Jalaj P. Jha

We got a feel of what a Device Context is in a earlier post WinAPI : Starting with Device Context. Let’s explore ahead.

We already learnt how to get the handle to the Device Context of a window using GetDC function.

All VB objects that have a Device Context associated with them allow you to get the handle to the device context by accessing the hDC property of the object say of the form.

Let’s take a few functions related to drawing on Device Contexts.

The smallest/basic geometrical shape is a point.

A point is drawn on a Device Context using SetPixel function.

Private Declare Function SetPixel Lib "gdi32" (ByVal hdc As Long, _
    ByVal x As Long, ByVal y As Long, ByVal crColor As Long) As Long

The function takes handle to the device context as the first parameter. The next two parameters are the X and Y co-ordinates where point is to be drawn. The last parameter is the color of the pixel which can be passed using RGB function.

Let’s get to draw a pixel on a given position on form within MouseMove event of the form.

Private Sub Form_MouseMove(Button As Integer, _
    Shift As Integer, X As Single, Y As Single)

        SetPixel Form1.hdc, X / Screen.TwipsPerPixelX, _
            Y / Screen.TwipsPerPixelX, RGB(0, 0, 0)

End Sub

Copy the codes above to the code section of form and execute. Whenever the MouseMove event is fired a pixel is drawn on the position below cursor.