fix: improve arc-tangency fitting and add layered engrave/cut passes for GravographIS

GeometrySimplifier/ArcFit now fit arcs that pass exactly through run
endpoints while balancing tangency error between trusted and estimated
directions, fixing arcs that previously bulged or broke tangent
continuity at fillet/compound-curve junctions.

GravographIS post processor gains per-layer (engrave/cut) tool passes
via a new GravographISPostConfig, so ENGRAVE/ETCH-tagged geometry runs
as a separate scribe pass with its own feed/depth and an operator
pause before the cut pass (spring-floated spindle needs a tool swap).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -120,3 +120,4 @@ Always keep `README.md` and `CLAUDE.md` up to date when making changes that affe
 - **Cut-off materialization lifecycle**: `CutOff` objects live on `Plate.CutOffs`. Each generates a `Drawing` (with `IsCutOff = true`) whose `Program` contains trimmed line segments. `Plate.RegenerateCutOffs(settings)` removes old cut-off Parts, recomputes programs, and re-adds them to `Plate.Parts`. Regeneration triggers: cut-off add/remove/move, part drag complete, fill complete, plate transform. Cut-off Parts are excluded from quantity tracking, utilization, overlap detection, and nest file serialization (programs are regenerated from definitions on load).
 - **User-defined G-code variables**: Programs can contain named variable definitions (`name = expression [inline] [global]`) referenced in coordinates with `$name`. Variables resolve to doubles at parse time for geometry/nesting. `VariableRefs` on `Motion`/`Feedrate` track the symbolic link so post processors can emit machine variable references. Cincinnati post maps non-inline variables to numbered machine variables (`#200+`) with descriptive comments. Global variables share a number across programs; local variables get per-drawing numbers. `ProgramReader` uses a two-pass parse (collect definitions, then parse G-code with substitution). `NestWriter` serializes definitions and `$references` back to text for round-trip fidelity.
 - **CAD import pipeline**: All "DXF → Drawing" conversion goes through `OpenNest.IO.CadImporter`. The UI form uses `Import` on file load (storing the mutable result in a `FileListItem`) and `BuildDrawing` on save (passing the user's current visible entities and bends). Console, MCP, API, and Training projects use `ImportDrawing` for headless conversion. This guarantees all callers produce drawings with the same shape: pierce-point `Source.Offset`, stable `SourceEntities` with GUIDs, `SuppressedEntityIds`, detected bends, and metadata.
+- **GravographIS engrave/cut passes**: The `OpenNest.Posts.GravographIS` post splits geometry by `LayerType` into ordered tool passes — engrave (`Scribe`) then cut (`Cut`/`Leadin`/`Leadout`); `Display` is skipped. `ConvertGeometry` tags DXF layers `ENGRAVE`/`ETCH` (lines, arcs, circles) as `Scribe`; the layer round-trips through `.nest` via `NestWriter`/`ProgramReader`. `NestPolylineExtractor.ExtractLayered` carries `LayerType` per polyline (splitting a continuous chain at any layer change); `GravographISPostProcessor.BuildPasses` groups them and `GravographISWriter.Write(IReadOnlyList<GravographPass>, …)` emits each pass at its own feed/depth, parking to origin and emitting an operator pause (motor off → aux off → `LB` console message → motor on) before any pass whose config has `PauseBefore`. Per-pass parameters live in `GravographISPostConfig` (an `IConfigurablePostProcessor` config with `Engrave`/`Cut` `LayerCutConfig` blocks), edited in the shared `PostProcessorConfigForm` PropertyGrid and persisted to JSON. The cut block pauses by default so the operator can swap/adjust the tool (the spring-floated spindle means programmed `DZ` depth is not the real cut depth).

OpenNest.Core/Converters/ConvertGeometry.cs

@@ -87,14 +87,15 @@ namespace OpenNest.Converters
 
             lastpt = endpt;
 
+            var layer = ClassifyLayer(arc);
             var sweep = System.Math.Abs(arc.SweepAngle());
             if (sweep < Tolerance.Epsilon || sweep.IsEqualTo(Angle.TwoPI))
             {
-                pgm.LineTo(endpt);
+                pgm.Codes.Add(new LinearMove(endpt) { Layer = layer });
             }
             else
             {
-                pgm.ArcTo(endpt, arc.Center, arc.IsReversed ? RotationType.CW : RotationType.CCW);
+                pgm.Codes.Add(new ArcMove(endpt, arc.Center, arc.IsReversed ? RotationType.CW : RotationType.CCW) { Layer = layer });
             }
 
             return lastpt;
@@ -107,7 +108,7 @@ namespace OpenNest.Converters
             if (startpt.DistanceTo(lastpt) > Tolerance.ChainTolerance)
                 pgm.MoveTo(startpt);
 
-            pgm.ArcTo(startpt, circle.Center, circle.Rotation);
+            pgm.Codes.Add(new ArcMove(startpt, circle.Center, circle.Rotation) { Layer = ClassifyLayer(circle) });
 
             lastpt = startpt;
             return lastpt;
@@ -118,13 +119,22 @@ namespace OpenNest.Converters
             if (line.StartPoint.DistanceTo(lastpt) > Tolerance.ChainTolerance)
                 pgm.MoveTo(line.StartPoint);
 
-            var move = new LinearMove(line.EndPoint);
-            if (string.Equals(line.Layer?.Name, "ETCH", System.StringComparison.OrdinalIgnoreCase))
-                move.Layer = LayerType.Scribe;
-            pgm.Codes.Add(move);
+            pgm.Codes.Add(new LinearMove(line.EndPoint) { Layer = ClassifyLayer(line) });
 
             lastpt = line.EndPoint;
             return lastpt;
         }
+
+        // Engrave/etch geometry maps to Scribe so the post processor can treat it as a
+        // separate tool pass; everything else keeps the move's default Cut layer.
+        private static LayerType ClassifyLayer(Entity geo)
+        {
+            var name = geo.Layer?.Name;
+            if (string.Equals(name, "ENGRAVE", System.StringComparison.OrdinalIgnoreCase) ||
+                string.Equals(name, "ETCH", System.StringComparison.OrdinalIgnoreCase))
+                return LayerType.Scribe;
+
+            return LayerType.Cut;
+        }
     }
 }

OpenNest.Core/Geometry/ArcFit.cs

@@ -57,13 +57,14 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Fits a circular arc constrained to be tangent to the given directions at both
-        /// the first and last points. The center lies at the intersection of the normals
-        /// at P1 and Pn, guaranteeing the arc departs P1 in the start direction and arrives
-        /// at Pn in the end direction. Uses the radius from P1 (exact start tangent);
-        /// deviation includes any endpoint gap at Pn.
+        /// Fits a circular arc that passes exactly through both the first and last points
+        /// while matching the given endpoint tangents as closely as possible. For any
+        /// circle through two points, the tangents at those points make equal mirrored
+        /// angles with the chord, so the achievable inscribed angle is the average of the
+        /// two requested ones — when the requested tangents are consistent with a single
+        /// circular arc, both are matched exactly.
         /// </summary>
-        internal static (Vector center, double radius, double deviation) FitWithDualTangent(
+        internal static (Vector center, double radius, double deviation) FitThroughEndpointsWithTangents(
             List<Vector> points, Vector startTangent, Vector endTangent)
         {
             if (points.Count < 3)
@@ -72,42 +73,39 @@ namespace OpenNest.Geometry
             var p1 = points[0];
             var pn = points[^1];
 
-            var stLen = System.Math.Sqrt(startTangent.X * startTangent.X + startTangent.Y * startTangent.Y);
-            var etLen = System.Math.Sqrt(endTangent.X * endTangent.X + endTangent.Y * endTangent.Y);
-            if (stLen < 1e-10 || etLen < 1e-10)
-                return (Vector.Invalid, 0, double.MaxValue);
-
-            // Normal to start tangent at P1 (perpendicular)
-            var n1x = -startTangent.Y / stLen;
-            var n1y = startTangent.X / stLen;
-
-            // Normal to end tangent at Pn
-            var n2x = -endTangent.Y / etLen;
-            var n2y = endTangent.X / etLen;
-
-            // Solve: P1 + t1*N1 = Pn + t2*N2
-            var det = n1x * (-n2y) - (-n2x) * n1y;
-            if (System.Math.Abs(det) < 1e-10)
-                return (Vector.Invalid, 0, double.MaxValue);
-
             var dx = pn.X - p1.X;
             var dy = pn.Y - p1.Y;
-            var t1 = (dx * (-n2y) - (-n2x) * dy) / det;
-
-            var cx = p1.X + t1 * n1x;
-            var cy = p1.Y + t1 * n1y;
-
-            // Use radius from P1 (guarantees exact start tangent and passes through P1)
-            var r1 = System.Math.Sqrt((cx - p1.X) * (cx - p1.X) + (cy - p1.Y) * (cy - p1.Y));
-            if (r1 < 1e-10)
+            var chordLen = System.Math.Sqrt(dx * dx + dy * dy);
+            if (chordLen < 1e-10)
                 return (Vector.Invalid, 0, double.MaxValue);
 
