William F Pokorny <ano### [at] anonymousorg> wrote:
> On 05/16/2016 12:22 PM, EdR wrote:
> > Hi,
> >
> > I'm having a problem rendering emitting objects, where transparent objects seem
> > to be blocking the light from others behind.
> >
> > It can be seen in the minimal code (part of a larger scene) below and in this
> > render https://www.dropbox.com/s/fki0av19wjwnbvu/minimal_bug_example.png?dl=0
> >
> > The left hand cylinder seems to obscure the right hand one, and I can't work out
> > why. Can anyone help?
> >
> > Thanks!
> >
> > ----
> > #include "colors.inc"
> > camera {
> >    location <15,0,40>
> >    look_at <5,0,0>
> >    angle 45
> > }
> >
> > //background
> > background { color Black }
> >
> > #declare thick = 1.5;
> > union {
> >    #for (ii, 8, 9)
> >      cylinder{
> >        (ii*thick*x), (((ii+1)*thick*x)-0.000001), 1
> >        pigment { Clear }
> >        hollow
> >
> >        interior {
> >          media {
> >            emission White * 0.03
> >          }
> >        }
> >      scale <1,12.4,12.4>
> >      }
> >    #end
> > }
> > ----
> >
> >
> >
> >
>
> I believe you are running into an internal a numerical cut off in the
> media sampling code(1). If you increase the overlap between cylinders,
> things work in my tests.
>
> In addition, it would be more normal to use a merge (which eliminates
> internal boundaries) when using media or transparency vs a union - but
> maybe you want some different effect. The pigment and hollow can also be
> applied to the entire merge/union instead of each cylinder when they are
> identical.
>
> My version of your code following some of the advice above :
>
>    merge {
>    //cylinder{ (1.5*x), ((3.0*x)-0.000001), 1 pigment { Red } }
>    //cylinder{ (3.0*x), ((4.5*x)-0.000001), 1 pigment { Green } }
>    //cylinder{ (1.5*x), ((3.0*x)-0.00001), 1 pigment { Clear } } // Fail
>    //cylinder{ (3.0*x), ((4.5*x)-0.00001), 1 pigment { Clear } }
>      cylinder{ (1.5*x), ((3.0*x)-0.0001), 1 pigment { Clear } } // OK
>      cylinder{ (3.0*x), ((4.5*x)-0.0001), 1 pigment { Clear } }
>      hollow
>
>      interior {
>        media {
>          emission White * 0.03
>        }
>      }
>    //scale <1,12.4,12.4>
>    }
>
> Bill P.
>
> (1) - How individual rays move through the shapes likely will also
> affect where this accuracy / cut off issue is seen. As would, I expect,
> changing the media sampling criteria.

Hi,

Thanks to you all. I did, indeed mean to have 0.000001 inside the bracket, but
the main issue was that 0.000001 was too small. It works fine with 0.001 as
suggested.

Thanks for all your help,
Ed

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