POV-Ray 3.6.1a.icl8

It seems to me that in an isosurface, one can substitute sin(f_th(x,y,z))
for x and cos(f_th(x,y,z)) for z, and that bears out in simple testing.

Next I try to construct a cylinder via (x*x + z*z - 1), which works like a
champ. Then when I substitute the sin/cos of f_th() for x and z, it goes
all hooey and I get a solid box with a bunch of speckles, regardless (it
seems) of max_gradient settings.

Anyone have any idea why?

Scene files follows, render at the resolution of your preference:

-----------------------------------------

#include "colors.inc"
#include "functions.inc"

camera {
  location  <2.0, 10.5, -10.0>
  look_at   <6.0, 0.0,  0.0>
}

light_source { <-30, 30, -30>  color rgb 1 }

plane { y, 0
  pigment { color rgb <0.7,0.5,0.3> }
}

#declare XTest= function{x}

#declare SinTest= function{ sin(f_th(x,y,z))}

#declare ZTest= function {z}

#declare CosTest= function{ cos(f_th(x,y,z))}

#declare XZTest= function{pow(XTest(x,y,z),2) +
                          pow(ZTest(x,y,z),2) - 1}

#declare SinCosTest= function{ pow(SinTest(x,y,z),2) +
                               pow(CosTest(x,y,z),2) - 1}

#declare Funcs = array[6] {function{XTest(x,y,z)},
                           function{SinTest(x,y,z)},
                           function{ZTest(x,y,z)},
                           function{CosTest(x,y,z)},
                           function{XZTest(x,y,z)},
                           function{SinCosTest(x,y,z)}}

#local i=0;
#while(i < 6)
        isosurface{
                function {Funcs[i](x,y,z)}
                contained_by {box {-1, 1}}
                open
                evaluate 0.6, 33, .7
                texture{pigment{color White}}
                translate i*3*x
        }
        difference{ box{-1,1} box {<-2,.1,-2>,2}
                    texture{pigment{color Blue}}
                    translate i*3*x}
        #local i=i+1;
#end

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