-            // Measure endpoint gap at Pn
-            var r2 = System.Math.Sqrt((cx - pn.X) * (cx - pn.X) + (cy - pn.Y) * (cy - pn.Y));
-            var endpointDev = System.Math.Abs(r2 - r1);
+            var ux = dx / chordLen;
+            var uy = dy / chordLen;
 
-            var interiorDev = MaxRadialDeviation(points, cx, cy, r1);
-            return (new Vector(cx, cy), r1, System.Math.Max(endpointDev, interiorDev));
+            // Inscribed angle between chord and tangent at each endpoint (mirrored at Pn)
+            var theta1 = SignedAngle(ux, uy, startTangent);
+            var theta2 = -SignedAngle(ux, uy, endTangent);
+            var theta = (theta1 + theta2) / 2;
+
+            // Nearly straight or degenerate (sweep would exceed ~356 degrees)
+            if (System.Math.Abs(theta) < 1e-3 || System.Math.Abs(theta) > System.Math.PI * 0.99)
+                return (Vector.Invalid, 0, double.MaxValue);
+
+            var halfChord = chordLen / 2;
+            var radius = halfChord / System.Math.Abs(System.Math.Sin(theta));
+            var d = -halfChord / System.Math.Tan(theta);
+
+            var cx = (p1.X + pn.X) / 2 + d * -uy;
+            var cy = (p1.Y + pn.Y) / 2 + d * ux;
+
+            return (new Vector(cx, cy), radius, MaxRadialDeviation(points, cx, cy, radius));
+        }
+
+        private static double SignedAngle(double ux, double uy, Vector to)
+        {
+            var len = System.Math.Sqrt(to.X * to.X + to.Y * to.Y);
+            if (len < 1e-10) return 0;
+            return System.Math.Atan2(ux * to.Y - uy * to.X, ux * to.X + uy * to.Y);
         }
 
         /// <summary>

OpenNest.Core/Geometry/GeometrySimplifier.cs

@@ -374,11 +374,8 @@ public class GeometrySimplifier
         var points = CollectPoints(entities, start, k);
         if (points.Count < 3) return null;
 
-        var startTangent = chainedTangent.IsValid()
-            ? chainedTangent
-            : new Vector(points[1].X - points[0].X, points[1].Y - points[0].Y);
-
-        var endTangent = GetExitDirection(entities[k]);
+        var startTangent = EstimateStartTangent(entities, start, points, chainedTangent);
+        var endTangent = EstimateEndTangent(entities, k, points);
         var (center, radius, dev) = TryFit(points, startTangent, endTangent);
         if (!center.IsValid()) return null;
 
@@ -386,8 +383,10 @@ public class GeometrySimplifier
         while (k + 1 <= runEnd)
         {
             var extPoints = CollectPoints(entities, start, k + 1);
-            var extEndTangent = GetExitDirection(entities[k + 1]);
-            var (nc, nr, nd) = extPoints.Count >= 3 ? TryFit(extPoints, startTangent, extEndTangent) : (Vector.Invalid, 0, 0d);
+            if (extPoints.Count < 3) break;
+
+            var extEndTangent = EstimateEndTangent(entities, k + 1, extPoints);
+            var (nc, nr, nd) = TryFit(extPoints, startTangent, extEndTangent);
             if (!nc.IsValid()) break;
 
             k++;
@@ -407,37 +406,172 @@ public class GeometrySimplifier
         return new ArcFitResult(center, radius, dev, points, k);
     }
 
-    private (Vector center, double radius, double deviation) TryFit(List<Vector> points, Vector startTangent, Vector endTangent)
+    private (Vector center, double radius, double deviation) TryFit(
+        List<Vector> points, TangentEstimate start, TangentEstimate end)
     {
-        // Try dual-tangent fit first (matches direction at both endpoints)
-        if (endTangent.IsValid())
+        foreach (var (center, radius, dev) in FitAttempts(points, start, end))
         {
-            var (dc, dr, dd) = ArcFit.FitWithDualTangent(points, startTangent, endTangent);
-            if (dc.IsValid() && dd <= Tolerance)
+            if (!center.IsValid() || dev > Tolerance)
+                continue;
+
+            // Check that the arc doesn't bulge away from the original line segments
+            var isReversed = SumSignedAngles(center, points) < 0;
+            var arcDev = MaxArcToSegmentDeviation(points, center, radius, isReversed);
+            if (arcDev > Tolerance)
+                continue;
+
+            return (center, radius, System.Math.Max(dev, arcDev));
+        }
+
+        return (Vector.Invalid, 0, 0);
+    }
+
+    /// <summary>
+    /// Yields fit attempts in preference order. A trusted tangent (chained from the
+    /// previous arc, an adjacent original arc, or a long straight edge) is enforced
+    /// exactly on its side; otherwise the tangency error is balanced between both
+    /// endpoints. The unconstrained mirror-axis fit is the last resort. Every attempt
+    /// passes exactly through both endpoints, so no gaps are introduced.
+    /// </summary>
+    private IEnumerable<(Vector center, double radius, double deviation)> FitAttempts(
+        List<Vector> points, TangentEstimate start, TangentEstimate end)
+    {
+        if (start.Trusted && !end.Trusted)
+        {
+            yield return ArcFit.FitWithStartTangent(points, start.Direction);
+            yield return ArcFit.FitThroughEndpointsWithTangents(points, start.Direction, end.Direction);
+            yield return FitWithEndTangent(points, end.Direction);
+        }
+        else if (end.Trusted && !start.Trusted)
+        {
+            yield return FitWithEndTangent(points, end.Direction);
+            yield return ArcFit.FitThroughEndpointsWithTangents(points, start.Direction, end.Direction);
+            yield return ArcFit.FitWithStartTangent(points, start.Direction);
+        }
+        else
+        {
+            yield return ArcFit.FitThroughEndpointsWithTangents(points, start.Direction, end.Direction);
+            yield return ArcFit.FitWithStartTangent(points, start.Direction);
+            yield return FitWithEndTangent(points, end.Direction);
+        }
+
+        yield return FitMirrorAxis(points);
+    }
+
+    /// <summary>
+    /// Fits an arc through both endpoints with an exact tangent at the last point,
+    /// by running the start-tangent fit on the reversed point sequence.
+    /// </summary>
+    private static (Vector center, double radius, double deviation) FitWithEndTangent(
+        List<Vector> points, Vector endTangent)
+    {
+        var reversed = new List<Vector>(points);
+        reversed.Reverse();
+        return ArcFit.FitWithStartTangent(reversed, new Vector(-endTangent.X, -endTangent.Y));
+    }
+
+    /// <summary>
+    /// An estimated tangent direction at a fit endpoint. Trusted estimates come from
+    /// exact geometry (a chained arc, an adjacent original arc, or a long straight
+    /// edge) and are enforced exactly; untrusted ones are derived from the polyline
+    /// vertices and only guide the fit.
+    /// </summary>
+    private readonly record struct TangentEstimate(Vector Direction, bool Trusted);
+
+    /// <summary>Segment-length ratio above which a neighboring line counts as a true
+    /// straight edge (rather than another chord of the tessellated curve).</summary>
+    private const double NeighborEdgeFactor = 3.0;
+
+    private static TangentEstimate EstimateStartTangent(
+        List<Entity> entities, int start, List<Vector> points, Vector chainedTangent)
+    {
+        if (chainedTangent.IsValid())
+            return new TangentEstimate(chainedTangent, true);
+
+        if (entities[start] is Arc startArc)
+            return new TangentEstimate(GetEntryDirection(startArc), true);
+
+        var firstChordLen = points[0].DistanceTo(points[1]);
+        if (start > 0)
+        {
+            var prev = entities[start - 1];
+            var prevEnd = prev switch { Line l => l.EndPoint, Arc a => a.EndPoint(), _ => Vector.Invalid };
+            if (prevEnd.IsValid() && prevEnd.DistanceTo(points[0]) < 1e-6)
             {
-                var isRev = SumSignedAngles(dc, points) < 0;
-                var aDev = MaxArcToSegmentDeviation(points, dc, dr, isRev);
-                if (aDev <= Tolerance)
-                    return (dc, dr, System.Math.Max(dd, aDev));
+                if (prev is Arc)
+                    return new TangentEstimate(GetExitDirection(prev), true);
+                if (prev is Line prevLine && prevLine.StartPoint.DistanceTo(prevLine.EndPoint) >= NeighborEdgeFactor * firstChordLen)
+                    return new TangentEstimate(GetExitDirection(prevLine), true);
             }
         }
 
