tomas mőller © 2000 speed-up techniques for real-time rendering or “what makes your real-time...

Tomas Mőller © 2000

Speed-up Techniques forSpeed-up Techniques forReal-Time RenderingReal-Time Rendering

or “What makes your real-time video game fly”or “What makes your real-time video game fly”

Tomas MöllerTomas MöllerChalmers University of Technology, SwedenChalmers University of Technology, Sweden

Speed-up Techniques forSpeed-up Techniques forReal-Time RenderingReal-Time Rendering

or “What makes your real-time video game fly”or “What makes your real-time video game fly”

Tomas MöllerTomas MöllerChalmers University of Technology, SwedenChalmers University of Technology, Sweden


MotivationMotivation

Graphics hardware 2x faster in 6 months!Wait… then it will be fast enough!NOT!We will never be satisfied

– Screen resolution: 2000x1000– Realism: global illumination– Geometrical complexity: no upper limit!


Keep this in mind during lecture…Keep this in mind during lecture…

Rendering pipeline: 3 stages– Application (CPU but also bandwidth from CPU

to graphics hardware)– Geometry (transform, lighting)– Rasterizer (fill, texture, per pixel ops)

To get faster overall performance:– Reduce time spent in bottleneck stage!


Overview of lectureOverview of lecture

The scene graphCulling techniquesLevel-of-detail rendering (LODs)Combination of techniquesCollision detectionResources and pointers


The Scene GraphThe Scene Graph DAG – directed acyclic graph

– Simply an n-ary tree without loops Uses: collision detection and faster rendering

internal node=


Contents of Scene GraphContents of Scene Graph

Leaves contains geometryEach node holds a

– Bounding Volume (BV) – pointers to children– possibly a transform

Examples of BVs: spheres, boxes The BV in a node contains all geometry

in its subtree


Scene graph exampleScene graph example

circles=BVsscene graph

root


Transforms in the Scene GraphTransforms in the Scene Graph Put a transform in each internal node Gives instancing, and hierarchical animation

Move right,Rotate 45°

Move up


Example: a running legExample: a running legA B CNo hierarchy:

one transform

21 3Hierarchy:3 transforms

Leg

Xform

Xform

Xform

Knee

Foot

Hip

Xform


Culling TechniquesCulling Techniques

“To cull” means “to select from group” In graphics context: do not process data

that will not contribute to the final imageThe “group” is the entire scene, and the

selection is a subset of the scene that we do not consider to contribute


Culling: OverviewCulling: Overview

Backface cullingHierarchical view-frustum cullingPortal cullingDetail cullingOcclusion culling


Culling examplesCulling examples

view frustum detail

backface

portal occlusion


Backface CullingBackface Culling

Simple technique to discard polygons that faces away from the viewer

Can be used for: – closed surface (example: sphere)– or whenever we know that the backfaces never

should be seen (example: walls in a room)

Two methods (screen space, eye space)Which stages benefits? Rasterizer, but

also Geometry (where test is done)


Backface culling (cont’d)Backface culling (cont’d)

Often implemented for you in the APIOpenGL: glCullFace(GL_BACK);How to determine what faces away?First, must have consistently oriented

polygons, e.g., counterclockwise

0

1

2

front facing0

1

2

back facing


How to cull backfacesHow to cull backfaces

screen space

1

0

2

front

01

2

back

front

backeye

eye space


View-Frustum CullingView-Frustum Culling

Bound every “natural” group of primitives by a simple volume (e.g., sphere, box)

If a bounding volume (BV) is outside the view frustum, then the entire contents of that BV is also outside (not visible)

Avoid further processing of such BV’s and their containing geometry


Can we accelerate VF culling Can we accelerate VF culling further?further?

Do what we always do in graphics…Use a hierarchical approach, e.g,

the scene graphWhich stages benefits?

– Geometry and Rasterizer – Bus between CPU and Geometry


Example of Hierarchical View Example of Hierarchical View Frustum CullingFrustum Culling

root

camera


Portal CullingPortal CullingImages courtesy of David P. Luebke and Chris Georges

Average: culled 20-50% of the polys in view Speedup: from slightly better to 10 times


Portal culling examplePortal culling example In a building from above Circles are objects to be rendered


Portal Culling AlgorithmPortal Culling Algorithm

Divide into cells with portals (build graph)For each frame:

– Locate cell of viewer and init 2D AABB to whole screen

– * Render current cell with VF cull w.r.t. AABB– Traverse to closest cells (through portals)– Intersection of AABB & AABB of traversed portal– Goto *


Portal overestimationPortal overestimation

To simplify:

actual portal overestimated portal


Portal Culling AlgorithmPortal Culling Algorithm

When to exit:– When the current AABB is empty– When we do not have enough time to render a

cell (“far away” from the viewer)

Also: mark rendered objectsWhich stages benefits?

