computing light vector direction from lightmap

[sky_blue]

I have generated the lightmap for both left and right eye from eye image.

let
the lightmap height is denoted by "height"
width is by "width"
assuming there is only one point light source in the map with
pixel color(255,255,255)
I have computed the light source position at light_u and light_v
now how can I compute the light direction?

I have got a code but cant convince myself with the computation.
would anyone plz help me to figure it out?

//compute the light direction

Vector light;
light.dy = ((float)light_u - (float)height / 2) * 2 / (float)height;
light.dx = -((float)light_v - (float)width / 2) * 2 / (float)width;
double factor = 1.0 - (double)(light.dx*light.dx + light.dy + light.dy);

if(factor < 0.0) factor = 0.0;
light.dz = (float)sqrt(factor);

light.Normalize();