ColorFunctionScaling with ParametricPlot3D & InterpolatingFunctions

Question

Part of this recent question involves coloring a ParametricPlot3D of three variables from a set of ODEs according to a fourth variable. I just figured out how to do that using the slot #4, but ColorFunctionScaling->True doesn't seem to behave properly.

First, solve the ODEs:

nsp = 4;

{r[1], r[2], r[3], r[4]} = {1, 0.72, 1.53, 1.27};
{k[1], k[2], k[3], k[4]} = {1, 1, 1, 1};
amat = {{1, 1.09, 1.52, 0}, {0, 1, 0.44, 1.36}, {2.33, 0, 1, 0.47}, {1.21, 0.51, 0.35, 1}};
Do[a[i, j] = amat[[i, j]], {i, nsp}, {j, nsp}];

eqns = Table[x[i]'[t] ==
  r[i]*x[i][t]*(1 - Sum[a[i, j]*x[j][t]/k[i], {j, nsp}]), {i, nsp}];
ics = Table[x[i][0] == 0.1, {i, nsp}];
unks = Table[x[i], {i, nsp}];

tmax = 10000;
sol = NDSolve[{eqns, ics}, unks, {t, 0, tmax}][[1]];

It looks good with ColorFunctionScaling->False:

ParametricPlot3D[Evaluate[{x[1][t], x[2][t], x[3][t]} /. sol], {t, 100, tmax}, 
  AxesLabel -> {"x1", "x2", "x3"}, PlotPoints -> 400, 
  ColorFunction -> (ColorData["SunsetColors", x[4][#4] /. sol] &), 
  ColorFunctionScaling -> False]

but totally weird with ColorFunctionScaling->True:

ParametricPlot3D[Evaluate[{x[1][t], x[2][t], x[3][t]} /. sol], {t, 100, tmax}, 
  AxesLabel -> {"x1", "x2", "x3"}, PlotPoints -> 400, 
  ColorFunction -> (ColorData["SunsetColors", x[4][#4] /. sol] &), 
  ColorFunctionScaling -> True]

We can see what it should look like by manually applying our own scaling directly to the ColorFunction:

max = NMaximize[{x[4][t] /. sol, 0 < t < tmax}, t, Method -> "RandomSearch"][[1]];
min = NMinimize[{x[4][t] /. sol, 0 < t < tmax}, t, Method -> "RandomSearch"][[1]];

ParametricPlot3D[Evaluate[{x[1][t], x[2][t], x[3][t]} /. sol], {t, 100, tmax}, 
  AxesLabel -> {"x1", "x2", "x3"}, PlotPoints -> 400, 
  ColorFunction -> (ColorData["SunsetColors", (x[4][#4] - min)/(max - min) /. sol] &), 
  ColorFunctionScaling -> False]

Bug or user error? :) v11.1 on MacOS.

Answer 1

Here is a simpler, but hopefully equally illustrative example:

{ParametricPlot3D[{Cos[t], Sin[t], 0}, {t, 0, 2 π}, 
                  ColorFunction -> (Hue[#4] &), ColorFunctionScaling -> True, 
                  PlotLabel -> "w/ scaling"], 
 ParametricPlot3D[{Cos[t], Sin[t], 0}, {t, 0, 2 π}, 
                  ColorFunction -> (Hue[#4] &), ColorFunctionScaling -> False, 
                  PlotLabel -> "w/o scaling"]} // GraphicsRow

Recalling that Hue[] is $1$-periodic, note that the circle on the left is colored by the color wheel exactly once, corresponding to an internal rescaling of the parameter to $[0,1]$, which is, coincidentally, Hue[]'s primary domain. The figure on the right, however, feeds the unscaled ($[0,2\pi]$) parameter values to Hue[], and this is manifested in the repeated bands of color.