> Consider this scene of a hollow sphere viewed from the inside:
>
>      difference {
>          sphere { 0, 100 }
>          sphere { 0, 99 }
>          pigment { rgb 1 }
>          finish { ambient .4 }
>      }
>
>      global_settings {
>          radiosity {
>              error_bound .1
>              always_sample off // default in 3.7
>          }
>      }
>
> Rendering produces dark splotches at the centers of the pretrace
> blocks. Blocks rendered earlier have darker splotches. They also differ
> in shape between renders even with +HR (but not with +WT1). Turning
> "always_sample" on removes them, as does increasing "count" past 1000.
>
> I wonder if this might be a bug, as 3.6 doesn't produce these artifacts
> regardless of "always_sample" and "count".
>
>
>
Changing pretrace_end to 0.01 make those spots disapear.

count 100 and pretrace_start 0.04 pretrace_end 0.02 also show an uniform 
surface.

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