enlarging and reducing the size of a circle using a scrollbar

[Kevmeister]

Hi all,

Im trying to enlarge a circle on a form using a scrollbar. I have managed to do this using the following code:

Private Sub DrawCircle(ByVal cp As Point, ByVal radius As Integer)
        Dim gr As Graphics
        gr = Panel1.CreateGraphics
        Dim rect As Rectangle = New Rectangle(cp.X - radius, cp.Y - radius, 2 * radius, 2 * radius)
        'gr.DrawEllipse(Pens.Black, rect)
        gr.DrawEllipse(Pens.Black, rect)
        gr.FillEllipse(Brushes.Red, rect)
        gr.Dispose()
    End Sub

Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll
        DrawCircle(New Point(TrackBar1.Value / 2, TrackBar1.Value / 2), TrackBar1.Value)

    End Sub

Thing is the code draws a circle on every scroll movement. You end up with a mass of circles. I want it to clear previous graphic and just show the circle that relates to the current scrollbar graphic, giving the impression of enlarging and shrinking. Any ideas? All help most appreciated.

[bergerkiller]

Hi.

You need to "clear" the graphics before you draw again, using:
gr.Clear(Color)

    Private Sub DrawCircle(ByVal cp As Point, ByVal radius As Integer)
        Dim gr As Graphics
        gr = Panel1.CreateGraphics
        gr.Clear(Panel1.BackColor)
        Dim rect As Rectangle = New Rectangle(cp.X - radius, cp.Y - radius, 2 * radius, 2 * radius)
        'gr.DrawEllipse(Pens.Black, rect)
        gr.DrawEllipse(Pens.Black, rect)
        gr.FillEllipse(Brushes.Red, rect)
        gr.Dispose()
    End Sub

    Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll
        DrawCircle(New Point(TrackBar1.Value / 2, TrackBar1.Value / 2), TrackBar1.Value)
    End Sub

[stateofidleness]

actually it's because he's declaring a new Graphics object each time, hence the multitude of circles. You need to declare the graphics object once, and maintain it.

Dim gr As Graphics is global

Then in your DrawCircle method, you would replace the "Dim gr As Graphics" line with "gr.Clear()"

[bergerkiller]

Yeah, but for the simplicity I just said that. While were at it anyways:
I recommend you to set the following line for the panel to true:

MyBase.DoubleBuffered = True

It would get rid of the "flashing" while drawing.
The flashing is caused by the "clear" sub, that causes the panel to invalidate and thus redraw.