Lighting Details

Classic three-point lighting uses key, fill, and back lights. Generally, the key light is the dominant source, the fill light serves as a secondary source to improve the visibility of regions that would otherwise be dark, and the back light illuminates edges. Each of these lights is directional. There can also be ambient light, which is nondirectional. A directional light can include diffuse and specular contributions: diffuse light is scattered from a surface equally in all directions, whereas specular light is reflected in a preferred direction.

In Chimera, several lighting modes are available:

ambient - ambient only
single - key + ambient
two-point - key + fill + ambient
three-point - key + fill + back + ambient

By default, only the key light gives specular highlights. The other lights have zero specular contributions and are treated as shadowless during raytracing. The lighting mode, light directions, and overall brightness and contrast can be controlled with the Lighting tool, the specular parameters of objects can be controlled in the Shininess section, and all of these as well as the colors and specular contributions of the lights can be adjusted with the lighting command.

Due to the many interacting parameters, the overall effect of changing some aspect of lighting can be unexpected. If adjusted at all, lighting should be the last thing adjusted before images are saved. See also: tips on preparing images

Combination Rules

“Object” will be used to refer to any item in the view. The illumination at a point on an object's surface due to a particular light source depends on several things:

the inherent color and intensity of the light
the direction of the light
the surface normal
for specular light only, the position of the viewer

The lights in Chimera do not attenuate with distance.

The specular and diffuse portions of each light can be treated as separate contributions. The color shown for a point on an object's surface is generated by the following process:

The object's color component values (RGB, meaning red, green, blue) are multiplied by the scaled diffuse color component values of each light.
Similarly, the object's specular color component values (spatially modulated by its specular sharpness and scaled by its specular reflectivity) are multiplied by the scaled specular color component values of each light.
The resulting sets of RGB values are added to generate the color shown for the point.

Multiplying color component values has the effect of intersection. For example, multiplying red (RGB values 1,0,0), and blue (0,0,1) gives black:

net red component = (1)(0) = 0
net green component = (0)(0) = 0
net blue component = (0)(1) = 0
(0,0,0) = black

Adding red and blue gives magenta:

net red component =  1 + 0  = 1
net green component =  0 + 0  = 0
net blue component =  0 + 1  = 1
(1,0,1) = magenta

UCSF Computer Graphics Laboratory / June 2012