Am 23.07.2012 22:52, schrieb Chaanakya:
> clipka <ano### [at] anonymousorg> wrote:
>> Am 23.07.2012 21:58, schrieb Chaanakya:
>>> "Chaanakya" <nomail@nomail> wrote:
>>>> Hey guys!  I just had a quick question...
>>>>
>>>> I was trying to generate the graph of this function (in Cartesian coordinates):
>>>>
>>>> -z + 1.00003 = (3x^2 + 3y^2)/(200000)
>>>>
>>>> However, when I render the following code, nothing shows up except for the
>>>> plane:
>> ...
>>>> isosurface {
>>>>     function {
>>>>       -y - ((3*pow(x,2) + 3*pow(z,2))/(200000)) + 1.00003
>>>>     }
>>>>     // contained_by { box { -2,2 } }
>>>>     pigment {
>>>>       color Red
>>>>     }
>>>> }
>> ...
>>> Even more strangely, when I use the equally valid function
>>>
>>> function {
>>>     y + 3*pow(x,2)/200000 + 3*pow(z,2)/200000 - 1.00003
>>> }
>>>
>>> I get a cube.  I think there's something fundamental about isosurfaces that I'm
>>> not understanding?  That is, how should I convert the function z = -3x^2/200000
>>> - 3y^2/200000 + 1.00003 into an isosurface?
>>
>> Do un-comment the "contained_by" line!
>>
>> At x=0,z=0 you have y = 1.00003, which is outside the default
>> contained_by object (box{1,1}), and even at the maximum x and z
>> (x=1,z=1) you have y = 1.00000, which just barely touches the box.
>>
>> I.e. the surface you defined is (for practical purposes) all outside
>> default container; so the inside of the default container is either
>> completely outside the isosurface (first version) so that you don't see
>> anything, or completely inside (second version) so that you simply see
>> the container's shape.
>
> I'm trying to figure out exactly what container I should use - if I use box {
> <-1,1,-1>,<1,1.00003,1> } I get nothing (I understand why).  If I use box {
> <-1,0.9,-1>,<1,1.00003,1> } I get a box.  How do I get the elliptic paraboloid
> to show up?

At the current dimensions, the curvature of the isosurface is simply too 
small to be noticeable

You'll need to use a much larger bounding container (and move back the 
camera a whole lot) to see anything. Something like 
contained_by{box{-20000,20000}}.

(BTW your elliptic paraboloid seems pretty circular to me.)

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