typedef void (* XPLMDrawWindow_f)(
                         XPLMWindowID         inWindowID,    
                         void *               inRefcon);    

A callback to handle 2-D drawing of your window. You are passed in your window and its refcon. Draw the window. You can use other XPLM functions from this header to find the current dimensions of your window, etc. When this callback is called, the OpenGL context will be set properly for 2-D window drawing.

Note: Because you are drawing your window over a background, you can make a translucent window easily by simply not filling in your entire window’s bounds.