"Kenneth" <kdw### [at] earthlinknet> wrote in message 
news:web.484abf7e784caf0d78dcad930@news.povray.org...
> "Chris B" <nom### [at] nomailcom> wrote:
>> "daviddoria" <nomail@nomail> wrote in message
>> news:web.4849a25ff5d589ac145cec320@news.povray.org...
>
>> #declare MyStart = <1, 1, 1>;
>> #declare MyDirection =  <0, 0, 0> - MyStart;
>
> The resulting MyDirection vector in this case would be <-1,-1,-1>...kind 
> of an
> odd direction in which to shoot a trace ray, *assuming* that the mesh 
> occupies
> some area on an x/z plane...i.e., horizontal. Far better would be to shoot 
> the
> trace ray straight 'down.'
>
> Let's assume that the mesh extends from <0,0,0> to <1,0,1>--like a POV-Ray
> height_field. Would it not be better to say
> #declare MyStart =
> <*some value between 0 and 1*,100,*some value between 0 and 1*>; // y is 
> just an
> // arbitrary large number, to place it well above the mesh
>
> #declare MyDirection = <0,-1,0>; // pointing straight down
>
> OR
> #declare MyDirection = <0,0,0> - <0, MyStart.y,0>; // using dot notation 
> to pick
> // one value out of a vector
>
> OR
> #declare MyDirection = <0, -MyStart.y,0>;
>
> These three MyDirection's all all equivalent...but <0,-1,0> is far easier 
> to
> write!
>
> Ken W.
>

Yes probably. I just adapted the example from the help. I assumed from his 
question that Dave would know the particular point that he wanted to shoot 
the ray from and the direction he wanted to shoot it in.

Regards,
Chris B.

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