Following all this radiosity talk, I have made a minor discovery. I think
Alain hinted that this was possible in a radiosity-related post last month,
so it's probably not totally new, but it seems that when using radiosity
lighting with transparency you get caustics naturally without any need for
photon-mapping!

Did anyone else know this?

#include "colors.inc"
global_settings {
  assumed_gamma 1
  max_trace_level 10
  radiosity {
    pretrace_start 0.08
    pretrace_end 0.01
    count 750
    nearest_count 2
    error_bound 0.1
    recursion_limit 12
    low_error_factor 0.5
    gray_threshold 0
    minimum_reuse 0.0001
    brightness 1
    adc_bailout 0.01/2 } }
camera {
  location <0, 5, -5>
  angle 45
  look_at <0, 0, 0> }

sphere { <10, 10, -10>, 3 pigment { color rgb 18 } finish { ambient 1 } }

difference {
  sphere { <0, 0, 0>, 1.5 }
  sphere { <0, 0, 0>, 1.4 }
  pigment { color Clear }
  finish { ambient 0 }
  interior { ior 1.5 } }
sphere {
  <0, 0.3, 0>, 0.3
  pigment { color Clear }
  finish { ambient 0 }
  interior { ior 1.5 } }
plane {
  <0, 1, 0>, 0
  pigment { color White }
  finish { ambient 0 } }

Good, eh?

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