On Sun, 9 Sep 2001 12:52:46 +0100, Mike Williams wrote:
>Most of the time 3.5b1 is a little faster, but occasionally, for some
>parametric isosurfaces, 3.5b1 is *phenomenally* slower.

Actually, it's probably phenomenally slower for most parametrics.  I would
be happy to correspond with you as to the details of why, if you think you
might have some suggestions that would help improve things.  We're lucky
it's even working at all in 3.5b1.

>a few holes, so I guess it needs a slightly higher max_gradient and will
>take even longer to render. (Evaluate isn't available for parametric
>isosurfaces, and I don't know how to defferentiate the Steiner Cross
>Cup, so I'm having to find a reasonable max_gradient by trial and
>error.)

Actually, what is called "max_gradient" for parametrics is the maximum of
the six partial derivatives dx/du, dy/du, dz/du, dx/dv, dy/dv, dz/dv.  
That might be easier to compute.  However, there is a bug in the current
implementation of parametric that leads to holes even if max_gradient is
big enough.  Making accuracy smaller helps with that, but also slows 
things down.

-- 
#local R=rgb 99;#local P=R-R;#local F=pigment{gradient x}box{0,1pigment{gradient
y pigment_map{[.5F pigment_map{[.3R][.3F color_map{[.15red 99][.15P]}rotate z*45
translate x]}]#local H=pigment{gradient y color_map{[.5P][.5R]}scale 1/3}[.5F
pigment_map{[.3R][.3H][.7H][.7R]}]}}}camera{location.5-3*z}//only my opinions

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