Visibility in Games Harald Riegler Visibility in Games

Visibility in Games Harald Riegler

Visibility in Games n What do we need it for? u Increase of rendering speed by removing unseen scene data from the rendering pipeline as early as possible u Reduction of data transfers to the graphics hardware u Current games would not be possible without visibility calculations Harald Riegler 2

Visibility methods n 2 very different categories: u Visibility from a region (Portals, PVS) n (Quake, Unreal, Severance and co. ) u Visibility from a point (Z-Buffer, BFC, . . . ) n Racing games, outdoor scenes, sports games etc. n We will focus on Point-Visibility here Harald Riegler 3

Traditional methods n Traditionally used: u Back-Face culling u Z-Buffering u View frustum culling (Quad or Octtree) n New methods are slowly breaking into games Harald Riegler 4

New scientific methods n Image-space occlusion culling u Hierarchical Z-Buffering u Hierarchical Occlusion Maps n Object-space occlusion culling u Hierarchical View Frustum culling u Hierarchical Back-Face culling n More methods in the paper Harald Riegler 5

Hierarchical Z-Buffer n Z-Buffer is arranged in an image pyramid n Scene is partitioned in an oct-tree n Oct-tree nodes are tested against the Z- Pyramid where pixels have the same size n Visible nodes serve as input for the next frame n Relies on HW visibility query Harald Riegler 6

HZB/Hierarchical occlusion maps Harald Riegler 7

Hierarchical occlusion maps n Potential occluders are pre-selected n These occluders are rendered to the occlusion map. The hierarchy can be built with MIP-Mapping HW n Depth test after occlusion test n Seperate depth estimation buffer Harald Riegler 8

Hierarchical View Frustum Culling n Speeds up VFC by testing only 2 box corners of a bounding box first. n Plane coherency during frame advancing n Test against VF-octants. n BB-Child masking Harald Riegler 9

Hierarchical Back-Face Culling n Partitions each model into clusters n Primitives in one cluster are: u Facing into similar directions u Lie close to each other n If the cluster fails the visibility test, all primitives in this cluster are culled Harald Riegler 10

Hierarchical Back-Face Culling Harald Riegler 11

The Real World (1) n Scientific approaches often too complicated n Science often uses models with hundreds of thousands of vertices, games don’t. (LOD) n Game developers “pick” ideas from different algorithms n Research has impact on hardware design! Harald Riegler 12

The Real World (2) n Parts of the HZB are used sometimes n Runtime-LOD is used as input for a simple HZB n VFC is almost always used. n HOM introduce too much overhead for games, and the z-buffer is there anyway n A simplified HBFC can be used Harald Riegler 13

The Real World (3) n PSX-One doesn’t even have a z-buffer n ATI’s Radeon has parts of a HZB (Called Hyper-Z) n GForce 2 only has a z-buffer n GForce 3 similar to Radeon, but supports HZB visibility query n Dreamcasts Power-VR 2 works pretty different (Infinite planes) Harald Riegler 14

A PSX Example n Iron Soldier 3 on PSX: u VFC based on a quad-tree u BFC u Painters algorithm Harald Riegler 15

Conclusions (1) n Visibility algorithms are used in many different applications u Occlusion culling u Shadow calculations u Radiosity u Volumetric lights n All these fields benefit from advances in visibility techniques Harald Riegler 16

Conclusion (2) n Occlusion culling will gain importance n More OC will be implemented in hardware n Discrepancies between science and gaming industry will alway exist due to different goals Harald Riegler 17

Hope you enjoyed the ride!