Line specials 777 - slope floor by anchors, 778 - slope ceiling by anchors,
779 - slope floor and ceiling by anchors.
Thing types 777 - floor anchors, 778 - ceiling anchors.
Summary.
These line specials slope the front or back sector's floor/ceiling by 'anchor'
things. Anchors are floor or ceiling specific and snap to the nearest vertex.
Using this method it is possible to create seamless slopes, without even
tagging.
Details.
Normal slope flags apply. To slope the backside sector, set No Climb on the
line. An anchor each is required to be near three of a sector's vertices in
order to slope it. More may be present but the order is undefined.