GAMBAS BOOK 3.19.1

The Clipper class (gb.clipper) implements clipping methods for polygons.

The Clipper class has these methods. Note the optional arguments, which are marked in blue:

Table 25.2.1.1.1 : Methods of the Clipper class

Each polygon or an arrangement of polygons must have a fill type when one of the following clipping methods is called: Difference, Union, Intersection or ExclusiveOr.

The constants for the polygon filling rules and the connection types are described here for the Clipper class.

Tables 25.2.1.2.1 : Constants of the Clipper class

You can find a project archive in the download area so that you can reproduce all the examples. You will also find the source texts used in the aforementioned chapter for the special polygons 'Rectangle' → Chapter 25.2.4 Classes Rect and RectF. For this reason, the complete example source texts are largely omitted. Only source code excerpts are presented and the results obtained are displayed. In principle, a picture is drawn, which is then displayed in a DrawArea. This is the only way to view the results when using the Clipper.Intersection, Clipper.Union, Clipper.ExclusiveOr or Clipper.Difference methods.

Example 1

In the first example, a triangle is presented as a concave polygon. A polygon is concave if all connecting lines in the polygon always lie inside the polygon.

• First, a polygon with four corners is created and the coordinates of a selected point are read out and displayed in different ways.
• A point is then removed using the Clipper.Remove(point_index) method and the reduced polygon is assigned to another polygon pTriangel. Its points are read out and displayed, as is the area of the polygon.
• Finally, the polygon is displayed.

Public Sub ScriptPolygon1()
  Dim iIndex As Integer  
  Dim pPolygon, pTriangle As New Polygon
  Dim Point As New PointF 
 
  pPolygon.Add(50, 100)      ' A
  pPolygon.Add(450, 30)      ' B
  pPolygon.Add(330, 240)     ' C
  pPolygon.Add(70, 123.4)    ' D
 
' Read out the 4th corner point (index=3) and display the coordinates Dx and Dy
  Point = pPolygon[3]
  Print "Dx = "; Point.X
  Print "Dy = "; Point.Y
  Print "Dx = "; pPolygon[3].X ' Alternative
  Print "Dy = "; pPolygon[3].Y
 
' The point D(Index=3) is removed from the polygon
  pPolygon.Remove(3, 1)
  pTriangle = pPolygon
 
' Read out all corner points and display the coordinates of the triangle
  For iIndex = 0 To pTriangle.Max
    Point = pTriangle[iIndex]
    Print "Point"; iIndex + 1; ".x = "; Point.X; " Punkt"; iIndex + 1; ".y = "; Point.Y
  Next
 
' Calculate and display the area of the polygon
  Print "The triangle ABC has an area of "; Round(pTriangle.Area, 2); " Units of area!"
 
  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(pTriangle, "f")
    Paint.AntiAlias = True
  Paint.End()
End

This is the output in the IDE console:

Dx = 70
Dy = 123,4
Dx = 70
Dy = 123,4
Point1.x = 50  Point1.y = 100
Point2.x = 450 Point2.y = 30
Point3.x = 330 Point3.y = 240
The triangle ABC has an area of 37800 Units of area!

Figure 25.2.1.3.1: Polygon 'Triangle'

Example 2

Only a convex polygon is presented here:

Figure 25.2.1.3.2: Convex polygon

The same polygons are always used for the next four examples - a pentagram and a rectangle. Then the 4 methods

• Clipper.Intersection,
• Clipper.Union,
• Clipper.ExclusiveOr and
• Clipper.Difference

are used for four different fill rules, which essentially determine the result.

Example 3 - Clipper.Intersection(..)

