The below scene demonstrates a bug in UVPov's sampling method 3. Render it and
the problem should be obvious. It seems to be related to the number of media
samples (more samples reduces the effect). It only manifests itself in areas
where the (non-black) background colour is visible.

Margus

//Begin scene

//lower background brightness and the problem becomes less visible
background{rgb .7}

camera{location <5,4,-8> look_at 0}

light_source{
    -z*1000 color rgb 5000
    spotlight point_at 0 radius .03 falloff .031
}

box{
    -1+z,1
    pigment{rgb x}
}

sphere{0,2000
    pigment{rgbf 1}
    finish{ambient 0 diffuse 0}
    interior{
        media{
            scattering{1 rgb .0002 extinction 1}
            intervals 1
            samples 1,1
            //samples 10,50 //more samples -> problem less visible
            method 3
        }
    }
    hollow
}

//The problem disappears on pixels occupied by the disc
disc{z*10,z,3 pigment{rgb z} finish{ambient .2} hollow}

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