diff --git a/src/geo/polygon.js b/src/geo/polygon.js
index c38453c0..f5c22757 100644
--- a/src/geo/polygon.js
+++ b/src/geo/polygon.js
@@ -1817,7 +1817,14 @@
                     ) {
                         return null;
                     } else if (debug) {
-                        console.log("union debug", { src, dst });
+                        console.log("union debug", {
+                            src,
+                            dst,
+                            d0: Math.abs(src[0].poly.area() - dst[0].poly.area()),
+                            d1: Math.abs(src[1].poly.area() - dst[1].poly.area()),
+                            m0: src[0].isEqual2D(dst[0]),
+                            m1: src[1].isEqual2D(dst[1])
+                        });
                     }
                     return null;
                 }
diff --git a/src/geo/wasm.js b/src/geo/wasm.js
index 5a9cc260..8820a503 100644
--- a/src/geo/wasm.js
+++ b/src/geo/wasm.js
@@ -10,7 +10,10 @@ if (!self.window) (function() {
     const factor = self.base.config.clipper;
     const geo = self.geo;
 
-    geo.poly = { offset : polyOffset };
+    geo.poly = {
+        union: polyUnion,
+        offset: polyOffset
+    };
 
     function log() {
         console.log(...arguments);
@@ -70,6 +73,27 @@ if (!self.window) (function() {
         return polyNest(out);
     }
 
+    function polyUnion(polys, z) {
+        let wasm = geo.wasm,
+            memat = wasm.malloc(1024 * 128),
+            writer = new DataWriter(wasm.heap, memat),
+            pcount = 0;
+        polys.forEach(poly => pcount += writePoly(writer, poly));
+        let resat = wasm.union(memat, pcount, pcount * factor),
+            reader = new DataReader(wasm.heap, resat),
+            out = [];
+        for (;;) {
+            let poly = readPoly(reader, z);
+            if (poly) {
+                out.push(poly);
+            } else {
+                break;
+            }
+        }
+        wasm.free(memat);
+        return polyNest(out);
+    }
+
     // nest closed polygons without existing parent / child relationships
     function polyNest(polys) {
         polys.sort((a,b) => {
@@ -120,14 +144,16 @@ if (!self.window) (function() {
             let heap = new DataView(exports.memory.buffer);
             let wasm = geo.wasm = {
                 heap,
+                exports,
                 memory: exports.memory,
-                start: exports._start,
-                offset: exports.poly_offset,
                 malloc: exports.mem_get,
-                free: exports.mem_clr
+                start: exports._start,
+                free: exports.mem_clr,
+                union: exports.poly_union,
+                offset: exports.poly_offset
             };
 
-            wasm.start();
+            // wasm.start();
         });
 
 })();
diff --git a/wasm/kiri-geo.cpp b/wasm/kiri-geo.cpp
index bb19c418..e04d8511 100644
--- a/wasm/kiri-geo.cpp
+++ b/wasm/kiri-geo.cpp
@@ -92,3 +92,53 @@ Uint32 poly_offset(Uint32 memat, Uint32 polys, float offset) {
 
     return resat;
 }
+
+__attribute__ ((export_name("poly_union")))
+Uint32 poly_union(Uint32 memat, Uint32 polys) {
+
+    using namespace ClipperLib;
+
+    Paths ins(polys);
+    Paths outs;
+    Uint32 pos = memat;
+    Uint16 poly = 0;
+
+    while (poly < polys) {
+        struct length16 *ls = (struct length16 *)(mem + pos);
+        Uint16 points = ls->length;
+        pos += 2;
+        while (points-- > 0) {
+            struct point32 *ip = (struct point32 *)(mem + pos);
+            pos += 8;
+            ins[poly] << IntPoint(ip->x, ip->y);
+        }
+        poly++;
+    }
+
+    Clipper clip;
+    clip.AddPath(ins[0], ptSubject, true);
+    clip.AddPath(ins[1], ptClip, true);
+    clip.Execute(ctUnion, outs, pftEvenOdd, pftEvenOdd);
+
+    Uint32 resat = pos;
+
+    for (Path po : outs) {
+        struct length16 *ls = (struct length16 *)(mem + pos);
+        ls->length = po.size();
+        pos += 2;
+        for (IntPoint pt : po) {
+            struct point32 *ip = (struct point32 *)(mem + pos);
+            ip->x = (int)pt.X;
+            ip->y = (int)pt.Y;
+            pos += 8;
+        }
+    }
+
+    // null terminate
+    struct length16 *ls = (struct length16 *)(mem + pos);
+    ls->length = 0;
+
+    clip.Clear();
+
+    return resat;
+}
diff --git a/wasm/kiri-geo.wasm b/wasm/kiri-geo.wasm
index a8169a9b..d4525db1 100755
Binary files a/wasm/kiri-geo.wasm and b/wasm/kiri-geo.wasm differ