GAMBAS BOOK 3.19.3

The Polygon class (gb.clipper) represents a polygon. This chapter introduces the properties and methods of the Polygon class.

The Polygon class has these four properties:

Table 25.2.2.1.1 : Properties of the Polygon class

All methods for the Polygon class are described here:

Table 25.2.2.2.1 : Methods of the Polygon class

Notes Simplify(..)

• This method is important for so-called non-simple polygons. These are polygons whose sides intersect and do not just touch at the corner points.
• A pentagram, for example, is a classic non-simple polygon. The fill rule then determines whether the pentagon that is created within the pentagram belongs to the “inside” of the polygon or the “outside”. When the polygon is constructed, the pentagon belongs to the interior first. The Simplify() method can then be called, if desired, with a suitable fill rule to remove the pentagon.

Notes Clean(..)

• Corners are semi-adjacent if there is exactly one other corner between them.
• The distance parameter is √2 by default, so that a corner is removed as soon as an adjacent or semi-adjacent corner exists whose x and y coordinates are no more than one unit of the coordinate system apart. If the corners are semi-adjacent, the corner between them is also removed.
• Distance is therefore a tolerance limit here. The Clean() method may make changes to the polygon in order to simplify it further. Distance also specifies how far points may be moved to allow greater simplification of the polygon.

This class can be created and used like an array:

Dim PolygonA, PolygonB As Polygon 

PolygonA = New Polygon    '-- Polygon with 0 corners p.d.
PolygonB = New Polygon(7) '-- Polygon with 7 corners

To define a polygon (pentagram) with 5 corner points (index 0..4) and read the number of corners, the orientation and the coordinates for all corner points from this polygon:

Public Sub ScriptPentagram()
  Dim iIndex As Integer  
  Dim Point As PointF 
  Dim pPolygon As New Polygon
 
  pPolygon.Add(128, 196)
  pPolygon.Add(412, 196) 
  pPolygon.Add(183, 27) 
  pPolygon.Add(270, 300) 
  pPolygon.Add(357, 27) 
 
  Print "Number of corner points = "; pPolygon.Count
  Print "Orientation = "; pPolygon.Orientation
 
' Read out all corner points and display the x,y coordinates of the corner points of the polygon
  For iIndex = 0 To pPolygon.Max
    Point = pPolygon[iIndex]
    Print "Point"; iIndex + 1; "(x) = "; Point.X; " Punkt"; iIndex + 1; "(y) = "; Point.Y
  Next
 
  GenerateNewPicture()
  SetPictureBorder()  
 
  Paint.Begin(hPicture)   
 
    Paint.Translate(xTranslate, yTranslate)
    Paint.Scale(xScale, yScale) ' +y ▲
    Paint.AntiAlias = False
    DrawCoordinateSystem() ' +y ▲      
    Paint.Brush = Paint.Color(Color.Red)
    DrawPolygon(pPolygon, "s") '-- Argument 's' → draw lines, 'f' fill area
 
  Paint.End()
 
End

Here you can see the output in the console of the IDE:

Number of corner points = 5
Orientation = False

Punkt1(x) = 128 Punkt1(y) = 196
Punkt2(x) = 412 Punkt2(y) = 196
Punkt3(x) = 183 Punkt3(y) = 27
Punkt4(x) = 270 Punkt4(y) = 300
Punkt5(x) = 357 Punkt5(y) = 27

The images of the special polygon (→ pentagram) were drawn with all connecting lines on the one hand and as an image with the fill colour red on the other:

Figure 25.2.2.3.1: Pentagram (connecting lines)

Figure 25.2.2.3.2: Pentagram area