+from the portion of the screen underlying the blitspin window, or from
+the system's video input, or from a random file on disk, as indicated by
+the \fIgrabDesktopImages\fP, \fIgrabVideoFrames\fP,
+and \fIchooseRandomImages\fP options in the \fI~/.xscreensaver\fP file;
+see
+.BR xscreensaver-demo (1)
+for more details.