CShake <cshake+pov### [at] gmailcom> wrote:
> One potential issue in using this method is that many people tend to
> define their pov 'base unit' differently, some with the whole scene
> bounded by box{-10,10} while others could have a wine glass itself 200
> units tall.
> I propose that when the epsilon value (min intersection distance) is
> set, it could be determined in a somewhat variable sense:
> 1) Set it to the standard 1e-6 by default and let the user change it in
> global_settings{}, call it 'coincidence_distance' or something. I think
> this would be best.

As a matter of fact, there already is a constant with similar use in POV: The
MIN_ISECT_DEPTH (currently hard-coded to 1e-4) is the minimum distance from a
ray's origin (camera or an earlier intersection) to accept an intersection; any
intersection closer than that is currently ignored, probably mainly to avoid
"re-intersecting" at the ray's origin.

I was already thinking about making some of POVs internal key distances
accessible to the user. However, there is some risk of breaking the established
balance between those constants (and possibly even some more hidden in remote
parts of the code).

> 2) Set it to be a function of some aspect of the scene during parsing,
> be it the largest finite object, the distance between the camera
> location and look_at, sky sphere radius, whatever. This might not work
> because you'd want to use the value before the scene is fully parsed.

Actually... no, that would not be a problem. With the approach I have in mind,
the "extension" of the wine (I just stick to that example) will not be done
during scene parsing, but during tracing.

But I guess attempting to auto-tune those key distances would be faced with too
many unknowns. Distance between camera and look_at seems like the best
reference value to me for such a purpose (or focal plane if focal blur is used
in the shot), but still someone might e.g. have chosen look_at just because the
direction was ok, without paying attention to its distance. Or a shot may have
look_at point somewhere in the distance, but highly detailed stuff visible in
the foreground.

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