source files: xgtm.[ch], term/*.trm, gtplot.h
(GT-CLEAR-VIEWPORT) (SEND <gtm:terminal> :CLEAR-VIEWPORT)
This command clears the viewport to the current color. It also clears the Zbuffer. If you are in zbuffer mode (most 3D programs should be), you will want to call this at the beginning of each frame, to clear the Zbuffer to infinity. Otherwise, things from the last frame will hide the things you are currently drawing, even if you have erased the visible images of the old things. This can be puzzling :).