> Great, now I can get something pretty close :)
>
> It is Fn(x,z,y) that I used for color_map, btw.
>
> Maybe the last question: in a user-defined pattern, does the output variable (Fn
> in this example) give value_n [0,1] for color_map? I experimented with it a bit,
> got somewhat unexpected results...
>
>     // the basic functions for the beam brightness
>     #declare FALLOFF  = 1; // just an example, toy with it
>     #declare a = 0.35;
>     #declare b = 0;
>     #declare c = 1;
>     #declare FnRadial = function(r) {a*exp(-(r-b)*(r-b)/2/c/c)}
>     #declare FnAxial  = function(z) { pow(FALLOFF,z) }
>
>     // helper function to compute radius from x,y
>     #declare FnR = function(x,y) { sqrt(x*x+y*y) }
>
>     // the resulting 3D function
>     #declare Fn = function(x,y,z) { FnRadial(FnR(x,y)) * FnAxial(z) }
>
>
> object {gauss
>    hollow
>    pigment { color rgbf<1, 1, 1, 1> }
>    interior {
>      media {
>        emission    color rgb < 1.0, 0.0, 0.0>*5
>        density{ function {Fn(x,z,y)}
>                  color_map {
>                    [0 color rgb <0.0, 0.0, 0.0>]
>                    [0.15 color rgb <0.1, 0.05, 0.0>]
>                    [0.32 color rgb <0.3, 0.05, 0.0>]
>                    [0.42 color rgb <0.5, 0.05, 0.0>]
>                    [0.52 color rgb <0.7, 0.05, 0.0>]
>                    [0.67 color rgb <0.9, 0.05, 0.0>]
>                    [1.0 color rgb <1.0, 0.05, 0.0>]
>                           }
>               } // end density
>       }//end media
>      } //end interior
>     scale <1,1.5,1>*0.5
>     translate<0,2.0,0.0>
> }
>
>

Your FnAxial function have a value of zero along the Z axis that climb 
to 1 at a radius of 1. It's similar to the wood pattern.
Make sure that your Gauss object extend along the Z axis.

As your map stand now, it's black on the axis and return rgb<1,0.05,0> 
on the edge.
To get a bright spot in the center fading to black at the edge, you 
should invert the colours of your map.

As your emission colour is pure red, the green component of your map 
have absolutely no effect. Mabe it would be beter to use emission rgb 5.
The final colour is the product of the value of emission and the entry 
of the color_map.


Another alternative don't use media nor any function. You can use a 
plain light_source with the cylindrical attribute.
You now control the bright spot using "radius", the overall width with 
"falloff" and the curve with "tightness"
radius and falloff are expressed in degree.

This would give you a nice smooth illumination:

light_source{10*y rgb<5,0,0> cylindrical
	point_at 0
	radius 0 //no notable "hot spot"
	falloff 1 // total width angle
	tightness 4 // gives a nice curve
// range from 1 to 100
}

The actual diameter og the beam is dictated by falloff and the lenght of 
the location to point_at vector.

Now, you can use some scattering media to make the beam visible if you 
want or need.


Alain

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