-        // Fall back to start-tangent-only, then mirror axis
-        var (center, radius, dev) = ArcFit.FitWithStartTangent(points, startTangent);
-        if (!center.IsValid() || dev > Tolerance)
-            (center, radius, dev) = FitMirrorAxis(points);
-        if (!center.IsValid() || dev > Tolerance)
-            return (Vector.Invalid, 0, 0);
-
-        // Check that the arc doesn't bulge away from the original line segments
-        var isReversed = SumSignedAngles(center, points) < 0;
-        var arcDev = MaxArcToSegmentDeviation(points, center, radius, isReversed);
-        if (arcDev > Tolerance)
-            return (Vector.Invalid, 0, 0);
-
-        return (center, radius, System.Math.Max(dev, arcDev));
+        var chord = new Vector(points[1].X - points[0].X, points[1].Y - points[0].Y);
+        if (points.Count >= 3)
+            return new TangentEstimate(EstimateVertexTangent(points[0], points[1], points[2], chord), false);
+        return new TangentEstimate(chord, false);
     }
 
+    private static TangentEstimate EstimateEndTangent(List<Entity> entities, int k, List<Vector> points)
+    {
+        if (entities[k] is Arc endArc)
+            return new TangentEstimate(GetExitDirection(endArc), true);
+
+        var lastChordLen = points[^1].DistanceTo(points[^2]);
+        if (k + 1 < entities.Count)
+        {
+            var next = entities[k + 1];
+            var nextStart = next switch { Line l => l.StartPoint, Arc a => a.StartPoint(), _ => Vector.Invalid };
+            if (nextStart.IsValid() && nextStart.DistanceTo(points[^1]) < 1e-6)
+            {
+                if (next is Arc nextArc)
+                    return new TangentEstimate(GetEntryDirection(nextArc), true);
+                if (next is Line nextLine && nextLine.StartPoint.DistanceTo(nextLine.EndPoint) >= NeighborEdgeFactor * lastChordLen)
+                    return new TangentEstimate(GetExitDirection(nextLine), true);
+            }
+        }
+
+        var chord = new Vector(points[^1].X - points[^2].X, points[^1].Y - points[^2].Y);
+        if (points.Count >= 3)
+            return new TangentEstimate(EstimateVertexTangent(points[^1], points[^2], points[^3], chord), false);
+        return new TangentEstimate(chord, false);
+    }
+
+    /// <summary>
+    /// Estimates the curve tangent at a polyline vertex from the circle through it and
+    /// its two nearest neighbors. A raw chord direction is off from the true tangent by
+    /// half the chord's subtended angle; the circumcircle estimate removes that bias.
+    /// Falls back to the travel direction when the three points are collinear.
+    /// </summary>
+    private static Vector EstimateVertexTangent(Vector at, Vector b, Vector c, Vector travel)
+    {
+        var d = 2 * (at.X * (b.Y - c.Y) + b.X * (c.Y - at.Y) + c.X * (at.Y - b.Y));
+        if (System.Math.Abs(d) < 1e-14)
+            return travel;
+
+        var sqA = at.X * at.X + at.Y * at.Y;
+        var sqB = b.X * b.X + b.Y * b.Y;
+        var sqC = c.X * c.X + c.Y * c.Y;
+        var cx = (sqA * (b.Y - c.Y) + sqB * (c.Y - at.Y) + sqC * (at.Y - b.Y)) / d;
+        var cy = (sqA * (c.X - b.X) + sqB * (at.X - c.X) + sqC * (b.X - at.X)) / d;
+
+        var tangent = new Vector(-(at.Y - cy), at.X - cx);
+        if (tangent.X * travel.X + tangent.Y * travel.Y < 0)
+            tangent = new Vector(-tangent.X, -tangent.Y);
+        return tangent;
+    }
+
+    /// <summary>
+    /// Returns the entry direction (tangent at start point) of an entity.
+    /// </summary>
+    private static Vector GetEntryDirection(Entity entity) => entity switch
+    {
+        Line line => new Vector(line.EndPoint.X - line.StartPoint.X, line.EndPoint.Y - line.StartPoint.Y),
+        Arc arc => arc.IsReversed
+            ? new Vector(System.Math.Sin(arc.StartAngle), -System.Math.Cos(arc.StartAngle))
+            : new Vector(-System.Math.Sin(arc.StartAngle), System.Math.Cos(arc.StartAngle)),
+        _ => Vector.Invalid,
+    };
+
     /// <summary>
     /// Computes the tangent direction at the last point of a fitted arc,
     /// used to chain tangent continuity to the next arc.

OpenNest.IO/Dxf.cs

@@ -242,6 +242,7 @@ namespace OpenNest.IO
             public ExportContext()
             {
                 Document = new CadDocument();
+                Document.Header.Version = ACadVersion.AC1018;
 
                 CutLayer = new Layer("Cut") { Color = new Color(1) };
                 RapidLayer = new Layer("Rapid") { Color = new Color(5) };

OpenNest.Posts.GravographIS/GravographISPostConfig.cs

@@ -0,0 +1,81 @@
+using System.ComponentModel;
+using OpenNest.CNC;
+
+namespace OpenNest.Posts.GravographIS
+{
+    /// <summary>
+    /// Cut parameters for one kind of pass (engrave or cut). Edited in the post
+    /// configuration PropertyGrid and persisted to JSON.
+    /// </summary>
+    [TypeConverter(typeof(ExpandableObjectConverter))]
+    public sealed class LayerCutConfig
+    {
+        [DisplayName("Feed (mm/sec)")]
+        [Description("XY and Z feed for this pass. Patches the VS and VZ wire commands.")]
+        public int FeedMmPerSec { get; set; } = 10;
+
+        [DisplayName("Depth (inches)")]
+        [Description("Programmed Z plunge (DZ). Note: the spring-floated spindle means this does not set actual cut depth — tool protrusion does.")]
+        public double Depth { get; set; } = 0.25;
+
+        [DisplayName("Pause Before")]
+        [Description("Stop the spindle and prompt the operator before this pass begins, so the tool can be swapped/adjusted.")]
+        public bool PauseBefore { get; set; }
+
+        [DisplayName("Pause Message")]
+        [Description("Message shown on the controller during the pause.")]
+        public string PauseMessage { get; set; } = "";
+
+        public override string ToString() => $"{FeedMmPerSec} mm/s, {Depth:0.###}\"" + (PauseBefore ? ", pause" : "");
+    }
+
+    /// <summary>
+    /// Configuration for the Gravograph IS post processor: one <see cref="LayerCutConfig"/>
+    /// per cut kind. The engrave block applies to <see cref="LayerType.Scribe"/> paths,
+    /// the cut block to <see cref="LayerType.Cut"/>/<see cref="LayerType.Leadin"/>/<see cref="LayerType.Leadout"/>.
+    /// The cut block carries the tool-change pause by default.
+    /// </summary>
+    public sealed class GravographISPostConfig
+    {
+        [Category("Engrave (Scribe)")]
+        [DisplayName("Engrave")]
+        [Description("Parameters for engrave/scribe geometry (text).")]
+        public LayerCutConfig Engrave { get; set; } = new LayerCutConfig
+        {
+            FeedMmPerSec = 10,
+            Depth = 0.25,
+            PauseBefore = false,
+            PauseMessage = "",
+        };
+
+        [Category("Cut")]
+        [DisplayName("Cut")]
+        [Description("Parameters for cut geometry (outlines). Pauses for a tool change by default.")]
+        public LayerCutConfig Cut { get; set; } = new LayerCutConfig
+        {
+            FeedMmPerSec = 3,
+            Depth = 0.25,
+            PauseBefore = true,
+            PauseMessage = "Change tool",
+        };
+
+        /// <summary>
+        /// Returns the cut config a polyline of the given layer should use, or null
+        /// if the layer is non-cutting (<see cref="LayerType.Display"/>) and should be skipped.
+        /// </summary>
+        public LayerCutConfig ConfigFor(LayerType layer)
+        {
+            switch (layer)
+            {
+                case LayerType.Scribe:
+                    return Engrave;
+                case LayerType.Cut:
+                case LayerType.Leadin:
+                case LayerType.Leadout:
+                    return Cut;
+                default:
+                    return null;
+            }
+        }
+    }
+}

