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LOGICAL FUNCTION INQBXV (XWCMIN,XWCMAX,YWCMIN,YWCMAX,ZWCMIN,
X ZWCMAX)
C$ (Inquire Box Visibility)
C$ Given the world coordinate extents of a 3-D box, determine
C$ the box visibility in the current viewport. The function
C$ value returned is .FALSE. if the box is entirely invisible,
C$ and .TRUE. if some or all of it is visible in the current
C$ viewport. Clipping is performed only according to the
C$ window, back plane, and front plane clipping options
C$ currently in effect, and is suppressed for any coordinate
C$ whose box range is not larger than zero.
C$
C$ If a clipping option is disabled for x or y, the visibility
C$ test is made against the unit NDC coordinate range on the
C$ grounds that any object entirely out of unit NDC space is
C$ invisible on any display device. Clipping may be totally
C$ disabled for the z (depth) coordinate, since a display
C$ device may choose to ignore out-of-range z values,
C$ resulting in a visible image. If only a box edge is
C$ coincident with the viewport, the box is considered
C$ invisible, and .FALSE. is returned.
C$
C$ This function is primarily useful for polygon, polyline,
C$ polymarker, and filling primitives which can pass it the
C$ world coordinates of the bounding box, and quickly
C$ determine whether a more detailed clip or fill is in order.
C$ The required conversion of world coordinates to normalized
C$ device space coordinates and tests against the viewport
C$ limits can then be localized entirely in this single
C$ routine.
C$ (06-APR-83)