INFO: Definition of a Region (10212)

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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().