OpenNest.Posts.GravographIS/GravographISPostProcessor.cs

@@ -1,16 +1,30 @@
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.IO.Ports;
+using System.Text.Json;
+using System.Text.Json.Serialization;
 using System.Threading;
+using OpenNest.Geometry;
 
 namespace OpenNest.Posts.GravographIS
 {
     /// <summary>
     /// IPostProcessor implementation for the Gravograph IS8000. <see cref="Post(Nest, Stream)"/>
     /// writes the binary HPGL bytes. For serial streaming, use <see cref="Stream(Nest, string, Handshake, CancellationToken)"/>.
+    ///
+    /// Geometry is split by <see cref="OpenNest.CNC.LayerType"/> into an engrave pass
+    /// (Scribe) and a cut pass (Cut), each with its own feed/depth from <see cref="Config"/>.
+    /// The cut pass pauses by default so the operator can swap/adjust the tool.
     /// </summary>
-    public sealed class GravographISPostProcessor : IPostProcessor
+    public sealed class GravographISPostProcessor : IConfigurablePostProcessor
     {
+        private static readonly JsonSerializerOptions JsonOptions = new()
+        {
+            WriteIndented = true,
+            Converters = { new JsonStringEnumConverter() }
+        };
+
         public string Name => "Gravograph IS8000";
         public string Author => "OpenNest";
         public string Description => "Gravograph IS8000 mechanical engraver (binary HPGL over serial)";
@@ -23,14 +37,53 @@ namespace OpenNest.Posts.GravographIS
 
         public bool AllowReverse { get; set; } = true;
 
+        public GravographISPostConfig Config { get; }
+
+        object IConfigurablePostProcessor.Config => Config;
+
+        public GravographISPostProcessor()
+        {
+            var configPath = GetConfigPath();
+            if (File.Exists(configPath))
+            {
+                var json = File.ReadAllText(configPath);
+                Config = JsonSerializer.Deserialize<GravographISPostConfig>(json, JsonOptions)
+                    ?? new GravographISPostConfig();
+            }
+            else
+            {
+                Config = new GravographISPostConfig();
+                SaveConfig();
+            }
+        }
+
+        public GravographISPostProcessor(GravographISPostConfig config)
+        {
+            Config = config ?? throw new ArgumentNullException(nameof(config));
+        }
+
+        public void SaveConfig()
+        {
+            var configPath = GetConfigPath();
+            var json = JsonSerializer.Serialize(Config, JsonOptions);
+            File.WriteAllText(configPath, json);
+        }
+
+        private static string GetConfigPath()
+        {
+            var assemblyPath = typeof(GravographISPostProcessor).Assembly.Location;
+            var dir = Path.GetDirectoryName(assemblyPath);
+            var name = Path.GetFileNameWithoutExtension(assemblyPath);
+            return Path.Combine(dir, name + ".json");
+        }
+
         public void Post(Nest nest, Stream outputStream)
         {
             if (nest == null) throw new ArgumentNullException(nameof(nest));
             if (outputStream == null) throw new ArgumentNullException(nameof(outputStream));
 
-            var polylines = Extractor.Extract(nest);
-            var prepared = PolylinePrePass.Prepare(polylines, StitchTolerance, AllowReverse);
-            new GravographISWriter(WriterOptions).Write(prepared, outputStream);
+            var passes = BuildPasses(Extractor.ExtractLayered(nest));
+            new GravographISWriter(WriterOptions).Write(passes, outputStream);
         }
 
         public void Post(Nest nest, string outputFile)
@@ -39,6 +92,52 @@ namespace OpenNest.Posts.GravographIS
             Post(nest, fs);
         }
 
+        /// <summary>
+        /// Groups layer-tagged polylines into ordered tool passes: engrave (Scribe)
+        /// first, then cut. Each group is stitch/reverse-optimized independently.
+        /// Geometry whose layer maps to no config (Display) is skipped. When only one
+        /// group is present, a single pass is returned (and so the writer emits no pause).
+        /// </summary>
+        public IReadOnlyList<GravographPass> BuildPasses(IEnumerable<LayeredPolyline> polylines)
+        {
+            if (polylines == null) throw new ArgumentNullException(nameof(polylines));
+
+            var engrave = new List<IReadOnlyList<Vector>>();
+            var cut = new List<IReadOnlyList<Vector>>();
+
+            foreach (var poly in polylines)
+            {
+                if (poly == null) continue;
+                var block = Config.ConfigFor(poly.Layer);
+                if (block == null)
+                    continue; // non-cutting (Display) geometry
+                if (ReferenceEquals(block, Config.Engrave))
+                    engrave.Add(poly.Points);
+                else
+                    cut.Add(poly.Points);
+            }
+
+            var passes = new List<GravographPass>();
+            if (engrave.Count > 0)
+                passes.Add(MakePass(Config.Engrave, engrave));
+            if (cut.Count > 0)
+                passes.Add(MakePass(Config.Cut, cut));
+
+            return passes;
+        }
+
+        private GravographPass MakePass(LayerCutConfig block, List<IReadOnlyList<Vector>> polylines)
+        {
+            return new GravographPass
+            {
+                Polylines = PolylinePrePass.Prepare(polylines, StitchTolerance, AllowReverse),
+                FeedMmPerSec = block.FeedMmPerSec,
+                DepthInches = block.Depth,
+                PauseBefore = block.PauseBefore,
+                PauseMessage = block.PauseMessage ?? "",
+            };
+        }
+
         /// <summary>
         /// Buffers the encoded job in memory, then streams it to the named COM port.
         /// </summary>

OpenNest.Posts.GravographIS/GravographISWriter.cs

@@ -1,10 +1,30 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Text;
 using OpenNest.Geometry;
 
 namespace OpenNest.Posts.GravographIS
 {
+    /// <summary>
+    /// One tool pass: a run of polylines cut at a single feed/depth, optionally
+    /// preceded by an operator pause (to swap or adjust the tool). The Gravograph
+    /// post builds one pass for engrave and one for cut.
+    /// </summary>
+    public sealed class GravographPass
+    {
+        public IEnumerable<IReadOnlyList<Vector>> Polylines { get; set; }
+
+        public int FeedMmPerSec { get; set; }
+
+        public double DepthInches { get; set; }
+
+        /// <summary>When true, park to origin and prompt the operator before this pass.</summary>
+        public bool PauseBefore { get; set; }
+
+        public string PauseMessage { get; set; } = "";
+    }
+
     /// <summary>
     /// Encodes polylines (in inches) into the Gravograph IS8000 native "binary HPGL"
     /// wire format. The byte stream is byte-exact against captures from GravoStyle'98.
@@ -84,12 +104,40 @@ namespace OpenNest.Posts.GravographIS
         public void Write(IEnumerable<IReadOnlyList<Vector>> polylines, Stream output)
         {
             if (polylines == null) throw new ArgumentNullException(nameof(polylines));
+
+            // A single pass at the configured feed/depth — byte-identical to the
+            // original single-group output (no transitions, no pause).
+            Write(new[]
+            {
+                new GravographPass
+                {
+                    Polylines = polylines,
+                    FeedMmPerSec = Options.FeedMmPerSec,
+                    DepthInches = Options.DepthInches,
+                    PauseBefore = false,
+                    PauseMessage = "",
+                },
+            }, output);
+        }
+
+        /// <summary>
+        /// Writes the full byte stream for an ordered list of tool passes. The preamble
+        /// carries the first pass's feed/depth; each later pass emits an inline feed
+        /// (and depth, if changed) and, when <see cref="GravographPass.PauseBefore"/> is
+        /// set, parks to the operator origin and emits an operator pause before cutting.
+        /// </summary>
+        public void Write(IReadOnlyList<GravographPass> passes, Stream output)
+        {
+            if (passes == null) throw new ArgumentNullException(nameof(passes));
             if (output == null) throw new ArgumentNullException(nameof(output));
 
+            var firstFeed = passes.Count > 0 ? passes[0].FeedMmPerSec : Options.FeedMmPerSec;
+            var firstDepth = passes.Count > 0 ? passes[0].DepthInches : Options.DepthInches;
+
             var preamble = (byte[])PreambleTemplate.Clone();
-            PatchOperand(preamble, (byte)'V', (byte)'S', (short)Options.FeedMmPerSec);
-            PatchOperand(preamble, (byte)'V', (byte)'Z', (short)Options.FeedMmPerSec);
-            PatchOperand(preamble, (byte)'D', (byte)'Z', DepthInStepsAsInt16());
+            PatchOperand(preamble, (byte)'V', (byte)'S', (short)firstFeed);
+            PatchOperand(preamble, (byte)'V', (byte)'Z', (short)firstFeed);
+            PatchOperand(preamble, (byte)'D', (byte)'Z', DepthInStepsAsInt16(firstDepth));
             output.Write(preamble, 0, preamble.Length);
 
