Texture and Color

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Subsections

Texture and Color

Simple Texture Characteristics

The surface textures applied to an object drastically alter its overall appearance, making textures and color one of the most important topics in this manual. As with many other renderers, textures can be declared and associated with a name so that they may be used over and over again in a scene definition with less typing. If a texture is only need once, or it is unique to a particular object in the scene, then it may be declared along with the object it is applied to, and does not need a name.

The simplest texture definition is a solid color with no image mapping or procedural texture mapping. A solid color texture is defined by the AMBIENT, DIFFUSE, SPECULAR, OPACITY and COLOR parameters. The AMBIENT parameter defines the ambient lighting coefficient to be used when shading the object. Similarly, the DIFFUSE parameter is the relative contribution of the diffuse shading to the surface appearance. The SPECULAR parameter is the contribution from perfectly reflected rays, as if on a mirrored surface. OPACITY defines how transparent a surface is. An OPACITY value of 0.0 renders the object completely invisible. An OPACITY value of 1.0 makes the object completely solid, and non-transmissive. In general, the values for the ambient, diffuse, and specular parameters should add up to 1.0, if they don't then pixels may be over or underexposed quite easily. These parameters function in a manner similar to that of other ray tracers. The COLOR parameter is an RGB triple with each value ranging from 0.0 to 1.0 inclusive. If the RGB values stray from 0.0 to 1.0, results are undefined. In the case of solid textures, a final parameter, TEXFUNC is set to zero (integer).

Texture Declaration and Aliasing

To define a simple texture for use on several objects in a scene, the TEXDEF keyword is used. The TEXDEF keyword is followed by a case sensitive texture name, which will subsequently be used while defining objects. If many objects in a scene use the same texture through texture definition, a significant amount of memory may be saved since only one copy of the texture is present in memory, and its shared by all of the objects. Here is an example of a solid texture definition:

 TEXDEF MyNewRedTexture   
    AMBIENT 0.1 DIFFUSE 0.9 SPECULAR 0.0 OPACITY 1.0 
    COLOR 1.0 0.0 0.0  TEXFUNC 0

When this texture is used in an object definition, it is referenced only by name. Be careful not to use one of the other keywords as a defined texture, this will probably cause the parser to explode, as I don't check for use of keywords as texture names.

When a texture is declared within an object definition, it appears in an identical format to the TEXDEF declaration, but the TEXTURE keyword is used instead of TEXDEF. If it is useful to have several names for the same texture (when you are too lazy to actually finish defining different variations of a wood texture for example, and just want to be approximately correct for example) aliases can be constructed using the TEXALIAS keyword, along with the alias name, and the original name. An example of a texture alias is:

  TEXALIAS MyNewestRedTexture  MyNewRedTexture

This line would alias MyNewestRedTexture to be the same thing as the previously declared MyNewRedTexture. Note that the source texture must be declared before any aliases that use it.

Image Maps and Procedural Textures

Image maps and procedural textures very useful in making realistic looking scenes. A good image map can do as much for the realism of a wooden table as any amount of sophisticated geometry or lighting. Image maps are made by wrapping an image on to an object in one of three ways, a spherical map, a cylindrical map, and a planar map. Procedural textures are used in a way similar to the image maps, but they are on the fly and do not use much memory compared to the image maps. The main disadvantage of the procedural maps is that they must be hard-coded into Tachyon when it is compiled.

The syntax used for all texture maps is fairly simple to learn. The biggest problem with the way that the parser is written now is that the different mappings are selected by an integer, which is not very user friendly. I expect to rewrite this section of the parser sometime in the near future to alleviate this problem. When I rewrite the parser, I may also end up altering the parameters that are used to describe a texture map, and some of them may become optional rather than required.

Texture Mapping Functions
Value for TEXFUNC	Mapping and Texture Description
0	No special texture, plain shading
1	3D checkerboard function, like a rubik's cube
2	Grit Texture, randomized surface color
3	3D marble texture, uses object's base color
4	3D wood texture, light and dark brown, not very good yet
5	3D gradient noise function (can't remember what it look like
6	Don't remember
7	Cylindrical Image Map, requires ppm filename
8	Spherical Image Map, requires ppm filename
9	Planar Image Map, requires ppm filename

Here's an example of a sphere, with a spherical image map applied to its surface:

SPHERE
  CENTER 2.0  0.0 5.0   
  RAD 2.0 
  TEXTURE 
    AMBIENT  0.4 DIFFUSE  0.8 SPECULAR 0.0  OPACITY 1.0 
    COLOR 1.0 1.0 1.0  
    TEXFUNC 7 /cfs/johns/imaps/fire644.ppm  
      CENTER 2.0 0.0 5.0
      ROTATE 0.0 0.0 0.0 
      SCALE  2.0 -2.0 1.0

Basically, the image maps require the center, rotate and scale parameters so that you can position the image map on the object properly

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