So, I did some more testing, wrote a simple, standalone sample scene, and found
yet another curious set of circumstances.

And it looks like probably what's happening is another case of floating point
math error, similar to what I experienced way back when trying to use fractional
loop iterators.

Sometimes the 4-value version works (as expected), and sometimes it doesn't.

Always a headache, with the simplest things.

- BW



#version 3.8;
global_settings {assumed_gamma 1}
default {finish {diffuse 1}}


#declare E = 0.001;

#declare Zoom = 700;
camera {
 orthographic
 location <0.5, 0.5, -50>
 right x*image_width/Zoom
 up y*image_height/Zoom
 look_at <0.5, 0.5, 0>
 rotate y*0
}

sky_sphere {pigment {rgb 0}}

light_source {<1, 1, -10> rgb 1}

#declare LW = 0.01;
#declare f_XLines = function {pigment {gradient y scale 0.1}}
#declare XLines = function {select (f_XLines (x, y, z).red-LW, 1, 0)}
#declare f_YLines = function {pigment {gradient x scale 0.1}}
#declare YLines = function {select (f_YLines (x, y, z).red-LW, 1, 0)}
#declare Grid = function {XLines (x, y, z) + YLines (x, y, z)}

box {0, 1+E pigment {function {Grid (x, y, z)}} }



#declare Select3 = function (N) {
 select (N*10-2, 0.1,
  select (N*10-5, 0.4,
   select (N*10-7, 0.6,
    0.9
   )
  )
 )
}

// In this version, when N-Val = 0 --- when N=Val,
// then the repeated value, argument B, should get returned.
#declare Select4 = function (N) {
 select (N-0.2, 0.1, 0.1,
  select (N-0.5, 0.4, 0.4,
   select (N-0.7, 0.6, 0.6,
    0.9
   )
  )
 )
}

#declare Line = 0.0015;

#declare Step = 0.05;
union {
  #for (X, 0, 1, Step)

   #local Y = Select3 (X);

   #debug concat( "X = ", str(X, 0, 3),  " Y = ", str(Y, 0, 3), "\n")

   #local Current = <X, Y, 0>;
   sphere {Current Line}
   #if (X > 0)
    cylinder {Last, Current, Line}
   #end
   #local Last = Current;
  #end

 pigment {rgb x}
}

#debug "\n===========================================\n"

union {
  #for (X, 0, 1, Step)

   #local Y = Select4 (X);

   #debug concat( "X = ", str(X, 0, 3),  " Y = ", str(Y, 0, 3), "\n")

   #local Current = <X, Y, 0>;
   sphere {Current Line}
   #if (X > 0)
    cylinder {Last, Current, Line}
   #end
   #local Last = Current;
  #end

 pigment {rgb z}
}

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