Wasn't it Rafal 'Raf256' Maj who wrote:
>Hi,
>I want to groove out (cut out) from isosurface shape (that reassembles an 
>box) a text (or other height-field-shape)

Use difference in exactly the same way that you would with an ordinary
box, except that you need to specify a suitable max_trace within the
isosurface.

difference {
  isosurface {
    function {F(x,y,z)}
    max_gradient 1.7
    contained_by{sphere{0,1}} open
    max_trace 3
  }
  text {
    ttf "crystal.ttf", "GROOVE",0.2,0
    rotate x*90
    translate <-1.5,0.1,-0.5>
    scale 0.3
  }
  pigment {rgb 1}
}

Hint: If you can't see your groove, try temporarily changing
"difference" into "union" so you can see which direction you have to
translate it in order to get it to intersect the correct part of your
isosurface.

It's probably not a good idea to try to make your text or heightfield
into an isosurface function if it has sharp edges since you're likely to
get infinite function gradients.

-- 
Mike Williams
Gentleman of Leisure

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