Panel: Rays vs. Rasters
When Will Ray-Tracing Replace Rasterization?
Organizer/Moderator : Brad Grantham
Panelists : Kurt Akeley, Brad
Grantham, David Kirk, Tim
Purcell, Larry Seiler, Philipp Slusallek
Brad Grantham started of the discussion with the
guideline that interactivity was the focus. When things are easy
& cheep and "good enough" they will take over. He
also observed that in raytracing, the image pixels are the outer
loop and the geometry is the inner loop. And the rasterizers were
rich in features such as bump mapping, reflections, motion blur,
Philipp Slusallek took the strongest position
that raytracing would become dominant because it was the right conceptual
model. It supports the basic physics and the algorithm is so simple.
He showed several images and videos. One had all
of the usual things you see in such a picture, the mirror ball,
reflections, translucency, but also a volume (UNC skull), and the
dragon light field. The rays just keep passing through material
- even if the material is processed by different renderers. They
just work via plug & play. The demos Phillip showed were running
at 2 frames per second on fairly vanilla hardware.
OpenRT is an API for ray-tracing similar to OpenGL(tm)
Tim Purcell showed how graphics chips, aka GPUs
or VGUs, have become stream processors. The streams flow in and
through kernels that have access to global data. This structure
can be used to write a ray-tracer on the graphics chip. Tim was
the first to say that renderers will use hybrids of rasters and
rays. He showed a demo of using rasters for the basic geometry and
rays for shadows.
David Kirk started by pondering "People
who believe they cannot use computers to do rendering, what do they
He believes that the renderers are hybrids. He believes that GPUs
are increasing in power at greater than Moore's law. The is mostly
due to the memory coherency, streaming, embarrassing parallelism,
and deep pipelining.
Kurt Akeley said that ray-tracing would not overtake
rasterizers in his lifetime. looked at other claims: Volume Rendering
will overtake geometry (Jim Kajiya, 1991), and IBR will replace
3D rendering. What happens it that the raster/texel engine adapts
to absorb those techniques. Half of the NV30 is occupied with the
depth buffer and the texture sampling to achieve that.
Rasterizers have become very precise and now use
infinitely thin rays to generate samples. The removes the argument
that rays are better quality.
Larry Seiler pointer out that Cobol is still around,
and more relevant - 2D bit BLTs are what these chips do most of
the time and that is not going to change. Neither will geometry
and rasters. However ray-trace like techniques are being used to
achieve more than straight rasters: NPR, blur, reflection, refraction.
Larry emphasized what others have said: GPU will
not become general purpose because we need to keep the streaming
model to keep them fast.
Realism is not the end goal - portraying information
or telling a story is the goal.
Larry ended with a note from Mark Twain: "Rumors
of rasterizations death have been greatly exaggerated!"
During questions, Tom Duff noted that ILM is changing
their renderer from having geometry as the inner loop to the outer
loop. The must do this because the amount of geometry they need
keeps expanding faster than the memory capacity of the hardware.