# lit - vs

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

## Syntax

lit dst, src |
---|

where

- dst is the destination register.
- src is a source register.

## Remarks

Vertex shader versions | 1_1 | 2_0 | 2_x | 2_sw | 3_0 | 3_sw |
---|---|---|---|---|---|---|

lit | x | x | x | x | x | x |

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 eight 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 - vs and exp - vs combination, or no more than approximately one significant bit for an 8-bit color component.

## Related topics