Global variable access in shading contexts

Global variable access in shading contexts

Each shading context is responsible for setting or modifying different variables. For example, the displacement context can modify the position of the surface being rendered, while the light context is responsible for setting the color of the light source. The Read/Write access of the global variables for each context is summarized in this table.

Variable	Type	Description	Displacement	Surface	Light	Shadow	Fog
Cf	vector	The final color for the surface. The vector represents the RGB color for the surface.	-	W	-	-	R/W
Of	vector	The final opacity for the surface. A value of {1,1,0} will be opaque in red/green, but let through blue light from behind. See opacity vs. alpha.	-	W	-	-	R/W
Af	float	The final alpha for the surface. This is the value which is placed in the alpha channel of the output image. See opacity vs. alpha.	-	W	-	-	R/W
P	vector	The position of the point on the surface being shaded. In light or shadow shaders, the P variable contains the point on the light source. Although P is modifiable in the surface context, changing P will not affect rendering of the surface, only shading.	R/W	R/W*	R	R	R
Pz	float	The Z component of the point being shaded.	R	R	-	-	R
Ps	vector	In light & shadow shaders, this is the position of the point on the surface being illuminated.	-	-	R	R	-
I	vector	-	R	R	R	R
Eye	vector	The position of the eye.	-	R	R	R	R
s, t	float	The parametric S and T (sometimes called U and V) coordinates on the surface being shaded.	R	R	R	R	R
dPds, dPdt	vector	The delta between the current position (P) and the position of the neighboring micropolygon vertex along S and T. You can use these vectors to get the approximate length of micropolygon edges.	R	R	R	R	R
N	vector	The shading normal for the surface.	vector	R/W	R/W	R	R	R
Ng	vector	The geometric normal for the surface. This normal represents the “true” normal of the surface being shaded. For example, with Phong shading, the N variable represents the interpolated normal, while the Ng variable represents the true polygon normal	R	R	R	R	R
Cl	vector	Light color.	-	R/W	W	R/W	R/W
L	vector	The vector from the point on the surface to the light source. The length of this vector represents the distance to the light source. See how the L variable is initialized.	-	R	R/W	R	R
Lz	vector	The Z-axis in the space of the light. This is a unit vector.	-	-	R	R	-