Effects, and Why They Matter
white paper focuses on a new, proprietary 3D rendering technology
and the problems it solves. The technology, from 3dfx, is the
It’s designed to bring a new level of realism to 3D graphics on
the consumer PC.
quest for real-time, full-motion photorealism in 3D graphics is
the holy grail for the 3D industry. The ultimate goal is to
eliminate all visual cues that distinguish synthesized — meaning
computer-generated — scenes from photographs or video clips
taken of physically real scenes.
this level of photorealism on a single photograph, or a single
frame of a full-motion clip, is relatively straightforward, in
that the required rendering algorithms are widely understood.
Achieving it on a series of frames to produce a full motion video
clip is also fairly straightforward, at least in situations that
allow the luxury of taking all the time you need to produce each
frame, then joining them together. Reaching this level of
photorealism in real-time, generating each frame quickly enough to
produce full motion, photorealistic scenes as needed in an
interactive game or simulation, for example, is a challenge that
has yet to be met on consumer platforms.
gap between the level of photorealism possible in scenes rendered
offline compared to scenes rendered in real-time is dramatic. The
best example of this gap can be found at the beginning of almost
every game that ships with a pre-rendered video introduction.
These cinematic introductions typically run for several minutes.
They are meant to introduce the game’s starting scenario and
draw the player into
the game. These intros are painstakingly created frame-by-frame
offline by artists, then assembled into full motion clips.
However, the scenes in the actual game, created in real-time
during game-play, rarely match the introductions for
photorealistic effect. The difference can leave users
disappointed, as the quality of photorealism falls noticeably once
the game starts.
T-Buffer allows several key digital effects for improving
photorealism in real-time 3D graphics rendering. Until now, these
effects have not been available on consumer-level PCs at real-time
frame rates. Chief among these are full-scene spatial
anti-aliasing (to remove the roughness or outright jaggedness from
lines or polygon edges over an entire visible scene), motion blur
(to add a realistic blur to moving objects that are otherwise
defined more sharply than they would be if they were real objects
actually photographed on chemical film) and depth of field (to add
visual cues that help define the distance to each object in a
scene by way of different levels of sharpness, or degrees of
focus, at different depths).
that the first two rendering techniques — spatial anti-aliasing
and motion blur — are non-issues in chemical photography.
Chemical film records edges without adding any jaggedness; motion
blur is the natural result of photographing moving objects at
shutter speeds of 1/250th of a second or slower. Adding
these effects to a rendered scene is a way to make the scene
appear much more realistic.
third rendering technique — depth of field — is also a natural
result of the way cameras work in the real world, but it
differs from the first two in that it’s a standard consideration
in photography and cinematography. In fact, directors regularly
take advantage of this effect. It lets them draw the audience’s
attention to one part of the screen or another simply by changing
the camera’s focus. If near objects are in focus, objects
further away from the camera become blurry. If far objects are in
focus, the objects nearer to the camera become blurry. The
audience’s attention automatically goes to whatever is in focus.
come back to these rendering techniques, and some others as well,
in more depth later in this paper. For the moment, it’s enough
to know that a key benefit of each of these effects is to make the
image on screen more realistic — meaning that it will better
match what the viewer sees in real life, in photographs, and in
Precursors to the T-Buffer
(A Short history)
of the features built into 3D controller chips for the consumer PC
today are derived from rendering algorithms developed for high end
workstations. As PCs became faster and more sophisticated, the
once costly workstation solutions eventually made their way into
the inexpensive PC systems we have today. To fully understand the
problems the T-Buffer technology addresses, it is useful to take a
brief look at some of the breakthrough algorithms developed first
for these high end workstations.
1984, Robert Cook, Thomas Porter, and Loren Carpenter of the
computer division of Lucasfilm Ltd. published a seminal paper,
called Distributed Ray Tracing, which focused on many of the same problems
the T-Buffer addresses. At the time the paper was written, an
earlier technique known as ray tracing was able to render sharp
shadows, reflections, and refracted light in computer-generated
images by determining precise ray directions.
However, the original ray tracing algorithm was limited in its
ability to create blurry effects or soft effects, such as the soft
edges of most real-world shadows, or the depth of field effect, as
shown in Figure 1.
reason for the limitation was that ray tracing is subject to
aliasing artifacts. The breakthrough in distributed ray tracing
was that it combined the basic concept of ray tracing with a new
technique. The resulting technique offered a new approach to
synthesizing images that made it far easier to create blurred
effects, such as soft shadows, motion blur, and depth of field.
