Need help with writing shader

[PhilB]

Hello all,
I am trying to use raymarching, thus I've written an interior volume shader which basically should return a value equal to the length of the ray going through object (I understand Pixie implements interior volume shader, right ?).
Unfortunately, I only get a completely black image. Could anyone point my mistake ?
Below, the shader script and the RIB file. BTW, I use Pixie 2.2.5.
Thanks
PhilB

**** My shader ****
volume thickness ()
  {
  point  frontP = P - I;
  point  currP = frontP;
  float  totalDist = length(I);
  float  accumulatedValue = 0;
  float stepSize=0.1;
  float currDist=0;

  while(currDist <= totalDist)
    {
    // Notice that I/totalDist is the normalized vector I
    // multiplying by a float (currDist) is a vector aligned with I
    currP = frontP + (currDist/totalDist) * I;

    accumulatedValue += stepSize;

    // Prepare for the next iteration of the loop
    currDist += stepSize;
    }
  // Modify the output colors.
  Ci = accumulatedValue;
  }

**** My RIB file calling the above shader ****
Display "sphere3.tif" "framebuffer" "rgb"
Projection "perspective" "fov"
Translate 0 0 30
WorldBegin
  Atmosphere "thickness"
  Opacity 0.1 0.1 0.5
  Sphere 10 -10 10 360
WorldEnd

[PhilB]

Sorry, the RIB file should read:
Display "sphere3.tif" "framebuffer" "rgb"
Projection "perspective" "fov"
Translate 0 0 30
WorldBegin
  Interior "thickness"
  Sphere 10 -10 10 360
WorldEnd

[PhilB]

OK, maybe this is not the right place for this request…
Would someone be kind enough to point me towards the right forum to post questions regarding interior volume shaders ?
TIA
Phil