gdk:window-create-similar-image-surface (window format width height scale)

window -- a gdk:window object to make the new surface similar to
format -- a value of the cairo:format-t enumeration for the format of the new surface
width -- an integer for the width of the new surface
height -- an integer for the height of the new surface
scale -- an integer for the scale of the new surface, or 0 to use same as window

A newly allocated cairo:surface-t instance. The caller owns the surface and should call the cairo:surface-destroy function when done with it. This function always returns a valid pointer, but it will return a "nil" surface if the surface is in an error state or any other error occurs.

Create a new image surface that is efficient to draw on the given window. Initially the surface contents are all 0, transparent if contents have transparency, black otherwise.

The width and height of the new surface are not affected by the scaling factor of the window, or by the scale argument. They are the size of the surface in device pixels. If you wish to create an image surface capable of holding the contents of the window you can use:

(let* ((win (gtk:widget-window window))
       (scale (gdk:window-scale-factor win))
       (width (* scale (gdk:window-width win)))
       (height (* scale (gdk:window-height win)))
       (surface nil))
  ;; Create the image surface
  (setf surface
        (gdk:window-create-similar-image-surface win
                                                 :rgb24
                                                 width height
                                                 scale))
  ...
  (cairo:surface-destroy surface))  

Note that unlike the cairo:surface-create-similar-image function, the new device scale of the surface is set to scale, or to the scale factor of window if the scale argument is 0.