1 KIPA Game Engine Seminars Jonathan Blow Seoul, Korea November 30, 2002 Day 5

2 Stencil Shadow Visibility Culling (clarification from yesterday…) There are two rendering passes, “shadow” and “normal”, and visibility detection is separate for each, but there are dependencies

3 GameCube rendering source code example (Wanted to show one yesterday but didn’t have it available)

4 Explanation of double-sort algorithm for networking Use a global sorted list for the server, along with one list for each client (on whiteboard: show how to use combined information to speed up object search) These lists only change incrementally, so keeping them sorted should be cheap

5 Further Work on Filtered Variance for Networking (quick summary of yesterday’s subjects) Presentation of solution that I said I hadn’t figured out yet, yesterday –Easy to rotate variance matrix into the frame of the viewpoint –No loner need approximation for z

6 Raycasting against The World First navigate a BSP tree or something. Then you need to collide against meshes in the world, probably. Mesh representation can make for some interesting issues…

7 Triangle Strip Meshes Want to iterate over the whole mesh, ignoring degenerate faces Since you want to do this frequently, it’s good to abstract that into an iterator class (looking at code)

8 PC vs. UMA Console On a PC, vertex buffers may be stored in video RAM (if application is high- performance) On a UMA console (e.g. Xbox), vertex buffers exist in common memory

9 PC Version is Less Efficient Maybe you have two copies of all vertex data –Expensive in terms of RAM Maybe you lock the vertex/index buffers to traverse them. –Expensive in terms of CPU

10 Cross-Platform Issues Looking at mesh geometry is very common –Raycasting, decals, dynamic bounding volumes… So it needs to be done from many places in the engine So we need to be able to inspect mesh geometry in a cross-platform way –Because we don’t want to write all that code several times… too much complication!

11 This makes me sad It means we need to introduce abstractions that are forced by the PC… All applications get access to vertex memory by calling a lock / unlock function. –But on consoles this usually is not necessary! –So console games are a little bit more complicated than they would be otherwise

12 Looking at the code… In the engine, the class “Fast_Rendering_Vertex_Data” provides this abstraction. Systems deep in the engine like skeletal animation, lock this buffer in order to save the deformed vertices On some platforms, the lock is a “no-op”

13 But it gets worse… Because locking the buffers may be expensive on a platform like the PC You want to try to optimize the core engine to minimize locks, even though locks are “no-ops” on some platforms –Example: Animating vertices, then updating vertex normals … sometimes we don’t need normals!

14 Difference in Philosophy: Xbox vs. GameCube Xbox wants you to lock vertex buffers –So that it can invalidate cache on the graphics processor –But software is more complicated due to lock/unlock calls GameCube lets you modify vertex and index buffers freely –But you must remember to invalidate the cache memory yourself Though it is easy to get this wrong as well I am not sure which is better

15 Visibility

16 Three main categories of visibility algorithms “Blind” –PVS View Space –Portals, occluders, horizon computation Screen Space –Span buffer, coverage mask, hierarchical z-buffer

17 Indoor vs Outdoor Rendering Different techniques applicable to each Indoor engine: Half-Life –Counter-Strike screenshot examples of “fake outdoors” Outdoor engine: Starsiege TRIBES –Screenshots

18 Indoor vs Outdoor Rendering But of course we want to make worlds that have both indoors and outdoors Some games have engines that switch between them (like Tribes) This is not a good thing; we want a unified rendering method.

19 High Average Frame Rate vs. Consistent Frame Rate Consistent frame rate is more important! If I am standing in some place and looking in some direction and I get 60fps, that is not very good if my frame rate drops to 15fps when I turn left –Example: a room in a first person shooter with a lot of objects in one corner (quick demo)

20 Some Outdoor Renderers Try to Use Visibility Culling Occluders Horizon detection-based culling (examples)

21 But, Occlusion Culling does not make much sense for outdoors Outdoor scenes are relatively uncontrolled; Definition of an outdoor engine is that you can go up on top of a big mountain and look around If frame rate must be fast in that situation, what is the point of trying to make it faster? Trying to make things faster in occluded cases will make the mountain case slower

22 But, Occlusion Culling is critical for indoors It’s the only way modern games like Doom 3 are fast What this means is…

23 Unification won’t really happen for a while. Until we come up with an occlusion culling method that is simple, and fast in the case where nothing is occluded; Or we come up with a way to render indoor areas without occlusion culling, and without suffering any speed losses.

24 Screenspace Method: Span Buffer Something we used a lot in software rendering Unreal used this (newer Unreal-engine games do not) –It did not scale well with hardware rendering –I think Unreal actually used span portal visibility (explained after we talk about portals)

25 Screenspace Method: Triage Coverage Masks Proposed by Ned Greene, paper: “Hierarchical Polygon Tiling with Coverage Masks”

26 Screenspace Method: Hierarchical Z-Buffer Also proposed by Ned Greene, paper: “Hierarchical Z-Buffer Visibility” Uses frame coherence for increased speed I do not like frame coherence

27 Why I Dislike Frame Coherence Games want a stable frame rate (not just a fast frame rate!) If the frame rate becomes slightly slower, the viewpoint moves further = less coherence Because there’s less coherence, the next frame will draw even slower This causes a feedback loop (the frame rate plummets)

28 Frame Coherence Frame Coherence is relied upon heavily by many of the algorithms I like to pick on –(like the ROAM terrain rendering algorithm)

29 Academic Research Frame coherence is thought of as a standard way to speed up algorithms in the academic world This is one of several ways in which the academic community is “out to lunch” Frame coherence is good for batch rendering; it is not suitable for most interactive rendering!

30 My Recommendations For runtime visibility in games, use an algorithm like PVS or portals. Don’t use a screenspace method. Screenspace methods are better for software rendering. These days we are making some tools that effectively do software rendering, so screenspace methods may apply –The normal map generation algorithm Actually doesn’t need occlusion culling, but it’s an example of software rendering “coming back” in preprocessing tools