Alain <aze### [at] qwertyorg> wrote:

> > Alain<aze### [at] qwertyorg>  wrote:

> >>
> >>> camera {
> >>>
> >>>       #declare xS = 0;
> >>>       #declare yS = 1;
> >>>       #declare zS = 0;
> >>>       up<xS,yS,zS>
> >>>       right x*image_width/image_height
> >>>
> >>>       angle 1
> >>>       location<0,0,-2000>
> >>>       look_at (Position - 9*x)
> >>> }
> >>> .....
> >>> light_source {                    // far enough to avoid problems ...
> >>>     0
> >>>     color White
> >>>     translate<20000, 30000, -10000>
> >>> }
> >>>
> >>> For zCam<= -1958.5 or zCam>= 2003.5, all the defects in my scene (where there
> >>> are other SORs, affected by the same problem) are gone.
> >>>
> >>> Gilles
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >> You place your camera at a large distance and use a very small angle
> >> that effectivelt removes perspective. That, in turn, can cause
> >> calculation errors. Those errors will cause the apearance of various
> >> artefacts.
> >>
> >> Try placing your camera closer and use the orthographic projection. It
> >> effectively simulate a camera set at infinity.
> >>
> >>
> >> Alain
> >
> >
> > I can imagine that (this is also a reply to Warp) floating point has a limiting
> > accuracy, which may be critical when you deal with infinitely thin (right ?)
> > surfaces like SOR.
> > Of course, without looking at the algorithm, one cannot say more ...
> > I am nevertheless slightly amazed by the fact that the defect vanish when you
> > move the camera closer or FARTHER from the object ...
> >
> > Gilles
> >
> >
> >
>
> That's the problem with floating numbers. There are situations where
> they introduce some random noise in the results, while, at other times,
> you get exact results.
>
> If you appen to work with values that can be represented exactly within
> the precision available, all is well. If, at least some values can't be
> represented exactly in binary, then you get rounding errors.
> If you use very small values with large ones, or very large with small,
> you get precision and underflow errors in addition to the rounding ones.
>
> You can also notice that the defects will move, change or vanish totaly
> if you rotate your object, or rotate the camera around it, while keeping
> the same distance.
>
> Also, in your case, if you scale the object, the result WILL change.
>
>
> Alain

That's true ... sigh ...
I remember the good old times of the double and triple precision specifications
....
.... which did not prevent you from running into problems to be handled, at some
point !
Gilles

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