Effects: Drop Shadow has Darkness, the object's own colors with black added

The reference app's Drop Shadow has a Color and a Darkness, the
percentage of black added to the object's own colors. Drop Shadow had
only a Color, so every shadow was one color. Choosing one or the other,
and the defaults, are VectorCraft's own.

Drop Shadow now takes `shade` ("color" or "darkness", default "color")
and `darkness` (%, default 100). Its dialog and the Appearance panel's
inline editor show Color and Darkness as radio buttons. With Darkness
d, a CMYK document keeps the object's cyan, magenta and yellow and
raises its black from K to K + d × (1 − K), and an RGB document
multiplies each channel by 1 − d, so in RGB 0% gives the object's
color and 100% gives black. A shadow saved without these keys draws as
before. SVG export writes a Darkness shadow with feComponentTransfer
and SVG import reads it back; CSS export leaves it out.

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

crates/effects/src/lib.rs

@@ -308,8 +308,8 @@ pub fn effect_catalog() -> Vec<EffectInfo> {
             "stylize.dropShadow",
             "Drop Shadow…",
             STYLIZE,
-            "{mode: blend mode (\"multiply\"), opacity: % (75), x: pt (7), y: pt (7), blur: pt (5), color: \"#rrggbb\" (\"#000000\")}",
-            json!({"mode": "multiply", "opacity": 75.0, "x": 7.0, "y": 7.0, "blur": 5.0, "color": "#000000"}),
+            "{mode: blend mode (\"multiply\"), opacity: % (75), x: pt (7), y: pt (7), blur: pt (5), shade: \"color\"|\"darkness\" (\"color\"), color: \"#rrggbb\" (\"#000000\"), darkness: % (100)} (shade \"darkness\": the shadow is the object's own colors with `darkness` percent of black added instead of `color`: in a CMYK document their cyan, magenta and yellow stay and their black K becomes K + darkness × (1 − K); in an RGB document each channel is multiplied by 1 − darkness)",
+            json!({"mode": "multiply", "opacity": 75.0, "x": 7.0, "y": 7.0, "blur": 5.0, "shade": "color", "color": "#000000", "darkness": 100.0}),
         ),
         r(
             "stylize.innerGlow",

crates/effects/src/raster.rs

@@ -21,6 +21,9 @@ pub enum RasterFx {
         dy: f64,
         blur: f64,
         color: Color,
+        /// Darkness (0..1): the shadow is the object's own colors with this share of black added
+        /// instead of `color`; `None` when it is `color`.
+        darkness: Option<f32>,
     },
     OuterGlow {
         mode: BlendMode,
@@ -111,6 +114,9 @@ pub fn raster_effects(effects: &[Effect]) -> Vec<RasterFx> {
                     dy: num(&p, "y", 7.0).clamp(-1e4, 1e4),
                     blur,
                     color: color_param(&p, Color::BLACK),
+                    darkness: text(&p, "shade", "color")
+                        .eq_ignore_ascii_case("darkness")
+                        .then(|| (num(&p, "darkness", 100.0) / 100.0).clamp(0.0, 1.0) as f32),
                 },
                 "stylize.outerGlow" => RasterFx::OuterGlow {
                     mode: blend_mode(text(&p, "mode", "screen")),

crates/effects/src/tests.rs

@@ -565,11 +565,12 @@ fn raster_effects_parse_and_outset() {
     let r = raster_effects(&list);
     assert_eq!(r.len(), 5);
     match &r[0] {
-        RasterFx::DropShadow { mode, opacity, dx, dy, blur, color } => {
+        RasterFx::DropShadow { mode, opacity, dx, dy, blur, color, darkness } => {
             assert_eq!(*mode, vectorcraft_doc::color::BlendMode::Multiply);
             assert!(close(*opacity as f64, 0.5, 1e-6));
             assert_eq!((*dx, *dy, *blur), (10.0, -4.0, 6.0));
             assert_eq!(color.to_rgb(), [1.0, 0.0, 0.0]);
+            assert_eq!(*darkness, None);
         }
         other => panic!("{other:?}"),
     }
@@ -578,6 +579,20 @@ fn raster_effects_parse_and_outset() {
     assert_eq!(outset(&[], vectorcraft_geom::Rect::ZERO), 0.0);
 }
 
+#[test]
+fn drop_shadow_darkness_replaces_its_color_when_chosen() {
+    let darkness = |params: Value| match raster_effects(&[fx("stylize.dropShadow", params)]).as_slice() {
+        [RasterFx::DropShadow { darkness, .. }] => *darkness,
+        other => panic!("{other:?}"),
+    };
+    assert_eq!(darkness(json!({"shade": "darkness", "darkness": 60})), Some(0.6));
+    assert_eq!(darkness(json!({"shade": "darkness"})), Some(1.0));
+    assert_eq!(darkness(json!({"shade": "darkness", "darkness": 250})), Some(1.0));
+    // Color is the default: a Darkness value alone leaves the shadow its color.
+    assert_eq!(darkness(json!({"darkness": 60})), None);
+    assert_eq!(darkness(json!({})), None);
+}
+
 #[test]
 fn blend_mode_names() {
     use vectorcraft_doc::color::BlendMode;