GPU Pro 5 Area Lights - opengl

Question

i'm trying to implement the area lights described in GPU Pro 5 in GLSL, but i'm having some trouble with the projections.
here is the shader code i'm currently using for diffuse lightning:
vec3 linePlaneIntersection
(
vec3 linePoint, vec3 lineNormal,
vec3 planeCenter, vec3 planeNormal
)
{
float t = (dot(planeNormal, planeCenter - linePoint) / dot(planeNormal, lineNormal));
return linePoint + lineNormal * t;
}
vec3 projectOnPlane(vec3 point, vec3 planeCenter, vec3 planeNormal)
{
float distance = dot(planeNormal, point - planeCenter);
return point - distance * planeNormal;
}
vec3 diffuse_color(vec3 p, vec3 surfaceDiffuseColor, vec3 n)
{
// for point p with normal n
vec3 directionToLight = normalize(light.position.xyz - p);
vec3 planeNormal = directionToLight;
planeNormal = light.orientation.xyz;
// intersect a ray from p in direction nII with light plane,
// creating point pI
vec3 nII = n;
if(dot(n, light.orientation.xyz) > 0.0)
{
// light plane points away from n, skew in direction of light plane
nII = -light.orientation.xyz;
}
vec3 pI = linePlaneIntersection(p, nII, light.position.xyz, planeNormal);
// project point p on the light plane, creating point pII
vec3 pII = projectOnPlane(p, light.position.xyz, planeNormal);
// create halfway vector h between ppI and ppII
vec3 ppI = pI - p;
vec3 ppII = pII - p;
vec3 h = normalize(ppI + ppII);
// intersect ray from p in direction h with the light plane,
// creating point pd
vec3 pd = linePlaneIntersection(p, h, light.position.xyz, planeNormal);
// treat vector ppd as light vector for diffuse equation
vec3 ppd = normalize(pd - p);
// distance from point p to point pI on dArea
float r = distance(p, pI);
// angle between light vector ppd and surface normal n
float cosP = clamp(dot(ppd, n), 0.0, 1.0);
// angle between surface normal and light plane orientation normal
float cosO = clamp(dot(n, -light.orientation.xyz), 0.0, 1.0);
float dArea = light.dAreaRadiance.x;
float radiance = light.dAreaRadiance.y;
return radiance * surfaceDiffuseColor * cosP * cosO * dArea / (r * r);
}
the light has the position {0, 100, 0} and the orientation {0, -1, 0}.
if i use the light orientation as the plane normal for the projections, the light always comes directly from the top, even when i'm changing the position on the x axis.
when i use the direction to the light position as the plane normal, it seems to work, but i'm pretty sure it is still not correct.