# What is the meaning of the last argument passed to the ColorFunction of a ParametricPlot3D?

According to the docs, the meaning of the arguments passed to ParametricPlot3D is $x, y, z, u, v$. This is 5 arguments: the Cartesian coordinates and the two parameters. Similarly, for ParametricPlot it's $x, y, u, v$.

Let's look at how many arguments are actually passed to the ColorFunction:

Reap[ParametricPlot3D[{Cos[u] Cos[v], Sin[u] Cos[v], Sin[v]},
{u, 0, 2 Pi}, {v, -Pi/2, Pi/2}, ColorFunction -> ((Sow[{##}]; Gray) &)]]

If you evaluate this, you'll notice that actually 6 arguments are passed to the colour function, but the last one is always zero. If we use the one-parameter form of ParametricPlot3D, then still six arguments will be passed, but now the last two are zero.

Reap[ParametricPlot3D[{Cos[u], Sin[u], 0}, {u, 0, 2 Pi},
ColorFunction -> ((Sow[{##}]; Gray) &)]]

Similarly, the colour function of ParametricPlot receives 5 arguments instead of 4.

What is the meaning of the last argument passed to the ColorFunction of these functions? Does it have any use? Does ParametricPlot3D have a form where the last argument of the colour function will not be zero and has an application?

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I just tested with RegionPlot3D and plain Plot: Both also add an extra 0. argument. – celtschk May 16 '12 at 12:20
"Reserved for future use"? – Sjoerd C. de Vries May 16 '12 at 12:52
@Sjoerd Well, they could always just add one more argument ... – Szabolcs May 21 '12 at 14:58
Interesting -- this explians why a color function defined per the ColorFunction docs with 5 arguments doesn't work!! mycolor[x_, y_, z_, u_, v_] := Red; ParametricPlot3D[{Cos[u] Cos[v], Sin[u] Cos[v], Sin[v]}, {u, 0, 2 Pi}, {v, -Pi/2, Pi/2}, ColorFunction -> (mycolor[##] &)] produces an 'uncolored' figure.. Apologies I cant see how to post a comment vs an 'answer' here.. – george2079 Aug 20 '12 at 22:07