Never tried this myself, but try using "once" and "frequency" on a radial
map.
Another possibility : radial texture_map with something like:
[0.00 texture{myImageMap}]
[0.25 texture{myImageMap}]
[0.25 texture{Invisible}]}]
[1.00 texture{Invisible}]
I hope this helps,
Povingly
Philippe
