INTRODUCTION
TO
COMPUTER
GRAPHIC S
Realism in Computer Graphics
•
John Hughes, and
Andy van Dam
•
Significantly updated
in 2001 and 2002 by
John Alex (former
123 TA and Pixarian,
who got his Ph.D. at
MIT)
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Realism in Computer Graphics
Roadmap
•
We tend to mean physical realism
•
How much can you deliver?
•
•
–
what medium? (still images, movie/video special effects,
etc.)
–
what resources are you willing to spend? (time, money,
processing power)
How much do you want or need? Depends on:
–
content (movies, scientific visualization, etc)
–
users (experts vs. novice)
The many categories of realism:
–
geometry and modeling
–
rendering
–
behavior
–
interaction
•
Many techniques for achieving varying amounts
of realism within each category
•
Achieving realism usually requires trade-offs
–
realistic in some categories, not in others
–
concentrate on the aspects most useful to your
application
INTRODUCTION
TO
COMPUTER
Realism and Media
GRAPHIC S
(1/2)
What is “realism”?
– King Kong (1933) vs. King Kong (2005)
•
In the early days of computer graphics, focus
was primarily directed towards producing still
images
– “realism” typically meant approaching
“photorealism.” Goal: to accurately reconstruct a
scene at a particular slice of time
– Emphasis placed on accurately modeling
geometry and light reflection properties of
surfaces
•
With the increasing production of animated
graphics (commercials, movies, special effects,
cartoons) new standard of “realism” important:
•
Behavior over time:
– character animation
– natural phenomena: cloth, fur, hair, skin, smoke,
water, clouds, wind
– Newtonian physics: things that bump, collide,
fall, scatter, bend, shatter, etc.
• Some of which is now calculated on a dedicated
physics card! (eg. AGEIA PhysX)
INTRODUCTION
TO
COMPUTER
Realism and Media
GRAPHIC S
(2/2)
Real-time vs. Non-real-time
•
“Realistic” static images and animations are
rendered in batch and viewed later. They often
take hours per frame to produce. Time is a
relatively unlimited resource
•
In contrast, other apps emphasize real-time output:
– graphics workstations: data visualization, 3D
design
– video games
– virtual reality
•
Any media that involves user interaction (e.g., all
of the above) also requires real-time interaction
handling
Rendered image
Real-time interaction
INTRODUCTION
TO
COMPUTER
Trade-offs
GRAPHIC S
(1/5)
Cost vs. Quality
•
Many computer graphics media (e.g., film
vs. video vs. CRT)
•
Many categories of realism to consider (far
from exhaustive):
– geometry
– behavior
– rendering
– interaction
•
Worst-case scenario: must attend to all of
these categories within an extremely limited
time-budget
•
Optimal balance of techniques for achieving
“realism” highly depends on context of use:
– medium
– user
– content
– resources (especially hardware)
•
We will elaborate on these four points
next…
INTRODUCTION
TO
COMPUTER
Trade-offs
•
GRAPHIC S
(2/5)
Medium
– Different media --> different needs
– Consider: doctor examining x-rays:
• if examining static transparencies, resolution
and accuracy matter most
• if doctor is interactively browsing a 3D
dataset of the patient’s body online, may
want to sacrifice resolution or accuracy for
faster navigation and ability to zoom in at
higher resolution on regions of interest
•
User
– Expert vs. novice users
– Data visualization:
• novice may see a clip of data visualization on
the news, doesn’t care about fine detail (e.g.,
weather maps)
• expert at workstation will examine details and
stumble over artifacts and small errors—
“expertise” involves acute sensitivity to small
fluctuations in data, anomalies, patterns,
features
– in general, “what does the user care (most)
about?”
INTRODUCTION
TO
COMPUTER
Trade-offs
GRAPHIC S
(3/5)
Content
– movie special-effects pack as much astonishment as
possible into budget: every trick in book
– conversely, CAD model rendering typically elides
detail for clarity, and fancy effects interfere with
communication
– Scientific visualizations show artifacts and holes in
data, don’t smooth them out.