Public Sub ScriptCIntersection()
  Dim pPentagramm, pRectangle, hP As New Polygon
 
  pPentagramm.Add(128, 196)
  pPentagramm.Add(412, 196) 
  pPentagramm.Add(183, 27) 
  pPentagramm.Add(270, 300) 
  pPentagramm.Add(357, 27) 
 
  pRectangle.Add(170, 230)
  pRectangle.Add(370, 230)
  pRectangle.Add(370, 60)
  pRectangle.Add(170, 60)
 
  GenerateNewPicture()
  SetPictureBorder()  
  Paint.Begin(hPicture)   
    Paint.Translate(xTranslate, yTranslate)
    Paint.Scale(xScale, yScale) ' +y ▲
    Paint.AntiAlias = False
    DrawCoordinateSystem() ' +y ▲       
 
    Paint.Background = Color.DarkGreen
    For Each hP In Clipper.Intersection([pPentagramm], [pRectangle], sbFillRule.Value) ' 0..3
        DrawPolygon(hP)
    Next
 
    Paint.Brush = Paint.Color(Color.Red)
    DrawPolygon(pRectangle, "s")
    Paint.Brush = Paint.Color(Color.Blue)
    DrawPolygon(pPentagramm, "s")    
  Paint.End() 
 
End

Figure 25.2.1.3.3: Intersection - Filling rule selection (0..3)

Figure 25.2.1.3.4: Intersection - Filling rule 0 (FillEvenOdd)

Figure 25.2.1.3.5: Intersection - Filling rule 1 (FillNonZero)

Example 4 - Clipper.Difference(..)

Figure 25.2.1.3.6: Difference - Filling rule 0 (FillEvenOdd)

Figure 25.2.1.3.7: Difference - Filling rule 2 (FillPositive)

Example 5 - Clipper.ExclusiveOr(..)

Figure 25.2.1.3.8: ExclusiveOr - Filling rule 0 (FillEvenOdd)

Figure 25.2.1.3.9: ExclusiveOrder - Filling rule 1 and 3

Example 6.1 - Clipper.Union(..)

To illustrate the use of the Clipper.Union(..) method, only one image is shown because the images for filling rule 0, 1 and 3 are the same:

Figure 25.2.1.3.10: Union - fill rule 0 (FillEvenOdd)

Example 6.2 - Clipper.Union(..)

Using the Clipper.Union(..) method, you can, for example, combine shapes such as a rectangle and a triangle to form an arrow (union of polygons) and view them as a unit - as a geometric object.

Figure 25.2.1.3.11: Use: Shapes from the image editor of the Gambas IDE

Figure 25.2.1.3.12: A triangle and a rectangle

Figure 25.2.1.3.13: An arrow (envelope curve)

Use this source code to create the figure → 25.2.1.3.12, which also allows you to draw the (commented out) alternative:

Public Sub ScriptPolygon2()
  Dim iIndex As Integer
  Dim Point As New PointF
  Dim pPolygon, pTriangle, hP As New Polygon
  Dim aPolygon As Polygon[]
 
  pPolygon.Add(100, 170)  ' A
  pPolygon.Add(360, 170)  ' B
  pPolygon.Add(360, 90)   ' C
  pPolygon.Add(100, 90)   ' D
 
  pTriangle.Add(360, 230) ' P 
  pTriangle.Add(480, 130) ' Q
  pTriangle.Add(360, 30)  ' R
 
  aPolygon = Clipper.Union([pPolygon], [pTriangle], 0)
 
  GenerateNewPicture()
  SetPictureBorder()  
  Paint.Begin(hPicture)   
    Paint.Translate(xTranslate, yTranslate)
    Paint.Scale(xScale, yScale) ' +y ▲
    Paint.AntiAlias = False
    DrawCoordinateSystem() ' +y ▲      
    Paint.Brush = Paint.Color(Color.DarkGreen)
 
  ' Rectangle and triangle
    Paint.Brush = Paint.Color(Color.Blue)
    DrawPolygon(pPolygon, "s")
    Paint.Brush = Paint.Color(Color.Red)
    DrawPolygon(pTriangle, "s")
 
  ' Arrow as a unit
  ' For Each hP In aPolygon
  '   DrawPolygon(hP, "s")
  ' Next    
  Paint.End()
 
End