Accumulated Bounding Rectangle
The accumulated bounding rectangle is the smallest rectangle enclosing the portion of a window or client area affected by recent drawing operations. An application can use this rectangle to conveniently determine the extent of changes caused by drawing operations. It is sometimes used in conjunction with LockWindowUpdate to determine which portion of the client area must be redrawn after the update lock is cleared.
An application uses the SetBoundsRect function (specifying DCB_ENABLE) to begin accumulating the bounding rectangle. The system subsequently accumulates points for the bounding rectangle as the application uses the specified display device context. The application can retrieve the current bounding rectangle at any time by using the GetBoundsRect function. The application stops the accumulation by calling SetBoundsRect again, specifying the DCB_DISABLE value.