GXInitLightAttnA

C Specification

#include <revolution/gx.h>

void GXInitLightAttnA(
GXLightObj* lt_obj,
f32         a0,
f32         a1,
f32         a2 );

Arguments

`lt_obj`	pointer to a light object
`a0`	angular attenuation coefficient
`a1`	angular attenuation coefficient
`a2`	angular attenuation coefficient

Return Values

None.

Description

This function sets coefficients used in the lighting attenuation calculation in a given light object. The application must allocate memory for the light object. The arguments a0, a1 and a2 are used for angular (spotlight) attenuation. The attenuation function is:

atten = clamp0( a2 * cos(theta)^2 + a1 * cos(theta) + a0 )
k2 * d^2 + k1 * d + k0

Here cos(theta) is the cosine of the angle between the normal vector of the light and the vector from the light's position to the vertex. d is the distance from the light position to the vertex. The coefficients k0 through k2 can be set using GXInitLightAttnK. You can set both the a0 through a2 and k0 through k2 coefficients with GXInitLightAttn. The light color will be multiplied by the atten factor when the attenuation function for the color channel referencing this light is set to GX_AF_SPOT (see GXSetChanCtrl).

All coefficients are represented with a 20-bit floating point format (1-bit sign, 8-bit exponent, and 11-bit significand) in the hardware, as for other arguments associated with vertex lighting. Be aware of possible rounding errors as a result.

Be aware that this function does not load any hardware registers directly. To load a light object into a hardware light, use GXLoadLightObjImm or GXLoadLightObjIndx.

The function GXInitLightSpot can conveniently be used to set the angle attenuation coefficients based on several spotlight types. The function GXInitLightDistAtten can be used to set the distance attenuation coefficients using one of several common attenuation functions.

Revision History