A Few Things about Graphics
Jian Huang
Computer Science
University of Tennessee
Graphics
• Graphics studies light transport in 3D scene,
sometimes over time as well.
• Example applications abound nowadays in
movies, MTVs, …
• In a nutshell, it’s some math and methods to
compute such math efficiently on processors,
memory banks and buses (lots of parallelism)
• Well, everything should start with that
Rendering Equation
The Rendering Equation
• I(x,x’) = intensity passing from x’ to x
• g(x,x’) = geometry term (1, or 1/r2, if x visible
from x’, 0 otherwise)
• e(x,x’) = intensity emitted from x’ in the direction
of x
• r(x,x’,x’’) = scattering term for x’ (fraction of
intensity arriving at x’ from the direction of x’’
scattered in the direction of x)
• S = union of all surfaces
It’s not analytically solvable
• So, instead of simply doing the math, can we use
computer to compute this equation in some
simplified version (oftentimes, immensely)?
• When approximations are made, priorities are set
accordingly to what is the most appealing to viewer
given the current hardware capability
• The consumer of the result is human being, so the
measurement of good and bad is not as
conventional as other computer science disciplines
What does it take to get that
equation done?
• First, one has to model the scene
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What color model is used (RGB, HSV)
Surfaces as geometrical meshes (volumes …)
Materials as lighting and texturing parameters
Light sources
• Second, compute the result for a screen device as it would
appear on a retina – the rendering process
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The graphics pipeline
Speed and high quality (30 fps, 60 fps, 120 fps)
Framebuffer: 320x240, 640x480, …, 1024x768, … 6000x6000.
Popular engines use OpenGL or DX, D3D libraries
Geometric Primitives
• All geometric primitives are specified by
vertices
Transformations
• Modeling transformations
• build complex models by
positioning simple components
• Viewing transformations
• place virtual camera in the world
• transform from world coordinates to
eye coordinates
• Vary transformations over time creates
motion - Animation
EYE
OBJECT
WORLD
Affine Transformation in 3D
• Translation
• Rotate
• Scale
• Shear
1.
2.
3.
4.
5.
6.
3D Models - The Mesh
Assembled
scene
A Virtual World
• Objects
– Shape: meshed geometry (and topology)
– Appearance: lighting, shading and texture
• Scene
– Camera and viewing
– Light position
– Scene composition
Appearance
• Having just the geometry, objects don’t look
right yet.
• The first step is to use light sources
• The second step is to put textures on
Multiple Light Source
• IL: light intensity
• For multiple light sources
– Repeat the diffuse and specular calculations for each light source
– Add the components from all light sources
– The ambient term contributes only once
• The different reflectance coefficients can differ.
– Simple “metal”: ks and kd share material color,
– Simple plastic: ks is white
• Remember, when cosine is negative lighting term is zero!
Un-lit
Smooth Shaded
What is a Texture?
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Color
Specular ‘color’ (environment map)
Normal vector perturbation (bump map)
Displacement mapping
Transparency
...
Rendering
Textures Make A Difference
• Good textures, when applied correctly,
make a world out of nothing!
Examples
Examples (cont.)
Examples (cont.)
Before and After “Good” Texture
Before and After “Good” Texture
More
Every Renderer is a State
Machine
• All rendering attributes are encapsulated in
the as states
• Transformations
• Viewing
• Rendering styles
• Shading
• Lighting
• Texture mapping
The Punch Line
• Graphics models the world with geometry and
appearance attributes
• The rendering process is extremely computationintensive. What processor won Processor-Of-TheYear award last year?
• The graphics state is complicated to maintain.
• The requirement is high interactivity
Distributed Graphics
• Systems for distributing the shared graphical state
of multi-display, multi-person, distributed,
interactive applications
Distributed Graphics
• Two key problems:
– The graphics database needs to be distributed
• Replication is a standard go-around
– The graphics states are distributed
• This is the tricky part
Distributed Graphics
• Two major categories:
– Data can be replicated (let’s do this!)
– Data can not be replicated
• Dependent on the choice
– Drastically different approach
– Specific pros and cons
• Let’s make sure graphics states are shared
properly
Distributed vs. Monolithic Apps
• Distributed control
– In a non-trivial distributed application, different components need
to be notified of changes to the distributed state
• Interactivity
– Network latency bandwidth limitations make updates to distributed
state much slower. For performance, need to perform some
operations locally
• Local variations
– There are times when a shared graphical scene may need to be
modified locally