Recycling pixels on the screen using raytracing to fill in holes
I am a huge fan of raytracing.
It allows graphics code to be written much
more elegantly and effectively,
and I believe it is better
(for complex enough scenes) than the paradigm of
calculating PVS sets before rendering and then
rasterizing the appropriate geometry.
If you ask most graphics programmers,
they will tell you that raytracing is slower
than rasterizing.
This is true (for the current scenes being
rendered in real time today) because of the
relatively low ratio of the number of objects in
the world to the number of pixels on the screen.
That is, given that there are a million pixels on
the screen, even a small amount of complex computations
per pixel would make the raytracing process slower
than simply telling the graphics card to rasterize
a million objects.
But what about when there are a billion objects?
Or a trillion?
One might say that a scene with a trillion objects
is contrived and that the amount of time required
for paging would cripple the system anyway.
However, we can imagine enormous scenes that do not
take up all that much memory if they involve repetitions
of geometry.
For example, in the above picture of the reflective
spheres, the amount of memory needed to store the
sceene was sublinear in the number of spheres!
It is in these sceenes in which raytracing is truly
beneficial.
How do computer games, such as Quake or Half-Life
rasterize complex sceenes in real-time?
The secret is that, ahead of time, they break the
level up into chunks and they compute which chunks
are visible from every other chunk.
This is a difficult task, because it is harder
to determine what is visible from a region than
what is visible from a single point (that latter is
all raytracing has to do).
Without this precomputed visibility information,
rasterizing a scene in real time would be more time
consuming (and in many cases quite wasteful).
Thus, the real battle is not between Rasterizing and
Raytracing, but between
Rasterizing + Precomputing Visibility
Information and
Raytracing
(raytracing solves the
visibility problem automatically every frame).
In sceenes in which effectively precomputing visibily
information is infeasible, raytracing has
a significant advantage.
