Clipping Regions - Win32 Programmer's Reference

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Clipping Regions

A clipping region is one of the graphic objects that an application can select into a device context (DC). It is typically rectangular. Some device contexts provide a predefined or default clipping region while others do not. For example, if you obtain a device context handle from the BeginPaint function, the DC contains a predefined rectangular clipping region that corresponds to the invalid rectangle that requires repainting. However, if you obtain a device context handle by calling the CreateDC or GetDC function, the DC does not contain a default clipping region. For more information about device contexts returned by the BeginPaint function, see Painting and Drawing. For more information about device contexts returned by the CreateDC and GetDC functions, see Device Contexts.

Applications can perform a variety of operations on clipping regions. Some of these operations require a handle identifying the region and some do not. For example, an application can perform the following operations directly on a device context's clipping region:

Determine whether graphics output appears within the region's borders by passing coordinates of the corresponding line, arc, bitmap, text, or filled shape to the PtVisible function.
Determine whether part of the client area intersects a region by calling the RectVisible function.
Move the existing region by a specified offset by calling the OffsetClipRgn function.
Exclude a rectangular part of the client area from the current clipping region by calling the ExcludeClipRect function.
Combine a rectangular part of the client area with the current clipping region by calling the IntersectClipRect function.

After obtaining a handle identifying the clipping region, an application can perform any operation that is common with regions; for example:

Combining a copy of the current clipping region with a second region by calling the CombineRgn function.
Compare a copy of the current clipping region to a second region by calling the EqualRgn function.
Determine whether a point lies within the interior of a copy of the current clipping region by calling the PtInRegion function.

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