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A Few Things about Graphics

This article provides an overview of graphics, including rendering equations, modeling scenes, rendering processes, lighting and texturing, and the challenges of distributed graphics systems.

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A Few Things about Graphics

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  1. A Few Things about Graphics Jian Huang Computer Science University of Tennessee

  2. 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

  3. 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

  4. 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

  5. What does it take to get that equation done? • First, one has to model the scene • 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 • 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

  6. Geometric Primitives • All geometric primitives are specified by vertices

  7. EYE OBJECT WORLD 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

  8. Affine Transformation in 3D • Translation • Rotate • Scale • Shear

  9. 3D Models - The Mesh Assembled scene

  10. A Virtual World • Objects • Shape: meshed geometry (and topology) • Appearance: lighting, shading and texture • Scene • Camera and viewing • Light position • Scene composition

  11. 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

  12. 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!

  13. Un-lit

  14. Smooth Shaded

  15. What is a Texture? • Color • Specular ‘color’ (environment map) • Normal vector perturbation (bump map) • Displacement mapping • Transparency • ...

  16. Rendering

  17. Textures Make A Difference • Good textures, when applied correctly, make a world out of nothing!

  18. Examples

  19. Examples (cont.)

  20. Examples (cont.)

  21. Before and After “Good” Texture

  22. Before and After “Good” Texture

  23. More

  24. Every Renderer is a State Machine • All rendering attributes are encapsulated in the as states • Transformations • Viewing • Rendering styles • Shading • Lighting • Texture mapping

  25. The Punch Line • Graphics models the world with geometry and appearance attributes • The rendering process is extremely computation-intensive. What processor won Processor-Of-The-Year award last year? • The graphics state is complicated to maintain. • The requirement is high interactivity

  26. Distributed Graphics • Systems for distributing the shared graphical state of multi-display, multi-person, distributed, interactive applications

  27. 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

  28. 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

  29. 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

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