Pattern edit preview: two dimmed copy groups around centre tile

Review feedback on #3498: dimmed neighbours share one opacity composite
before the centre tile and one after, preserving overlap paint order
without per-copy compositing.

Co-authored-by: nrq6993-a11y <nrq6993-a11y@users.noreply.github.com>

crates/doc/src/pattern.rs

@@ -307,6 +307,23 @@ impl PatternDef {
         }
         out
     }
+    /// Preview copies drawn before and after the centre tile in [`Self::preview_offsets_in_paint_order`],
+    /// for one dimmed composite each (the centre tile is drawn at full strength between them).
+    pub fn preview_offsets_dimmed_sides(&self) -> (Vec<Vec2>, Vec<Vec2>) {
+        let mut before = vec![];
+        let mut after = vec![];
+        let mut past_center = false;
+        for o in self.preview_offsets_in_paint_order() {
+            if o.x == 0.0 && o.y == 0.0 {
+                past_center = true;
+            } else if past_center {
+                after.push(o);
+            } else {
+                before.push(o);
+            }
+        }
+        (before, after)
+    }
     /// Instances covering `region` (document space) as plain nodes for a pattern painted with
     /// `paint_xf` (pattern space → document), fresh-id-less (id 0). Used by exporters and Distort
     /// Pattern Fills; Expand uses [`Self::instances_in_with`].
@@ -768,6 +785,9 @@ mod tests {
         d2.overlap = Overlap { right_in_front: true, bottom_in_front: true };
         assert!(close(*d2.preview_offsets_in_paint_order().last().unwrap(), Vec2::new(10.0, 10.0)));
         assert_eq!(def.preview_offsets_in_paint_order().len(), 9);
+        let (before, after) = def.preview_offsets_dimmed_sides();
+        assert_eq!(before.len() + after.len(), 8);
+        assert!(before.iter().all(|o| *o != Vec2::ZERO) && after.iter().all(|o| *o != Vec2::ZERO));
     }
 
     #[test]

crates/render/src/pattern.rs

@@ -22,7 +22,7 @@ use std::sync::{Arc, Weak};
 use vectorcraft_color::cms::Cms;
 use vectorcraft_doc::pattern::{Overlap, PatternDef, TileType};
 use vectorcraft_doc::{Document, Node};
-use vectorcraft_geom::{Affine, Rect, Shape};
+use vectorcraft_geom::{Affine, Rect, Shape, Vec2};
 use vello_cpu::peniko::{Extend, ImageQuality, ImageSampler};
 use vello_cpu::{Pixmap, RenderContext};
 
@@ -299,6 +299,22 @@ pub fn render_pattern_swatch(doc: &Document, name: &str, size: u32) -> Option<Re
 }
 
 impl Renderer {
+    /// Dimmed pattern-editing preview copies at `offsets`.
+    fn draw_dimmed_pattern_copies(&mut self, ctx: &mut RenderContext, f: &Frame, dim: f32, offsets: &[Vec2], children: &[Arc<Node>]) {
+        if offsets.is_empty() {
+            return;
+        }
+        let comp = crate::group::Composite { opacity: dim, ..Default::default() };
+        self.group(ctx, f, comp, &mut |r, c, fr| {
+            for o in offsets {
+                let frame = Frame { view: fr.view * Affine::translate(*o), visible: fr.visible - *o, ..*fr };
+                for a in children {
+                    r.draw_arc(c, &frame, a);
+                }
+            }
+        });
+    }
+
     /// Pattern editing mode: draw the tile layer, its dimmed copies and the tile edge instead of
     /// the document. Returns false when not in pattern editing mode.
     pub(crate) fn draw_pattern_edit(&mut self, ctx: &mut RenderContext, f: &Frame) -> bool {
@@ -308,20 +324,10 @@ impl Renderer {
         let children: Vec<Arc<Node>> = layer.children().cloned().unwrap_or_default();
         if layer.visible && !children.is_empty() {
             let dim = (def.dim_copies / 100.0).clamp(0.0, 1.0);
-            for o in def.preview_offsets_in_paint_order() {
-                let is_center = o.x == 0.0 && o.y == 0.0;
-                if is_center {
-                    self.draw_arc(ctx, f, layer);
-                } else {
-                    let comp = crate::group::Composite { opacity: dim, ..Default::default() };
-                    self.group(ctx, f, comp, &mut |r, c, fr| {
-                        let frame = Frame { view: fr.view * Affine::translate(o), visible: fr.visible - o, ..*fr };
-                        for a in &children {
-                            r.draw_arc(c, &frame, a);
-                        }
-                    });
-                }
-            }
+            let (before, after) = def.preview_offsets_dimmed_sides();
+            self.draw_dimmed_pattern_copies(ctx, f, dim, &before, &children);
+            self.draw_arc(ctx, f, layer);
+            self.draw_dimmed_pattern_copies(ctx, f, dim, &after, &children);
         } else {
             self.draw_arc(ctx, f, layer);
         }