12.9. Glossary of Texturing Terminology   

Aliasing

Aliasing occurs when the screen sampling rate is too low to accurately display object detail. That is, the colour regions are smaller than, or are not properly captured by, the screen pixels. See the section on Displaying Images on the Screen for more information.

Bump Map

Bump maps operate on the same principles as texture maps except the values refer to an elevation or surface normal value rather than a shading intensity (colour). When used in conjunction with shading or raytracing operations they give the impression of a bumpy surface even though the geometric definition of the surface is flat.

Convolution

Convolution is the process in signal (or texture) filtering where the pixel intensity is calculated by averaging weighted sub-pixel values.

Digitizing (textures)

Real-world textures can be digitized as shading intensity values and then applied to surface geometry to simulate that real-world object. For example a brick surface could be digitized, and the texture map applied repeatedly to a a flat surface to give the impression of a brick wall, making the screen image highly realistic.

Domain

the domain of a function is the set of values which it can be applied to. For example if the domain of a function is the unit square then the x and y inputs it accepts are those between 0 and 1.

Facets

Objects are defined geometrically as a set of polygonal surfaces, or facets. Each facet is made up of a set of vertices and edges. Facets are the surfaces to which texture maps and shading algorithms are applied. Individual facets are generally not discernible in complex objects once shading algorithms have been applied, but simple objects such as cubes have obvious facets ( refer to Progressions in Image Realism to see the facets of a complex image and how they cannot be distinguished once sophisticated shading has taken place).

Filtering

Filtering is the process of resampling an object at an appropriate sampling rate and combining the resulting intensity values into one, which is displayed by the screen pixel. This is done when object detail is too great to be accurately displayed by the screen device. See Aliasing.

Flat Shading

Flat shading is the process of obtaining a colour value for each individual facet of an object based on the lighting, surface colour and facet orientation. The entire facet is then shaded in that colour.

Interpolation

Interpolation is the process of assigning a value at some point based on the known values at some end-points and the distance from those end-points.

Pixels

Pixels refers to the smallest unit of measurement. Most commonly used to refer to screen pixels, which are the smallest display units.

Polygonal Surfaces

See facets.

Rendering

Rendering is the process of shading all the objects and the possibly background in a scene. There are a variety of shading methods that can be used, some as simple as filling a region with a colour.

Shading Parameter (p)

the shading parameter (p) is used to alter or determine the shading intensity at a given point. It can be determined from a texturing function or by looking up a tabulated texture map. The value can be simply used as the shading intensity at that point or can be used in a function to alter the existing intensity.

Sub-pixel

Often when the output device can not provide sufficiently high resolution, output values are calculated for sub-pixels, which are then combined via some convolution technique. Sub-pixels can be simply thought of as a set of pixels within a pixel.

Surface Normal

the surface normal is a vector projected perpendicularly from the surface and is used to determine the orientation of the surface. The surface normal is obtained using cross products of two edges of the facet.

Surface Parameterization

Surface parameterization is the process of mapping a 2D image onto a 3D object. See the section on Surface Parameterization for more information.

Texel

a texel is the smallest unit on a texture map. It can be described as a pixel on the texture map. Texture values (for example the value of a shading parameter) are calculated for individual texels.

Texture

texture can be described as any surface detail. The term is usually used to refer to colour variation in computer graphics.

Texturing Function

the texturing function is used to obtain a value for some shading parameter at given (x,y) co-ordinates. Co-ordinates from the object space must first be transformed into the domain of the texuturing function. See the section on Texture Synthesis information.

Texture Maps

A texture map is a tabulated set of values which can be used to determine shading values on an object's surfaces. Often the effect is of draping the texture map over the surface of the object. The texture map can take the visual form of a picture or pattern.

Texture Synthesis

Texture synthesis is the process of creating textures from a mathematical function. Refer to the section on Texture Synthesis for a complete explanation.

Tilable Textures

Tilable textures can be placed side by side to create a larger, seamless, texture map. They are used to create infinite texture spaces from finite sources as well as to create repetetive patterns. See the section on Tilable vs Non-Tilable Textures for examples and information.

Transformation

Transformations are used in texture mapping to transform a point in object space either into the domain of a texturing function or into a point in a table of texture values. That is, a point on the surface of an object corresponds to a point on the texture map. The correspondence is determined by the transformation used.

Unit Square

the unit square is used as a domain for many texturing functions. It is the set of all points (x,y) for which x and y are between 0 and 1 inclusive.

Wireframe

wireframe images are those which display the vertices of all the facets making up an object. That is, facets are displayed without any surface shading. See the section on Progressions in Image Realism for more information and an example.