line: Add center anchor

src/draw/shapes.typ

@@ -14,6 +14,7 @@
 #import "/src/anchor.typ" as anchor_
 #import "/src/mark.typ" as mark_
 #import "/src/mark-shapes.typ" as mark-shapes_
+#import "/src/polygon.typ"
 #import "/src/aabb.typ"
 
 #import "transformations.typ": *
@@ -504,6 +505,7 @@
 /// 
 /// = Anchors
 ///   Supports path anchors.
+///   The `"center"` anchor is calculated for triangles or closed polygons if all vertices share the same z value.
 ///
 /// - ..pts-style (coordinates, style): Positional two or more coordinates to draw lines between. Accepts style key-value pairs.
 /// - close (bool): If true, the line-strip gets closed to form a polygon
@@ -566,10 +568,17 @@
       close: close
     )
 
+    // Find center for simple polygons, might return none
+    let center = if close {
+      polygon.simple-centroid(pts)
+    }
+
     // Get bounds
     let (transform, anchors) = anchor_.setup(
-      auto,
-      (),
+      name => {
+        if name == "center" { return center }
+      },
+      if center != none { ("center",) } else { () },
       name: name,
       transform: ctx.transform,
       path-anchors: true,

src/polygon.typ

@@ -57,3 +57,36 @@
     none
   }
 }
+
+// Calculate triangle centroid
+#let triangle-centroid(points) = {
+  let (mx, my, mz) = points.first()
+  let n = points.len()
+  for p in points {
+    let (x, y, z) = p
+    mx += x / n
+    my += y / n
+    mz += z / n
+  }
+  return (mx, my, mz)
+}
+
+// Calculate the centroid of a line, triangle or simple polygon
+// Formulas:
+//   https://en.wikipedia.org/wiki/Centroid
+#let simple-centroid(points) = {
+  return if points.len() == 2 {
+    vector.lerp(..points, .5)
+  } else if points.len() == 3 {
+    calc-triangle-centroid(points)
+  } else if points.len() > 3 {
+    // Skip polygons with multiple z values
+    let z = points.first().at(2, default: 0)
+    if points.any(p => p.at(2) != z) {
+      return none
+    }
+
+    let a = signed-area(points)
+    return (cx/(6*a), cy/(6*a), z)
+  }
+}