– Geometry, Rasterizer, and Bus

Source (for Performer): http://www.cs.virginia.edu/~luebke/


Detail CullingDetail Culling

Idea: objects whose projected BV occupy less than N pixels are culled

This is an approximative algorithm as the things you cull away may actually contribute to the final image

Advantage: trade-off quality/speedWhich stages benefits?

– Geometry, Rasterizer, and Bus


Example of Detail CullingExample of Detail Culling

detail culling OFFNot much difference, but 80-400% fasterGood when moving

detail culling ON

Images courtesy of ABB Robotics Products, created by Ulf Assarsson


ProjectionProjection

Projection gets halved when distance is doubled

eye


Projection (cont’d)Projection (cont’d)

dot(d, (c-v)) is distance along d p=nr/dot(d, (c-v)) is estimation of projected

radius p2 is the area

(eye) v

(near plane) n

d (normalized view direction)

r

c


Occlusion CullingOcclusion Culling

Main idea: Objects that lies completely “behind” another set of objects can be culled

Hard problem to solve efficiently


ExampleExample

final image

Note that “Portal Culling” is an algorithm for occlusion culling


Occlusion culling algorithmOcclusion culling algorithm

Use some kind of occlusion

representation OR

for each object g do:

if( not Occluded(OR ,g)) render(g);

update(OR ,g); end;end;


Occlusion culling algorithm Occlusion culling algorithm exampleexample

Process from front to back Maintain an occlusion horizon (yellow)


Occlusion culling algorithm Occlusion culling algorithm exampleexample

To process tetrahedron (which is behind grey objects):– find axis-aligned box of projection– compare against occlusion horizon

culled


Occlusion culling algorithm Occlusion culling algorithm exampleexampleWhen an object is considered visible:Add its “occluding power” to the

occlusion representation


Level-of-Detail RenderingLevel-of-Detail Rendering Use different levels of detail at different

distances from the viewer More triangles closer to the viewer


LOD renderingLOD rendering Not much visual difference, but a lot faster

Use area of projection of BV to select appropriate LOD


Scene graph with LODsScene graph with LODs

Car chair


Far LOD renderingFar LOD rendering

When the object is far away, replace with a quad of some color

When the object is really far away, do not render it (detail culling)!

Which stages benefits? – Geometry, Rasterizer, and Bus


Combinations of TechniquesCombinations of Techniques

Not trivial to combine!VF Cull + LOD == minimal if serious

about performance Indoors: LOD + portal cullingOutdoors (cities, suburbia): VF Cull +

LOD + occlusion culling


Real-Time Rendering?Real-Time Rendering? In computer graphics, “real-time” is used

in a soft way: say >30 fps for most frames

In other contexts, it’s a tougher requirement: the framerate must never be <30 fps, i.e., constant framerate

What can we do?– Reactive LOD algorithm– Reactive detail culling– Reactive visual quality


Collision DetectionCollision Detection

To get interesting and more realistic interaction between geometric objects in an application

Cannot test each triangle against each other: O(n*m)

Alas, need smarter approachesUse BVs because these are cheap to testUse a scene graph (hiearchy)

– For efficiency use only one triangle per leaf


How to collide two scene graphs?How to collide two scene graphs? Three cases: Reach two internal nodes’ BVs

– If not overlap, then exit– If overlap, then descend into the children of the internal

node with largest volume

Reach an internal node and a triangle– Descend into the internal node

If you reach two triangles– Test for overlap

Start recursion with the two roots of the scene graphs


Bounding Volumes in Collision Bounding Volumes in Collision DetectionDetection

Wanted: tight fitting BV, and fast to test for overlap

Common choices are– Spheres– Axis aligned box– Oriented box


Triangle-triangle overlap testTriangle-triangle overlap test

Compute the line of intersection between the two planes

Compute the intersection interval between this line and triangles – gives two intervals

If intervals overlap then triangles overlap!


Simpler collision detectionSimpler collision detection Only shoot rays to find collisions, i.e.,

approximate an object with a set of rays Cheaper, but less accurate Can use the scene graph again or a BSP tree


Can you compute the time of a Can you compute the time of a collision?collision?

Move ball, test for hit, move ball, test for hit… can get “quantum effects”!

In some cases it’s possible to find closed-form expression: t=s/v

v

s


Collision detection resourcesCollision detection resources

Check out www.realtimerendering.comRAPID: uses OBBsSOLID: handles deformable objectsEberly: code for dynamic CD

– where no quantum effects can occur– Each object has a velocity direction


Resources and PointersResources and Pointers

http://www.realtimerendering.comJournal of Graphics Tools

– http://www.acm.org/jgt/– source for projected screen area of box– intersection test routines– occlusion culling

http://www.magic-software.com

tomas mőller © 2000 speed-up techniques for real-time rendering or “what makes your real-time...

Documents

sweden slide

subtree slide

culling techniques

bvs scene graph root

culling occlusion

portal culling

culling examples

eye eye space slide