diff --git a/src/geo/polygon.js b/src/geo/polygon.js
index fd476a96..95a9d5c9 100644
--- a/src/geo/polygon.js
+++ b/src/geo/polygon.js
@@ -704,6 +704,10 @@
         return !this.open;
     };
 
+    PRO.appearsClosed = function() {
+        return this.first().isEqual(this.last());
+    };
+
     PRO.setClockwise = function() {
         if (!this.isClockwise()) this.reverse();
         return this;
diff --git a/src/mode/cam/topo.js b/src/mode/cam/topo.js
index 63bb8c40..2528e458 100644
--- a/src/mode/cam/topo.js
+++ b/src/mode/cam/topo.js
@@ -73,7 +73,7 @@
 
             // return the touching z given topo x,y and a tool profile
             function toolTipZ(x,y) {
-                let profile = toolOffset,
+                const profile = toolOffset,
                     sx = stepsx,
                     sy = stepsy,
                     xl = sx - 1,
@@ -101,7 +101,7 @@
             }
 
             function push_point(x,y,z) {
-                let newP = newPoint(x,y,z);
+                const newP = newPoint(x,y,z);
                 if (lastP && lastP.z === z) {
                     if (curvesOnly) {
                         end_poly();
@@ -138,27 +138,27 @@
                     gridy,
                     gridi, // index
                     gridv, // value
-                    x, y, tv, ltv;
+                    i, il, j, jl, x, y, tv, ltv;
 
                 // for each Y slice, find z grid value (x/z swapped)
-                for (let j=0, jl=slices.length; j<jl; j++) {
-                    let slice = slices[j],
-                        lines = slice.lines;
+                for (j=0, jl=slices.length; j<jl; j++) {
+                    const slice = slices[j];
+                    const lines = slice.lines;
                     gridy = 0;
                     // slices have x/z swapped
                     for (y = minY; y < maxY && gridy < stepsy; y += resolution) {
                         gridi = gridx * stepsy + gridy;
                         gridv = data[gridi] || 0;
                         // strategy using raw lines (faster slice, but more lines)
-                        for (let i=0, il=lines.length; i<il; i++) {
-                            let line = lines[i], p1 = line.p1, p2 = line.p2;
+                        for (i=0, il=lines.length; i<il; i++) {
+                            const line = lines[i], p1 = line.p1, p2 = line.p2;
                             if (
                                 (p1.z > zMin || p2.z > zMin) && // one endpoint above 0
                                 (p1.z > gridv || p2.z > gridv) && // one endpoint above gridv
                                 ((p1.y <= y && p2.y >= y) || // one endpoint left
                                  (p2.y <= y && p1.y >= y)) // one endpoint right
                             ) {
-                                let dy = p1.y - p2.y,
+                                const dy = p1.y - p2.y,
                                     dz = p1.z - p2.z,
                                     pct = (p1.y - y) / dy,
                                     nz = p1.z - (dz * pct);
diff --git a/src/mode/fdm/prepare.js b/src/mode/fdm/prepare.js
index 899e9222..59b30b0e 100644
--- a/src/mode/fdm/prepare.js
+++ b/src/mode/fdm/prepare.js
@@ -409,12 +409,24 @@
 
         const opts = options || {};
         const tools = opts.tools || {};
-        const thin = opts.thin || false;
+        const flat = opts.flat;
+        const thin = opts.thin && !flat;
         const ckspeed = opts.speed !== false;
         const moveColor = opts.move >= 0 ? opts.move : 0xaaaaaa;
         const printColor = opts.print >= 0 ? opts.print : 0x777700;
         const layers = [];
 
+        const moveOpt = {
+            face: moveColor,
+            line: flat ? 1 : moveColor,
+            opacity: flat ? 0.5 : 1
+        };
+        const printOpt = {
+            face: printColor,
+            line: flat ? 1 : printColor,
+            opacity: flat ? 0.5 : 1
+        };
+
         const maxspd = levels.map(level => {
             return level.map(o => o.speed || 0).reduce((a, v) => Math.max(a,v));
         }).reduce((a, v) => Math.max(a, v)) + 1;
@@ -422,6 +434,7 @@
         // for reporting
         self.worker.print.maxSpeed = maxspd - 1;
         self.worker.print.thinColor = thin;
+        self.worker.print.flatColor = flat;
 
         let lastOut = null;
         let current = null;
@@ -490,15 +503,26 @@
                 pushPrint(lastOut.tool, current)
             }
             output
-                .setLayer(opts.other || 'move', moveColor, opts.moves !== true)
+                .setLayer(opts.other || 'move', moveOpt, opts.moves !== true)
                 .addPolys(moves, { thin: true, z: opts.z });
             Object.values(prints).forEach(array => {
-                array.forEach(poly => { if (poly.length > 1) output
-                    .setLayer(opts.action || 'print', printColor)
-                    .addPolys([ poly ], thin ? { thin, z: opts.z, color: poly.color } : {
-                        offset: array.width, height, z: opts.z,
-                        color: { face: poly.color, line: poly.color }
-                    })
+                array.forEach(poly => {
+                    if (poly.appearsClosed()) {
+                        poly.setClosed();
+                        poly.points.pop();
+                        poly.length--;
+                    }
+                    output
+                    .setLayer(opts.action || 'print', printOpt)
+                    .addPolys([ poly ],
+                        thin ? { thin, z: opts.z, color: poly.color } :
+                        flat ? {
+                            flat, z: opts.z, color: poly.color,
+                            outline: true, offset: array.width, open: poly.open  } :
+                        {
+                            offset: array.width, height, z: opts.z,
+                            color: { face: poly.color, line: poly.color }
+                        })
                 });
             });
 
diff --git a/src/mode/fdm/slice.js b/src/mode/fdm/slice.js
index 76643e51..c15ae15f 100644
--- a/src/mode/fdm/slice.js
+++ b/src/mode/fdm/slice.js
@@ -30,7 +30,11 @@
 
     function vopt(opt) {
         if (opt) {
-            if (isFlat) { opt.flat = true; return opt }
+            if (isFlat) {
+                opt.flat = true;
+                opt.outline = true;
+                return opt
+            }
             if (isThin) return null;
         }
         return opt;
@@ -71,9 +75,17 @@
             view = widget.mesh && widget.mesh.newGroup ? widget.mesh.newGroup() : null;
 
         isFlat = false;
-        isThin = settings.controller.thinRender;
+        isThin = !isFlat && settings.controller.thinRender;
         offset = nozzleSize / 2;
 
+        if (isFlat) {
+            Object.values(COLOR).forEach(color => {
+                color.flat = true;
+                color.line = 1
+                color.opacity = 0.5;
+            });
+        };
+
         if (!(sliceHeight > 0 && sliceHeight < 100)) {
             return ondone("invalid slice height");
         }