Clipping Regions

You can use clipping regions to restrict your output to a specified subregion of the client area. To use a clipping region, you must select it into the device context associated with the display device.

Clipping is used in Windows CE in a variety of ways. Word processing and spreadsheet applications clip keyboard input to keep it from appearing in the margins of a page or spreadsheet. Computer-aided design (CAD) and drawing applications clip graphics output to keep it from overwriting the edges of a drawing or picture.

Some device contexts provide a predefined or default clipping region. For example, the device context created by the BeginPaint contains a predefined rectangular clipping region that corresponds to the invalid rectangle that needs to be repainted. However, the device contexts created by the CreateDC and GetDC functions contain empty clipping regions; clipping is only done to keep graphics output in the window's client area.

You 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, you can perform the following operations directly on a device context's clipping region.

• Determine whether part of the client area intersects a region by calling the RectVisible 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, you can perform any operation that is common with regions, such as:

• Combine 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.