https://invisible-island.net/xterm/xtoolkit/
XtAddExposureToRegion - merge exposure events into a region
#include <X11/Intrinsic.h>
void XtAddExposureToRegion(XEvent *event, Region region);
event Specifies a pointer to the Expose or GraphicsExpose event.
region Specifies the region object (as defined in <X11/Xutil.h>).
The XtAddExposureToRegion function computes the union of the rectangle
defined by the exposure event and the specified region. Then, it
stores the results back in region. If the event argument is not an
Expose or GraphicsExpose event, XtAddExposureToRegion returns without
an error and without modifying region.
This function is used by the exposure compression mechanism (see Sec-
tion 7.9.3).
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtAddExposureToRegion(3)