This tutorial shows what you need to do to have 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: