Real-time 3D Graphics - Virginia Techcourses.cs.vt.edu/cs5754/lectures/rtgraphics.pdf · Graphics pipeline Object definition Compose scene Define view parameters Perform culling Clip

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Real-time 3D Graphics

Doug A. Bowman

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(C) 2008 Doug Bowman, Virginia Tech 2

Outline

(Very) brief review of 3D image synthesis

Real-time 3D graphics for VEs

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Rendering 3D scenes

Most 3D graphics is based on scenescomposed of objects built of polygons.

Goal: from scene specification and viewingspecification, produce a 2D image of thescene with proper 3D perspective bydetermining the color for each pixel on thedisplay

3D teapot animation…

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Overview of 3D image synthesis(rendering)

3D model

2D projection

Pixel values

Displayed image

Includes scene definition, camera properties,lighting characteristics, etc.

projection

rasterization

display

Process of mapping the desired 3D primitivesinto a 2D space

2D versions of 3D primitives

Process of changing primitive definitions intopixel locations/colors

Color value for each pixel on the screen, storedin the frame buffer

Process of taking information from framebuffer and producing physical representationon a display device

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3D Viewing Example

XZ

Y

Object model

X

Y

Z

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Coordinate systems

Right- vs. left-handed World vs. object (object

trees, scenegraphs) World vs. camera Screen/display

Transformations (mappings)between CSs

Right-handedCS

X

Y

Z

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Graphics pipeline

Object definition

Compose scene

Define view parameters

Perform culling

Clip to 3D view

volume

Hidden surface removal

Rasterization

Shading

Local space

3D screen space

View space

World space

Display space

Modeling transformation

Viewing transformation

Perspectiveprojection

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For each vertex:transform object toworld coords.

Each M represents atransformation matrix.

Where is chair2relative to room?

room

table rug

chair1 chair2

M1 M2

M4M3

M0

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Graphics pipeline

Object definition

Compose scene


Perform culling

Clip to 3D view

volume


Rasterization

Shading

Local space

3D screen space

View space

World space

Display space




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Transform world to camera coords. Where are the objects with respect to the camera?

Viewing transformation animation…

View frustum

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Clipping

Process of removing polygons and parts ofpolygons that fall outside the view frustum

Basically, comparing each vertex withplane equations for the six view frustumsides

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Projections

Projection: mapping 3D to 2D spaceCenter of projection = camera = eyeProjection plane = viewplane: 2D plane

onto which objects are projected

Perspective projection

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Mathematics of a one-pointprojection

xprojected = dx/z= x/(z/d)

yprojected = dy/z= y/(z/d)

d

P(x, y, z)

Projection animation…

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Graphics pipeline

Object definition

Compose scene


Perform culling

Clip to 3D view

volume


Rasterization

Shading

Local space

3D screen space

View space

World space

Display space




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Hidden-surface elimination

Problem: we only need to displaypolygons that are visible

Example approaches:z-buffer (hardware)BSP treeSpatial subdivision

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Shading

We can increase realism by adding shading/lighting, butmore complex approaches take longer to calculate

constant flat Gouraud

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Rasterization

Once vertices have been projected, we onlyneed to “fill in” the primitives between thevertices

Rasterization (sometimes called scanconversion) is the process of determining whichpixels are part of a primitive and “turning on”those pixels.

Polygon fill animation…

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Graphics pipeline

Object definition

Compose scene


Perform culling

Clip to 3D view

volume


Rasterization

Shading

Local space

3D screen space

View space

World space

Display space




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What is real time?

“Fast enough” that the user canPerceive smooth motionInteract with the environment

Often stated in frames per second (fps) –number of views of the scene rendered inone second

A frame rate of approx. 10-30 fps is theminimum for “real-time”

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Speed vs. Quality tradeoff

More complex models ⇒ slower renderingWe must manage this tradeoff to achieve

maximum quality with an acceptable framerate

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Techniques for real-time 3D graphics

Polygonal simplificationView cullingOcclusion cullingLevel of detail managementTexture substitutionLightmaps

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Polygonal simplification

Decrease the number of polygons without sacrificingvisual quality (meet polygon budget)

30944 triangles 2502 triangles 621 triangles 251 triangles

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Level of detail management

LOD: change the complexityof the model/imagedynamically to maintain real-time performance

Ex: use simplified modelswhen objects are at a greatdistance

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Level of detail - another example

Matchcharacteristics ofhuman eye

Use high detail infovea, low detail inperiphery

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View culling

Not the same as clipping!

Easy: remove all objects that are behindthe plane of the camera

Harder: remove all objects outside theview frustum

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Occlusion culling

Hidden-surface elimination techniques: stillrender all polygons (e.g. z-buffer)

Occlusion culling: remove polygons fromconsideration before rendering

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Texture substitution (imposters)

Textures cancause objects toappear muchmore detailedthan they actuallyare

Problem: whenthe user is close

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Lightmaps

Pre-calculate lighting for a scene, thentexture lighting onto objects

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Lightmapped VEs

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Other graphics issues for VEs

Stereo renderingOff-axis projections Inserting the user into the scenegraphParallel / distributed rendering

Documents

Real-time 3D Graphics - Virginia Techcourses.cs.vt.edu/cs5754/lectures/rtgraphics.pdf · Graphics pipeline Object definition Compose scene Define view parameters Perform culling Clip