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Clipping Regions

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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, when you obtain a device context handle by calling the GetDC function with a NULLhWnd parameter, or by calling the CreateDC 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, such as:

  • 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.