Question

How come when I call the SetPixel function to set a pixel in iTunes, it does shows me the pixel in that specific application (like it should), whilst when calling the same function for Windows Media Player it doesn’t show at all. Also, this function doesn’t seem to work for fullscreen windows either. Why this great diversity? Any explanation would be greatly appreciated. Thanks in advance.

Answer 1

As mentioned, it’s very easy not to get the result you are expecting because it’s not your window, and you ultimately have no control over what it does!

Two things you could do (one obviously being better than the other) are:

• Use GetDC (NULL) to get a handle to the screen device context. Use ClientToScreen to calculate the position of your pixel on the screen. Call SetPixel with that handle. This will get cleared fairly quickly, though.

• Create your own top-level, popup, transparent (to messages), layered window with an appropriate colour key. Use ClientToScreen to calculate the position of the screen, and possibly ScreenToClient to make sure it goes properly onto your window (which should theoretically have the same client dimensions as the screen). Draw the pixel onto your window and you will be in control of when it leaves.

If using the second, your extended window style should be:

WS_EX_TOPMOST | WS_EX_TRANSPARENT | WS_EX_LAYERED

and your normal window style should be:

WS_POPUP | WS_VISIBLE

The dimensions can be found using GetSystemMetrics with the values SM_CXSCREEN and SM_CYSCREEN.

To set the colour key, use SetLayeredWindowAttributes:

SetLayeredWindowAttributes (hwnd, RGB (red, green, blue), 0, LWA_COLORKEY);

Where red, green, and blue make up the background colour of your window, and should never appear as an actual colour. That way your background will not be drawn, giving an unnoticeable difference to the user, but when anything is drawn, it will show up.