How to Correctly Create Arc Segments from Polylines in C#

Introduction

If you've ever found yourself puzzled by how to extract arc segments from polylines in BricsCAD while developing with C#, you're not alone. This common issue arises when trying to re-create arc segments as standalone arc entities. In this article, we'll explore how to handle this situation effectively.

Understanding Arc Segments in Polylines

Arc segments in a polyline can be tricky because they often include a bulge, which affects how the arc behaves in your drawings. When you convert a polyline arc segment to a standalone arc, it’s crucial to ensure that the properties (like start and end angles) are correctly calculated.

Common Problems with Arc Conversion

One frequent issue is the mismatch between the generated arc's starting and ending angles versus those manually constructed. For instance, in the provided example:

• Generated Arc:
  • Start angle: 0.00
  • End angle: 143.11
• Manually Constructed Arc:
  • Start angle: 356.38
  • End angle: 139.48

This discrepancy can lead to incorrect placements of the arcs in your drawings, which is problematic when performing actions based on their geometry.

Step-by-Step Solution to Correct Arc Placement

Here’s how you can ensure that an arc segment from a polyline is created correctly:

1. Get the Arc Segment from Polyline

First, ensure you are fetching the correct arc segment from the polyline correctly:

if (pline.GetSegmentType(i) == _AcDb.SegmentType.Arc)
{
    _AcGe.CircularArc2d arcSeg = pline.GetArcSegment2dAt(i);

    // Logic for further processing
}

2. Calculating Start and End Angles

You need to calculate the starting and ending angles. These angles must reflect the orientation of the arc within your drawing context. To do this, convert the bulge to angles:

double bulge = arcSeg.Bulge;
double startAngle = Math.Atan2(arcSeg.StartPoint.Y - arcSeg.Center.Y, arcSeg.StartPoint.X - arcSeg.Center.X);
double endAngle = startAngle + (2 * Math.PI * Math.Abs(bulge));

if (bulge < 0)
{
    // For counter-clockwise arcs, reverse start and end angles if necessary.
    startAngle += Math.PI;
    endAngle += Math.PI;
}

3. Create the Arc with Correct Angles

Use the computed angles in the creation of the arc entity, ensuring they are correctly adjusted if required:

using (_AcDb.Arc segAsArc = new _AcDb.Arc( 
    center,
    normal,
    arcSeg.Radius,
    startAngle,
    endAngle))
{
    if (segAsArc.Length >= minCreaseLength)
    {
        DrawCounterFromArc(segAsArc, indentLength, counterWidth, btr, acTrans);
        segAsArc.Layer = "0";
        btr.AppendEntity(segAsArc);
        acTrans.AddNewlyCreatedDBObject(segAsArc, true);
    }
}

4. Validate the Arc Position

After creating the arc, verify its position in your drawing to ensure it matches the original polyline segment. You can visually check or calculate its endpoints to validate correctness.

Frequently Asked Questions

Q: Why do my arcs not match the original segments?

A: Mismatches often occur due to incorrect angle calculations or transformations. Always ensure the angles are calculated from the correct geometry references.

Q: Do I need to adjust angles for negative bulge values?

A: Yes, negative bulge indicates a clockwise arc versus the counter-clockwise orientation usually expected. Adjust angles as necessary to match the geometric properties of your arc.

Conclusion

Handling arc segments from polylines accurately is vital for creating geometrically correct CAD drawings in BricsCAD. By understanding how to compute and interpret the start and end angles of arcs, you can ensure precise geometry in your applications. Remember to validate the created arcs against the original polyline to avoid discrepancies and improve your CAD programming skills.

Reference the detailed article for more insights: Converting Polylines to ACDbArc.