acad-hatches

name	acad-hatches
description	Hatch patterns, gradient fills, boundary loops, associativity, hatch styles

AutoCAD Hatches and Gradient Fills

Use this skill when creating or modifying hatch patterns, solid fills, or gradient fills in drawings.

Creating a Hatch Pattern

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = (BlockTableRecord)tr.GetObject(
        db.CurrentSpaceId, OpenMode.ForWrite);

    Hatch hatch = new Hatch();
    hatch.SetDatabaseDefaults();
    hatch.Normal = Vector3d.ZAxis;
    hatch.Elevation = 0.0;

    // Step 1: Add to database FIRST (required before SetHatchPattern and AppendLoop)
    btr.AppendEntity(hatch);
    tr.AddNewlyCreatedDBObject(hatch, true);

    // Step 2: Set pattern AFTER AppendEntity, BEFORE AppendLoop
    hatch.SetHatchPattern(HatchPatternType.PreDefined, "ANSI31");
    hatch.PatternScale = 1.0;
    hatch.PatternAngle = 0.0;

    // Step 3: Set Associative BEFORE AppendLoop
    hatch.Associative = true;

    // Step 4: Add boundary loop
    ObjectIdCollection boundaryIds = new ObjectIdCollection();
    boundaryIds.Add(polylineId);  // closed polyline as boundary
    hatch.AppendLoop(HatchLoopTypes.External, boundaryIds);

    // Evaluate to generate hatch lines
    hatch.EvaluateHatch(true);

    tr.Commit();
}

Boundary Loop Types

From Entity IDs (most common)

ObjectIdCollection outerBoundary = new ObjectIdCollection();
outerBoundary.Add(outerPolylineId);
hatch.AppendLoop(HatchLoopTypes.External, outerBoundary);

// Add island (inner boundary)
ObjectIdCollection innerBoundary = new ObjectIdCollection();
innerBoundary.Add(innerCircleId);
hatch.AppendLoop(HatchLoopTypes.Default, innerBoundary);

From Polyline Vertices (with bulge)

Point2dCollection vertices = new Point2dCollection();
vertices.Add(new Point2d(0, 0));
vertices.Add(new Point2d(100, 0));
vertices.Add(new Point2d(100, 100));
vertices.Add(new Point2d(0, 100));

DoubleCollection bulges = new DoubleCollection();
bulges.Add(0); bulges.Add(0); bulges.Add(0); bulges.Add(0);

hatch.AppendLoop(HatchLoopTypes.External | HatchLoopTypes.Polyline,
    vertices, bulges);

From 2D Curve Edges

Curve2dCollection edges = new Curve2dCollection();
IntegerCollection edgeTypes = new IntegerCollection();

edges.Add(new LineSegment2d(new Point2d(0, 0), new Point2d(100, 0)));
edgeTypes.Add((int)HatchEdgeType.Line);  // 1

edges.Add(new CircularArc2d(
    new Point2d(100, 0), new Point2d(100, 50), new Point2d(100, 100)));
edgeTypes.Add((int)HatchEdgeType.CircularArc);  // 2

edges.Add(new LineSegment2d(new Point2d(100, 100), new Point2d(0, 0)));
edgeTypes.Add((int)HatchEdgeType.Line);

hatch.AppendLoop(HatchLoopTypes.External, edges, edgeTypes);

Using HatchLoop Object

HatchLoop loop = new HatchLoop(HatchLoopTypes.External);
// loop is read-only after construction; use for inspection:
// loop.Curves, loop.Polyline, loop.LoopType, loop.IsPolyline

hatch.AppendLoop(loop);

Loop Management

int loopCount = hatch.NumberOfLoops;
HatchLoopTypes loopType = hatch.LoopTypeAt(0);
HatchLoop loop = hatch.GetLoopAt(0);

// Insert loop at specific position
hatch.InsertLoopAt(1, HatchLoopTypes.Default, boundaryIds);
hatch.InsertLoopAt(1, hatchLoop);

// Remove loop
hatch.RemoveLoopAt(0);

// Get associated objects for a loop
ObjectIdCollection assocIds = hatch.GetAssociatedObjectIdsAt(0);
ObjectIdCollection allAssocIds = hatch.GetAssociatedObjectIds();
hatch.RemoveAssociatedObjectIds();

