The routine gSetViewportClipSwitch() determines the setting of window limits after a call to gSetViewport2D(). When sw = GON, after a call to gSetViewport2D() the window limits are restricted to the viewport limits, and therefore it will not be possible to draw outside these limits. In these circumstances the setting of a viewport effectively becomes a mapping and clipping operation. If a call is made to gSetViewportClipSwitch(GOFF), any viewport setting will act simply as a mapping operation and window limits may extend outside the viewport limits.
Note that gSetViewportClipSwitch() only affects subsequent calls to gSetViewport2D().