Chambers <ben### [at] pacificwebguycom> wrote:
> Warp wrote:
> > Chambers <ben### [at] pacificwebguycom> wrote:
> >> You're dealing with two numbers, one of them being approximately 20
> >> million times the other.  It's a known aspect of the hardware we use
> >> that you should keep your ratios smaller than this to avoid precision
> >> errors.
> >
> >   Where exactly are you getting that number?
>
> Location of 1M, radius of 0.05.  Since floating point uses, well, a
> floating exponent, you're fine mixing numbers of the same scale, but
> objects of extremely different scales won't necessarily have the same
> precision when used together.
Yes I see, note it is actually the accuracy that varies: precision is
constant at "double precision".

>
> I assumed that it was this mixing of scales that caused the error
> (similar to what's explained here:

That's reasonable and it does cause accuracy problems. Here the problem is
more that the shift away from the origin by 1M means that all coordinates
have less accuracy: the coordinate space has lower resolution.
Note: I am deliberately causing accuracy problems to measure the effects.
It is only in this case that the effect is not completely explainable (to
me).
I know it is caused by loss of accuracy but not precisely why the strange
truncation is occurring as opposed to the more common effects such as
shifting of relative object positions (and the consequent changes in
rendering), z buffer tearing/clipping etc.

chris

> http://tag.povray.org/povQandT/misconceptions.html, look under "Sunlight
> Simulation"), as I didn't know about POV's internal limit on
> intersection distances.
>
> ...Chambers

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