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  Add a non-origin sphere example to Scale (Message 1 to 4 of 4)  
From: Phil Cook
Subject: Add a non-origin sphere example to Scale
Date: 26 Apr 2004 16:29:49
Message: <opr62n2cc2p4ukzs@news.povray.org>
Someone else has just been caught out by 'moving' objects when using 
scale, I was wondering if adding a non-origin sphere to the example would 
highlight this, perhaps:

Scale also alters the vector components of an object

For example

  sphere { <1,1,1>, 1
   scale 5> // Evaluates as <5,5,5> so uniformly scale
       // by 5 in every direction.
       // but also multiplies the vector location
	// resulting in a translation to <5,5,5>
  }

--
Phil Cook

-- 
All thoughts and comments are my own unless otherwise stated and I am 
happy to be proven wrong.


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From: Christoph Hormann
Subject: Re: Add a non-origin sphere example to Scale
Date: 26 Apr 2004 16:55:01
Message: <c6jeqs$7ss$1@chho.imagico.de>
Phil Cook wrote:
> Someone else has just been caught out by 'moving' objects when using 
> scale, I was wondering if adding a non-origin sphere to the example 
> would highlight this, perhaps:
> 
> Scale also alters the vector components of an object
> 
> For example
> 
>  sphere { <1,1,1>, 1
>   scale 5> // Evaluates as <5,5,5> so uniformly scale
>       // by 5 in every direction.
>       // but also multiplies the vector location
>     // resulting in a translation to <5,5,5>
>  }

See "2.2.7.2 Transformation Order" - without such a complete example but 
well explained IMO.

Apart from that your explanation is wrong.  A transform does not 
transform the radius and center of the sphere but the whole object.  In 
fact POV-Ray does not even transform the object but transforms the ray 
with the inverse transform - otherwise a lot of shapes could not be 
transformed arbitrarily at all.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 21 Mar. 2004 _____./\/^>_*_<^\/\.______


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From: Warp
Subject: Re: Add a non-origin sphere example to Scale
Date: 26 Apr 2004 18:47:34
Message: <408d5946@news.povray.org>
Christoph Hormann <chr### [at] gmxde> wrote:
> Apart from that your explanation is wrong.  A transform does not 
> transform the radius and center of the sphere but the whole object.

  Actually that's not completely true. POV-Ray performs some optimizations
in transformations in order to avoid making a transformation matrix.
If you eg. create a sphere and translate it, POV-Ray will change its
center.

  It's better to explain transformations in a more abstract level,
without going too deep into details. From the point of view of the
user it doesn't really matter which way transformations are applied
(ie. by modifying the object parameters, if possible, or by creating
a transformation matrix).

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -


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From: Phil Cook
Subject: Re: Add a non-origin sphere example to Scale
Date: 27 Apr 2004 09:04:48
Message: <opr63x4nh2p4ukzs@news.povray.org>
On Mon, 26 Apr 2004 18:53:48 +0200, Christoph Hormann 
<chr### [at] gmxde> wrote:

> Phil Cook wrote:
>> Someone else has just been caught out by 'moving' objects when using 
>> scale, I was wondering if adding a non-origin sphere to the example 
>> would highlight this, perhaps:
>>
>> Scale also alters the vector components of an object
>>
>> For example
>>
>>  sphere { <1,1,1>, 1
>>   scale 5> // Evaluates as <5,5,5> so uniformly scale
>>       // by 5 in every direction.
>>       // but also multiplies the vector location
>>     // resulting in a translation to <5,5,5>
>>  }
>
> See "2.2.7.2 Transformation Order" - without such a complete example but 
> well explained IMO.
>
> Apart from that your explanation is wrong.  A transform does not 
> transform the radius and center of the sphere but the whole object.  In 
> fact POV-Ray does not even transform the object but transforms the ray 
> with the inverse transform - otherwise a lot of shapes could not be 
> transformed arbitrarily at all.
>

The only transformation order I can see in the v3.5 help file is at 6.3.2 
which states:

"Similarly scaling after translation also moves an object unexpectedly. If 
you scale after you translate the scale will multiply the translate 
amount.
For example

   translate <5, 6, 7>
   scale 4

will translate to <20,24,28> instead of <5,6,7>. Be careful when 
transforming to get the order correct for your purposes."

But doesn't explicitly state what happens if you create an object at a 
non-origin point, if you take the position that doing so is the equivalent 
of creating an origin object then translating it then this makes sense, 
but it's not obvious.

If someone translated an object and then rotated it and complained that 
it's 'moved' you can tell them to look up rotate in the help file and read 
what is there; but you can't look up scale and get a similar answer. I 
just thought it would be better to keep this information together in an 
easier-to-find place as is done with rotate.

As for my explanation, if you want to change it to 'apparent' 
translation/transform or whatever, fine; but as Warp pointed out from the 
users perspective who cares, to them the object has just arbitrarily moved 
without an explanation under scale as to why.

--
Phil Cook

-- 
All thoughts and comments are my own unless otherwise stated and I am 
happy to be proven wrong.


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