William F Pokorny <ano### [at] anonymousorg> wrote:
>
> In the upper right is today's (v3.7/v3.8) true normal { quilted ... }
> result. It shows one of the longstanding issues with the quilted normal
> perturbation implementation. Specifically, if the shape's surface
> "lands" on any of the texture's axial "floors" during texture
> evaluation, you get a noisy result.
>
> The usual fix for that sort of problem is to apply a small positive or
> negative epsilon - somewhere - in the system being calculated so such
> landings are unlikely in practice.

Thanks for looking into this. I have always suspected that the quilted pattern
(at least as a 'normal') has some unexpected quirks.

Back in April 2018, I actually posted an animation test about this, with some
notes; perhaps it would prove helpful...

http://news.povray.org/povray.binaries.animations/thread/%3Cweb.5ae6126fe8495210a47873e10%40news.povray.org%3E/

I too found that I needed to add a slight positioning 'fudge factor' to the
pattern in my SDL code (like your internal epsilon tweaking) to avoid the
'noise' that you described. I also think that the pattern has some kind of
'discontinuity'-- in other words, that it does not seem to be uniform in its
behavior throughout its unit(?)-cube spatial volume. The effect is hard to
describe; maybe my animation will make it clear. I had assumed that the pattern
'spread out' or expanded from a central point-- so that it would perturb all six
faces of a cube in an equal way; but the visual result looks...odd.

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