Attenuation and Spotlight Terms

Microsoft DirectX 8.1 (C++)

Attenuation and Spotlight Terms

The diffuse and specular lighting components of the global illumination equation contain terms that describe light attenuation and the spotlight cone. These terms are described below.

Attenuation Term

The attenuation of a light depends on the type of light and the distance between the light and the vertex position. To calculate attenuation, use one of the following three equations.

Atten = 1, if the light is a directional light.
Atten = 0, if the distance between the light and the vertex exceeds the light's range.
Atten = 1/( att0_i + att1_i*d + att2_i*di²).

Parameter	Default value	Type	Description
att0i	(0,0,0,0)	FLOAT	Linear attenuation factor
att1i	(0,0,0,0)	FLOAT	Squared attenuation factor
att2i	(0,0,0,0)	FLOAT	Exponential attenuation factor
di	(0,0,0,0)	FLOAT	Distance from vertex position to light position

The att0, att1, att2 values are specified by the Attenuation0, Attenuation1, and Attenuation2 members of the D3DLIGHT8 structure.

The distance between the light and the vertex position is always positive.
di = ||L_dir||
where:

Parameter	Default value	Type	Description
L_dir	0.0	D3DVECTOR	Direction vector from vertex position to the light position

If di is greater than the light's range, that is, the Range member of a D3DLIGHT8 structure, Direct3D makes no further attenuation calculations and applies no effects from the light to the vertex. The dvAttenuation0, dvAttenuation1, and dvAttenuation2 values are the light's attenuation constants as specified by the members of a light object's D3DLIGHT8 structure. The corresponding structure members are Attenuation0, Attenuation1, and Attenuation2.

The attenuation constants act as coefficients in the formula—you can produce a variety of attenuation curves by making simple adjustments to them. You can set Attenuation0 to 1.0 to create a light that doesn't attenuate but is still limited by range, or you can experiment with different values to achieve various attenuation effects.

The attenuation at the maximum range of the light is not 0.0. To prevent lights from suddenly appearing when they are at the light range, an application can increase the light range. Or, the application can set up attenuation constants so that the attenuation factor is close to 0.0 at the light range. The attenuation value is multiplied by the red, green, and blue components of the light's color to scale the light's intensity as a factor of the distance light travels to a vertex.

Spotlight Term

Parameter	Type	Description
rho	N/A	Angle
phi	FLOAT	Penumbra angle of spotlight in radians. Range: [thetaI, p)
theta	FLOAT	Umbra angle of spotlight in radians. Range: [0, p)
falloff	FLOAT	Falloff factor. Range: (-infinity, +infinity)

where:

rho = norm(L_dcs)^.norm(L_dir)

Parameter	Default value	Type	Description
L_dcs	0.0	D3DVECTOR	Direction vector from origin to the light position in camera space
L_dir	0.0	D3DVECTOR	Direction vector from vertex position to the light position

After computing the light attenuation, Direct3D also considers: spotlight effects if applicable, the angle that the light reflects from a surface, and the reflectance of the current material to calculate the diffuse and specular components for that vertex. For more information, see Spotlight Model.