XLIB Programming Manual, Rel. 5, Third Edition

6.3 Regions

An X region is an arbitrary set of pixels on the screen. But usually a region is either a rectangular area, several overlapping or adjacent rectangular areas, or a general polygon. Regions are chiefly used to set the clip_mask member of the GC. XSetRegion() sets the clip_mask to a region so that output will occur only within the region. Using XSetRegion() is a lot easier than defining a single-plane pixmap with the desired size and shape and then using that bitmap to set the clip_mask with XSetClipMask(), and it is more flexible than the clip_mask you can set with XSetClipRectangles().

The most common use of setting the clip_mask to a region is to combine the rectangle from each of multiple contiguous Expose events on a single window into a single region and clip the redrawing to that region. This provides a performance improvement in some situations. See 3.2.13.1 Repainting the Window for more information and an example.

A region has an x and y offset, which is used internally when making calculations with regions (offsets for all regions have a common origin). The offset has an effect if the region is used as a clip_mask. When making a graphics request with the clip_mask of the GC set with XSetRegion(), the offset of the region is added to clip_x_origin and clip_y_origin to determine the placement of the region relative to the destination drawable.

Regions can be created with XCreateRegion() or XPolygonRegion(). XCreateRegion() creates an empty region to which rectangles ...