You can say "set isosamples 30,40" now to get different isosampling
rates for the first and second dummy variables (x,u resp. y,v). This
will affect surface-plots in hidden3d- and nohidden3d-mode. It gets
rid of the necessity of having to change samples in hidden3d-mode when
you actually want to change the isosamples for the first dummy
variable. For people that are lazy and for compatibility you can also
type "set isosamples 30". This will set the value to 30 in both
directions. For completeness I did the same with "samples". Thus you
can say "set samples 50,100" to get different sampling rates for each
dummy variable. This only affects surface-plots in nohidden3d-mode.
Again, you still can say "set samples 50". This will set both sampling
rates to 50. For 2d-plots the first value of samples is used.

Summary:

Suppose isosamples is set to 4,5 in hidden3d-mode. The grid evaluated
for hidden3d-plots will look like this (* denotes points that get
evaluated):

      |------ isosamples #1 -----|

  -   *--------*--------*--------*
  |   |        |        |        |
      |        |        |        |
  i   |        |        |        |
  s   *--------*--------*--------*
  o   |        |        |        |
  s   |        |        |        |
  a   |        |        |        |
  m   *--------*--------*--------*
  p   |        |        |        |
  .   |        |        |        |
      |        |        |        |
  #   *--------*--------*--------*
  2   |        |        |        |
      |        |        |        |
  |   |        |        |        |
  -   *--------*--------*--------*

Suppose isosamples is set to 4,5 and samples to 10, 9 in
nohidden3d-mode. The grid evaluated for hidden3d-plots will look like
this:

      |------ isosamples #1 -----|

  -   *--*--*--*--*--*--*--*--*--*   -
  |   |        |        |        |   |
      *        *        *        *   |
  i   |        |        |        |    
  s   *--*--*--*--*--*--*--*--*--*   s
  o   |        |        |        |   a
  s   *        *        *        *   m
  a   |        |        |        |   p
  m   *--*--*--*--*--*--*--*--*--*   l
  p   |        |        |        |   e
  .   *        *        *        *   s
      |        |        |        |    
  #   *--*--*--*--*--*--*--*--*--*   #
  2   |        |        |        |   2
      *        *        *        *    
  |   |        |        |        |   |
  -   *--*--*--*--*--*--*--*--*--*   -

      |------- samples #1 -------|


BTW: The examples above also show how the bottom grids will look like
both in noparametric- and in parametric-mode. Thus you can get a rough
idea of how the mapping from (u,v)-space to (x,y,z)-space works by
drawing a bottom grid in parametric-mode.

Best regards,

Carsten