Solid Fill

Hatch hatch = new Hatch();
hatch.SetDatabaseDefaults();
hatch.Normal = Vector3d.ZAxis;
hatch.SetHatchPattern(HatchPatternType.PreDefined, "SOLID");
// IsSolidFill will be true

Gradient Fills

Hatch gradient = new Hatch();
gradient.SetDatabaseDefaults();
gradient.Normal = Vector3d.ZAxis;
gradient.HatchObjectType = HatchObjectType.GradientObject;

// Set gradient type
gradient.SetGradient(GradientPatternType.PreDefinedGradient, "LINEAR");
// Other names: "CYLINDER", "INVCYLINDER", "SPHERICAL", "INVSPHERICAL",
//              "HEMISPHERICAL", "INVHEMISPHERICAL", "CURVED", "INVCURVED"

gradient.GradientAngle = Math.PI / 4;  // 45 degrees

// Two-color gradient
gradient.GradientOneColorMode = false;
GradientColor[] colors = new GradientColor[2];
colors[0] = new GradientColor(Color.FromRgb(255, 0, 0), 0.0f);   // start
colors[1] = new GradientColor(Color.FromRgb(0, 0, 255), 1.0f);   // end
gradient.SetGradientColors(colors);

// One-color gradient (single color with tint)
gradient.GradientOneColorMode = true;
gradient.ShadeTintValue = 0.5f;  // 0 = dark, 1 = light
gradient.GradientShift = 0.0f;

// Add boundary and evaluate
btr.AppendEntity(gradient);
tr.AddNewlyCreatedDBObject(gradient, true);
gradient.AppendLoop(HatchLoopTypes.External, boundaryIds);
gradient.EvaluateHatch(true);

Gradient methods:

• SetGradient(GradientPatternType type, string name) - set gradient pattern
• GetGradientColors() -> GradientColor[] - get color stops
• SetGradientColors(GradientColor[] colors) - set color stops
• EvaluateGradientColorAt(float value) -> Color - sample color at position (0-1)

Enums

HatchLoopTypes (flags): Default=0x0, External=0x1, Polyline=0x2, Derived=0x4, Textbox=0x8, Outermost=0x10, NotClosed=0x20, SelfIntersecting=0x40, TextIsland=0x80, Duplicate=0x100

HatchEdgeType: Line=1, CircularArc=2, EllipticalArc=3, Spline=4

HatchStyle: Normal=0 (fills between nested boundaries), Outer=1 (outermost boundary only), Ignore=2 (ignores internal boundaries)

HatchPatternType: UserDefined=0, PreDefined=1, CustomDefined=2

HatchObjectType: HatchObject=0, GradientObject=1

GradientPatternType: PreDefinedGradient=0, UserDefinedGradient=1

Gotchas

• Set the pattern type/name BEFORE adding loops and BEFORE calling EvaluateHatch
• AppendLoop must be called AFTER AppendEntity and AddNewlyCreatedDBObject - the hatch must be in the database first
• EvaluateHatch(true) underestimates line count (faster); EvaluateHatch(false) is accurate but slower
• For user-defined patterns, use PatternSpace instead of PatternScale; set PatternDouble = true for crosshatch
• Associative must be set to true BEFORE adding boundary loops for associativity to work
• Boundary loops must be closed - open boundaries cause EvaluateHatch to fail silently
• The outer boundary should use HatchLoopTypes.External; inner islands should use HatchLoopTypes.Default
• HatchStyle.Normal alternates fill between nested boundaries; Outer fills only the outermost; Ignore fills everything
• For gradients, set HatchObjectType = HatchObjectType.GradientObject before calling SetGradient
• PatternName and PatternType are read-only - use SetHatchPattern() to change them
• After changing PatternScale, PatternAngle, or PatternSpace, you must re-call SetHatchPattern(type, name) before calling EvaluateHatch() — otherwise the change has no visual effect
• BulgeVertex Bulge value: 0 = straight segment, positive = counterclockwise arc, negative = clockwise arc; tan(includedAngle/4)

Related Skills

• acad-polylines — polylines as hatch boundaries; vertex+bulge loop construction
• acad-layers — layer assignment for hatch entities