ID Number: Q10212
2.00 2.10 3.00
WINDOWS
Summary:
The term "region" is used throughout the Windows documentation;
however, the term is not defined. This article explains regions, how
they are used, and how a region is defined or specified.
More Information:
Unlike most graphics packages that can manipulate only simple
geometric structures (usually rectilinear), The Windows graphics
device interface (GDI) has the unique ability to gather an arbitrary
set of spatially coherent points into a structure called a region, and
perform complex yet rapid manipulations and calculations on such
structures. This feature not only makes standard programs simpler and
faster, it also allows operations to be performed that would otherwise
be nearly impossible (for example, it is fundamental to the
implementation of the Windows user interface).
Although most GDI function parameters are given in logical units and
then are converted to device units, region function parameters are
given in device units.
A region is defined by defining lines, shapes (such as rectangles and
ellipses), or other regions. The outline of a region should be one or
more closed loops. A region can be concave or convex, can consist of
one area or many disjoint areas, and can have "holes" in the middle.
Many calculations can be performed on regions. For example, given any
two regions, GDI can find their union, intersection, difference, and
exclusive. There is a set of graphic operations on regions to draw
them on the screen.
When a region is selected into a device context (DC), a new clipping
area is created. Any graphics sent to that DC will be clipped to the
region created. Regions are defined using CreateRectRgn(),
CreateEllipticRgn(), and CreatePolygonRgn().