Category : Printer + Display Graphics
Archive   : DEMO4.ZIP
Filename : SPLITS.DOC

Output of file : SPLITS.DOC contained in archive : DEMO4.ZIP
Depth Sorting and Splitting Planes
Written by Bernie Roehl, August 1992

Version 4.00 of the REND386 package has support for binary trees of
"splitting planes" to help reduce the number of visibility errors.

Visibility errors occur when polygons are not rendered in the proper
order. This is an inherent limitation of the depth-sorting approach, and
since depth-sorting is the only technique that will let us achieve
reasonable rendering speeds, visibility errors are a fact of life.

Splitting planes give you more precise control over rendering order,
and can get rid of some (but not all) object-object visibility errors.

To understand how visibility errors occur, and how splitting planes can help,
consider the following situation:

object1 /
/ object2


There are two objects (object1 and object2), one on each side of an opaque

From the position of the observer in the above diagram, object1 should be
obscured behind the wall, and object2 should be visible. The correct
drawing order is "object1, wall, object2".

However, since the farthest point on the wall is farther away than object1,
the depth-sorting algorithm will cause the wall to get drawn first; the
resulting order is "wall, object1, object2" (which is incorrect, since
object1 appears to be on the near side of the wall when it's not).

We can modify the depth sorting; let's say we sort by nearest point rather
than farthest. Clearly this will help in some cases; however, it
doesn't work for the situation above. What we'd have is just the opposite
of the original situation; the order would be "object1, object2, wall", so
object2 (which should be visible) isn't.

This is a situation where splitting planes can help. A splitting plane
is an infinite mathematical plane that breaks space into two areas, one
on each side of the plane. When we go to render the scene, we draw the
objects on the far side of the plane first, then objects "on" the plane,
then objects on the near side of the plane.

By introducing a splitting plane that runs along the wall, and rendering
objects on the "far" side of the wall first, we eliminate the problem in
the situation above; from where the observer is, object1 is on the "far"
side of the plane and therefore gets drawn first. The wall is "on" the
plane, and gets drawn next. Finally, object2 (which is on the "near"
side of the plane) gets drawn. The order is "object1, wall, object2",
which is what we want. If the observer moves to the other side of the
wall, the solution still works; object2 will be on the far side, and
will therefore be drawn first (followed by the wall, followed by object1).

Note that a splitting plane is a true mathematical plane (i.e. it's
infinitely large, and has no boundaries). It is *not* a polygon!
A splitting plane is defined by a point in the plane and by a normal to
the plane. The normal is a line pointing away from the plane: i.e. a
normal to a floor or ceiling (i.e. constant z coords) might be 0,0,1.
For a wall with constant x-coordinates, it would be 1,0,0. Any angle
can be specified.

A single splitting plane divides space into two "areas"; we can further
subdivide either or both of these with additional splitting planes. The
planes therefore define a binary tree, whose non-leaf nodes are the planes
and whose leaf nodes are "areas" (with which we can associate various
properties, including lists of objects in that area).

Note that the order of split definition is important. For example, four
splits would perhaps form the pattern:

2 3 1
2444443 or 1222222
2 3 1 3
111111111111 1444443
depending on the order in which they're defined.

Also, you should try to keep walls, ceilings, floors, etc. confined within
the area defined by their split. Otherwise vis errors are likely. It is
possible to write a program to define splits automatically, and divide walls;
so far this hasn't been done. (any volenteers?)

When an object moves, it has to be removed from its current location in the
splitting tree and re-inserted at the correct place; this imposes a (small)
overhead. Because the object is not actually divided, but "exists" on the
side determined by its center, the overhead is much smaller than with true
BSP trees; however, it means that split trees are not foolproof when it comes
to eliminating vis errors.

It's important to keep in mind that splitting planes are not a cure-all; even
with clever use of splitting planes one can still have visibility errors.
However, they can be drastically reduced.

By breaking non-convex objects into smaller, separate pieces, you can
eliminate some errors. By carefully setting the depth-sorting type field
for certain objects, you can eliminate still more errors. And by using
splitting planes, you can remove most of the remaining errors.

The combination of these techniques can result in scenes that can be rendered
at high speeds, with relatively few vis errors.

  3 Responses to “Category : Printer + Display Graphics
Archive   : DEMO4.ZIP
Filename : SPLITS.DOC

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

  2. This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

  3. But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: