[XP and Fup2 povray.unofficial.patches, since this seems to be a problem
of megapov only]

On Wed, 30 Aug 2000 11:35:46 -0700, ryan constantine wrote:
>"Peter J. Holzer" wrote:
>> 
>> On Tue, 25 Jul 2000 15:30:39 -0700, ryan constantine wrote:
>> >what i really want is to use cylinder type 4,
>> >however the results are not what i expect.  what i expect to see is all
>> >four corners with straight rods conecting them instead of the curved
>> >ones from type 2.  all i get, however are four horizontal rods.  i don't
>> >see any of the sphere-corners.  changing the right and up vectors has no
>> >effect.
>> 
>> Strange. It works (almost) fine for me (megapov 0.5a on linux).
                                           ^^^^^^^^^^^^^^^^^^^^^
Oops. I obviously used povray 3.1g when I tried that (I tried both with
several camera statements, but it seems I missed some combinations).

>could you post your camera code?

camera {
  cylinder 4
  angle 360
  location <0, 10, 0>
  right <20, 0, 0>  
  up <0, 6, 0> 
  direction <0, 0, 2.5>
  look_at <100, 10, 100>
  rotate <0, clock, 0>
}

With megapov the picture looks very strange. If the camera points
directly to one of the vertical rods, the rod fills the whole width of
the picture and I can see a mirror image of the other rods in it. If it
points between vertical rods, I see only the horizontal rods. 

Looks like the camera is much too narrow. 

Looking at the source, I see that this block has been added in megapov:

      /* 1999 July 10 Bugfix - as per suggestion of Gerald K. Dobiasovsky 
           added this if(Precompute_Camera_Constants) block */
      /* Get aspect ratio, normalize camera vectors  */
      if (Precompute_Camera_Constants)
      {
        VLength(lx, FCR);
        VLength(ly, FCU);

        /* The inverse of the usual aspect ratio */
        Camera_Aspect_Ratio = ly / lx;

        VNormalize(FCR, FCR);
        VNormalize(FCU, FCU);
        VNormalize(FCD, FCD);  /* If not normalized in "angle"-statement in parse.c */
      }
      /* ---- */

      /* 2000 May 8 Bugfix - added Camera_Aspect_Ratio */
      x0 /= Camera_Aspect_Ratio;

This is wrong for two reasons:

1) It tries to scale the cylinder in inverse proportion to its radius
(i.e, if you specified { up <0, 5, 0> right <20, 0, 0> } it gets transformed
into { up <0, 1, 0> right <4, 0, 0> }, which is totally unexpected.

2) The if (Precompute_Camera_Constants) block is obviously intended to
be entered only once (since it normalizes FCR and FCU,
Camera_Aspect_Ratio will always be 1 after the first pixel). 

Fixes:

1) Change 
        Camera_Aspect_Ratio = ly / lx;
   to
        Camera_Aspect_Ratio = lx;
    and 
      x0 /= Camera_Aspect_Ratio;
    to
      x0 *= Camera_Aspect_Ratio;

2) Add 
	Precompute_Camera_Constants = 0;
   at the end of the if-block.

This will restore the length of the right vector, but will normalize
the up vector (so the length of the up vector will be ignored, only its
direction counts).

If you just want the same behaviour as in povray 3.1g, remove the whole
normalization stuff. I don't see any problems with that.

	hp

-- 
   _  | Peter J. Holzer    | Nicht an Tueren mangelt es,
|_|_) | Sysadmin WSR       | sondern an der Einrichtung (aka Content).
| |   | hjp### [at] wsracat      |    -- Ale### [at] univieacat
__/   | http://www.hjp.at/ |       zum Thema Portale in at.linux

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