INTRODUCTION
TO
COMPUTER
Trade-offs
GRAPHIC S
(4/5)
Resources
– Intel 286 (1989)
• wireframe bounding boxes
– Microsoft Xbox360 (2006)
• Tri-core (3*3.2Ghz) system with onboard ATI
graphics hardware – capable of 1080p HDTV
output – complete with controllers for $350
– nVidia GeForce 8800GTX (2006)
• texture-mapped, environment-mapped, bumpmapped, shadow-mapped, 11 billion
vertices/second, subsurface scattering, stencilshadowed goodness for $600 fully loaded
– AGEIA PhysX (2005)
• Explosions, dust, cloth, smoke, fog, lifelike
character animation for an extra $150
INTRODUCTION
TO
COMPUTER
Trade-offs
GRAPHIC S
(5/5)
Computing to a time budget (“time-critical” algos)
•
A vast array of techniques have been developed
for generating “realistic” geometry, behavior,
rendering…
•
The “best” can often be traded for the “good” at
a much lower computational price
•
We call bargain-basement deals “hacks”
•
Some techniques use progressive refinement (or
its inverse, graceful degradation): the more time
we spend, the better output we get.
– Excellent for situations when we want the best
quality output we can get for a fixed period of time,
but we can’t overshoot our time limit (e.g., IVR
surgery!). Maintaining constant update rates is a
form of guaranteed “Quality of Service” (a
networking term).
– web image downloads
– progressive refinement for extremely large meshes
• see also slide 11 and 14…
http://www.equinox3d.com/renderer.html
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Digression - Definitions
•
Texture-Maps: map an image onto
surface geometry to create appearance
of fine surface detail. A high level of
realism may require many layers of
textures.
•
Environment-Maps: multiple images
(textures) which record global reflection
and lighting on object. These images are
resampled during rendering to extract
view- specific information which is then
applied as texture to object.
•
Bump-Maps: fake surface normals by
applying height field (intensities in the
map indicate height above surface).
From height field calculate gradient
across surface and use this to perturb
the surface normal.
•
Normal-Maps: similar to bump-maps.
Instead of using grayscale image to
calculate the normals, pre-generate
normals from high-resolution model and
store result in the low-resolution
polygonal model.
•
Shadow-Maps: generate shadow
texture by capturing silhouettes of
objects as seen from the light source.
Project texture onto scene. Note: must
recalculate for moving lights.
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Geometry
(1/4)
The Hacked
•
– Texture mapping: excellent way to fake fine
surface detail—more often used to fake geometry
than to add pretty colors
– more complicated texture mapping strategies such
as polynomial texture maps use image-based
techniques for added realism
•
The Good
– Polygonization: very finely
tessellated meshings of
curved surfaces
– Polys easily converted to
subdivisional surfaces
(right). More on this later.
– linear approximation
– massively hardwareaccelerated!
Mesh decimation:
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Geometry
(2/4)
The Best
• Splines
– no polygons at all!
Continuous mathematical
surface representations
(polynomials)
– 2D and 3D curved surfaces:
Non-Uniform Rational BSplines (NURBS)
– control points
– high order polynomials are
hard to work with
– used a lot in computer-aided
designs, engineering
• Implicit Surfaces
(blobbies)
– F(x,y,z) = 0
– add, subtract, blend
– relatively hard to render
(need to raytrace or convert
to polygon, both slow)
F(x,y,z) = ((x^2*(1-x^2)-y^2)^2+0.5*z^2-f*(1+b*(x^2+y^2+z^2)) = 0
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Geometry
The Best
(3/4)
• Subdivision
Surfaces
– subdivide triangles
into more triangles,
moving to a
continuous limit
surface
– elegantly avoid
gapping and tearing
between features
– support creases
– allow multi-resolution
deformations (editing
of lower resolution
representation of
surface)
From Pixar’s “Geri’s Game”
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Geometry
•
(4/4)
The Gracefully Degraded
– Level-of-Detail(LOD): as object gets farther
away from viewer, replace it with a lowerpolygon version or lower quality texture map.
Discontinuous jumps in model detail
– Mesh decimation: save polygons
Left: 30,392 triangles
Right: 3,774 triangles
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(1/9)
Good Hacks
•
Easily implemented in hardware: fast!
