Modelling

Ruth Aylett

Overview

Basic issues– Modelling packages– Low polygon modelling– Importing models

City modelling– Photogrammetry– Laser range finders– LIDAR

The great outdoors– Trees, plants, water etc

3D Authoring tools

Most models produced with generic 3D modelling packages offline

Some packages derive from film/animation:– Maya, 3DStudioMax, Softimage.

– creation of realistic creatures or backgrounds for film or gamesindustries.

– Not designed for creating real-time rendered models

Others from science/technology:– Matlab, PV-WAVE, AutoCAD.

– Not designed for realistic textures, lighting etc

All packages create objects, give them styles, assign behavioursand place them in 3D Cartesian space.

Other VR Software Tools

Transom Jack DI Guy, People Shop - Boston Dynamics Poser - Metacreations Reflex 7 - Reflex Research SBIR - Seamless Solutions Inc. Others...

3D Modelling Tools

Vizard - Worldviz Creator/Creator Pro - Presagis Tucan - Awaren Blender - shareware AC3D Modeller Maya 3D Studio MAX - Autodesk Truespace - Caligari VRML: ISB - Parallel Graphics Others...

3D Studio Max

3D Studio Max (or 3ds max) by Discreet– Probably biggest selling professional 3D content

creation package.

Many special purpose pluginssuch as Character Studio.

Still relatively expensive– PC onlyMuch used in film see:

http://www.groundzerofx.com/maxinfilm/

Low polygon modelling

Realtimeconstraints requireworking to a‘polygon budget’

– Seehttp://www.webreference.com/3d/lesson52/ for anexample

– Alsohttp://www.saunalahti.fi/~ops/tutorial/lowhead/lowhead.htm

Using level of detail (LOD)

Produce different versions of sameobject– Very low polygon versions to be used when

distant– 5000 polygons in one view is often plenty– Using texture can help

• But 64 bit by 64 bit often what is required

Obtaining texture images

Libraries from software– 3DS Max, Maya etc

Web– www.3dcafe.com

Camera with Photoshop editing

Texture photography

Orthogonal pictures are best (butphysically impossible)

If the camera has a zoom function– zoom in as far as possible– stand well back– this gives photos that are as close as

possible to being orthogonal

Texture photography

If the camera has a manual / automaticcontrol then use it– particularly important for white balance

A series of photos taken on automaticwill have a different setting for whitebalance for each photo and the colourswill not balance

Texture photography

Avoid using the flash if you can It is better to have light evenly

distributed across the object you arephotographing

Direct flash light is often uneven,particularly if close up

Photo editing in Photoshop

The best tools for texture editing are phototouch up packages like Adobe Photoshop

These packages allow the user to– Distort the texture

• perspective in photos taken at an angle

– Rub out unwanted features in the texture• passing traffic• undesirable reflections

– Create tileable textures The aim is to create a clean, orthogonal,

(maybe repeating) image

Importing models

Always an issue– Given use of separate modelling and runtime

software

Some formats are widely imported– 3ds and vrml

Geometry is not the only problem– Textures often problematic– Games engines may be idiosyncratic

• E.g Unreal Tournament has a ‘carve out’ approach tomodelling

Example: Modelling buildings

Use architects’ drawings Photogrammetry Laser scanning from models LIDAR data

Architects’ drawings

The obvious approach, BUT– Not always available– Require a long manual model construction– As-built not always as-designed

Photogrammetry

The process of deriving metricinformation about an object throughphoto measurements– Very old and trusted method of surveying– Basis of most current maps

Multiple photos of object or area fromdifferent angles– Min of three: facing, left and right

