"Kenneth" <kdw### [at] earthlinknet> wrote in message 
news:web.4bbf648570f2d07d65f302820@news.povray.org...
> Warp <war### [at] tagpovrayorg> wrote:
>> Dre <and### [at] gmailcom> wrote:
>> > Yet this does:
>>
>> > #declare norm = <0, 0, 0>;
>> > #declare startPoint =<xPos, 10, zPos>;
>> > #declare endPoint = <xPos, -10, zPos>;
>> > trace(object, startPoint, endPoint, norm)
>>
>>   I don't think it does (well, not in the way you want). The third 
>> parameter
>> of trace() specifies a direction, not an endpoint.
>>
>>   Probably you want the third parameter to be <0, -1, 0> because that's
>> the direction you are shooting the rays towards.
>>
>
> Warp's correct. (Also, I would assume that <0,-10,0> has the same 
> directional
> effect as <0,-1,0> )
>
> There's a big difference between <xpos,-10,zPos> and <0,-10,0> as a 
> *direction*
> vector. Let's say xPos=7 and zPos=5.  That produces a *direction* of
> <7,-10,5>--which certainly isn't straight down. The trace rays are 
> probably not
> even hitting the HF (and your placed object(s) are ending up at a <0,0,0>
> location by default, unless you're using Norm to 'stop' that in some way, 
> which
> I assume you are.)
>
> As has been mentioned:
> * The trace ray 'start' location needs to be somewhere *above* the
> HF...obviously  ;-)  Personally, I would use <xPos,1000,zPos> to be safe.
> * the 'start' vector is an actual location point; the trace ray direction 
> is a
> *directional* vector, NOT the location of a point, and shouldn't normally
> change--at least when tracing a HF.
>
> Ken
>
Thanks very much for the clarification.  That clears things up completely :)

I was getting the third parameter wrong, I thought it was an end point 
rather than a direction vector, I mentioned this in my reply to warp.

So much POV to learn, so little time!

Cheers Dre

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