Private Sub groupbox1_Paint(sender As Object, 
              e As System.Windows.Forms.PaintEventArgs) _
              Handles groupbox1.Paint
 
     Const xmax As Integer = 580
     Const ymax As Integer = 380
 
     Dim x, y, xr, yr, x1, y1 As Integer
 
     If Not Integer.TryParse(textbox1.Text, x) Then Exit Sub
     If Not Integer.TryParse(textbox2.Text, y) Then Exit Sub
 
     Rescale(x, y, xmax, ymax, xr, yr) 'Integer-Variante von oben
 
     x1 = CInt((600 - xr) / 2)
     y1 = CInt((400 - yr) / 2)
 
     Using myBrush As New SolidBrush(Color.LemonChiffon)
        e.Graphics.FillRectangle(myBrush, New Rectangle(x1, y1, xr, yr))
     End Using
End Sub

Private Sub Button1_Click(sender As Object, _
        e As System.EventArgs) Handles Button1.Click
 
   groupbox1.Invalidate()
 
End Sub