Place: DCS transparency interactions look only at what clipping paths show

Show DCS Transparency Interactions compared full object bounds, so a
transparent object over the part of a DCS image that its clipping path
hides marked the image. Bounds are now cut to the clipping paths of the
clip groups and clipping layers that hold each object.

crates/engine/src/cmd/links.rs

@@ -1939,15 +1939,21 @@ fn status_rank(status: &str) -> usize {
 /// holds them) or overlap an object that is, as Show DCS Transparency Interactions marks them.
 fn dcs_transparency(d: &Document) -> BTreeSet<NodeId> {
     /// Each visible DCS image's id, bounds and whether it is transparent, and the bounds of each
-    /// other visible object with its own transparency.
-    fn walk(d: &Document, n: &Node, held: bool, images: &mut Vec<(NodeId, Rect, bool)>, see_through: &mut Vec<Rect>) {
+    /// other visible object with its own transparency; bounds are cut to `clip`, the bounds of the
+    /// clipping paths of the clip groups and layers holding the object.
+    fn walk(d: &Document, n: &Node, held: bool, clip: Option<Rect>, images: &mut Vec<(NodeId, Rect, bool)>, see_through: &mut Vec<Rect>) {
         if !n.visible {
             return;
         }
         let own = n.shows_own_transparency();
+        // The part of the object that shows through `clip`; none when it is clipped away.
+        let shown = n.geometric_bounds().and_then(|b| match clip {
+            Some(c) => Some(c.intersect(b)).filter(|i| i.area() > 0.0),
+            None => Some(b),
+        });
         match &n.kind {
             NodeKind::Image(im) if d.images.get(&im.key).is_some_and(ImageBlob::is_separated) => {
-                if let Some(b) = n.geometric_bounds() {
+                if let Some(b) = shown {
                     images.push((n.id, b, held || own));
                     // A transparent DCS image is an object with its own transparency for the
                     // others it overlaps.
@@ -1956,16 +1962,25 @@ fn dcs_transparency(d: &Document) -> BTreeSet<NodeId> {
                     }
                 }
             }
-            _ if own => see_through.extend(n.geometric_bounds()),
+            _ if own => see_through.extend(shown),
             _ => {}
         }
-        for c in n.children().into_iter().flatten() {
-            walk(d, c, held || own, images, see_through);
+        let Some(children) = n.children() else { return };
+        // A clip group's or clipping layer's first child clips the others.
+        let clip = match &n.kind {
+            NodeKind::Group { clip: true, .. } | NodeKind::Layer { clip: true, .. } => match children.first().and_then(|c| c.geometric_bounds()) {
+                Some(c) => Some(clip.map_or(c, |o| o.intersect(c))),
+                None => clip,
+            },
+            _ => clip,
+        };
+        for c in children {
+            walk(d, c, held || own, clip, images, see_through);
         }
     }
     let (mut images, mut see_through) = (vec![], vec![]);
     for l in &d.layers {
-        walk(d, l, false, &mut images, &mut see_through);
+        walk(d, l, false, None, &mut images, &mut see_through);
     }
     images
         .into_iter()

crates/engine/src/tests/dcs.rs

@@ -657,6 +657,32 @@ fn dcs_transparency_marks_transparent_and_overlapped_dcs_images() {
     assert!(error(&mut s, "links.list", json!({ "dcsTransparency": "yes" })).contains("dcsTransparency"), "a flag that isn't true or false");
 }
 
+#[test]
+fn dcs_transparency_looks_only_at_the_part_of_an_image_its_clipping_path_shows() {
+    let dir = temp_dir("transparency-clipped");
+    let path = write(&dir, "pic.eps", &two_plates());
+    let mut s = session();
+    let r = s.execute("file.place", &json!({ "path": path, "at": [100, 100] })).unwrap();
+    let group = NodeId(r["ids"][0].as_u64().unwrap());
+    let image = image_of(&s, &r["ids"][0]).id;
+    let (clip, all) = {
+        let doc = &s.doc().unwrap().doc;
+        let NodeKind::Group { children, clip: true } = &doc.node(group).unwrap().kind else { panic!("a clipping group") };
+        (children[0].geometric_bounds().unwrap(), doc.node(image).unwrap().geometric_bounds().unwrap())
+    };
+    assert!(all.x1 - clip.x1 > 2.0, "the file's clipping path shows the left half: {clip:?} of {all:?}");
+    // A half-transparent square over the part of the image the clipping path hides.
+    let x = (clip.x1 + all.x1) / 2.0 - 1.0;
+    let over = s.execute("shape.rectangle", &json!({"x": x, "y": all.y0, "width": 2, "height": 2})).unwrap()["id"].as_u64().unwrap();
+    s.execute("transparency.set", &json!({"ids": [over], "opacity": 50})).unwrap();
+    let marked = |s: &mut Session| s.execute("links.list", &json!({ "dcsTransparency": true })).unwrap()["links"][0]["dcsTransparency"] == true;
+    assert!(!marked(&mut s), "the square is over the hidden part only");
+    s.execute("transparency.set", &json!({"ids": [over], "opacity": 100})).unwrap();
+    let shown = s.execute("shape.rectangle", &json!({"x": clip.x0, "y": all.y0, "width": 2, "height": 2})).unwrap()["id"].as_u64().unwrap();
+    s.execute("transparency.set", &json!({"ids": [shown], "opacity": 50})).unwrap();
+    assert!(marked(&mut s), "a square over the part that shows");
+}
+
 #[test]
 fn dcs_transparency_counts_a_transparent_dcs_image_over_another() {
     let dir = temp_dir("transparency-two");

docs/mcp.md

@@ -6705,7 +6705,8 @@ DCS Transparency Interactions, off by default) adds `dcsTransparency: true` to t
 and the panel shows a warning mark on them. VectorCraft's rule for it: the image, or a group or layer holding it, has
 an opacity below 100%, a blend mode other than Normal, an opacity mask or an effect, or its bounds overlap the
 bounds of another visible object that has one of those or a gradient with a transparent stop (in a fill, a stroke or
-a character's fill or stroke); a DCS image with transparency of its own counts as such an object. The check looks at
+a character's fill or stroke); a DCS image with transparency of its own counts as such an object. Bounds are cut to the
+bounds of the clipping paths of the clip groups and clipping layers holding an object. The check looks at
 every object in the layers each time the list is read. A `dcsTransparency` that is not true or false is an error.
 `links.goTo {id}` selects an image; `links.embed {ids?}` keeps linked files' pixels in the document (an image whose
 file is missing stays linked: relink it first); `links.unembed {id, path}` writes an embedded image to a file