I've been doing various things with conic-swept prisms, and I've been
getting some very unexpected results. I tried reducing my problem to a
simpler scene file, and though it did not come up with exactly the same
unexpected results as before, it's still not working as I expected. Here is
a simple example. With a normal perspective camera, it is a nice square
pyramid. With a directly overhead orthogonal view, however, the whole thing
appears green. Though I am not sure, from my previous experimentation I
believe that only the base is appearing, and not the walls of the pyramid,
similar to results one receives when using back-face culling with reversed
normals.

//camera {location <5, 7, 2> look_at <1,0,0>}
camera {orthographic location <0, 10, 0> look_at <0, 0, 0> up x*3 right z*4}
light_source {<5,5,5> rgb 1}

difference {
    prism {
        conic_sweep
        linear_spline
        0, 1, 5,
        <-1, 1>, <1, 1>, <1, -1>, <-1, -1>, <-1, 1>
        scale <1, -1, 1>
        translate y*1
    }

    pigment {
        gradient y

        color_map {
        [0.0 color <0, 1, 0> ]
        [1 color <0, 0, 1> ]
        }
        scale y*.2
    }
    finish {ambient .5}
}

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Oops, don't ask why I put it in a difference, I was doing a more complicated
example before. Now that part can be deleted.

"clum" <nomail@nomail> wrote:
> I've been doing various things with conic-swept prisms, and I've been
> getting some very unexpected results. I tried reducing my problem to a
> simpler scene file, and though it did not come up with exactly the same
> unexpected results as before, it's still not working as I expected. Here is
> a simple example. With a normal perspective camera, it is a nice square
> pyramid. With a directly overhead orthogonal view, however, the whole thing
> appears green. Though I am not sure, from my previous experimentation I
> believe that only the base is appearing, and not the walls of the pyramid,
> similar to results one receives when using back-face culling with reversed
> normals.
>
> //camera {location <5, 7, 2> look_at <1,0,0>}
> camera {orthographic location <0, 10, 0> look_at <0, 0, 0> up x*3 right z*4}
> light_source {<5,5,5> rgb 1}
>
> difference {
>     prism {
>         conic_sweep
>         linear_spline
>         0, 1, 5,
>         <-1, 1>, <1, 1>, <1, -1>, <-1, -1>, <-1, 1>
>         scale <1, -1, 1>
>         translate y*1
>     }
>
>     pigment {
>         gradient y
>
>         color_map {
>         [0.0 color <0, 1, 0> ]
>         [1 color <0, 0, 1> ]
>         }
>         scale y*.2
>     }
>     finish {ambient .5}
> }

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"clum" <nomail@nomail> wrote in message 
news:web.44eea9b5cc351d989bc721f00@news.povray.org...
> Oops, don't ask why I put it in a difference, I was doing a more 
> complicated
> example before. Now that part can be deleted.
>
> "clum" <nomail@nomail> wrote:
>> I've been doing various things with conic-swept prisms, and I've been
>> getting some very unexpected results. I tried reducing my problem to a
>> simpler scene file, and though it did not come up with exactly the same
>> unexpected results as before, it's still not working as I expected. Here 
>> is
>> a simple example. With a normal perspective camera, it is a nice square
>> pyramid. With a directly overhead orthogonal view, however, the whole 
>> thing
>> appears green. Though I am not sure, from my previous experimentation I
>> believe that only the base is appearing, and not the walls of the 
>> pyramid,
>> similar to results one receives when using back-face culling with 
>> reversed
>> normals.
>>
>> //camera {location <5, 7, 2> look_at <1,0,0>}
>> camera {orthographic location <0, 10, 0> look_at <0, 0, 0> up x*3 right 
>> z*4}
>> light_source {<5,5,5> rgb 1}
>>
>> difference {
>>     prism {
>>         conic_sweep
>>         linear_spline
>>         0, 1, 5,
>>         <-1, 1>, <1, 1>, <1, -1>, <-1, -1>, <-1, 1>
>>         scale <1, -1, 1>
>>         translate y*1
>>     }
>>
>>     pigment {
>>         gradient y
>>
>>         color_map {
>>         [0.0 color <0, 1, 0> ]
>>         [1 color <0, 0, 1> ]
>>         }
>>         scale y*.2
>>     }
>>     finish {ambient .5}
>> }
>
>
Hi Clum,

If you change the camera to
  camera {orthographic location <0.0000001, 100, 0> look_at <0, 0, 0> up x*3 
right z*4}
it should fix this particular problem (possibly not some of the others you 
elude to).

I can't recall the exact reason for this problem, but it's to do with having 
the camera directly over the look_at point, which I think used to raise an 
error in earlier versions of POV-Ray. I think it's something to do with 
divide-by-zero errors that arise quite a lot when calculating for that 
particular condition.

Regards,
Chris B.

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Thanks, that fixed all of my problems. I was trying to make an overhead 2-d
"map" of my scene, and all of the prisms weren't rendering correctly, but
now they are.

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