3D graphics eBook - Course Materials Repository

Surface normal 210
Normal in geometric optics
The normal is an imaginary line perpendicular to the surface [1] of an
optical medium. The word normal is used here in the mathematical
sense, meaning perpendicular. In reflection of light, the angle of
incidence is the angle between the normal and the incident ray. The
angle of reflection is the angle between the normal and the reflected
ray. That Normal force will then Be perpendicular to the surface
References
[1] "The Law of Reflection" (http:/ / www. glenbrook. k12. il. us/ gbssci/ phys/ Class/
refln/ u13l1c. html). The Physics Classroom Tutorial. . Retrieved 2008-03-31.
External links
• An explanation of normal vectors (http:/ / msdn. microsoft. com/
en-us/ library/ bb324491(VS. 85). aspx) from Microsoft's MSDN
• Clear pseudocode for calculating a surface normal (http:/ / www. opengl. org/ wiki/
Calculating_a_Surface_Normal) from either a triangle or polygon.
Texel
A texel, or texture element (also texture pixel) is the fundamental unit
of texture space, [1] used in computer graphics. Textures are represented
by arrays of texels, just as pictures are represented by arrays of pixels.
Texels can also be described by image regions that are obtained
through a simple procedure such as thresholding. Voronoi tesselation
can be used to define their spatial relationships. This means that a
division is made at the half-way point between the centroid of each
texel and the centroids of every surrounding texel for the entire texture.
The result is that each texel centroid will have a Voronoi polygon
surrounding it. This polygon region consists of all points that are closer
to its texel centroid than any other centroid. [2]
Rendering With Texels
When texturing a 3D surface (a process known as texture mapping) the
renderer maps texels to appropriate pixels in the output picture. On
modern computers, this operation is accomplished on the graphics
processing unit.
The texturing process starts with a location in space. The location can
be in world space, but typically it is in Model space so that the texture
moves with the model. A projector function is applied to the location to
Diagram of specular reflection
Voronoi polygons for a group of texels.
Two different projector functions.