             // Cumulative head position from the operator-set upper-left origin, in
@@ -105,45 +153,51 @@ namespace OpenNest.Posts.GravographIS
 
             var firstPolyline = true;
             var polyIndex = 0;
+            var currentFeed = firstFeed;
+            var currentDepth = firstDepth;
 
-            foreach (var poly in polylines)
+            for (var p = 0; p < passes.Count; p++)
             {
-                polyIndex++;
-                if (poly == null || poly.Count < 2)
-                    continue;
+                var pass = passes[p];
 
-                var (startX, startY) = ToWire(poly[0]);
-                WriteTravel(output,
-                    firstPolyline ? (byte)'D' : (byte)'P',
-                    firstPolyline ? (byte)'R' : (byte)'U',
-                    checked(startX - headX), checked(startY - headY),
-                    ref headX, ref headY, envelopeXSteps, envelopeYSteps, polyIndex);
-
-                // PD command + single records-follow flag, then one record per segment.
-                output.WriteByte(0xFF);
-                output.WriteByte(0xFD);
-                output.WriteByte((byte)'P');
-                output.WriteByte((byte)'D');
-                output.WriteByte(0x00);
-                output.WriteByte(0x00);
-
-                var prevX = startX;
-                var prevY = startY;
-                for (int i = 1; i < poly.Count; i++)
+                if (p > 0)
                 {
-                    var (cx, cy) = ToWire(poly[i]);
-                    var dx = checked(cx - prevX);
-                    var dy = checked(cy - prevY);
-                    EnsureEnvelope(headX + dx, headY + dy, envelopeXSteps, envelopeYSteps,
-                        polyIndex, segment: i, isTravel: false);
-                    WriteRecord(output, dx, dy);
-                    prevX = cx;
-                    prevY = cy;
-                    headX += dx;
-                    headY += dy;
+                    if (pass.PauseBefore)
+                    {
+                        // Park: lift Z, then rapid (pen-up) back to the operator origin so
+                        // the head is clear of the work while the tool is swapped.
+                        WriteLiftOnly(output);
+                        WriteTravel(output, (byte)'P', (byte)'U',
+                            checked(-headX), checked(-headY),
+                            ref headX, ref headY, envelopeXSteps, envelopeYSteps, polyIndex);
+                        WritePauseCore(output, pass.PauseMessage);
+                    }
+
+                    if (pass.FeedMmPerSec != currentFeed)
+                    {
+                        WriteCommand(output, (byte)'V', (byte)'S', (short)pass.FeedMmPerSec);
+                        WriteCommand(output, (byte)'V', (byte)'Z', (short)pass.FeedMmPerSec);
+                        currentFeed = pass.FeedMmPerSec;
+                    }
+
+                    if (pass.DepthInches != currentDepth)
+                    {
+                        WriteCommand(output, (byte)'D', (byte)'Z', DepthInStepsAsInt16(pass.DepthInches));
+                        currentDepth = pass.DepthInches;
+                    }
                 }
 
-                firstPolyline = false;
+                if (pass.Polylines == null) continue;
+
+                foreach (var poly in pass.Polylines)
+                {
+                    polyIndex++;
+                    if (poly == null || poly.Count < 2)
+                        continue;
+
+                    WritePolyline(output, poly, ref firstPolyline, ref headX, ref headY,
+                        envelopeXSteps, envelopeYSteps, polyIndex);
+                }
             }
 
             WriteLiftOnly(output);
@@ -156,6 +210,89 @@ namespace OpenNest.Posts.GravographIS
             output.Write(EndJobBytes, 0, EndJobBytes.Length);
         }
 
+        private void WritePolyline(Stream output, IReadOnlyList<Vector> poly,
+            ref bool firstPolyline, ref int headX, ref int headY,
+            int envelopeXSteps, int envelopeYSteps, int polyIndex)
+        {
+            var (startX, startY) = ToWire(poly[0]);
+            WriteTravel(output,
+                firstPolyline ? (byte)'D' : (byte)'P',
+                firstPolyline ? (byte)'R' : (byte)'U',
+                checked(startX - headX), checked(startY - headY),
+                ref headX, ref headY, envelopeXSteps, envelopeYSteps, polyIndex);
+
+            // PD command + single records-follow flag, then one record per segment.
+            output.WriteByte(0xFF);
+            output.WriteByte(0xFD);
+            output.WriteByte((byte)'P');
+            output.WriteByte((byte)'D');
+            output.WriteByte(0x00);
+            output.WriteByte(0x00);
+
+            var prevX = startX;
+            var prevY = startY;
+            for (int i = 1; i < poly.Count; i++)
+            {
+                var (cx, cy) = ToWire(poly[i]);
+                var dx = checked(cx - prevX);
+                var dy = checked(cy - prevY);
+                EnsureEnvelope(headX + dx, headY + dy, envelopeXSteps, envelopeYSteps,
+                    polyIndex, segment: i, isTravel: false);
+                WriteRecord(output, dx, dy);
+                prevX = cx;
+                prevY = cy;
+                headX += dx;
+                headY += dy;
+            }
+
+            firstPolyline = false;
+        }
+
+        // The operator pause, minus the leading lift/park which the caller emits.
+        // Stops the spindle (MC off), turns off aux, writes the console message, then
+        // restarts the spindle (MC on) so the job resumes when the operator presses start.
+        private static void WritePauseCore(Stream s, string message)
+        {
+            WriteCommandRaw(s, (byte)'M', (byte)'C', 0x00, 0x00); // motor off
+            WriteCommandRaw(s, (byte)'O', (byte)'U', 0xFF, 0xFB); // aux off
+            WriteCommandRaw(s, (byte)'O', (byte)'U', 0xFF, 0xFA); // aux off
+            WriteCommandRaw(s, (byte)'L', (byte)'B', 0x00, 0x00); // begin message
+            WriteMessagePackets(s, message);
+            WriteCommandRaw(s, (byte)'N', (byte)'R', 0x00, 0x01); // line terminator
+            WriteCommandRaw(s, (byte)'L', (byte)'B', 0x00, 0x01); // end message
+            WriteCommandRaw(s, (byte)'M', (byte)'C', 0x00, 0x01); // motor on
+        }
+
+        // Console label packets carry two ASCII chars each; an odd-length message is
+        // space-padded to a whole number of packets so widths stay 2 bytes.
+        private static void WriteMessagePackets(Stream s, string message)
+        {
+            if (string.IsNullOrEmpty(message)) return;
+
+            var chars = Encoding.ASCII.GetBytes(message);
+            for (var i = 0; i < chars.Length; i += 2)
+            {
+                var c0 = chars[i];
+                var c1 = (i + 1 < chars.Length) ? chars[i + 1] : (byte)0x20;
+                WriteCommandRaw(s, (byte)'L', (byte)'B', c0, c1);
+            }
+        }
+
+        private static void WriteCommand(Stream s, byte c0, byte c1, short value)
+        {
+            WriteCommandRaw(s, c0, c1, (byte)((value >> 8) & 0xFF), (byte)(value & 0xFF));
+        }
+
+        private static void WriteCommandRaw(Stream s, byte c0, byte c1, byte hi, byte lo)
+        {
+            s.WriteByte(0xFF);
+            s.WriteByte(0xFD);
+            s.WriteByte(c0);
+            s.WriteByte(c1);
+            s.WriteByte(hi);
+            s.WriteByte(lo);
+        }
+
         private const double StepsPerMm = 80.0;
 
         private void EnsureEnvelope(int wireX, int wireY,
@@ -181,11 +318,11 @@ namespace OpenNest.Posts.GravographIS
                 $"work envelope from upper-left origin. Refusing to emit the record.");
         }
 
-        private short DepthInStepsAsInt16()
+        private static short DepthInStepsAsInt16(double depthInches)
         {
-            var steps = (long)System.Math.Round(Options.DepthInches * StepsPerInch, MidpointRounding.AwayFromZero);
+            var steps = (long)System.Math.Round(depthInches * StepsPerInch, MidpointRounding.AwayFromZero);
             if (steps < short.MinValue || steps > short.MaxValue)
-                throw new ArgumentOutOfRangeException(nameof(Options.DepthInches), $"Depth {Options.DepthInches} in. → {steps} steps overflows int16.");
+                throw new ArgumentOutOfRangeException(nameof(depthInches), $"Depth {depthInches} in. → {steps} steps overflows int16.");
             return (short)steps;
         }
 

