SYNOPSIS
 #include <X11/extensions/Xdbe.h>
 
XdbeScreenVisualInfo *XdbeGetVisualInfo(
    Display  *dpy,
    Drawable *screen_specifiers,
    int      *num_screens)
 
DESCRIPTION
 This function returns information about which visuals support double buffering.  The argument 
num_screens specifies how many elements there are in the 
screen_specifiers list.  Each drawable in 
screen_specifiers designates a screen for which the supported visuals are being requested.  If 
num_screens is zero, information for all screens is requested.  In this case, upon return from this function, 
num_screens will be set to the number of screens that were found.  If an error occurs, this function returns NULL, else it returns a pointer to a list of 
XdbeScreenVisualInfo structures of length 
num_screens. The nth element in the returned list corresponds to the nth drawable in the 
screen_specifiers list, unless 
num_screens was passed in with the value zero, in which case the nth element in the returned list corresponds to the nth screen of the server, starting with screen zero.  The 
XdbeScreenVisualInfo structure has the following fields:
 
     int            count
     XdbeVisualInfo *visinfo
 
count specifies the number of items in 
visinfo. visinfo specifies a list of visuals, depths, and performance hints for this screen.
 
The 
XdbeVisualInfo structure has the following fields:
 
     VisualID visual
     int      depth
     int      perflevel
 
visual specifies one visual ID that supports double-buffering. 
depth specifies the depth of the visual. 
perflevel is a performance hint.
 
The only operation defined on a 
perflevel is comparison to a 
perflevel of another visual on the same screen.  The visual having the higher 
perflevel is likely to have better double-buffering graphics performance than the visual having the lower 
perflevel. Nothing can be deduced from the following: the magnitude of the difference of two 
perflevels, a 
perflevel value in isolation, or comparing 
perflevels from different servers.