[RESOLVED] help: find a pixel on screen by its color

[iPrank]

ive often seen the hDC thing in api calls but not entirely sure what it means.

in this example

lColor = GetPixel(GetWindowDC(GetDesktopWindow()), x, y )

does it mean it will only search pixels on the desktop

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and in this example

ColorMe = GetPixel(hDC, X, Y)

does it mean it will search pixels on the current screen?
or do i have to change the hDC to something else to do that?

hDc: This property is a Windows operating environment device context handle. The Windows operating environment manages the system display by assigning a device context for the Printer object and for each form and PictureBox control in your application. You can use the hDC property to refer to the handle for an object's device context. This provides a value to pass to Windows API calls.

You can think it as a drawing board. If your window/control/printer has an hDc, whatever you 'draw' on its hDc, it appears on the screen/printed page.

Note: Some controls like Label or Image controls don't have any DC. They are painted direcly on the form. They are called lightweight controls.
Again, some controls, like the Frame control, don't expose the hDC property in VB IDE. You'll not see it in the intellisence list or in the object browser. But you can access it using the GetDC API.

GetPixel gets the specified pixel of a given hDc (in other words, a particular pixel of a window)

For your own form/Picturebox etc, you can get the hDc from the .hDc property.
For getting the hDc of an external window we have 2 APIs:

GetDC: The GetDC function retrieves a handle to a display device context (DC) for the client area of a specified window or for the entire screen. You can use the returned handle in subsequent GDI functions to draw in the DC.

And,

GetWindowDC: The GetWindowDC function retrieves the device context (DC) for the entire window, including title bar, menus, and scroll bars. A window device context permits painting anywhere in a window, because the origin of the device context is the upper-left corner of the window instead of the client area.

You can find their full documentation in your MSDN CD or here

The desktop itself is a window. It has valid hWnd (Window Handel) and valid DC.
So, in order to get a particular pixel from it's DC, we need to first get its hWnd by using the GetDesktopWindow API.
Then we need to get its dc using the GetWindowDC API.
Finally, we need to get the pixel's color value using the GetPixel API.

If we break the code,
lColor = GetPixel(GetWindowDC(GetDesktopWindow()), x, y )
it will be like,

Dim lDesktopHwnd As Long
Dim lDesktopDc As Long
Dim lColor As Long
 
lDesktopHwnd = GetDesktopWindow() 'get hWnd of the desktop
lDesktopDc = GetWindowDC(lDesktopHwnd) 'get the hDc of the desktop
lColor = GetPixel(lDesktopDc, x, y) 'get the color of specified pixel

Hope it helps.

Edit: Hey ! You changed the complete post.