diff --git a/notes.md b/notes.md
index b442cc0d..39ffe91a 100644
--- a/notes.md
+++ b/notes.md
@@ -103,10 +103,10 @@
 * `F` maintain several part orientations + op chains in a single profile
 
 * `P` improve parser - do not require spaced tokens and support implied G0 / G1
-* `P` inner corner arc moves
+* `P` outer corner arc moves
 * `P` log Z interpolation for contour XYZ moves
 * `P` option to start with the smallest poly by area on layer change
-* `P` refactor slicing engine to use common core in geo
+* `P` refactor slicing engine to add minion threading
 * `P` decrease cutting speed when entire tool is engaged (start of roughing, rest machining)
 * `P` port arc code from FDM export to CAM export
 * `P` redo all path route / planning in prepare to account for terrain before camOut
diff --git a/release.md b/release.md
index 6386efa4..381348cb 100644
--- a/release.md
+++ b/release.md
@@ -6,21 +6,22 @@ Full docs @ https://docs.grid.space/projects/kiri-moto
 # Release 3.8
 
 * update server-side sample module code
-* add new fdm parameters, range overrides
 * various fixes and improvements to CAM indexing
 * various fixes and improvements to gcode variables
-* CAM move to save Z between ops, refresh spindle speed
 * improve gcode arc import decoding
-* improve trace line selection with zoom adaptive thresholds
-* CAM add origin offset optional parameters
-* remove auto-decimation on object import
+* removed auto-decimation on object import
+* FDM slicing memory reduction from team lychee
+* FDM add new parameters, range overrides
 * FDM fix raft line fill strategy
-* slicing memory reduction from team lychee
-* option to control first output point order
+* CAM improve trace line selection with zoom adaptive thresholds
+* CAM option to control first output point order
+* CAM move to save Z between ops, refresh spindle speed
+* CAM add origin offset optional parameters
 * CAM add control to omit initial tool change
 * CAM fix vertical face selection and step over defaults
 * CAM fix multi-part object import / grouping
 * CAM fix tracing nested polyline offset ordering
+* CAM refactor of contouring yields 2x - 20x speedup
 
 - refactor object/group storage model in Mesh:Tool
 - add omit pocket to rough and outline
diff --git a/src/kiri-mode/cam/client.js b/src/kiri-mode/cam/client.js
index 3540fed4..259d4646 100644
--- a/src/kiri-mode/cam/client.js
+++ b/src/kiri-mode/cam/client.js
@@ -232,7 +232,7 @@ CAM.init = function(kiri, api) {
     ], () => {
         if (!isCamMode) return;
         for (let op of current.process.ops) {
-            if (op.type === 'trace') {
+            if (op.type === 'trace' && !flipping) {
                 op.areas = {};
             }
         }
@@ -720,6 +720,8 @@ CAM.init = function(kiri, api) {
         if (axis === 'Y') {
             API.selection.rotate(0, Math.PI, 0);
         }
+        // clear traces cache
+        CAM.traces_clear();
         flipping = false;
         process.ops = op2;
         process.op2 = ops;
diff --git a/src/kiri-mode/cam/driver.js b/src/kiri-mode/cam/driver.js
index f22d4cd2..e2de2b28 100644
--- a/src/kiri-mode/cam/driver.js
+++ b/src/kiri-mode/cam/driver.js
@@ -75,6 +75,12 @@ kiri.load(api => {
                 ondone(ids);
             });
         };
+
+        CAM.traces_clear = function(ondone) {
+            kiri.client.send("cam_traces_clear", {}, () => {
+                // console.log({clear_traces: true});
+            });
+        };
     }
 
     if (kiri.worker) {
@@ -114,15 +120,28 @@ kiri.load(api => {
             send.done(faces);
         }
 
