Bob Hughes <inv### [at] aolcom> wrote:
: You used the 'fisheye' lens? Or not...?

  Or better the 'ultra_wide_angle'.
  An example of distortion made with the ultra_wide_angle camera can be
found at http://iki.fi/warp/pics/logo5.jpg

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

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Bob Hughes wrote in message <36C7FB77.D3C4B4A7@aol.com>...
>You used the 'fisheye' lens? Or not...?


Not.

#declare main_camera= camera {
   location  <0, 0, -1*pogsize.z>
   direction <0, 0,  0.4>
   up        <0, 1,  0>
   right   <4/3, 0,  0>
   look_at   <0, 0, 0>
   translate y*0.5
   }

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Nieminen Mika wrote in message <36c81e65.0@news.povray.org>...
>Bob Hughes <inv### [at] aolcom> wrote:
>: You used the 'fisheye' lens? Or not...?
>
>  Or better the 'ultra_wide_angle'.
>  An example of distortion made with the ultra_wide_angle camera can be
>found at http://iki.fi/warp/pics/logo5.jpg


I don't want to duplicate the effect.  I just wonder why the regular
perspective lens doesn't do that.  I thought that effect, which we se in a
camera, is an artifact of the wide perspective.

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John M. Dlugosz wrote:
> 
> Nieminen Mika wrote in message <36c81e65.0@news.povray.org>...
> >Bob Hughes <inv### [at] aolcom> wrote:
> >: You used the 'fisheye' lens? Or not...?
> >
> >  Or better the 'ultra_wide_angle'.
> >  An example of distortion made with the ultra_wide_angle camera can be
> >found at http://iki.fi/warp/pics/logo5.jpg
> 
> I don't want to duplicate the effect.  I just wonder why the regular
> perspective lens doesn't do that.  I thought that effect, which we se in a
> camera, is an artifact of the wide perspective.

I have found that the radial distortion you would expect often times
manifests itself more often with camera positions close to objects
and seems to deminish with distance. An example would be a tall object
nearly off screen and near to the camera while the main focus of the
scene is further back. You can really exagerate this (to me unwanted)
effect by adding a little tilt to the camera. Like placing it at y*5
and have your look_at set to y*1.

-- 
Ken Tyler

mailto://tylereng@pacbell.net

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Ken wrote in message <36C8CD3B.FF291F00@pacbell.net>...
>I have found that the radial distortion you would expect often times
>manifests itself more often with camera positions close to objects
>and seems to deminish with distance. An example would be a tall object
>nearly off screen and near to the camera while the main focus of the
>scene is further back. You can really exagerate this (to me unwanted)
>effect by adding a little tilt to the camera. Like placing it at y*5
>and have your look_at set to y*1.


I also find it an unwanted effect for my current effort, and upon studying
the original woodcut I found that the back end appears 1/3 the size of the
front end, and a little geometry told me that the camera needed to be half
that depth away from the front face to get that effect.  Then I "zoomed out"
(reduce the camera length vector) to get the whole scene into view.

When planning it, I thought I might have distortions and was contemplating
ways of dealing with it.  But lo and behold, there is =zero= distortion.  So
my understanding of what causes wide-angle distortion is flawed.

When I get unwanted perspective effects, I get the results I expect by
backing away and zooming in.  So some of it anyway agrees with my mental
model and real-world camera experience.

--John

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John M. Dlugosz heeft geschreven in bericht <36c7c155.0@news.povray.org>...
>A photograph using a wide angle lens will show distortions, such that
>buildings, which are vertical, appear bent.


In photography, with good quality lenses and with the film-plane and
object-plane parallel to each other there should be no distortions of the
kind you discribe. As soon as these planes are not parallel, the perspective
changes. Buildings start falling over. ( You can shift, tilt and rotate the
lens-plane any wat you want, it has no influence on the perspective).

When using fish-eye and super wide angle lenses, you wil have bend lines in
your picture. These are not perspective distortions, it is a result of the
central projection. If you watch the picture from the right distance the
perspective will appear normal. The normal viewing distance is the
focaldistance of the lens multiplied by the enlargement of the negative.
This goes for all pictures, not only for wide angle ones.

To prevent building from falling over, instead of tilting the camera, one
should keep the planes parallel and translate the lensplane up. This can be
done with fieldcameras or shift-objectives. In POV it can be done by
shearing the camera.

ingo

--
Met dank aan de muze met het glazen oog.

