diff --git a/src/geo/polygons.js b/src/geo/polygons.js
index 64e954f4..6d6a1bf0 100644
--- a/src/geo/polygons.js
+++ b/src/geo/polygons.js
@@ -299,13 +299,14 @@
             min = minArea || 0.1,
             out = [];
 
-        function filter(from, to) {
+        function filter(from, to = []) {
             from.forEach(function(poly) {
                 if (poly.area() >= min) {
                     to.push(poly);
                     out.push(poly);
                 }
             });
+            return to;
         }
 
         // expensive but worth it?
@@ -319,6 +320,9 @@
                 cleanClipperTree(ctre);
                 filter(fromClipperTree(ctre, z, null, null, min), outA);
             }
+            let woutA = filter(geo.poly.diff(setA, setB, z));
+            // outA.appendAll(woutA);
+            // console.log({outA, woutA});
         }
 
         if (outB) {
@@ -334,6 +338,9 @@
                 cleanClipperTree(ctre);
                 filter(fromClipperTree(ctre, z, null, null, min), outB);
             }
+            let woutB = filter(geo.poly.diff(setB, setA, z));
+            // outB.appendAll(woutB);
+            // console.log({outB, woutB});
         }
 
         return out;
diff --git a/src/geo/wasm.js b/src/geo/wasm.js
index 8820a503..f2e39483 100644
--- a/src/geo/wasm.js
+++ b/src/geo/wasm.js
@@ -11,6 +11,7 @@ if (!self.window) (function() {
     const geo = self.geo;
 
     geo.poly = {
+        diff: polyDiff,
         union: polyUnion,
         offset: polyOffset
     };
@@ -94,6 +95,29 @@ if (!self.window) (function() {
         return polyNest(out);
     }
 
+    function polyDiff(polysA, polysB, z) {
+        let wasm = geo.wasm,
+            memat = wasm.malloc(1024 * 128),
+            writer = new DataWriter(wasm.heap, memat),
+            pcountA = 0,
+            pcountB = 0;
+        polysA.forEach(poly => pcountA += writePoly(writer, poly));
+        polysB.forEach(poly => pcountB += writePoly(writer, poly));
+        let resat = wasm.diff(memat, pcountA, pcountB, (pcountA + pcountB) * 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) => {
@@ -149,6 +173,7 @@ if (!self.window) (function() {
                 malloc: exports.mem_get,
                 start: exports._start,
                 free: exports.mem_clr,
+                diff: exports.poly_diff,
                 union: exports.poly_union,
                 offset: exports.poly_offset
             };
diff --git a/wasm/kiri-geo.cpp b/wasm/kiri-geo.cpp
index e04d8511..cc3057c9 100644
--- a/wasm/kiri-geo.cpp
+++ b/wasm/kiri-geo.cpp
@@ -94,7 +94,7 @@ Uint32 poly_offset(Uint32 memat, Uint32 polys, float offset) {
 }
 
 __attribute__ ((export_name("poly_union")))
-Uint32 poly_union(Uint32 memat, Uint32 polys) {
+Uint32 poly_union(Uint32 memat, Uint32 polys, float offset) {
 
     using namespace ClipperLib;
 
@@ -116,9 +116,72 @@ Uint32 poly_union(Uint32 memat, Uint32 polys) {
     }
 
     Clipper clip;
-    clip.AddPath(ins[0], ptSubject, true);
-    clip.AddPath(ins[1], ptClip, true);
-    clip.Execute(ctUnion, outs, pftEvenOdd, pftEvenOdd);
+    clip.AddPaths(ins, ptSubject, true);
+    clip.Execute(ctUnion, outs);
+
+    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;
+}
+
+
+
+__attribute__ ((export_name("poly_diff")))
+Uint32 poly_diff(Uint32 memat, Uint32 polysA, Uint32 polysB, float offset) {
+
+    using namespace ClipperLib;
+
+    Paths inA(polysA);
+    Paths inB(polysB);
+    Paths outs;
+    Uint32 pos = memat;
+
+    for (Uint32 poly = 0; poly < polysA; poly++ ) {
+        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;
+            inA[poly] << IntPoint(ip->x, ip->y);
+        }
+        poly++;
+    }
+
+    for (Uint32 poly = 0; poly < polysB; poly++ ) {
+        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;
+            inB[poly] << IntPoint(ip->x, ip->y);
+        }
+        poly++;
+    }
+
+    Clipper clip;
+    clip.AddPaths(inA, ptSubject, true);
+    clip.AddPaths(inB, ptClip, true);
+    clip.Execute(ctDifference, outs, pftEvenOdd, pftEvenOdd);
 
     Uint32 resat = pos;
 
diff --git a/wasm/kiri-geo.wasm b/wasm/kiri-geo.wasm
index d4525db1..332c1ef1 100755
Binary files a/wasm/kiri-geo.wasm and b/wasm/kiri-geo.wasm differ