Light Source Shaders

In the chapter Other Kinds of Shader in book [3] you find a section about light shaders. In book [1] there are two chapters dealing with light shaders: The chapter Lighting and Shading gives an overview of light source shaders and the chapter A Gallery of Shaders shows the source code of the standard shaders. There you will find the following light source shaders explained:

• ambientlight⁴⁰

• distantlight⁴¹

• pointlight⁴²

• spotlight⁴³

In the book [2] there is section of chapter Illumination Models and Lights which deals with Light Sources and beside the standard light shaders you will find the source code for the following shaders:

• arealight⁴⁴

• shadowspot⁴⁵

The book [3] will show you some non-standard light shaders:

• nearfarlight: Is an example of a non-physical light.

• beamlight: Is a simplified version of the standard spotlight shader with sharp edges.

• colorlight: Shows how to modify the light color based on the "hsv" color space:

  ...
  lightcolor = color "hsv" (cosangle*10, 1, 1);
  ...
  

  Here are all the color coordinate systems supported in the shading language:

  
  

  Table 2: Color spaces

  Coordinate system	Description
  "rgb"	Red, green, blue
  "hsv"	Hue, saturation, value
  "hsl"	Hue, saturation, lightness
  "xyz", "XYZ"	CIE XYZ coordinates
  "YIQ"	NTSC coordinates

  
  

From the RIB file perspective you should know the following facts about lights. The RenderMan interface defines two types of light sources:

• Non-geometric lights are light sources for which there is a single ``ray'' of light from the light source to any given point on the surface and they are created with the RIB command LightSource.

• Geometric lights are light sources for which there are multiple rays of light, emanating from independent positions, streaming towards each point on the surface and they are created with the RIB command AreaLightSource. The easiest way to describe those positions is to associate some geometric shape with the light source.

You can make further distinctions based on the commands used in the light shader:

• An ambient light source is one with no illuminate or solar statement. See section 5.1 for an example.

• The illuminate construct needs a position and optional you can specify a direction axis and an angle to define a ``cone of light'' with the apex at position. See section 5.3 for the use of this construct in the standard pointlight shader by using only the position. See section 5.4 for the use of this construct with all possible parameters in the standard shader spotlight.

  illuminate(position [,axis,angle]) statement
  

• The solar construct restricts illumination to a range of directions without specifying a position for the source. The direction axis and the angle are optional. See section 5.2 for the use of this statement in the standard shader distantlight. In the book [1] you will find an example how to use the solar construct for a skylight shader which defines a hemisphere.

  solar([axis,angle]) statement
  

Table 5 on page shows a list of predefined light source shader variables.