Hi,

I need to compare POV images with some photos I took. The photo camera view
angles were 31.5 x 24.25 degrees. I would ask:

- perspective is the most appropriate camera type?

- in my .pov file I wrote:
  camera
{location  <0,3,0>         // i used scale 1:1 and the height of
                           // photocamera was 3 meters

angle 31.25

look_at   <0,0,0>}

and i noticed if i double camera height, halving camera angle, the generated
images are different among them. Why? Should I use up and right vectors? I used
camera location because i need to generate different images rotating the camera
location around the origin.
Thank you.

Francesco

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Francesco napsal(a):
> Hi,
> 
> I need to compare POV images with some photos I took. The photo camera view
> angles were 31.5 x 24.25 degrees. I would ask:
> 
> - perspective is the most appropriate camera type?
> 
> - in my .pov file I wrote:
>   camera
> {location  <0,3,0>         // i used scale 1:1 and the height of
>                            // photocamera was 3 meters
> 
> angle 31.25
> 
> look_at   <0,0,0>}
> 
> and i noticed if i double camera height, halving camera angle, the generated
> images are different among them. Why? Should I use up and right vectors? I used
> camera location because i need to generate different images rotating the camera
> location around the origin.
> Thank you.
> 
> Francesco
> 
> 
1) If you move the viewport you can not get the same view simply because 
   of the parallax.

2)if you only consider the ground plane - if you want this to be the 
same you need to half tan(angle/2) instead of the angle itself.
In the case of your angle you should use 15.92 degrees instead of 15.62.

You can use right/up instead and you may actually find it easier.
-- 
the ultimate time-killer:
+a0.0 +am2 +r9

Johnny D

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> 1) If you move the viewport you can not get the same view simply because
>    of the parallax.

what is the viewport? Cannot I set the POV camera the same as the photo camera?


> 2)if you only consider the ground plane - if you want this to be the
> same you need to half tan(angle/2) instead of the angle itself.
> In the case of your angle you should use 15.92 degrees instead of 15.62.

Thank you very much. The question is similar to the previous one: which setting
is the most appropriate to replicate my photo camera?

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Francesco napsal(a):
>> 1) If you move the viewport you can not get the same view simply because
>>    of the parallax.
> 
> what is the viewport? Cannot I set the POV camera the same as the photo camera?
> 
I mean the point where the camera rays are shot from. This roughly 
corresponds to the camera lens.

You can and probably should.
Add a sky vector when looking straight up or down, before the look_at 
vector. Otherwise Pov-ray does not know which way to roll the camera.

-- 
You know you've been raytracing too long when...
you start thinking up your own "You know you've been raytracing too long 
when..." sigs (I did).
-Johnny D

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What you need to do is set the size of the
output image to match the ratio of your photos,
then set the camera ratio with the right statement.

#local AX = 31.5;
#local AY = 24.25;
camera {
   location ...
   right x*(AX/AY)
   angle AX
   look_at ...
}

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Francesco <nas### [at] gmailcom> wrote:
> and i noticed if i double camera height, halving camera angle, the generated
> images are different among them.

  If you walk twice as far from the target and zoom twice as much and take
a photo, the resulting photo is different. I think that should be obvious.

-- 
                                                          - Warp

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"Tim Attwood" <tim### [at] comcastnet> wrote:
> #local AX = 31.5;
> #local AY = 24.25;
> camera {
>    location ...
>    right x*(AX/AY)
>    angle AX
>    look_at ...
> }


Thank you Tim.  I try to explain me better because I have still a doubt. The
photo camera angles were 31.5 x 24.25 and i placed it at 3 m height
(i took photos looking at the soil). Initially, in order to reproduce pov
images similar to the photos, i placed camera at 3 m from ground (<0,3,0> in
scale 1:1) setting camera angle=31.5. But i noticed i have better results if i
place camera at 15.6 m from ground setting camera angle=6 (i did not use
tan(angle), so I changed proportionally the two values). I would understand why
it happens (ok, it could also be due to my model, if it is not good it may works
better with different parameters from those of the photo camera).
camera).

Francesco

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> Thank you Tim.  I try to explain me better because I have still a doubt. 
> The
> photo camera angles were 31.5 x 24.25 and i placed it at 3 m height
> (i took photos looking at the soil). Initially, in order to reproduce pov
> images similar to the photos, i placed camera at 3 m from ground (<0,3,0> 
> in
> scale 1:1) setting camera angle=31.5. But i noticed i have better results 
> if i
> place camera at 15.6 m from ground setting camera angle=6 (i did not use
> tan(angle), so I changed proportionally the two values). I would 
> understand why
> it happens (ok, it could also be due to my model, if it is not good it may 
> works
> better with different parameters from those of the photo camera).
> camera).

POV acts like an ideal pinhole camera, at smaller angles
the perspective effect is reduced because the rays are closer
to being parallel. A RL camera uses lenses that correct for
perspective distortion somewhat, so you can only roughly
match the perspective distortion in some photo. If you just
want a flat picture with no distortion, use an orthographic
camera in POV.

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"Tim Attwood" <tim### [at] comcastnet> wrote:
> What you need to do is set the size of the
> output image to match the ratio of your photos,
> then set the camera ratio with the right statement.
>
> #local AX = 31.5;
> #local AY = 24.25;
> camera {
>    location ...
>    right x*(AX/AY)
>    angle AX
>    look_at ...
> }

If i use these parameters the distorsion is similar to that of the photo camera
or I need to know some other lens characteristic?

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> If i use these parameters the distorsion is similar to that of the photo 
> camera
> or I need to know some other lens characteristic?

With those parameters the POV camera would be
at about the same location in a virtual environment
as the real camera is in the matching real environment
in order to capture the same general image.
The distortion might be slightly different than the
RL camera.

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