Lighting

Two lights illuminate the view in Chimera. The key light is generally the dominant (brighter) source of light. The fill light generally serves as a secondary source; it improves the visibility of regions that would otherwise be dark. This scheme is based loosely on the classic three-point lighting method, but without the back light. The key and fill lights each have directionality and 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.

To convey the three-dimensional nature of a scene, the best direction for lighting is not along the line of sight (from right behind the viewer, like a headlamp), but oblique, somewhat from the side and above (from over the viewer's shoulder). Key and fill lights are often placed to converge from partially opposing directions, with the key light coming from a higher angle. Key lighting is usually 4-8 times brighter than fill lighting.

The directionalities and diffuse and specular parameters of the key and fill lights can be changed with the Lighting tool. The specular parameters inherent to viewed items can be adjusted with Shininess Control.

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.

"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 color and scale values in the Lighting tool)
• the direction of the light
• the surface normal
• for specular light only, the position of the viewer

The key/diffuse, fill/diffuse, key/specular, and fill/specular lights can be treated as four separate contributions. The color shown for a point on an object's surface is generated by the following process:

1. The object's color component values (RGB, meaning red, green, blue) are multiplied by the scaled diffuse color component values due to each light, yielding a set of RGB values for the key/diffuse light and a set for the fill/diffuse light.
   
   
2. Similarly, the object's specular color component values (spatially modulated according to the shininess and scaled by the brightness, see Shininess Control) are multiplied by the scaled specular color component values due to each light, yielding a set of RGB values for the key/specular light and a set for the fill/specular light.
   
   
3. The four sets of RGB values are added to generate the color shown for the point. When a scale factor (see the Lighting tool) is negative, however, the corresponding contribution is effectively subtracted. Light subtraction is nonphysical but possible in Chimera.

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

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