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I see someone knows a bit of photography here.
I hadn't tried the "shear" you mention (does that work? I'll find out),
in fact I hadn't tried the offset parallel lens -- ie. create lens
object and translate off center of the camera look_at -- kind of thing
yet either. Will have to take a look and see how both these methods look
in POV-Ray. Aside from doing decimal direction perspective before like
John M. D., I haven't done too much camera play since most times I keep
to a usual look in a scene anyhow.


ingo wrote:
> 
> To prevent building from falling over, instead of tilting the camera, one
> should keep the planes parallel and translate the lensplane up. This can be
> done with fieldcameras or shift-objectives. In POV it can be done by
> shearing the camera.
> 
> ingo
> 
> --
> Met dank aan de muze met het glazen oog.

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/POVring.htm
 mailto:inv### [at] aolcom?PoV

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Bob Hughes heeft geschreven in bericht <36CC952C.90EB54DE@aol.com>...
>I see someone knows a bit of photography here.
>I hadn't tried the "shear" you mention (does that work? I'll find out),
>in fact I hadn't tried the offset parallel lens -- ie. create lens
>object and translate off center of the camera look_at -- kind of thing
>yet either. Will have to take a look and see how both these methods look
>in POV-Ray. Aside from doing decimal direction perspective before like
>John M. D., I haven't done too much camera play since most times I keep
>to a usual look in a scene anyhow.
>

An example of camera shearing for perpective correction. Put -uv in the
commandline when transforming the camera.

light_source {
  <500, 500,-500>
  color rgb <1, 1, 1>
}

#declare Shear= transform {
   matrix <  1,  0,  0,
                   0,  1,  0.3333,
                   0,  0,  1,
                   0,  0,  0 >
}

camera {
  location  <0.0, 0.01,-1.5>
  look_at   <0.0, 0.50, 0.0>
  angle 95
   //    transform Shear
}

plane

  y, 0
  texture {
    pigment {
        checker color rgb <1, 1, 1> color rgb <0, 0, 1>
        scale 0.25
    }
  }
}
box {
  <-0.5,0,-0.5>,<0.5,1,0.5>
  pigment {rgb 1}
}
box {
  <-0.5,0,-0.5>,<0.5,1,0.5>
  scale 0.5
  translate <0.75,0,-0.5>
  pigment {rgb 1}
}


--
Met dank aan de muze met het glazen oog.

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Thanks, but now I don't get to struggle with that alone, darn it all ;)


ingo wrote:
> 
> An example of camera shearing for perpective correction. Put -uv in the
> commandline when transforming the camera.
> 
> light_source {
>   <500, 500,-500>
>   color rgb <1, 1, 1>
> }
> 
> #declare Shear= transform {
>    matrix <  1,  0,  0,
>                    0,  1,  0.3333,
>                    0,  0,  1,
>                    0,  0,  0 >
> }
> 
> camera {
>   location  <0.0, 0.01,-1.5>
>   look_at   <0.0, 0.50, 0.0>
>   angle 95
>    //    transform Shear
> }
> 
> plane
> 
>   y, 0
>   texture {
>     pigment {
>         checker color rgb <1, 1, 1> color rgb <0, 0, 1>
>         scale 0.25
>     }
>   }
> }
> box {
>   <-0.5,0,-0.5>,<0.5,1,0.5>
>   pigment {rgb 1}
> }
> box {
>   <-0.5,0,-0.5>,<0.5,1,0.5>
>   scale 0.5
>   translate <0.75,0,-0.5>
>   pigment {rgb 1}
> }
> 
> --
> Met dank aan de muze met het glazen oog.

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/POVring.htm
 mailto:inv### [at] aolcom?PoV

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>An example of camera shearing for perpective correction. Put -uv in the
>commandline when transforming the camera.



Some additional info on camera shearing.

camera

location <0,0,-3>
look_at <0,0,0>
matrix < 1, 0, A
               0, 1, B
              C, D, 1
               0, 0, 0 >
}
box { -0.5,0.5 }

The matrix should be last in the camera statement.
Use -uv in the commandline

A: The effect is as if you are rotating the filmplane around the Y-axis
(pov). With +A the right side of the film wil be closer to the box and wil
be seen much bigger than the left side. You will see only the front of the
box, not the sides.

B: As if rotating around the X-axis. Same situation as with A but the top or
bottom wil be enlarged.

C: Here the lensplane and film plane are translated horizontal to opposite
sides. You will see a perfect scare front plane of the cube and also one of
its sides. (this one in combination with A is perfect for advertising,
stretching the limo even further).

D: Here the two planes are translated vertical and in opposite direction.
This makes it possible to see the front and top or bottom of the cube. Again
the front is a perfect square. (C+D is perfect for detergent ads, a perfect
rectangular frontside and you can also see the top and right side of the
package).


ingo

--
Met dank aan de muze met het glazen oog.

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