POV-Ray : Newsgroups : povray.bugreports : sphere slicing problem : Re: sphere slicing problem Server Time
23 Oct 2021 23:46:55 EDT (-0400)
  Re: sphere slicing problem  
From: William F Pokorny
Date: 2 Nov 2019 03:38:10
Message: <5dbd3262$1@news.povray.org>
On 11/1/19 1:39 PM, William F Pokorny wrote:
> On 11/1/19 10:16 AM, jr wrote:
...
>>
>> I now think that if, perhaps, there is a relatively simple and 
>> accurate way of
>> calculating camera parameters for both camera types such that the 
>> object is seen
>> in exactly the same size/position, perhaps one could merge the 
>> results.  any
>> thought(s) welcome.
>>
> 
> Maybe... 
...

Woke up this morning with a thought. If you're willing to render each 
sample as three frames your idea might fly using two perspective cameras.

While in general using whatever you are using set up wise for best 
results:

Using a perspective camera down in the first frame of a sample; A 
perspective camera up in the second frame of a sample; In the last frame 
use the results of the of the first two sample frames for planar 
pigments which themselves would be used in a user defined pigment 
applied to a plane with no lights but an ambient 1 finish and an 
orthographic camera.

In frame 3/3 of each sample in the scan the user defined pigment would 
be something like:

#declare PigmentMaxUpDownPerspective = pigment {
     user_defined {
         function { max(FnUp(x,y,z).r,FnDown(x,y,z).r) },
         function { max(FnUp(x,y,z).g,FnDown(x,y,z).g) },
         function { max(FnUp(x,y,z).b,FnDown(x,y,z).b) },
         ,
     }
}

There is planar-position distortion due the perspective camera. You 
would want to keep the angle small(1) to limit this. Further for each 
sample the up / down cameras should be equal distance to the middle of 
each sample's slice. In other words, the camera positions should be 
based upon the middle of each sample slice.

Aside: A user defined perspective camera simultaneously up and down an 
option too - say alternating rows at twice the vertical height - could 
be rendered in one frame. You'd have to be willing to scale down 
vertically only with some post render program/process.

Bill P.

(1) - Small perspective camera angles will tend to work against you in 
being more parallel - more like the orthographic - some balancing 
required. Small perspective camera angles also require the camera be 
further away. So, I'll also mention there are accuracy issues especially
with higher order polynomial shapes when the camera rays start far away.
The accuracy issue is improved if you use my updated solver branch, but 
still significantly present. The sphere is essentially order 2, so with 
it your OK accuracy wise even at large distances away for the camera.


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