Recently Bald Eagle has been playing with elliptical tori and he created 
a parametric one. I picked it up to play with the shape - but also to 
perhaps dig into why parametrics so slow.

Like many folks, initially grabbed Mike Williams's stuff and ran it - 
years ago. Quickly agreed parametrics are extremely slow.  I confess to 
20 years plus of almost completely ignoring the things...

Well turns out the parametric's max_gradient is not like the 
isosurface's max_gradient and the default of 1.0 looks to be a bad 
choice.  I've not run all the parametric scenes I have, but looks mostly 
you want to run parametrics with gradients well below 1.0. Something 
like 0.001 looks pretty good as a start. Only run into two very 
complicated parametrics needing max gradients at 1.0 or above. Most, a 
LOT smaller.

I've also set accuracy to 0.0005 below. The default 0.001 a little high 
for today's monitors. Expect further tuning possible. I only took a 
couple turns at them.

So in my mind parametric now use-ably slow, instead of never-use slow. :-)

Bill P.

---
Parametric scenes from Mike Williams's site on my 2 core i3.
-----------
povr2 <scene>.pov +a0.3 +am2 +r3  (800x600)

Param00.pov     46.082 ->  10.204 ---> -77.86%  (2.7s elapsed)
Param01.pov  * 411.856 ->  12.808 ---> -96.89%  (3.4s elapsed)
Param02.pov     19.163 ->  16.746 ---> -12.61%  (4.4s elapsed)
Param04.pov  1 515.145 -> 788.846 ---> +53.13%  (198s)
param05.pov    877.672 ->  91.962 ---> -89.52%  (23.6s)
param06.pov    268.503 ->  59.404 ---> -77.88%  (15.2s)
param07.pov    331.665 ->  71.517 ---> -78.44%  (18.2s)
param09.pov     37.754 ->  15.670 ---> -58.49%  (4.1s)
param10.pov     32.087 ->  20.735 ---> -35.38%  (5.4s)
param11.pov     84.101 ->  19.407 ---> -76.92%  (5.1s)
param11a.pov   426.047 ->  63.336 ---> -85.13%  (16.3s)

param12.pov  These last all param.inc examples.
param13.pov
param14.pov
param15.pov

(*) Had artifacts at shipped settings. Not at updated. Updates running 
with higher accuracy generally.

(1) See (*). Removed no_shadow modifier from both. Trying povr raw_wave 
with the granite pattern and no .grey so forced to v3.8. Not apples to 
apples plus color shifts. If drop back to default 0.001 accuracy better 
match original (shadow tolerance issues) at 460s cpu (-10%).

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