modeling 3d objects with polygons - virginia techcs4204/lectures/polygonal... · 2008. 3. 26. ·...
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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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(C) Doug Bowman, Virginia Tech, 2008 3
Terminology
Polygon soup: ageneral set ofunstructured polygonsused to define a scene
Polygonal mesh: a setof connected polygonsthat together form asurface
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(C) Doug Bowman, Virginia Tech, 2008 4
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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(C) Doug Bowman, Virginia Tech, 2008 5
Methods of creating polygonalmeshes
Build mesh by hand
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(C) Doug Bowman, Virginia Tech, 2008 6
Methods of creating polygonalmeshes
Tesselate atheoretical smoothsurface Tesselation: the
process of creating apolygonalapproximation froma smooth surface
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(C) Doug Bowman, Virginia Tech, 2008 7
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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(C) Doug Bowman, Virginia Tech, 2008 8
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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(C) Doug Bowman, Virginia Tech, 2008 10
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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(C) Doug Bowman, Virginia Tech, 2008 11
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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(C) Doug Bowman, Virginia Tech, 2008 12
Texture substitution Textures
(“imposters”) cancause objects toappear much moredetailed than theyactually are
Problem: when theuser is close
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(C) Doug Bowman, Virginia Tech, 2008 13
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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(C) Doug Bowman, Virginia Tech, 2008 14
Face normals vs. vertexnormals
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(C) Doug Bowman, Virginia Tech, 2008 15
Practical polygonal modeling
Modeling tools 3D Studio Max Maya AutoCAD …
File formats OBJ, DXF, 3DS, FLT, DWG, …
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(C) Doug Bowman, Virginia Tech, 2008 16
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