From PixieWiki

[edit] Soft Raytraced Shadows

In order to have soft raytraced shadows, you can either use the shadow or transmission calls with blur parameter set to something other than 0 (default).

For example, if you change (in shadowdistant.sl)

vis = (1 - shadow(shadowname,Ps));

with

vis = (1 - shadow(shadowname,Ps,"blur",0.1));

you should get the following picture:

A blur value 1 means that the light source is hemispherical. The blur value 0 means the light source is pure directional.

Notice the noisy shadow penumbra because the default number of samples for shadow call is 1.

If we use the following line:

vis = (1 - shadow(shadowname,Ps,"blur",0.1,"samples",50));

we get the following picture which looks much better: