

Title  Draw a nontriangular Sierpinski gasket (fractal) 
Description  This example shows how to draw a nontriangular Sierpinski gasket (fractal) in Visual Basic 6. 
Keywords  Sierpinski gasket, fractal, random 
Categories  Graphics 


When the user leftclicks, the program saves the point to use as a corner. When the user rightclicks, the program calls subroutine PlotPoints to draw the fractal.
PlotPoints picks a random corner to use as a starting point. It the repeatedly picks a new random corner and moves halfway from the current point to this corner, and plots the resulting point. When this process repeats enough times, the program draws a Sierpinski gasket.
If you cover and expose the form, the program draws the same Sierpinski gasket.


Private m_Running As Boolean
Private m_NumPoints As Integer
Private m_CornerX() As Single
Private m_CornerY() As Single
Private Const RADIUS As Single = 60
Private Sub Form_Load()
Randomize
End Sub
Private Sub Form_MouseDown(Button As Integer, Shift As _
Integer, X As Single, Y As Single)
If m_Running Then
' Stop running.
m_Running = False
m_NumPoints = 0
Else
' Left or right button?
If Button = vbLeftButton Then
' If this is the first point,
' erase the previous picture.
If m_NumPoints = 0 Then Me.Cls
' Save the point.
m_NumPoints = m_NumPoints + 1
ReDim Preserve m_CornerX(0 To m_NumPoints  1)
ReDim Preserve m_CornerY(0 To m_NumPoints  1)
m_CornerX(m_NumPoints  1) = X
m_CornerY(m_NumPoints  1) = Y
Me.Circle (X, Y), RADIUS
Else
' See if we have enough points.
If m_NumPoints < 2 Then
' We need more points.
MsgBox "You must leftclick at least two " & _
"points before rightclicking.", _
vbExclamation, "Need More Points"
Else
' Start.
m_Running = True
PlotPoints
End If
End If
End If
End Sub
Private Sub Form_Resize()
Dim wid As Single
wid = Me.ScaleWidth  2 * Label1.Left
If wid < 120 Then wid = 120
Label1.Width = wid
End Sub
Private Sub PlotPoints()
Dim i As Integer
Dim last_x As Single
Dim last_y As Single
' Pick a starting point.
i = Int(Rnd * m_NumPoints)
last_x = m_CornerX(i)
last_y = m_CornerY(i)
' Plot points until the user clicks again.
Do While m_Running
' Pick the next corner.
i = Int(Rnd * m_NumPoints)
' Move halfway from the current point
' to the new corner.
last_x = (last_x + m_CornerX(i)) / 2
last_y = (last_y + m_CornerY(i)) / 2
Me.PSet (last_x, last_y)
' Check for events.
DoEvents
Loop
End Sub
Private Sub Form_Unload(Cancel As Integer)
End
End Sub


Experiment with different corner points. Try a triangle, rectangle (you may need to use code to position the points exactly), and pentagon. Try a rectangle with three extra points at the same spot in the middle.
For more information on drawing fractals and other graphics, see my book Visual Basic Graphics Programming.





