Ok, since my previous silly question was about piecewise functions 
rather than #switch per se (that is, case was supposed to be used to 
make function piecewise), I think I'll publish a solution for initial 
problem here, although it's not directly related to #switch.

Initially, I wanted piecewise linear function, as most simple to edit. 
Resulting solution looks somewhat like this:

#declare scl = function(c, lin, hin, lout, hout) {
     (c-lin)/(hin-lin) * (hout-lout) + lout
   }  // Linear rescale function from lin..hin to lout..hout range

#declare map_4 = function(c) {      // Piecewise rescaling example start
   (c <= 0.7) * scl(c, 0, 0.7, 0,1)  // rescale 0-0.7 to 0-1
   + (c > 0.7 & c <= 0.9) * scl(c, 0.7, 0.9, 1, 0.5)  // rescale 0.7-0.9 
to 1-0.5
   + (c > 0.9) * scl(c, 0.9, 1, 0.5, 1)  // rescale 0.9-1 to 0.5-1
   }  // Piecewise example end

yes, I managed to overcome my laziness and pack rescaling into separate 
scl function with simplified syntaxis.

But later I realized (yes, good ideas always come to me later) that 
piecewise interpolation is somewhere in POVRay already. For example, in 
splines. After a long attempts to understand what's written in manual I 
came up with the following:

#declare interpol = function {
   spline {
     linear_spline
     0.0, <0.0,0,0>
     0.7, <1.0,0,0>
     0.9, <0.5,0,0>
     1.0, <1.0,0,0>
     }
   }

#declare map_5 = function(c) {interpol(c).u}

and well, map_5 seem to give the same result as map_4 above, while being 
much easier to edit.

Surely, all this is too simple and obvious for people here, but well, at 
least next time I need interpolation, I can find it in this NG archives :-)

-- 
Ilyich the Toad
https://dnyarri.github.io/

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