1: The depth of field effect shows objects at different
distances at different degrees of focus. In this image, the
woman in the foreground is in focus, while the trees in the
background are out of focus, or blurry. This different focus
depending on distance is a natural result of the way camera lens work.
This effect is difficult to emulate using standard 3D
all practical purposes, this was the first time that these, and
were available for computer-generated images. These newly
available effects brought 3D computer graphics to a new level of
realism, but also created new obstacles to overcome.
of the problems that needed to be solved was how to render these
new effects in real-time – at a high frame and fill rate. That
problem was addressed in 1990 by Paul Haeberli and Kurt Akeley of
Silicon Graphics Computer Systems, in their paper, The
Accumulation Buffer: Hardware Support for High-Quality Rendering.
Accumulation Buffer, which we discuss in more detail later in this
white paper, is a system architecture that provides an additional
buffer to integrate multiple renderings of a scene. Although it
was originally conceived to be used for spatial anti-aliasing, it
also provides a way to achieve such effects as soft shadows,
motion blur, and depth of field. (See Figure 2 for an example of
spatial aliasing.) Most important, the Accumulation Buffer can
provide these effects in real time.
these issues solved for high end workstations, the next natural
step was to bring similar technologies to consumer PCs.
The Problems the T-Buffer
many new 3D rendering technologies, the primary purpose of
3dfx’s T-Buffer technology is to improve image quality. The
ultimate goal of computer-generated 3D graphics is to create such
realistic images that the computer monitor effectively becomes a
window into a lifelike, fully interactive, 3D world. By
definition, that lifelike world should not show any artifacts that
serve as a distraction and break the illusion of reality. The
T-Buffer lets PCs take another key step in that direction.
challenge is exactly how to narrow this gap between
computer-generated 3D graphics and what users typically see in
real life, photography, and motion pictures. The T-Buffer attempts
to narrow the gap considerably by offering real-time hardware
acceleration of spatial anti-aliasing, motion blur, depth of
field, and some other, closely-related effects.
is important to realize that rendering with full-scene spatial
anti-aliasing, motion blur, depth of field, and related techniques
can make the difference between allowing the viewer to perceive
the image as realistic, and jarring him or her out
of a willing
suspension of disbelief.
That phrase is most often associated with the state of mind a
fiction writer strives to maintain in readers or a director
strives to maintain in movie viewers. However, it applies just as
well to the state of mind a viewer should be in when viewing a 3D
world on screen for interactive entertainment, game playing, or
simulations. With this in mind, here’s a look at the specific
digital effects the T-Buffer addresses.
Full Scene Spatial
basic concept of spatial anti-aliasing applies to almost every
kind of computer-based
image. The undesirable artifacts on everything from jagged text
characters in a word processor to the stair-stepping of
near-horizontal lines in a 3D scene can be characterized as
spatial aliasing artifacts. Spatial anti-aliasing is a technique
for smoothing those jagged edges. However, there is more to
spatial aliasing than just jagged edges, and more to spatial
anti-aliasing than simply smoothing those edges.
today’s consumer level 3D graphic boards which only support
primitive types of spatial anti-aliasing, you will not only still
see jagged edges as a result of spatial aliasing, but you will see
or popping, polygons.
2: The left side of this figure shows aliasing in the top and bottom edges of the window
view of a sunset in Quake2, with no anti-aliasing. The image on the right side is a
rendering of the same scene with anti-aliasing applied. Screenshots from Quake2 ă 1998 id software,
problem shows when a small, thin object, like a light pole in the
distance, repeatedly appears and disappears as the object moves
relative to the screen, or when the object sometimes shows as a
solid line and sometimes as a broken line. Both jagged (or
stair-stepping) edges and popping polygons fall under the general
category of spatial aliasing artifacts. Spatial anti-aliasing is a
technique intended to reduce these visual anomalies. Figure 3
shows the dramatic visual difference between a spatially aliased
image and a spatially anti-aliased image.
better understand what causes aliasing, it’s important to know a
little about how computer generated 3D graphics are created.
Rendering a digital image involves a process called sampling,
and the technical definition of aliasing is artifacts
caused by under-sampling.
getting into details, the need for sampling grows from the reality
of memory restrictions, bandwidth restrictions, and the like.