•
Use polygons
•
Only calculate lighting at polygon vertices, from
point lights
•
For non-specular (i.e., not perfectly reflective),
opaque objects, most light comes directly from the
lights (“locally”), and not “globally” from other
surfaces in the scene
•
Local lighting approximations
•
•
–
diffuse Lambertian reflection: only accounts for angle
between surface normal and vectors to the light source.
–
fake specular spots on shiny surfaces: Phong lighting
Global lighting approximations
–
introduce a constant “ambient” lighting term to fake an
overall global contribution
–
reflection: environment mapping
–
shadows: shadow mapping
Polygon interior pixels shaded by simple color
interpolation: Gouraud shading
–
Phong shading: evaluate some lighting functions on a perpixel basis, using interpolated surface normal.
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(2/9)
Example: Doom 3
•
Few polygons (i.e., low geometric complexity
(a.k.a. scene complexity))
•
Purely local lighting calculations
•
Details created by texturing everything with
precomputed texture maps
•
–
surface detail
–
smoke, contrails, damage and debris
–
even the lighting and shadows are done with textures
–
“sprites” used for flashes and explosions
Bump mapping in hardware
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(3/9)
The Best
•
Global illumination: find out where all the light
entering a scene comes from, where and how much it
is absorbed, reflected or refracted, and all the places
it eventually winds up
•
We cover Ray-tracing (specular) and Radiosity
(diffuse), neither is physically accurate.
•
IBR. Not geometry-based, explained later under
Image-Based Rendering.
•
Early method: Ray-tracing. Avoid forward tracing
infinitely many light rays from light sources to eye.
Work backwards to do viewer/pixel-centric rendering:
shoot viewing rays from viewer’s eyepoint through
each pixel into scene, and see what objects they hit.
Return color of object struck first. If object is
transparent or reflective, recursively cast ray back
into scene and add in reflected/refracted color Nong Li, 2006
–
Turner Whitted, 1980
–
moderately expensive to solve
–
“embarrassingly parallel”—can
use parallel computer or
networked workstations
–
models simple lighting equation (e.g., ambient, diffuse
and specular) for direct illumination but only perfectly
specular reflection for indirect (global) illumination
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(4/9)
The Best: Ray Tracing (cont.)
•
Ray-tracing good for: shiny, reflective,
transparent surfaces such as metal, glass,
linoleum. Can produce:
–
sharp shadows
–
a caustic: “the envelope of light rays reflected or
refracted by a curved surface or object, or the
projection of that envelope of rays on another
surface, e.g., the patches of bright light overlaying
the shadow of the glass” (Wikipedia)
•
Can do volumetric effects, caustics with
extensions such as “photon maps”
•
Can look “computerish” if too many such effects
are in one scene (relatively rare in daily life)
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(5/9)
The Best: Radiosity (Energy Transport) - Diffuse
•
Scene-centric rendering. Break scene up into
small surface patches and calculate how much light
from each patch contributes to every other patch.
Circular problem: some of patch A contributes to
patch B, which contributes some back to A, which
contributes back to B, etc. Very expensive to
solve—iteratively solve system of simultaneous
equations
– viewer-independent—batch preprocessing step
followed by real-time, view-dependent display
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(6/9)
The Best: Radiosity (cont.)
•
Good for: indirect (soft) lighting, color bleeding,
soft shadows, indoor scenes with matte surfaces.
As we live most of our lives inside buildings with
indirect lighting and matte surfaces, this
technique looks remarkably convincing
•
Even better results can be obtained by combining
radiosity with ray-tracing
– Various methods for doing this. Looks great!
Really expensive!
www.povray.com
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(7/9)
The Gracefully Degraded Best
•
Selectively ray-trace. Usually only a few
shiny/transparent objects in a given ray-traced
scene. Can perform local lighting equations on
matte objects, and only ray-trace the pixels that
fall precisely upon the shiny/transparent objects
•
Calculate radiosity at vertices of the scene once,
and then use this data as the vertex colors for
Gouraud shading (only works for diffuse colors in
static scenes)
raytrace
http://www.okino.com/conv/imp_jt.htm
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(8/9)
The Real Best: Sampling Realistically
•
The Kajiya rendering equation (covered in CS224 by
Spike) describes this in exacting detail
– very expensive to compute!
