KIPA Game Engine Seminars

1. KIPA Game Engine Seminars Day 4 Jonathan Blow Seoul, Korea November 29, 2002

2. High-Level Networking(continued) • Review of yesterday…

3. Deciding What To Transmit • Limited bandwidth to fit all entity updates into • Apportion this out into slices somehow? • Can we do this without adding latency? A hard problem! • Need to accept the fact that the client won’t be perfectly updated about everything, always a little bit wrong • Approach network communications as an error minimization problem

4. Deciding What To Transmit • Need a metric for the amount of error between object on client and server • Position, orientation, state variables • Probably attenuated by distance to deal with viewpoint issues! • What about a rocket with contrail attached? • Record the world state that you sent to the client, and diff it against current state • A lot of memory! • A lot of CPU!

5. There is a lot of coherence between clients’ error functions • We ought to be able to exploit that • Example of objects that move a lot • They will have high error in all world views • Similarly for objects that move slowly • How do we detect “a lot of motion”? • Should not use distance traveled per frame • example on whiteboard

6. Detecting “a lot of motion” • Idea: Neighborhood bounding box • Too quantized; how do we decide when to move the center? • Idea: Bounding sphere with moving center • How do we compute this without holding big arrays of data? • Also, too anisotropic • (we are picky because we pay for bandwidth!)

7. Why we want anisotropy • For distant objects, we care most about motion parallel to the view plane • Motion orthogonal to that plane only produces small changes in perspective size • (graph of 1/z on whiteboard)

8. Variance of a vector • Also: “Covariance” of the vector components • “Variance/Covariance Matrix” of components • (demo) • Can be filtered, like scalars, to approximate where something has been over different periods of time

9. Summary of Variance Methods • Characterized by ellipsoid • Find ellipsoid by eigenvalues/eigenvectors of outer product matrix • These variances can be treated intuitively like mass (tensor of inertia, in physics)

10. Derivation of Variance Recentering • Allows us to filter the variance of the vector, and transform that relative to a filtered position, in order to visualize • (derivation on whiteboard)

11. Code Inspection • Covariance2, openGL demo app • Covariance3 • discuss finding eigenvectors of 2x2 versus 3x3 matrix

12. Do we need the eigenvectorsfor networking? • Perhaps not! • First, discussion of how we would use the eigenvectors • But instead of back-projecting, can we forward-query? • A simple query is very cheap • (example on whiteboard)

13. For the global sort,it’s even easier • The product of the eigenvalues is the determinant of the matrix • Volume of ellipsoid! • The sum of eigenvalues is the trace of the matrix • Useful in approximating eccentricity of ellipsoid; ratio of volume to ideal volume of a sphere with radius of (1/3) tr M

14. Shaders

15. Shaders • Some people are confused by marketing hype to think shaders are new… • They have been around for a long time in software rendering • In hardware, the fixed-function pipeline provided shader functionality.

16. Interesting idea:Deferred Shading • Only write iterated vertex parameters into the frame buffer • Perform complicated shading operations in a post-pass • If multipass rendering, vertex shader will run less often • But technique is of limited use? http://www.delphi3d.net/articles/viewarticle.php?article=deferred.htm

17. Early Game Shader Language:Quake3 shaders.txt • Goal: to abstract away the number of texture stages in a graphics card’s pipeline • Earlier cards had 1, 2, or 3 stages • Also: enable level designers to create shaders by hand

18. Normalization Cube Map • Promoted by Mark Kilgard of Nvidia • An interesting idea for early shaders, but outdated now? • With more shader instructions we can actually run a fast normalization function • Does not require texture memory or a texture input slot! • Cube maps are still useful for parameterizing arbitrary functions over the sphere

19. Xbox / Old PC’s texture pipeline

20. GameCube’s texture pipeline

21. Early DirectX vertex / pixel shaders • (Version 1.0, 1.1) • Did not do much you couldn’t already do in fixed function pipeline • But, an important step toward paradigm of programmability

22. OpenGL vs. DirectX:Extensions vs. Control • OpenGL provides extensions • DirectX is about Microsoft creating a “standard”