Because of these limitations, less image data can be stored than
ideally should be stored. The solution is to sample the source
image data at regular intervals and store the resulting,
incomplete set of data. Because some of the source data gets
discarded, this process can result in visual artifacts such as
aliasing. To alleviate this problem, rendering techniques often
try to increase the sample rate to store more of the original
image data. Supersampling is a technique for increasing the sample
useful in the context of discussing the T-Buffer to make the
distinction between spatial anti-aliasing and anti-aliasing in
general. As it happens, almost all the digital effects the
T-Buffer makes possible are, in a strict technical sense,
variations on general anti-aliasing. When discussing spatial
anti-aliasing, we’re talking
about removing the jagged polygon edges and minimizing polygon
popping. The T-Buffer, however, can also perform temporal
anti-aliasing (supersampling in time to add motion blur) and focal
anti-aliasing (supersampling in focal length to add depth of
also important to distinguish between full-scene
spatial anti-aliasing and lesser forms of spatial anti-aliasing
that deal only with edges (including lines). Only full-scene
spatial anti-aliasing eliminates all spatial aliasing artifacts,
including both jagged edges and popping polygons. Some 3D boards
available today claim to support spatial anti-aliasing, but there
are numerous limitations to the implementations: either the scene
needs to be sorted (essentially arranging 3D slices by depth to
remove hidden surfaces) or the edges need to be flagged and
re-rendered. Either approach puts a large demand on the system’s
CPU. As a result, software developers have chosen to maintain high
frame rates rather than take advantage of currently available
“non real-time” spatial anti-aliasing technologies. The
T-Buffer is the first consumer technology to offer real-time,
full-scene spatial anti-aliasing.
traditional computer generated images, a given frame showing an
object in motion will render that object with crisp, clean edges
in each frame. When viewing a full motion version of the scene
based on these images, the result is an unrealistic strobing
effect (much like watching someone move underneath a strobe
light). This strobing effect is quite different from the
continuous, fluid motion of moving objects in real life. Strobing
artifacts can also be thought of as temporal aliasing. The T-Buffer
addresses this issue by adding support for real-time motion blur.
Figure 3 (top): The aliasing in the image to the left shows
as both jagged, or stair-stepping, lines and as broken
lines. The jagged lines
the edges of the mountains
the upper left of the scene. The broken lines show in the light
poles in the lower right.
Figure 3 (bottom): Both types of artifacts are dramatically
improved with spatial anti-aliasing in the
version of the same scene to the right. Screenshots
from Barrage ă 1998 Mango Grits, Inc.
better understand what causes temporal aliasing, it helps to
understand how static images are put together in sequence to form
a full motion version. As already mentioned, aliasing refers to
those artifacts that result from undersampling, which is to say
there is more data that needs to be stored than can be stored.
temporal aliasing, the strobing effect is caused by an object’s
motion not being sampled frequently enough. So instead of fluid
movement the eye sees jerky movement from one frame to the next.
The most common way to reduce this strobing artifact is to
increase the number of snapshots
of an object’s motion. The problem with this brute-force
approach for real-time 3D rendering is that it dramatically raises
the performance requirements to unreasonable levels for a
T-Buffer technology allows for a much more cost effective approach
to this problem by averaging multiple, discrete motion data
together. This adds a motion blur to moving objects in each frame
within a scene. And instead of seeing one discrete movement at a
time, the eye sees multiple samples of movement at once. The
result is a substantially improved motion effect which removes the
strobing problem that’s so typical of computer-generated 3D graphics.
Figure 4 shows an example of a single frame with motion blur
an example of how much more visually appealing motion blurred
sequences are, consider Hollywood movies. Movie projectors update
each frame at only 24 frames per second. However, if you tried to
play a computer game that updated at 24 frames per second you
would find it jerky and utterly unrealistic.
difference stems from the way cameras take pictures, by holding a
shutter open for some period of time, to let light fall on the
film, while the moving object continues to move. This
automatically adds motion blur as a natural result of the process.
And the motion blur, recorded in each frame of the film, lets the
audience experience the movie comfortably at 24 frames per second.
In comparison, a non-motion-blurred computer-generated image has
to update frames at 2 to 3 times that rate to avoid the strobing
effect. The T-Buffer technology allows for extremely high frame
rates combined with real-time motion blur for an unparalleled
Figure 4: The motion blur effect gives a
sense of movement even in a still image like this.
Copyright ©1984, the Association for Computing Machinery. Permission
text as appropriate to be added by 3dfx.