OpenNest.Posts.GravographIS/NestPolylineExtractor.cs

@@ -1,15 +1,35 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using OpenNest.CNC;
 using OpenNest.Geometry;
 
 namespace OpenNest.Posts.GravographIS
 {
+    /// <summary>
+    /// A polyline together with the <see cref="LayerType"/> of the moves that
+    /// produced it. The Gravograph post groups by layer to emit separate engrave
+    /// and cut passes (with a tool-change pause between them).
+    /// </summary>
+    public sealed class LayeredPolyline
+    {
+        public LayeredPolyline(List<Vector> points, LayerType layer)
+        {
+            Points = points;
+            Layer = layer;
+        }
+
+        public List<Vector> Points { get; }
+
+        public LayerType Layer { get; }
+    }
+
     /// <summary>
     /// Lifts polylines out of an OpenNest <see cref="Nest"/> for the Gravograph
     /// backend. Walks each <see cref="Part"/>'s <see cref="Program"/>, breaks
-    /// polylines at rapid moves, and tessellates arcs to a chord-deviation
-    /// tolerance (the wire format takes line segments only).
+    /// polylines at rapid moves and at <see cref="LayerType"/> changes, and
+    /// tessellates arcs to a chord-deviation tolerance (the wire format takes
+    /// line segments only).
     /// </summary>
     public sealed class NestPolylineExtractor
     {
@@ -17,13 +37,31 @@ namespace OpenNest.Posts.GravographIS
 
         /// <summary>
         /// Extracts polylines from every non-cutoff part in every plate of the nest,
-        /// returning them in plate coordinates (inches).
+        /// returning them in plate coordinates (inches). Layer information is dropped;
+        /// use <see cref="ExtractLayered(Nest)"/> to keep it.
         /// </summary>
         public List<List<Vector>> Extract(Nest nest)
+        {
+            return ExtractLayered(nest).Select(p => p.Points).ToList();
+        }
+
+        /// <summary>
+        /// Extracts polylines for a single part without layer information.
+        /// </summary>
+        public List<List<Vector>> ExtractPart(Part part)
+        {
+            return ExtractPartLayered(part).Select(p => p.Points).ToList();
+        }
+
+        /// <summary>
+        /// Extracts layer-tagged polylines from every non-cutoff part in every plate,
+        /// in plate coordinates (inches). Each polyline is layer-uniform.
+        /// </summary>
+        public List<LayeredPolyline> ExtractLayered(Nest nest)
         {
             if (nest == null) throw new ArgumentNullException(nameof(nest));
 
-            var result = new List<List<Vector>>();
+            var result = new List<LayeredPolyline>();
 
             foreach (var plate in nest.Plates)
             {
@@ -40,17 +78,17 @@ namespace OpenNest.Posts.GravographIS
         }
 
         /// <summary>
-        /// Extracts polylines for a single part. Public so callers driving the
-        /// writer directly (e.g. from a console one-off) can use it.
+        /// Extracts layer-tagged polylines for a single part. Public so callers
+        /// driving the writer directly (e.g. from a console one-off) can use it.
         /// </summary>
-        public List<List<Vector>> ExtractPart(Part part)
+        public List<LayeredPolyline> ExtractPartLayered(Part part)
         {
-            var list = new List<List<Vector>>();
+            var list = new List<LayeredPolyline>();
             ExtractPart(part, list);
             return list;
         }
 
-        private void ExtractPart(Part part, List<List<Vector>> sink)
+        private void ExtractPart(Part part, List<LayeredPolyline> sink)
         {
             var program = part.Program;
             if (program == null) return;
@@ -67,6 +105,7 @@ namespace OpenNest.Posts.GravographIS
             var offset = part.Location;
             var pos = new Vector(0, 0);
             List<Vector> current = null;
+            var currentLayer = LayerType.Cut;
 
             foreach (var code in program.Codes)
             {
@@ -77,17 +116,14 @@ namespace OpenNest.Posts.GravographIS
                 {
                     case RapidMove rapid:
                     {
-                        FlushCurrent(sink, ref current);
+                        FlushCurrent(sink, ref current, currentLayer);
                         pos = rapid.EndPoint;
                         break;
                     }
 
                     case LinearMove linear:
                     {
-                        if (current == null)
-                        {
-                            current = new List<Vector> { pos + offset };
-                        }
+                        StartOrSplit(sink, ref current, ref currentLayer, linear.Layer, pos + offset);
                         var end = linear.EndPoint;
                         current.Add(end + offset);
                         pos = end;
@@ -96,10 +132,7 @@ namespace OpenNest.Posts.GravographIS
 
                     case ArcMove arc:
                     {
-                        if (current == null)
-                        {
-                            current = new List<Vector> { pos + offset };
-                        }
+                        StartOrSplit(sink, ref current, ref currentLayer, arc.Layer, pos + offset);
                         TessellateArc(pos, arc, offset, ArcChordToleranceInches, current);
                         pos = arc.EndPoint;
                         break;
@@ -107,13 +140,33 @@ namespace OpenNest.Posts.GravographIS
                 }
             }
 
-            FlushCurrent(sink, ref current);
+            FlushCurrent(sink, ref current, currentLayer);
         }
 
-        private static void FlushCurrent(List<List<Vector>> sink, ref List<Vector> current)
+        // Ensures `current` is an open polyline whose layer matches `moveLayer`,
+        // seeded at `seed` (the current pen position). When the layer changes
+        // mid-chain the previous polyline is flushed and a new one begins at the
+        // shared seam vertex so engrave and cut passes stay geometrically continuous.
+        private static void StartOrSplit(List<LayeredPolyline> sink, ref List<Vector> current,
+            ref LayerType currentLayer, LayerType moveLayer, Vector seed)
+        {
+            if (current == null)
+            {
+                current = new List<Vector> { seed };
+                currentLayer = moveLayer;
+            }
+            else if (moveLayer != currentLayer)
+            {
+                FlushCurrent(sink, ref current, currentLayer);
+                current = new List<Vector> { seed };
+                currentLayer = moveLayer;
+            }
+        }
+
+        private static void FlushCurrent(List<LayeredPolyline> sink, ref List<Vector> current, LayerType layer)
         {
             if (current != null && current.Count >= 2)
-                sink.Add(current);
+                sink.Add(new LayeredPolyline(current, layer));
             current = null;
         }
 

OpenNest.Tests/Converters/ConvertGeometryLayerTests.cs

@@ -0,0 +1,70 @@
+using System.Linq;
+using OpenNest.CNC;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Converters;
+
+public class ConvertGeometryLayerTests
+{
+    private static Program ProgramFor(Entity entity)
+    {
+        var shape = new Shape();
+        shape.Entities.Add(entity);
+        return ConvertGeometry.ToProgram(shape);
+    }
+
+    [Fact]
+    public void AddLine_EngraveLayer_TagsScribe()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddLine_EtchLayer_TagsScribe()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("etch") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddArc_EngraveLayer_TagsScribe()
+    {
+        var arc = new Arc(new Vector(0, 0), 1, 0, System.Math.PI / 2) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(arc);
+
+        var arcs = pgm.Codes.OfType<ArcMove>().ToList();
+        Assert.NotEmpty(arcs);
+        Assert.All(arcs, m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddCircle_EngraveLayer_TagsScribe()
+    {
+        var circle = new Circle(0, 0, 1) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(circle);
+
+        var arcs = pgm.Codes.OfType<ArcMove>().ToList();
+        Assert.NotEmpty(arcs);
+        Assert.All(arcs, m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddLine_DefaultLayer_StaysCut()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("0") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Cut, m.Layer));
+    }
+}

OpenNest.Tests/Geometry/GeometrySimplifierTests.cs

@@ -131,6 +131,111 @@ public class GeometrySimplifierTests
         Assert.IsType<Arc>(result.Entities[6]);
     }
 