Processing the data

Manual identification of approx 12points in all 3 views– Use of accurate knowledge of focal length

and negative size– Can calculate exact distance of camera

from scene

Laser rangefinding

Use of laser beam to gather distanceinformation– time-of-flight: the round trip travel time of a single

pulse is measured and distance calculated basedon the speed of light

– Can also capture reflectance data

Laser Scanning Range Finder

Real World Capture Conversion of range data

– Range 2.5m - 80+m– Accuracy 2mm - 10+cm

The Liverpool Project

Use laser scanner to scan the physicalmodel

Create VE model of a model

The Process -Build the Model Select area to scan

– 1 overall - 5 perspectives @ 4mm intervals– 4 sections - 4 perspectives each @ 2mm intervals

Nearly 2 million points in the clouds

Segmentation– Hierarchy - areas, buildings, planes– Fit surfaces

10,000 Polygons

The Liverpool Model

Creating the dinosaurs from the BBC TVseries using Laser Scanning

Advantages: Fast,Accurate, Can beautomated

Disadvantage:Expensive, Largeoutput file, Can behazardous

LIDAR

Light Detection And Ranging– Airbourne laser rangefinding– Using known position, altitude, orientation

of plane

Data collection

Collects position (x,y) and elevation (z)– At pre-determined intervals– Accuracy depend son flying height, laser beam

diameter, GPS quality, post-processing– Accuracy up to +- 15cms elevation– No need to select specific scanning targets

The great outdoors

Human artefacts are the easiest thing tomodel– Lots of planes, often orthogonal– Thus low-polygon models

But grass? Bushes? Trees? Flowers?– One polygon for each blade of grass? Several for

each leaf? Water? Clouds? Fire? Flesh? Fur? Hair?

Cheap methods

All involve compromises with appearance Water:

– Use a water texture: of course it doesn’t move• Sunken blue shape

– Create some polygons and a colour animation• This can look like flow

Trees

Billboard with tree texture on it– Looks the same from any angle

Two polygons at 90 degrees with tree textureon them

There are commercially available models And a few free ones: e.g xfrog public plant

• See http://web.inf.tu-dresden.de/ST2/cg/downloads/publicplants/

More sophisticated methods

Water, fire, rain, snow– Particle system: see later lecture– Included in some games engines like UT

Trees– Use fractals: Lindenmayer systems (L-

systems) are method of choice

L-systems

A type of fractal– Invented in 1968 by Aristid Lindenmayer to model

biological growth Constructed from line segments using rules

specified in drawing commands– Start with an initial string– Axiom, transformation rules are applied a specified

number of times– Produce the final command string– Which is used to draw the image

How it works

L-systems are parallel rewriting systems. L-systems consist of:

– an alphabet (V)– an axiom (a non-empty “seed” string V+)– Production rules of the form:

V => V* (predecessor => successor)

Rules replace predecessors with successors. Identity production rule is the default.

L-system example

Alphabet:{a, b}

Axiom (“seed”):b

Production rules:a => abb => a

A variant of the Koch curve which uses only right-angles.– variables : F– constants : + −– start : F– rules : (F → F+F−F−F+F)

Here, F means "draw forward", + means "turn left90°", and - means "turn right 90°"–

n = 0: F

n = 1: F+F-F-F+F

Example: the Koch Curve

n = 2: F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-

F+F+F+F-F-F+F

n = 3:F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-F+F+F+F-F-F+F+ F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-F+F+F+F-F-F+F- F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-F+F+F+F-F-F+F- F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-F+F+F+F-F-F+F+ F+F-F-F+F+F+F-F-F+F-F+F-F-F+F-F+F-F-F+F+F+F-F-F+F

A fractal plant variables : X F

– constants : + −– start : X– rules : (X → F-[[X]+X]+F[+FX]-X),(F → FF)– angle : 25º

Here,– F means "draw forward",– - means "turn left 25º", and– + means "turn right 25º".– X does not correspond to any drawing action and is

used to control the evolution of the curve.– [ corresponds to saving the current values for position

and angle, which are restored when the corresponding ]is executed

Using L-systems

Modelling software available– See

http://www.vterrain.org/Plants/plantsw.html for a list of some

– E.g shareware xfrog plant modeller• http://www.vterrain.org/Plants/Xfrog/index.htm

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