Bob Armstrong wrote:
>  Playing with variations on this form , I guess there are
>  issues of syntax and allowed elisions that I don't understand
>  yet . When
> 
>   plane { z , 5
>    pigment { checker
>     < .6 , 1 , .6 > ,
>     < 1 , .6 , 1 >  }
>    }
> 
>  works , I don't see why
> 
>   plane { z , 5
>    pigment { checker
>     pigment { checker rgb 1 rgb  0 scale 1/12 } ,
>    < 1 , .6 , 1 >  }
>   }
> 
>  doesn't . I would naively expect a principle of substitution .

checker does either take two colors (as in your first example), or two 
pigments. Your second example is one full pigment and one color, but 
that's not valid. You can replace the color with a full pigment 
statement and it should work:

plane { z , 5
   pigment {
     checker
     pigment { checker rgb 1 rgb  0 scale 1/12 },
     pigment { rgb < 1 , .6 , 1 >  }
}


HTH,
Florian

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