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Modeling 3D objects withpolygons

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(C) Doug Bowman, Virginia Tech, 2008 2

Why polygons? Simple mathematical description Standard 3D graphics primitive All graphics packages optimized for polygon

throughput Most 3D graphics algorithms assume a

polygon-based scene Common polygon algorithms implemented in

hardware In the end, everything (well, almost

everything) is a polygon

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Terminology

Polygon soup: ageneral set ofunstructured polygonsused to define a scene

Polygonal mesh: a setof connected polygonsthat together form asurface

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More terminology

3D polygonal model: a 3D object madeup entirely of polygons

3D polygonal modeling: the process ofbuilding a 3D object by specifying thepolygons that make up that object NOTE: you can build a 3D polygonal model

without using 3D polygonal modeling!

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Methods of creating polygonalmeshes

Build mesh by hand

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Methods of creating polygonalmeshes

Tesselate atheoretical smoothsurface Tesselation: the

process of creating apolygonalapproximation froma smooth surface

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Methods of creating polygonalmeshes

Extrude a 2Dpolygon, curve,etc. Extrusion: the

process of movinga 2D cross-sectionthrough space tocreate a 3D solid

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Methods of creating polygonalmeshes

Revolve/sweep a 2Dpolygon or curve Revolution: the

process of rotating a2D cross-section aboutan axis to create a 3Dsolid

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(C) Doug Bowman, Virginia Tech, 2008 9

Problems with polygonalmodels

They approximate smoothly curvingsurfaces

Tradeoff between realism and efficiency Lots of polygons: good approximation,

slow to process Few polygons: fast processing, poor

approximation

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Polygonal simplification Decrease the number of polygons without

sacrificing visual quality (meet polygonbudget)

30944 triangles 2502 triangles 621 triangles 251 triangles

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Level of detail (LOD)techniques

LOD: change thecomplexity of themodel/imagedynamically tomaintain real-timeperformance

Ex: use simplifiedmodels when objectsare at a great distance

“Popping” problem

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Texture substitution Textures

(“imposters”) cancause objects toappear much moredetailed than theyactually are

Problem: when theuser is close

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Rendering polygonal meshes Simply use traditional raster graphics

techniques Proper lighting depends on proper

normals Face normal: the normal to the plane in which

a polygon lies Vertex normal: the normal to the underlying

surface (being approximated by a polygonalmesh) at a particular vertex

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Face normals vs. vertexnormals

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Practical polygonal modeling

Modeling tools 3D Studio Max Maya AutoCAD …

File formats OBJ, DXF, 3DS, FLT, DWG, …

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Alternatives to polygonalmodeling As we noted, almost everything is a polygon

eventually, but the modeling process doesnot have to be based on polygons

Instead, we can model objects using smoothhigher-order surfaces, and only convert topolygons in order to render

Also, we can model volumes instead ofsurfaces

Next: modeling with curves/surfaces