•
Previous techniques were different approximations to
the full rendering equation
•
Photon mapping provides pretty good approximation to
the equation.
•
Led to the development of path-tracing: point sampling
the full rendering equation
•
Eric Veach’s Metropolis Light Transport is a faster way
of sampling the full rendering equation (converge to
accurate result of the rendering equation)
•
New research in combining MLT and photon mapping
Rendered using MLT, all light comes from the other room
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Techniques—Rendering
(9/9)
Side Note—Procedural Shading
•
Complicated lighting effects can be obtained
through use of procedural shading languages
– provides nearly infinite lighting possibilities
– global illumination can be faked with low
computational overhead
– usually requires skilled artist to achieve decent
images
•
Pixar’s Renderman
•
Procedural shading is now in hardware
– Any card you can buy today has programmable
vertex and pixel shaders
• Cg (nVidia)
• GLSL (OpenGL)
• HLSL (Microsoft)
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Procedural Shading
• A number of advanced
rendering techniques are
shaders implemented on the
GPU in real-time
– High Dynamic Range
Rendering
– Subsurface Scattering
– Volumetric Light Shafts
– Volumetric Soft Shadows
– Parallax Occlusion Mapping
– and many more!
• You will implement some
simple shaders later in the
semester
INTRODUCTION
TO
COMPUTER
GRAPHIC S
High Dynamic Range Rendering
•
Lighting calculations can exceed 0.0 to 1.0 limit
– allows more accurate reflection and refraction
calculations
• sun might have a value of 6,000 instead of 1.0
– clamped to 1.0 at render time
– unless using HDR monitor: BrightSide
– requires more resources:
• 16 or 32 bit values instead of 8-bit for RGB
With HDRR
Without HDRR
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Subsurface Scattering
• Advanced technique for rendering
translucent materials (skin, wax, milk, etc)
– light enters material, bounces around,
some exits with new properties
– hold your hand up to a light, you’ll
notice a “reddish” glow
http://graphics.ucsd.edu/~henrik/
Real-time
versions!
No SSS
Whole
Skim
www.nvidia.com
http://graphics.ucsd.edu/~henrik/
http://www.crytek.com/
INTRODUCTION
TO
COMPUTER
GRAPHIC S
“Godrays”
• Volumetric light shafts are
produced by interactions
between sunlight and the
atmosphere
– Can be faked on GPU as
translucent volumes
Cry-tek Game Engine: http://www.crytek.com/technology.html
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Volumetric Soft Shadows
• Volume is computed from
perspective of light source
– objects that fall within volume
are occluded
Cry-tek Game Engine: http://www.crytek.com/technology.html
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Parallax Occlusion Mapping
• Provides sense of depth on
surfaces with relief
– brick wall, stone walkway
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Image-Based Rendering
(1/2)
A Different Approach:
•
Image-based rendering (IBR) is only a few years
old. Instead of spending time and money modeling
every object in a complex scene, take photos of it.
You’ll capture both perfectly accurate geometry and
lighting with very little overhead
•
Analogous to image compositing in 3-D
•
Dilemma: how to generate views other than the one
photo you took. Various answers.
•
Part of new area of Computational Photography
The Hacked
•
QuickTimeVR
– Stitch together multiple photos
taken from the same location at
different orientations. Produces
cylindrical or spherical map which
allows generation of arbitrarily
oriented views from that one
position.
– generating multiple views:
discontinuously jump from one
precomputed viewpoint to the
next. In other words, can’t
reconstruct missing (obscured)
information
Brown
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Image-Based Rendering
(2/2)
The Best
•
Plenoptic modeling: using multiple
overlapping photos, calculate depth
information from image disparities.
Combination of depth info and surface color
allows on-the-fly reconstruction of “best guess”
intermediary views between the original
photo-positions
http://www.cs.unc.edu/~ibr/pubs/mcmillanplenoptic/plenoptic-abs_s95vid.mpg
•
Lightfield rendering: sample the path and
color of many light rays within a volume
(extremely time-consuming pre-processing
step!). Then interpolate these sampled rays to
place the camera plane anywhere within the
volume and quickly generate a view.
– Treats images as 2d slices of a 5d function –
position (xyz) and direction (theta, phi on sphere)
– Drawbacks: have to resample for any new
geometry.
