> I've just come across an unusual bit of behaviour when rendering a large
> number of similar isosurfaces.
> When I use the code:
>> #declare Index = 0;
>> #while (Index<29)
>> 	isosurface {
>> 		function {f_rounded_box(x, y, z, 0.1, 1.19, 0.3, 0.4)+f_granite(x+rand(Seed),
>> 		y+rand(Seed), z+rand(Seed))*0.09}
> I get the message:
>> Parse Error: Expected 'function identifier', } found instead
> However, when I use:
>> #declare Index = 0;
>> #while (Index<29)
>> #local RX=rand(Seed);
>> #local RY=rand(Seed);
>> #local RZ=rand(Seed);
>> 	isosurface {
>> 		function {f_rounded_box(x, y, z, 0.1, 1.19, 0.3, 0.4)+f_granite(x+RX,
>> 		y+RY, z+RZ)*0.09}
> Everything works as expected.
> What's happening? Surely Pov's only got to look up the next number in
> the random stream which can't be much different from looking up the
> values of the declared variables.
> 
> John (scratching his head in confusion)

There is a big difference:
When you trace an isosurface, you take several samples inside the 
container, each time evaluating the function. You must take several 
samples for each ray that enters the container untill you actualy find a 
surface.

In the first case, you'd need to fetch a new random value EACH time you 
sample the container to see if you've found the surface. This makes for 
several random values for each ray. This would makes the isosurface 
looks like a ball of dust if it was allowed. As you progress toward the 
surface, one sample detect that it's should be well ahead, then, the 
next sample detects that the surface that should still be ahead is now 
beheind. Most of the time, the surface can't be found and you pass right 
trough your object.

In the second case, you get a random value ONCE for each direction. The 
random value is constent everywhere, and shifts the surface in a random, 
but constant, direction.



Alain

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