lit

Provides partial support for lighting by calculating lighting coefficients from two dot products and an exponent.

lit dest, src

Registers

dest

Destination register.

src

Input source register.

To learn more about registers, see Registers.

Remarks

The source vector is assumed to contain the values shown in the following pseudocode.

src.x = N*L        ; The dot product between normal and direction to light
src.y = N*H        ; The dot product between normal and half vector
src.z = ignored    ; This value is ignored
src.w = exponent   ; The value must be between –128.0 and 128.0


// The following code fragment shows the operations performed.
dest.x = 1;
dest.y = 0;
dest.z = 0;
dest.w = 1;

float power = src.w;
const float MAXPOWER = 127.9961f;
if (power < -MAXPOWER)
    power = -MAXPOWER;          // Fits into 8.8 fixed point format
else if (power > MAXPOWER)
    power = -MAXPOWER;          // Fits into 8.8 fixed point format

if (src.x > 0)
{
    dest.y = src.x;
    if (src.y > 0)
    {
        // Allowed approximation is EXP(power * LOG(src.y))
        dest.z = (float)(pow(src.y, power));
    }
}

Reduced precision arithmetic is acceptable in evaluating the destination y component (dest.y) An implementation must support at least 8 fraction bits in the power argument. Dot products are calculated with normalized vectors, and clamp limits are -128 to 128.

Error should correspond to a logp and expp combination, or no more than approximately 1 least significant bit (LSB) for an 8-bit color component.