Depth of Field
amateur photographer whose camera includes a focus control and
f-stop settings has at least some familiarity with depth
of field issues. Put a camera close to a window pane covered with
raindrops, focus it to show the raindrops well, and most of the
world beyond the window pane will show as an unrecognizable blur.
Change the focus to show the house across the street, and the
raindrops will essentially disappear from view. (Figure 5 shows an
example of the depth of field effect.)
human eye works in much the same way, although people typically
aren't aware of the blurry parts in their field of view. Move your
eye close to that same window with raindrops, focus on the
raindrops, and the world beyond the raindrops will be blurry. Look
at the house across the street and the nearby window will turn
and TV directors take advantage of this depth of field effect all
the time. With an actor in the foreground, for example, the camera
may focus on the actor first, then change focus to something
happening in the background. The foreground becomes blurry as the
action in the background comes into visual focus, and the
audience’s attention automatically moves to the part of the
screen that's in focus. Similarly, when two characters are at
different distances from the camera, the camera focus — and with
it, the audience's attention — may shift from one character to the
other as the conversation goes back and forth. Alternatively, the
camera may stay focused on one character while the other is
talking, to focus the audience's attention on the first
Figure 5: The version of this image on the
left shows all the crayons in focus, from the closest to the
furthest away. The image on the right adds a depth of field
effect that keeps the crayons relatively close to the viewer
in focus, while blurring the closest crayons, and those
furthest away. This different focus at different depths
gives a visual cue to three dimensionality.
addition to being able to use the depth of field effect to focus a
viewer's attention on some particular area of the screen, the
effect gives important visual cues to the distance between two
objects in a scene or between the viewer and each object.
computer generated images today, however, don't show any visual
cues to distance, and they don't allow for using this effect to
draw the viewer's attention to certain areas of the screen. They
show both near and far objects in focus unless you add the
appropriate blurriness though a depth of field effect. The
T-Buffer technology brings the capability of offering real-time
depth of field effects for the first time to the consumer PC.
Other effects (Soft Shadows
and Reflectance Blur)
T-Buffer allows several other effects as well. Two in particular
are worth mention: soft shadows and reflectance blur. Some
games and 3D applications are now starting to take advantage of
hardware features such as Stencil Buffers to render shadows. The
result, however, looks synthetic and is disappointing to the user
in that the shadows are drawn with unrealistically sharp edges. In
real life, shadows normally have soft edges as in the shadow in
Figure 6 (these soft edges are also called blurry penumbras). The
T-Buffer can be used to generate much more realistic soft shadow
effects in real-time on consumer PCs.
blur (also called soft reflectance) is another natural visual
phenomenon. In the real world, there are some semi-glossy
surfaces, like polished wood or brushed stainless steel, that will
reflect objects with different degrees of focus, depending on how
close the object is to the surface. Hold a pencil perpendicular to
the surface, for example, and the part of the pencil that's
closest to the surface will reflect in sharp focus, but the
reflection will become increasingly blurry for parts of the pencil
that are further and further away from the surface, as shown in
Figure 6. Using today’s 3D accelerators, the entire reflection
of the pencil will be in sharp focus. In contrast, the T-buffer
can be used to simulate reflectance blur in real-time on consumer
What all these effects have
Full-scene spatial anti-aliasing,
motion blur, depth of field effects, soft shadows, and reflectance
blur all have one important thing in common: From a strict
technical sense, they are all variations on anti-aliasing. As
already mentioned, aliasing is technically defined as artifacts
caused by under-sampling. More generally, aliasing artifacts are
the visual anomalies that result from needing more data stored and
displayed than actually can be stored and displayed.
T-Buffer allows acceleration of general anti-aliasing by providing
more snapshots (for lack of a better term) of the source image
data to yield better overall visual quality. For
full-scene spatial anti-aliasing this means providing more samples
in the 2D (XY) space to remove the roughness, or jaggedness, from
polygon edges. For motion blur, it means providing more samples over time to remove the strobing effect for
fast-moving objects. And for depth of field effects it means
providing more focal (or “camera focus”) samples to allow 3D
rendering to function much more like a real-world camera. Soft
shadows and reflectance blur effects are also based on similar
techniques using multiple samplings.
aspect these new 3D rendering features have in common is that —
with the exception of full-scene spatial anti-aliasing — they
are relatively subtle (Spatial anti-aliasing is far from subtle.)