INTRODUCTION
TO
COMPUTER
Temporal Aliasing
GRAPHIC S
(1/3)
Stills vs. Animation
•
At first, computer graphics researchers
thought, “If we know how to make one still
frame, then we can make an animation by
stringing together a sequence of stills”
•
They were wrong. Long, slow process to learn
what makes animations look acceptable
•
Problem: reappearance of spatial aliasing
•
Individual stills may contain aliasing artifacts
that aren’t immediately apparent or irritating
–
impulse may be to ignore them
•
Sequential stills may differ only slightly in
camera or object position. However, these
slight changes are often enough to displace
aliasing artifacts by a distance of a pixel or
two between frames
•
Moving or flashing pixel artifacts are
alarmingly noticeable in animations. Called
the “crawlies”. Edges and lines may ripple,
but texture-mapped regions will scintillate like
a tin-foil blizzard
•
How to fix crawlies: use traditional filtering to
get rid of spatial artifacts in individual stills
INTRODUCTION
TO
COMPUTER
Temporal Aliasing
GRAPHIC S
(2/3)
• Moiré patterns change at
different viewpoints
– in animation, produces a
completely new artifact as a
result of aliases
Moiré
pattern
aliased
antialiased
• Can we anti-alias across
frames?
INTRODUCTION
TO
COMPUTER
Temporal Aliasing
GRAPHIC S
(3/3)
Motion Blur
•
Another unforeseen problem in animation:
temporal aliasing
•
Much like spatial aliasing problem, only over time
– if we sample a continuous function (in this case,
motion) in too few steps, we lose continuity of
signal
•
Quickly moving objects seem to “jump” around if
sampled too infrequently
•
Solution: motion blur. Turns out cameras
capture images over a relatively short interval of
time (function of shutter speed). For slow
moving objects, the shutter interval is sufficiently
fast to “freeze” the motion, but for quickly
moving objects, the interval is long enough to
“smear” object across film. This is, in effect,
filtering the image over time instead of space
•
Motion blur a very important
cue to the eye for maintaining
illusion of continuous motion
•
We can simulate motion blur
in rendering by taking
weighted average of series of
samples over small time
increments
INTRODUCTION
TO
COMPUTER
Techniques—Behavior
GRAPHIC S
(1/4)
Modeling the way the world moves
•
Cannot underestimate the importance of
behavioral realism
– we are very distracted by unrealistic behavior
even if the rendering is realistic
– good behavior is very convincing even when the
rendering is unrealistic (e.g., motion capture
data animating a stick figure still looks very
“real”)
– most sensitive to human behavior – easier to get
away with faking ants, toys, monsters, fish etc.
•
Hand-made keyframe animations
– professional animators often develop an intuition
for the behavior of physical forces that
computers spend hours calculating
– “cartoon physics” sometimes more convincing or
more appealing than exact, physically-based,
computer calculated renderings
– vocabulary of cartoon effects:
anticipation, squash,
stretch, follow-through, etc.
INTRODUCTION
TO
COMPUTER
Techniques—Behavior
GRAPHIC S
(2/4)
The Best
•
Motion-capture
– sample positions and orientations of motiontrackers over time.
• Trackers usually attached to joints of human
beings performing complex actions.
• Once captured, motion extremely cheap to play
back: no more storage required than a keyframe
animation.
• Irony: one of cheapest methods, but provides
excellent results
– usually better than keyframe animations and
useful for a variety of characters with similar
joint structure (e.g., Brown → Chad Jenkins,
Michael Black)
– “motion synthesis”: a recent hot topic – how to
make new animations out of the motion capture
data that you have.
INTRODUCTION
TO
COMPUTER
Techniques—Behavior
GRAPHIC S
(3/4)
The Best (cont.)
•
Physics simulations (kinematics for rigidbody motion, dynamics for F = ma)
– Hugely complex modeling problem
– expensive, using space-time constraints,
inverse kinematics, Euler and Runge-Kutta
integration of forces, N2-body problems.
These can take a long time to solve
– looks fairly convincing…but not quite real (yet)
INTRODUCTION
TO
COMPUTER
Techniques—Behavior
GRAPHIC S
(4/4)
The Gracefully Degraded
•
Break laws of physics (hopefully
imperceptibly)
– Simplify numerical simulation: consider fewer
forces, use bounding boxes instead of precise
collision detection, etc.