+    [Fact]
+    public void Analyze_FilletBetweenTangentLines_ArcIsTangentToLines()
+    {
+        // A 90-degree fillet (r=0.3, center origin, 270deg..360deg CCW) between two
+        // long tangent lines, approximated by 8 chords whose interior vertices bulge
+        // radially outward within tolerance (simulates real DXF tessellation noise).
+        var r = 0.3;
+        var deltas = new[] { 0.0, 0.002, 0.003, 0.0035, 0.0035, 0.0035, 0.003, 0.002, 0.0 };
+        var pts = new List<Vector>();
+        for (var i = 0; i <= 8; i++)
+        {
+            var ang = OpenNest.Math.Angle.ToRadians(270 + 11.25 * i);
+            var radius = r + deltas[i];
+            pts.Add(new Vector(radius * System.Math.Cos(ang), radius * System.Math.Sin(ang)));
+        }
+
+        var shape = new Shape();
+        shape.Entities.Add(new Line(new Vector(-2, -r), pts[0]));
+        for (var i = 0; i < pts.Count - 1; i++)
+            shape.Entities.Add(new Line(pts[i], pts[i + 1]));
+        shape.Entities.Add(new Line(pts[^1], new Vector(r, 2)));
+
+        var simplifier = new GeometrySimplifier { Tolerance = 0.004 };
+        var candidates = simplifier.Analyze(shape);
+
+        Assert.Single(candidates);
+        var arc = candidates[0].FittedArc;
+
+        // Arc must pass exactly through the run's boundary vertices (no gaps)
+        Assert.True(arc.StartPoint().DistanceTo(pts[0]) < 1e-6);
+        Assert.True(arc.EndPoint().DistanceTo(pts[^1]) < 1e-6);
+
+        // Arc must be tangent to the adjacent straight edges at its endpoints
+        var startDelta = AngleBetweenDeg(ArcTangentAt(arc, arc.StartPoint()), new Vector(1, 0));
+        var endDelta = AngleBetweenDeg(ArcTangentAt(arc, arc.EndPoint()), new Vector(0, 1));
+        Assert.True(startDelta < 0.3, $"Arc start not tangent to incoming line: off by {startDelta:F3} deg");
+        Assert.True(endDelta < 0.3, $"Arc end not tangent to outgoing line: off by {endDelta:F3} deg");
+    }
+
+    [Fact]
+    public void Analyze_CompoundCurve_AdjacentArcsAreTangentAtJunction()
+    {
+        // Two tangent-continuous arcs of different radii (r=0.2 sweeping 60deg, then
+        // r=0.6 sweeping 40deg), tessellated into chords with slight radial noise.
+        // The fitted arcs must stay tangent-continuous at their junction.
+        var c1 = new Vector(0, 0);
+        var r1 = 0.2;
+        var deltas1 = new[] { 0.0, 0.001, 0.0005, -0.0005, -0.001, -0.0005, 0.0 };
+        var pts = new List<Vector>();
+        for (var i = 0; i <= 6; i++)
+        {
+            var ang = OpenNest.Math.Angle.ToRadians(10 * i);
+            var radius = r1 + deltas1[i];
+            pts.Add(new Vector(c1.X + radius * System.Math.Cos(ang), c1.Y + radius * System.Math.Sin(ang)));
+        }
+
+        // Second arc center along the junction radius so tangents match at the junction
+        var junctionAngle = OpenNest.Math.Angle.ToRadians(60);
+        var u = new Vector(System.Math.Cos(junctionAngle), System.Math.Sin(junctionAngle));
+        var r2 = 0.6;
+        var c2 = new Vector(c1.X + u.X * (r1 - r2), c1.Y + u.Y * (r1 - r2));
+        var deltas2 = new[] { 0.0, 0.001, -0.001, 0.0005, -0.0005, 0.0 };
+        for (var i = 1; i <= 5; i++)
+        {
+            var ang = OpenNest.Math.Angle.ToRadians(60 + 8 * i);
+            var radius = r2 + deltas2[i];
+            pts.Add(new Vector(c2.X + radius * System.Math.Cos(ang), c2.Y + radius * System.Math.Sin(ang)));
+        }
+
+        var shape = new Shape();
+        for (var i = 0; i < pts.Count - 1; i++)
+            shape.Entities.Add(new Line(pts[i], pts[i + 1]));
+
+        var simplifier = new GeometrySimplifier { Tolerance = 0.004 };
+        var candidates = simplifier.Analyze(shape);
+
+        Assert.Equal(2, candidates.Count);
+        var arcA = candidates[0].FittedArc;
+        var arcB = candidates[1].FittedArc;
+
+        // Arcs must share the junction vertex exactly
+        Assert.True(arcA.EndPoint().DistanceTo(arcB.StartPoint()) < 1e-6);
+
+        // Tangent continuity across the junction
+        var junctionDelta = AngleBetweenDeg(ArcTangentAt(arcA, arcA.EndPoint()), ArcTangentAt(arcB, arcB.StartPoint()));
+        Assert.True(junctionDelta < 0.3, $"Tangent break of {junctionDelta:F3} deg at arc-arc junction");
+    }
+
+    private static Vector ArcTangentAt(Arc arc, Vector pt)
+    {
+        var ang = System.Math.Atan2(pt.Y - arc.Center.Y, pt.X - arc.Center.X);
+        return arc.IsReversed
+            ? new Vector(System.Math.Sin(ang), -System.Math.Cos(ang))
+            : new Vector(-System.Math.Sin(ang), System.Math.Cos(ang));
+    }
+
+    private static double AngleBetweenDeg(Vector v1, Vector v2)
+    {
+        var l1 = System.Math.Sqrt(v1.X * v1.X + v1.Y * v1.Y);
+        var l2 = System.Math.Sqrt(v2.X * v2.X + v2.Y * v2.Y);
+        var dot = (v1.X * v2.X + v1.Y * v2.Y) / (l1 * l2);
+        dot = System.Math.Max(-1, System.Math.Min(1, dot));
+        return System.Math.Acos(dot) * 180.0 / System.Math.PI;
+    }
+
     [Fact]
     public void Apply_DynaPanDxf_NoGapsAfterSimplification()
     {

OpenNest.Tests/GravographIS/GravographISPostConfigTests.cs

@@ -0,0 +1,51 @@
+using OpenNest.CNC;
+using OpenNest.Posts.GravographIS;
+
+namespace OpenNest.Tests.GravographIS;
+
+public class GravographISPostConfigTests
+{
+    [Fact]
+    public void Defaults_EngraveBlock_NoPause_FasterFeed()
+    {
+        var config = new GravographISPostConfig();
+
+        Assert.Equal(10, config.Engrave.FeedMmPerSec);
+        Assert.False(config.Engrave.PauseBefore);
+    }
+
+    [Fact]
+    public void Defaults_CutBlock_PausesToChangeTool()
+    {
+        var config = new GravographISPostConfig();
+
+        Assert.Equal(3, config.Cut.FeedMmPerSec);
+        Assert.True(config.Cut.PauseBefore);
+        Assert.Equal("Change tool", config.Cut.PauseMessage);
+    }
+
+    [Theory]
+    [InlineData(LayerType.Scribe)]
+    public void ConfigFor_Scribe_ReturnsEngraveBlock(LayerType layer)
+    {
+        var config = new GravographISPostConfig();
+        Assert.Same(config.Engrave, config.ConfigFor(layer));
+    }
+
+    [Theory]
+    [InlineData(LayerType.Cut)]
+    [InlineData(LayerType.Leadin)]
+    [InlineData(LayerType.Leadout)]
+    public void ConfigFor_CutLayers_ReturnCutBlock(LayerType layer)
+    {
+        var config = new GravographISPostConfig();
+        Assert.Same(config.Cut, config.ConfigFor(layer));
+    }
+
+    [Fact]
+    public void ConfigFor_Display_ReturnsNull_SoItIsSkipped()
+    {
+        var config = new GravographISPostConfig();
+        Assert.Null(config.ConfigFor(LayerType.Display));
+    }
+}

OpenNest.Tests/GravographIS/GravographISPostProcessorTests.cs

@@ -0,0 +1,70 @@
+using System.Collections.Generic;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.Posts.GravographIS;
+
+namespace OpenNest.Tests.GravographIS;
+
+public class GravographISPostProcessorTests
+{
+    private static LayeredPolyline Poly(LayerType layer, params Vector[] pts)
+        => new LayeredPolyline(new List<Vector>(pts), layer);
+
+    [Fact]
+    public void BuildPasses_EngraveAndCut_OrdersEngraveFirstThenCutWithPause()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(1, 0)),
+            Poly(LayerType.Scribe, new Vector(0, 0), new Vector(0, 1)),
+        });
+
+        Assert.Equal(2, passes.Count);
+
+        Assert.Equal(post.Config.Engrave.FeedMmPerSec, passes[0].FeedMmPerSec);
+        Assert.False(passes[0].PauseBefore);
+
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[1].FeedMmPerSec);
+        Assert.True(passes[1].PauseBefore);
+        Assert.Equal("Change tool", passes[1].PauseMessage);
+    }
+
+    [Fact]
+    public void BuildPasses_CutOnly_IsSinglePass()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(1, 0)),
+        });
+
+        Assert.Single(passes);
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[0].FeedMmPerSec);
+    }
+
+    [Fact]
+    public void BuildPasses_SkipsDisplayGeometry()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Display, new Vector(0, 0), new Vector(1, 0)),
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(0, 1)),
+        });
+
+        Assert.Single(passes);
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[0].FeedMmPerSec);
+    }
+
+    [Fact]
+    public void Config_IsExposedThroughConfigurableInterface()
+    {
+        var post = new GravographISPostProcessor(new GravographISPostConfig());
+        OpenNest.IConfigurablePostProcessor configurable = post;
+        Assert.Same(post.Config, configurable.Config);
+    }
+}

