"Philippe-H Cadet" <del### [at] videotronca> wrote in message
news:3a41ac3d@news.povray.org...

> As I can see Povray is not vertex based, anyways I goin to try avoid this
> metal illusion
> that a shape or 3d object can have a middle.
>

It's a very common beginners misconception. I suspect because beginners
start with the simple shapes such as sphere etc. which can easily behave as
though they do have a centre (as well as having their position defined with
just one set of co-ordinates).

For me, it wasn't until I started working with blobs that I began to
understand what was going on.

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Tom Melly <tom### [at] tomandlucouk> wrote:
: It's a very common beginners misconception. I suspect because beginners
: start with the simple shapes such as sphere etc.

  This is probable the most common reason.
  It's easy to believe that a beginner (and even a not so beginner) could
think something like this:

  Ok, I make a
sphere
{ <1,2,3>, 1
  texture { Wood }
}
  This means that the center of the sphere is at <1,2,3> and its radius is 1.
  Now I'll rotate it around its center so that I get a rotating ball:

sphere
{ <1,2,3>, 1
  texture { Wood }
  rotate <0,45,0>
}

  Oops! What happened? Why it didn't rotate around its center but it moved
to a totally different place?

  The user has confused the center coordinate of the sphere with the
rotation axis center. It's quite natural to think that the center of the
sphere is the center of the rotation axis.
  However, when the user makes something like this, he will be quite
confused:

union
{ sphere { <-.5,0,0>, 1 }
  sphere { <1,0,.5>, 1 }
  texture { Wood }
  rotate <0,45,0>
}

  "Uh... What is the center of this object now? Is it <-.5,0,0>? Is it
<1,0,.5>? Is it somewhere in the middle? What does the rotation actually
do to this object?"

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Tom Melly <tom### [at] tomandlucouk> wrote:
: Warp's info is the most accurate, but possibly not the most comprehensible
: to a novice.

  What?! What?! That was just outrageous!

  ;)

: The best way, IMHO, to avoid muddles is to always create objects with a
: point that rests at <0,0,0>. It doesn't matter which point (e.g. the center
: of a sphere or its base), just as long as you know which point.

  It's good to know how to rotate around a certain point, which may not
necessarily be <0,0,0>.
  For example, it may be possible that the user wants to rotate the object
first around one point and then around another different point.

: Secondly, always scale, then rotate and finally translate (there are many
: exceptions to this, but you will generally recognise them as exceptions).

  It should be noted that it's not a mandatory rule to have one scale, one
rotate and one translate per object and in that order.
  It's perfectly possible to, for example, first scale, then rotate, than
scale again, then translate and then rotate again. There's practically no
limit in the amount and combinations of transformations.
  Of course it requires experience to handle a complex combination of
transformations.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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"Warp" <war### [at] tagpovrayorg> wrote in message
news:3a41f228@news.povray.org...
>
> union
> { sphere { <-.5,0,0>, 1 }
>   sphere { <1,0,.5>, 1 }
>   texture { Wood }
>   rotate <0,45,0>
> }
>
>   "Uh... What is the center of this object now? Is it <-.5,0,0>? Is it
> <1,0,.5>? Is it somewhere in the middle? What does the rotation actually
> do to this object?"

Exactly - it was running into this kind of issue with blobs that I realised
a) pov didn't work how I thought it did and b) the notion of objects having
a "center" was wrong.

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