– Decrease number of time steps used for Euler
integration
Bounding Box
INTRODUCTION
TO
COMPUTER
Real-time Interaction
GRAPHIC S
(1/6)
Frame Rate (CRT)
•
•
•
•
Video refresh rate is independent of scene update
rate (frame rate), should be >=60Hz to avoid
flicker
–
refresh rate is the number of times per second that
a CRT scans across the entire display surface
–
includes the vertical retrace time, during which the
beam is on its way back up (and is off)
–
must swap buffers while gun is on its way back up.
Otherwise, get “tearing” when parts of two different
frames show on the screen at the same time
–
to be constant, frame rate must then be the output
refresh rate divided by some integer (at 60Hz
output refresh rate, can only maintain 60, 30, 20,
15, etc. frames per second constantly)
–
refresh rate not an issue with LCD screens:
continuous light stream, no refresh occurs
Frame rate equals number of distinct images
(frames) per second
Good: frame rate is as close to refresh rate as
possible
Best: frame rate is close to constant
–
–
humans perceive changes in frame rate (jerkiness)
fundamental precept of “real-time:” guarantee
exactly how long each frame will take
–
polygonal scan conversion: close to constant, but
not boundable, time
–
raytracing: boundable time, but image quality varies
wildly
INTRODUCTION
TO
COMPUTER
Real-time Interaction
GRAPHIC S
(2/6)
Frame Rate (cont.)
•
Insufficient update rates can cause temporal
aliasing—the breakup of continuity over time –
jerky motion
•
Temporal aliasing not only ruins behavioral
realism but destroys the illusion of immersion in
IVR.
•
How much temporal aliasing is ‘bad’?
–
in a CAD-CAM program, a 10 frame-per-second
update rate may be acceptable because the scene is
relatively static, usually only the camera is moving
–
in video games and simulations involving many
quickly moving bodies, a higher update rate is
imperative: most games aim for 60 fps but 30 is
often acceptable.
–
motion blur is expensive in real-time graphics
because it requires calculation of state and complete
update at many points in time
Without motion blur
With motion blur
INTRODUCTION
TO
COMPUTER
Real-time Interaction
GRAPHIC S
(3/6)
Frame Rate and latency
•
Frame time is the period over which a frame
is displayed (reciprocal of frame rate)
•
Problem with low frame rates is usually
“latency,” not smoothness
•
Latency (also known as “lag”) in a real-time
simulation is the time between an input
(provided by the user) and its result
– best: latency should be kept below 10ms or
there is noticeable lag between input and
result
– noticeable lag affects interaction and task
performance, especially for an interactive
“loop”
– large lag causes potentially disastrous results;
a particularly nasty instance is IVR-induced
“cyber sickness” which causes fatigue,
headaches and even nausea
– lag for proper task performance on non-IVR
systems should be less than 100ms
INTRODUCTION
TO
COMPUTER
Real-time Interaction
GRAPHIC S
(4/6)
Frame Rate and Latency (cont.)
•
Imagine a user that is constantly feeding inputs
to the computer
•
Constant inputs are distributed uniformly
throughout the frame time, collect and process
one (aggregate) input per frame
•
Average time between input and next frame is ½
of frame time
•
Average latency = ½ frame time
•
•
–
at 30Hz, average latency is 17ms>10ms
–
at 60Hz, average latency is 8.3ms<10ms
–
therefore, frame rate should be at least 60Hz
Must sample from input peripherals at a
reasonable rate as well
–
often 10-20 Hz suffices, as the user’s motion takes
time to execute
–
high-precision and high-risk tasks will of course
require more (Phantom (haptic) does 1000 Hz!)
–
in Cave many users prefer 1–2Hz (especially if it has
geometrical accuracy) to 5–10Hz; somehow it is less
disconcerting
Separate issue: flat-panel display hardware
–
During fast-paced gaming, LCD must maintain a
response time < 10ms to avoid “ghosting”
•
Latest monitors provide as little as 4ms!