No matter how subtle they are, however, they can make the
difference between offering a reasonably good computer-generated
image, and one that approaches being indistinguishable from a
photograph. Leave them out, and a viewer with an untrained eye
will know that the scene looks less than fully realistic, even
though he or she may not be able to spot what’s wrong with the
image. The T-Buffer bridges this realism gap to let consumer PCs
achieve far more photorealistic images at real-time rendering
Figure 6: Most shadows in the real
world have soft edges as in the shadow of the paper clip
shown here. Many semi-glossy surfaces show the kind of
reflection shown here, with the reflection showing as
increasingly blurry for those parts of the object farther
and farther away from the reflecting surface. Copyright
©1984, the Association
for Computing Machinery. Permissions
to be added by 3dfx.
How The T-Buffer Works
best understand the advantages of the T-Buffer technology it’s
important to understand how it works and how it differs from the
Accumulation Buffer that preceded it.
there are some variations on Accumulation Buffer implementation,
the rendering technique normally uses the following steps:
the Accumulation Buffer.
the back buffer and render the full scene in the back buffer.
the rendered scene to the Accumulation Buffer, with weighting
(this is known as the accumulate step).
steps 2 and 3 as many times as needed. Each repetition
combines a new version of the image with the image already in
the Accumulation Buffer, so the final image is created as a
weighted average result of different images. It's the
combination of the multiple images that adds the effects.
with scaling, the Accumulation Buffer to the back buffer, and
flip (or swap) buffer contents. The result becomes the
Accumulation Buffer offers an elegant, but expensive, solution to
one of the obstacles to rendering realistic images. However,
it’s somewhat limited in consumer applications because it’s so
expensive. The T-Buffer offers substantially the same
capabilities, and in some cases better performance, at consumer
Although the T-Buffer does essentially the same thing as
the Accumulation Buffer in that it combines multiple different
images to create digital effects, it reaches the final result
The steps for creating these
effects using the T-Buffer are quite simple:
the back buffer(or buffers)
multiple images into the back buffer(or buffers)
the front and back buffer for display
both the Accumulation Buffer and the T-Buffer, the key steps that
add the multiple renderings need to be repeated as many times as
necessary. It happens
that repeating these steps quickly reaches the point of
diminishing returns, with the first few repetitions doing most of
the work to increase the realism of the final result, and
additional repetitions adding less and less improvement.
a rule of thumb, for the best possible image quality, you need to
integrate 16 to 30 renderings of slightly different images. With
current technology, however, it’s extremely difficult to reach
this many repetitions at real-time frame rates. Fortunately,
between 4 and 8 repetitions typically yield a high quality image
for adding these effects in real time, and both the Accumulation
Buffer and the T-Buffer support this mode of operation.
of the distinguishing characteristics of the T-Buffer is that it
does not require an accumulate step nor a copy from an
Accumulation Buffer to the back buffer. Rather, the hardware
automatically combines the various T-Buffer contents when
a particular image is ready to be displayed on the computer
second distinguishing feature of the T-Buffer as compared to the
Accumulation buffer is that it lets you apply particular effects
to only a selected portion of the image. Consider, for example, a
still background with a moving car that needs motion blur added.
For any given number of renderings, with the Accumulation Buffer
you would need to render the entire image each time. With the
T-Buffer you can render the still background once, then repeat the
rendering on the car only. Since fewer triangles and pixels are
involved in the additional renderings, the T-Buffer offers an
automatic performance boost compared to the Accumulation Buffer.
T-Buffer opens the door to digital effects that have never before
been possible in real-time on consumer PCs. The most important of
these is the addition of full-scene
spatial anti-aliasing at real-time frame rates. Of all the
remaining problems to be solved in 3D rendering on consumer PCs,
this is the most important. It will, by itself, do more than any
other step forward to help preserve the willing suspension of
disbelief that is central to the enjoyment of interactive
T-Buffer also opens the door to more effects that we have covered
here, because the T-Buffer is a tool, rather than a technique.
3dfx fully expects that software developers will take advantage of
this new technology to add effects beyond the ones it was
originally developed to accelerate. In fact, we have already
discovered another interesting effect that’s best described as
“double vision” and is readily accelerated by the T-Buffer
technology. With such flexibility of the T-Buffer technology,
combined with the creative minds of developers, imagery and
special effects never seen before in real-time on consumer PCs are
only a short time away.