PDF import: a knockout group in a form of its own opacity reads back as one group

PDF export writes a translucent knockout group as a form with its opacity around the /K group. PDF import now reads that pair back as one knockout group with the opacity, blend mode and isolation, instead of a translucent group holding a knockout group.

crates/pdf/src/import/builder.rs

@@ -387,6 +387,21 @@ impl<'p> Builder<'p> {
                         only = fold_stop_opacity(&only, &m).unwrap_or_else(|| only.with_mask(Some(Box::new(m))));
                     }
                     Some(only)
+                } else if let [only] = children.as_slice()
+                    && matches!(only.kind, NodeKind::Group { clip: false, .. })
+                    && only.knockout == Knockout::On
+                    && only.opacity >= 0.999
+                    && only.blend == BlendMode::Normal
+                    && !only.isolate
+                    && only.extras().mask.is_none()
+                    && mask.is_none()
+                    && !knockout
+                {
+                    // A knockout group drawn in a form that carries its opacity, blend mode or
+                    // isolation: one knockout group with them.
+                    let mut only = (**only).clone();
+                    (only.opacity, only.blend, only.isolate) = (opacity, blend, isolate);
+                    Some(only)
                 } else {
                     let mut g = Node::group(self.id(), children);
                     (g.opacity, g.blend, g.isolate) = (opacity, blend, isolate);

crates/pdf/src/tests/blend.rs

@@ -175,6 +175,32 @@ fn an_isolated_knockout_group_is_one_isolated_pdf_knockout_group() {
     }
 }
 
+/// A translucent knockout group is written as a form with its opacity around the `/K` group, and
+/// reads back as one knockout group with that opacity and its isolation.
+#[test]
+fn a_translucent_knockout_group_reads_back_as_one_group() {
+    for isolate in [false, true] {
+        let mut d = knockout_squares(3, false, isolate);
+        let g = d.layers[0].children().unwrap()[1].id;
+        d.node_mut(g).unwrap().opacity = 0.5;
+        let top = d.node(g).unwrap().children().unwrap()[2].id;
+        d.node_mut(top).unwrap().blend = BlendMode::Multiply;
+        let back = import(&export(&d, &PdfOptions::default()).unwrap()).unwrap();
+        let mut found = vec![];
+        back.walk(|n| {
+            if matches!(n.kind, vectorcraft_doc::NodeKind::Group { .. }) {
+                found.push((
+                    n.knockout == vectorcraft_doc::Knockout::On,
+                    n.isolate,
+                    (n.opacity * 100.0).round(),
+                    n.children().map_or(0, |c| c.len()),
+                ));
+            }
+        });
+        assert_eq!(found, [(true, isolate, 50.0, 3)], "isolate {isolate}");
+    }
+}
+
 /// Elements whose opacity defines their knockout shape knock out less than a PDF knockout group
 /// knocks out: each is drawn through one soft mask of the elements above it, so that 200 of them
 /// still read back, every one of them.