INTRODUCTION
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COMPUTER
Real-time Interaction
GRAPHIC S
(5/6)
Rendering trade-offs
•
Frame rate should be at least 60Hz. 30
hurts for very interactive applications (e.g.,
video games)
– only have 16.7ms (frame time) to render
frame, must make tradeoffs
– IVR often falls short of this ideal
•
What can you get done in 16.7ms?
•
Do some work on host (pre-drawing)
•
Best: multiprocessor host and graphics
cards
– accept and integrate inputs throughout frame
(1 CPU)
– update database (1+CPUs)
• swap in upcoming geometry and texture
• respond to last rendering time (adjust level of
detail)
• test for intersections and respond when they
occur
• update view parameters and viewing transform
– do coarse view culling, scenegraph optimizing
(1 CPU per view/pipe)
INTRODUCTION
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COMPUTER
Real-time Interaction
GRAPHIC S
(6/6)
Rendering trade-offs (cont.)
•
•
Do rest of work (as much as possible!) on specialized
graphics hardware
Best (and hacked): multipass
– combine multiple fast, cheap renders into one goodlooking one
– full-screen anti-aliasing (multi-sampling and T-buffer,
which blends multiple rendered frames)
– Quake III uses 10 passes to hack “realistic rendering”
• 1-4 bump mapping
• 5-6 diffuse lighting, base texture
• 7 specular lighting
• 8-9 emissive lighting, volumetric effects
• 10 screen flashes (explosions)
– Doom 3 uses shaders to avoid most of these passes
RGB mimap representation
(Same image, pre-filtered at
multiple levels)
•
Good (enough) lighting and shading
– Phong/Blinn lighting and Phong
shading models
– tons of texturing, must filter
quickly and anisotropically by using mipmaps.
Will learn more about textures in a future lecture
INTRODUCTION
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COMPUTER
GRAPHIC S
Raising the Bar
Improving standards over time
•
Bigger view, multiple views
– engage peripheral vision
– multiple projectors
• caves, spherical, cylindrical and dome screens
• render simultaneously (1+CPUs and graphics
pipelines per screen)
– Brown’s Cave uses a 48-node linux cluster
– requires distortion correction and edge blending
– stereo rendering (double frame rate)
•
•
•
•
•
We rarely have the patience for last year’s special
effects, much less the last decade’s
The quality of “realism” increases with every new
technique invented
“Tron” - a convincing virtual reality?
“Lord of the Rings” looks realistic now, but for how long?
What will realism look like next year?
INTRODUCTION
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COMPUTER
GRAPHIC S
Non-Photorealistic Rendering
One last digression
•
Artistic rendering—trying to evoke handdrawn or hand-painted styles, such as
charcoal sketching, pen and ink illustration,
or oil painting
•
For certain applications, elision of some
details and exaggeration of others can be
helpful (mechanical illustration, scientific
visualization, etc.)
•
Non-realism is also used in behavior
(cartoon physics), interaction (paintbrushes,
virtual tricorder and other virtual widgets in
IVR), geometry (Monsters, Inc.)
•
Examples of non-realism:
– Finding Nemo
– some research has investigated “cartoon
physics” and other kinds of exaggerated
motion and behavior
•
Strategic use of non-realism is a new field
with many opportunities
INTRODUCTION
TO
COMPUTER
GRAPHIC S
Non-Photorealistic rendering
Examples
• Cartoon Shading –
you’ll be doing this
in modeler!
• WYSIWYG NPR –
Draw strokes right
onto 3d models,
paper by Spike and
other Brownies!
• NPR allows for more
expressive
rendering than
traditional lighting
INTRODUCTION
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COMPUTER
GRAPHIC S
Brownies and NPR
Important SIGGRAPH papers!
•
Brown and it graduates have become identified
with some of the hottest research in non-realistic
rendering: David Salesin, Cassidy Curtis, Barbara
Meier, Spike, David Laidlaw, Lee Markosian,
Aaron Hertzmann, and Adam Finkelstein are all
pioneers in the field.
INTRODUCTION
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COMPUTER
GRAPHIC S
Brown’s Expertise in Realism
•
Geometric modeling (Spike et al.)
•
Animation (Barb Meier)
•
Rendering for IVR (David et al.)
•
Interaction for IVR (Andy et al.)
•
NPR (Spike et al.)