sl globals

Input	Cs	user-requested surface color	`color`
Input	Os	user-requested opacity	`color`
Output	Ci	resulting surface color	`color`
Output	Oi	resulting opacity	`color`
Basic Geom	P	poistion in world space	`point`
	Ng	geometric normal vector	`point`
	N	"shading" normal (may change due to displacement)	`point`
Lights*	I	direction from surface to light source	`point`
Lights*	Cl	color of light	`color`
Cameras	I	direction of ray (usually from camera)	`point`
Cameras	E	position of camera	`point`
Surface	u, v	surface params	`float`
	s, t	texture coordinates [0,1]	`float`
	du, dv	change in u, v	`float`
	dPdu, dPdv	change in P wrt u, v	`point`

* used within the illuminance() construct