve's stops: a stitching function of more than the two ends.
+        assert!(pdf.contains("/FunctionType 3"), "{what}: inks on the curve");
+        assert!(!warnings.iter().any(|w| w.contains("spot")), "{what}: {warnings:?}");
+    }
+    // The curve starts at the first stop's inks: no cyan, Ink at 100 %.
+    run(&mut s, "paint.editGradient", with(ink_to_cyan.clone(), json!({"method": "perceptual"})));
+    let (pdf, _) = pdf_of(&s);
+    assert!(pdf.contains("/C0[0 1]"), "the first stop's inks");
+    // A radial one too.
+    run(&mut s, "paint.editGradient", with(ink_to_cyan, json!({"method": "perceptual", "kind": "radial"})));
+    let (pdf, _) = pdf_of(&s);
+    assert!(!pdf.contains("/Subtype/Image") && pdf.contains("/ShadingType 3") && pdf.contains("/DeviceN[/Cyan/Ink]/DeviceCMYK"), "radial");
+}
+
 #[test]
 fn spot_gradients_drawn_as_images_warn() {
     let (mut s, _) = setup();
     let spot_warning = |w: &[String]| w.iter().any(|w| w.contains("spot colors in gradients exported as images"));
-    let ink_to_cyan = json!({"stops": [{"offset": 0, "swatch": "Ink"}, {"offset": 1, "color": {"c": 1, "m": 0, "y": 0, "k": 0}}]});
+    let ink_to_green = json!({"stops": [{"offset": 0, "swatch": "Ink"}, {"offset": 1, "color": "#00ff00"}]});
     // More stops than the export finds tints for: an image, as before.
     let n = 4097;
     let stops: Vec<Value> = (0..n)
@@ -250,7 +278,7 @@ fn spot_gradients_drawn_as_images_warn() {
         })
         .collect();
     for (what, edit) in [
-        ("Perceptual Ink → cyan", with(ink_to_cyan, json!({"method": "perceptual"}))),
+        ("Perceptual Ink → RGB green", with(ink_to_green, json!({"method": "perceptual"}))),
         ("dithered Classic Ink → paper", with(ink_to_white(100.0), json!({"dither": true, "method": "classic"}))),
         ("dithered Perceptual Ink → paper", with(ink_to_white(100.0), json!({"dither": true, "method": "perceptual"}))),
         ("Perceptual Ink and paper, 4,097 stops", json!({"stops": stops, "dither": false, "method": "perceptual"})),