OpenNest.Tests/GravographIS/GravographISWriterTests.cs

@@ -178,6 +178,101 @@ public class GravographISWriterTests
         Assert.Equal(-GravographISWriter.StepsPerInch, dy);
     }
 
+    [Fact]
+    public void Passes_PauseBeforeCut_EmitsPauseSequenceBetweenGroups()
+    {
+        var engrave = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(1, 0) } };
+        var cut = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(0, -1) } };
+        var passes = new List<GravographPass>
+        {
+            new GravographPass { Polylines = engrave, FeedMmPerSec = 10, DepthInches = 0.25 },
+            new GravographPass { Polylines = cut, FeedMmPerSec = 3, DepthInches = 0.25, PauseBefore = true, PauseMessage = "Hi" },
+        };
+
+        using var ms = new MemoryStream();
+        new GravographISWriter(new GravographISWriterOptions
+        {
+            EnvelopeGuardEnabled = false,
+            ReturnToOriginAtEnd = false,
+        }).Write(passes, ms);
+        var bytes = ms.ToArray();
+
+        var mcOff = IndexOf(bytes, 0, (byte)'M', (byte)'C', 0x00, 0x00);
+        var ouFb = IndexOf(bytes, mcOff, (byte)'O', (byte)'U', 0xFF, 0xFB);
+        var ouFa = IndexOf(bytes, ouFb, (byte)'O', (byte)'U', 0xFF, 0xFA);
+        var lbBegin = IndexOf(bytes, ouFa, (byte)'L', (byte)'B', 0x00, 0x00);
+        var lbMsg = IndexOf(bytes, lbBegin, (byte)'L', (byte)'B', (byte)'H', (byte)'i');
+        var nr = IndexOf(bytes, lbMsg, (byte)'N', (byte)'R', 0x00, 0x01);
+        var lbEnd = IndexOf(bytes, nr, (byte)'L', (byte)'B', 0x00, 0x01);
+        var mcOn = IndexOf(bytes, lbEnd, (byte)'M', (byte)'C', 0x00, 0x01);
+
+        Assert.True(mcOff >= 0, "motor-off (MC 0000) not found");
+        Assert.True(mcOff < ouFb && ouFb < ouFa && ouFa < lbBegin && lbBegin < lbMsg
+            && lbMsg < nr && nr < lbEnd && lbEnd < mcOn,
+            "pause commands out of order");
+
+        // Resume sets the cut feed inline (VS 0x0003) after the motor restarts.
+        var vsCut = IndexOf(bytes, mcOn, (byte)'V', (byte)'S', 0x00, 0x03);
+        Assert.True(vsCut > mcOn, "cut feed not set after resume");
+    }
+
+    [Fact]
+    public void Passes_PauseOddMessage_SpacePadsLastPacket()
+    {
+        var passes = new List<GravographPass>
+        {
+            new GravographPass { Polylines = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(1, 0) } }, FeedMmPerSec = 10 },
+            new GravographPass { Polylines = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(0, -1) } }, FeedMmPerSec = 3, PauseBefore = true, PauseMessage = "abc" },
+        };
+
+        using var ms = new MemoryStream();
+        new GravographISWriter(new GravographISWriterOptions { EnvelopeGuardEnabled = false, ReturnToOriginAtEnd = false }).Write(passes, ms);
+        var bytes = ms.ToArray();
+
+        var lbAb = IndexOf(bytes, 0, (byte)'L', (byte)'B', (byte)'a', (byte)'b');
+        var lbCPad = IndexOf(bytes, lbAb, (byte)'L', (byte)'B', (byte)'c', 0x20);
+        Assert.True(lbAb >= 0, "first message packet 'ab' not found");
+        Assert.True(lbCPad > lbAb, "odd packet not space-padded to 'c '");
+    }
+
+    [Fact]
+    public void Passes_DifferentFeeds_NoPause_EmitsInlineFeedChangeNoMessage()
+    {
+        var passes = new List<GravographPass>
+        {
+            new GravographPass { Polylines = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(1, 0) } }, FeedMmPerSec = 10 },
+            new GravographPass { Polylines = new List<IReadOnlyList<Vector>> { new[] { new Vector(0, 0), new Vector(0, -1) } }, FeedMmPerSec = 3, PauseBefore = false },
+        };
+
+        using var ms = new MemoryStream();
+        new GravographISWriter(new GravographISWriterOptions { EnvelopeGuardEnabled = false, ReturnToOriginAtEnd = false }).Write(passes, ms);
+        var bytes = ms.ToArray();
+
+        Assert.True(IndexOf(bytes, 0, (byte)'V', (byte)'S', 0x00, 0x03) >= 0, "inline cut feed change missing");
+        Assert.True(IndexOfCmd(bytes, (byte)'L', (byte)'B') < 0, "no LB message expected without a pause");
+    }
+
+    private static int IndexOf(byte[] bytes, int from, byte c0, byte c1, byte hi, byte lo)
+    {
+        for (var i = System.Math.Max(0, from); i <= bytes.Length - 6; i++)
+        {
+            if (bytes[i] == 0xFF && bytes[i + 1] == 0xFD && bytes[i + 2] == c0 &&
+                bytes[i + 3] == c1 && bytes[i + 4] == hi && bytes[i + 5] == lo)
+                return i;
+        }
+        return -1;
+    }
+
+    private static int IndexOfCmd(byte[] bytes, byte c0, byte c1)
+    {
+        for (var i = 0; i <= bytes.Length - 4; i++)
+        {
+            if (bytes[i] == 0xFF && bytes[i + 1] == 0xFD && bytes[i + 2] == c0 && bytes[i + 3] == c1)
+                return i;
+        }
+        return -1;
+    }
+
     private static void AssertOperand(byte[] bytes, byte c0, byte c1, byte hi, byte lo)
     {
         for (var i = 0; i < bytes.Length - 5; i++)

OpenNest.Tests/GravographIS/NestPolylineExtractorTests.cs

@@ -34,4 +34,45 @@ public class NestPolylineExtractorTests
         Assert.Equal(new Vector(0.25, 47.75), poly[3]);
         Assert.Equal(new Vector(0.25, 46.75), poly[4]);
     }
+
+    [Fact]
+    public void ExtractPartLayered_SplitsContinuousChainAtLayerChange()
+    {
+        // A single continuous chain (no rapid) that switches from Scribe to Cut
+        // must be split into two layer-uniform polylines sharing the seam vertex,
+        // so the post can emit a tool-change pause between engrave and cut.
+        var program = new Program(Mode.Absolute);
+        program.Codes.Add(new LinearMove(1, 0) { Layer = LayerType.Scribe });
+        program.Codes.Add(new LinearMove(2, 0) { Layer = LayerType.Scribe });
+        program.Codes.Add(new LinearMove(2, 1) { Layer = LayerType.Cut });
+        program.Codes.Add(new LinearMove(3, 1) { Layer = LayerType.Cut });
+
+        var drawing = new Drawing("Mixed", program);
+        var part = new Part(drawing, new Vector(0, 0));
+
+        var polylines = new NestPolylineExtractor().ExtractPartLayered(part);
+
+        Assert.Equal(2, polylines.Count);
+
+        Assert.Equal(LayerType.Scribe, polylines[0].Layer);
+        Assert.Equal(new[] { new Vector(0, 0), new Vector(1, 0), new Vector(2, 0) }, polylines[0].Points);
+
+        Assert.Equal(LayerType.Cut, polylines[1].Layer);
+        Assert.Equal(new[] { new Vector(2, 0), new Vector(2, 1), new Vector(3, 1) }, polylines[1].Points);
+    }
+
+    [Fact]
+    public void ExtractPartLayered_UniformChain_IsSinglePolyline()
+    {
+        var program = new Program(Mode.Absolute);
+        program.Codes.Add(new LinearMove(1, 0));
+        program.Codes.Add(new LinearMove(1, 1));
+
+        var part = new Part(new Drawing("Cut", program), new Vector(0, 0));
+
+        var polylines = new NestPolylineExtractor().ExtractPartLayered(part);
+
+        Assert.Single(polylines);
+        Assert.Equal(LayerType.Cut, polylines[0].Layer);
+    }
 }