Chambers <bdc### [at] yahoocom> wrote:
> "Eamon Caddigan" <eca### [at] uiucedu> wrote in message
> news:40686aa5$1@news.povray.org...
>> In scenes rendered with a negative camera direction, the scene appears
>> flipped about the y-axis. Using the pinhole camera model, however, it
>> would seem that the image should be flipped about both the x- and
>> y-axes. Am I missing something important?
>>
>> -Eamon
> 
> How did you give the camera a "negative" direction?  The answer will
> determine how to flip the axis.

One scene has the following camera definition:
camera { 
  location <0.0000, 0.0000, -2.0000>
  look_at <0.0000, 0.0000, -0.0000>
  up <0.0000, 6.0000, 0.0000>
  right <8.0000, 0.0000, 0.0000>
  direction <-0.0000, -0.0000, -4.0000> 
}

If the direction vector begins at the location point, it's pointing
"away" from the look_at point. Maybe the problem is my comprehension of
POV-Ray's camera model.

> That being said, try putting "up -y" in the camera statement.

Changing up vector in the example above to <0.0000, -6.0000, 0.0000>
simply flips the image about the y-axis again, resulting in the same
image obtained using a positive camera direction. I also tried flipping
the right vector, and both vectors, but nothing gave the results I was
expecting. I even tried adding a sky vector and rearranging the order of
the camera options, but that didn't do anything either.

I'll be the first to admit that placing the image plane /behind/ the
view point is an odd thing to do, and I'm willing to accept that POV-Ray
just won't behave exactly like a pinhole camera (mirroring the image
about both axes) in this case. I just wanted to make sure that I wasn't
missing anything before I gave up.

Thanks for the reply,
Eamon

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