Remarks:
This tutorial is intended for people who already have a working knowledge of XSI

2) increase the subdivisions U 18, V 18

3) Get->Property->Color at Vertices

4) Get->Property->Paint Vertex Color Tool

5) Paint on the sphere

6) Ctrl-w opens the paint options

7) in the Vertex color painting tab you can change the opacity , color, and bleeding options.
Bleeding is a sort of "spread-tha-cool-color" and can be set to polygons or vertex. Hard to explain ...best is you try it

here are 2 tips:

- by pressing the right mouse button on the sphere you have a color picker

- by holding the e-key and the midlle mouse button you can adjust the opacity by scrolling

8) now that you have experimented with vertex painting we will use it for painting a transparency map

9) paint a black and white pattern a bit like this one (white will be transparent and black will not)

10) now we are going to connect the vertex values to the transparency map

11) open the render tree

12) get a Vertex RGBA node (under Node->Texture-> vertex RGBA) and connect it to the transparency of the phong node:

by connecting a blue color to the phong node and adding 2 shadow-casting spots,
you should get a result a bit like this one :

that's it! it's finished: