translate processing code into glsl

Hello,

I have written this algorithm with Processing and I'm trying to rewrite it in glsl. I think I'm close to the solution but I'm missing something because I don't get the same result:

code to translate :

PVector camPos = cameraPos.get();    
PVector v  = PVector.sub(pos, psCenter);

float sn = -camPos.dot(v);

float sd = camPos.mag();

sd *= sd;

camPos.mult(sn / sd);

PVector isec = PVector.add(pos, camPos);

float dist = isec.dist(pos);

Can you find a mistake below ?

  vec3 camPos = cameraPosition;

  vec3 v = vertex.xyz - psCenter;

  //----- calculate sn -----//

  float sn = dot(-camPos, v);

  //----- calculate sd -----//

  float sd = length(camPos);
  sd = sd * sd;

  //----- calculate isec -----//

  camPos *= sn/sd;

  vec3 isec = vertex.xyz + camPos;

  float dist = distance(isec, vertex.xyz);

  float distToFocalPlane = dist;