-        kiri.worker.cam_traces = function(data, send) {
+        kiri.worker.cam_traces = async function(data, send) {
             const { settings, single } = data;
             const widgets = Object.values(kiri.worker.cache);
-            const fresh = widgets.filter(widget => CAM.traces(settings, widget, single));
+            const fresh = [];
+            for (let widget of widgets) {
+                if (await CAM.traces(settings, widget, single)) {
+                    fresh.push(widget);
+                }
+            }
+            // const fresh = widgets.filter(widget => CAM.traces(settings, widget, single));
             send.done(kiri.codec.encode(fresh.map(widget => { return {
                 id: widget.id,
                 traces: widget.traces,
             } } )));
         };
+
+        kiri.worker.cam_traces_clear = function(data, send) {
+            for (let widget of Object.values(kiri.worker.cache)) {
+                delete widget.traces;
+            }
+            send.done({});
+        };
     }
 
 });
diff --git a/src/kiri-mode/cam/ops.js b/src/kiri-mode/cam/ops.js
index 4f4381b1..7e2fc7b1 100644
--- a/src/kiri-mode/cam/ops.js
+++ b/src/kiri-mode/cam/ops.js
@@ -974,7 +974,7 @@ class OpTrace extends CamOp {
                     } else {
                         poly = [ poly ];
                     }
-                    for (let pi of poly)
+                    for (let pi of POLY.flatten(poly, [], true))
                     if (down) {
                         let zto = pi.getZ() - thru;
                         if (zThru && similar(zto,0)) {
diff --git a/src/kiri-mode/cam/slice.js b/src/kiri-mode/cam/slice.js
index 35ddcc08..f273528f 100644
--- a/src/kiri-mode/cam/slice.js
+++ b/src/kiri-mode/cam/slice.js
@@ -331,7 +331,7 @@ CAM.addDogbones = function(poly, dist, reverse) {
     }
 };
 
-CAM.traces = function(settings, widget, single) {
+CAM.traces = async function(settings, widget, single) {
     if (widget.traces && widget.trace_single === single) {
         // do no work if cached
         return false;
@@ -365,12 +365,13 @@ CAM.traces = function(settings, widget, single) {
             let z = poly.getZ();
             for (let i=0, il=traces.length; i<il; i++) {
                 let trace = traces[i];
+                let dz = Math.abs(z - trace.getZ());
                 // only compare polys farther apart in Z
-                if (Math.abs(z - trace.getZ()) > 0.01) {
+                if (dz < 0.01) {
                     continue;
                 }
                 // do not add duplicates
-                if (traces[i].isEquivalent(poly)) {
+                if (traces[i].isEquivalent(poly) && dz < 1) {
                     return;
                 }
             }
@@ -378,9 +379,9 @@ CAM.traces = function(settings, widget, single) {
         });
     };
     let opts = { each: oneach, over: false, flatoff: 0, edges: true, openok: true };
-    slicer.slice(indices, opts);
+    await slicer.slice(indices, opts);
     opts.over = true;
-    slicer.slice(indices, opts);
+    await slicer.slice(indices, opts);
     widget.traces = traces;
     widget.trace_single = single;
     return true;
diff --git a/src/kiri-run/worker.js b/src/kiri-run/worker.js
index 7cd381af..1feafd10 100644
--- a/src/kiri-run/worker.js
+++ b/src/kiri-run/worker.js
@@ -647,11 +647,15 @@ kiri.worker = {
     }
 };
 
+function is_async(fn) {
+    return fn.constructor.name === "AsyncFunction";
+}
+
 dispatch.send = (msg, direct) => {
     self.postMessage(msg, direct);
 };
 
-dispatch.onmessage = self.onmessage = function(e) {
+dispatch.onmessage = self.onmessage = async function(e) {
     let time_recv = time(),
         msg = e.data || {},
         run = dispatch[msg.task],
@@ -683,7 +687,7 @@ dispatch.onmessage = self.onmessage = function(e) {
     if (run) {
         try {
             let time_xfer = (time_recv - msg.time),
-                output = run(msg.data, send),
+                output = is_async(run) ? await run(msg.data, send) : run(msg.data, send),
                 time_send = time(),
                 time_proc = time_send - time_recv;