DETAILED DESCRIPTION Generally, computer graphics systems represent dynamic images by displaying data on the screen of a cathode ray tube (CRT).
Typically, the CRT screen displays a rectangular array of thousands of individual picture elements (pixels).
To activate displays, the visual characteristic such as the color, intensity and depth of each pixel is defined by associated pixel data.
In traditional three-dimensional graphics systems, three-dimensional images (containing a variety of objects or primitives) are generated for representation on the two-dimensional CRT screen.
The operation involves defining a three-dimensional model space (e.
g.
, a frustum of vision from a predefined viewpoint) and assigning sections of the model space (called component pixel frustums) to the pixels of the CRT screen.
During a process known as polygon rendering, the pixel data is calculated according to the characteristics of objects or primitives, e.
g.
, three-dimensional polygons, that lie in that pixel's frustum.
The degree to which a given polygon is displayed by a given pixel depends on the intersection of the pixel's frustum with the polygon.
The intersection defines an area, called a projected pixel footprint, on the polygon.
Accordingly, each pixel displays the combined visual effects such as color, shade and transparency defined within the projected pixel footprint on all the polygons in that pixel's frustum.
The visual characteristics of a polygon may be enhanced by texturing the polygon.
Texture is analogous to a digital wallpaper that is applied to surfaces, e.
g.
, polygons.
Texture can represent changes in any spatially varying surface parameter and is typically used to represent changes in intensity, color, opacity, or thematic content (such as surface material type).
The process of applying texture patterns to surfaces is generally referred to as "texture mapping" and is a well known and widely used technique in computer graphics.
For example, see U.
S.
Pat.
No.
4,855,943 (System For Texturing Computer Graphics Images, Robinson) and the textbook Computer Graphics: Principles and Practice
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Image Analysis 
