Using Layered Windows

Using Layered Windows

To have a dialog box come up as a translucent window, first create the dialog as usual. Then, on WM_INITDIALOG, set the layered bit of the window's extended style and call SetLayeredWindowAttributes with the desired alpha value. The code might look like this:

// Set WS_EX_LAYERED on this window 
SetWindowLong(hwnd, 
              GWL_EXSTYLE, 
              GetWindowLong(hwnd, GWL_EXSTYLE) | WS_EX_LAYERED);

// Make this window 70% alpha
SetLayeredWindowAttributes(hwnd, 0, (255 * 70) / 100, LWA_ALPHA);

Note that the third parameter of SetLayeredWindowAttributes is a value that ranges from 0 to 255, with 0 making the window completely transparent and 255 making it completely opaque. This parameter mimics the more versatile BLENDFUNCTION of the AlphaBlend function.

To make this window completely opaque again, remove the WS_EX_LAYERED bit by calling SetWindowLong and then ask the window to repaint. Removing the bit is desired to let the system know that it can free up some memory associated with layering and redirection. The code might look like this:

// Remove WS_EX_LAYERED from this window styles
SetWindowLong(hwnd, 
              GWL_EXSTYLE,
              GetWindowLong(hwnd, GWL_EXSTYLE) & ~WS_EX_LAYERED);

// Ask the window and its children to repaint
RedrawWindow(hwnd, 
             NULL, 
             NULL, 
             RDW_ERASE | RDW_INVALIDATE | RDW_FRAME | RDW_ALLCHILDREN);