Drawing on a PictureBox

[RyanSmith]

Hi, I have switched to .NET only recently and I am having some trouble with the picturebox and drawing on it over an image. The aim is to have a picturebox with an image, then using a checkbox, be able to show and hide graphics over the image. In this case I am trying to draw a crosshair at the centre of the image. The examples I have come across on the web have been minimal help. I am also unclear about the paint event for the picturebox and the form. I suspect I should be calling the draw the crosshair sub in one or both of these events ... as well as in the checkbox event? I have created a simple, to show my problem. My hope is someone can set me straight here. Below is the code from my sample form.

Public Class Form1

Private Sub Button1\_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    'LOAD A IMAGE INTO THE PICTUREBOX
    PictureBox1.Load("SampleImage.jpg")
End Sub

Private Sub CheckBox1\_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged
    If CheckBox1.Checked = True Then
        'Show the Cental Crosshair
        DisplayCentralXhair()
    Else
        'Do not show (erase?) the Central Crosshair
        RemoveCentralXhair()
    End If
End Sub

Private Sub DisplayCentralXhair()
    Dim mypen As Pen
    Dim aGraphics As Graphics

    mypen = New Pen(Color.Red, 1)
    aGraphics = PictureBox1.CreateGraphics()

    aGraphics.DrawLine(mypen, 182, 137 - 5, 182, 137 + 5)
    aGraphics.DrawLine(mypen, 182 - 5, 137, 182 + 5, 137)

End Sub

Private Sub RemoveCentralXhair()
    'Can I clear the Graphics drawn on the picturebox and retain the image below ?
    'Do I have to reload the image ?

End Sub

End Class

[Luc Pattyn]

Hi, this article[^] could point you in the right direction. BTW: the code shown is C#, the principles are valid in all .NET languages though. :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]

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I only read code that is properly indented, and rendered in a non-proportional font; hint: use PRE tags in forum messages

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[RyanSmith]

Thanks for the example Luc. Unfortunately, I am still not clear with my example if I have to redraw the image when I want to remove the graphics I have drawn ? Or can I clear the graphics without effecting the image below ? Cheers Ryan

[Luc Pattyn]

A PictureBox has two layers: the image inside (which you can only access through the PictureBox.Image property), and whatever you draw on top of the PB (and best in the Paint handler). These two are not related at all (the image is below the glass plate, your scribbles are on top of the glass plate). I would use PB.Refresh() to erase the latter, if I ever were to use a PB that is. :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]

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I only read code that is properly indented, and rendered in a non-proportional font; hint: use PRE tags in forum messages

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[RyanSmith]

Thanks Luc, The physical description "glass plate" you described has reinforced my own interpretation of how this PB works. The .Refresh() helped too. I suspect you would not choose a PB to do this task ? Thanks again. Ryan