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:
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:
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.
One or more values passed in screen_specifiers is not a valid drawable.
DBE, XdbeAllocateBackBufferName(), XdbeBeginIdiom(), XdbeDeallocateBackBufferName(), XdbeEndIdiom(), XdbeFreeVisualInfo(), XdbeGetBackBufferAttributes(), XdbeQueryExtension(), XdbeSwapBuffers().