Defining Geometry in 3-D


Modeling is the process of describing an object or scene so that we can construct an image of it.


Points & polygons







A cube described in .obj file format


g

v -2.500000 2.500000 2.500000
v -2.500000 -2.500000 2.500000
v 2.500000 -2.500000 2.500000
v 2.500000 2.500000 2.500000
v -2.500000 2.500000 -2.500000
v -2.500000 -2.500000 -2.500000
v 2.500000 -2.500000 -2.500000
v 2.500000 2.500000 -2.500000
# 8 vertices

g cube

f 1 2 3 4
f 8 7 6 5
f 4 3 7 8
f 5 1 4 8
f 5 6 2 1
f 2 6 7 3
# 6 elements


Primitives

Primitives are the fundamental geometric entities within a given data structure.






Rendering - The process of computing a two dimensional image using a combination of a three-dimensional database, scene characteristics, and viewing transformations. Various algorithms can be employed for rendering, depending on the needs of the application.


Tessellation - The subdivision of an entity or surface into one or more non-overlapping primitives. Typically, renderers decompose surfaces into triangles as part of the rendering process.


Sampling - The process of selecting a representative but finite number of values along a continuous function sufficient to render a reasonable approximation of the function for the task at hand.


Level of Detail (LOD) - To improve rendering efficiency when dynamically viewing a scene, more or less detailed versions of a model may be swapped in and out of the scene database depending on the importance (usually determined by image size) of the object in the current view.


Polygons and rendering




Surface normal - a vector that is perpendicular to a surface and ``outward'' facing





With polygonal databases:


A cylinder described in VRML

#VRML V2.0 utf8 CosmoWorlds V1.0

Group {
children Transform {
children Shape {
appearance Appearance {
material Material {
}

}

geometry Cylinder {
}

}

translation 0.0 0.0 0.0

}

}


Surface models















Constructive solid geometry and boolean operations


Metaballs (blobby surfaces)


Isosurfaces


Binary Space Partitioning - for modeling/rendering volumetric data


Grouping and hierarchies


Back to course outline

This file was last modified on January 22, 2002.