William F Pokorny <ano### [at] anonymousorg> wrote:
> On 2/20/21 11:50 AM, Norbert Kern wrote:

> Took me a second to recognize the Vermeer - it's mirrored...

Thank you, but the vermeer only seems to be mirrored. It seems so, because it is
a picture in a mirror. To be sure I made a test with an old mirror...

> As I suspect, you suspect, given your code comments, what you are seeing
> is the normal inversion I was talking some about while fixing the
> quilted pattern for povr last fall. Something which happens often enough
> when you play with bump sizes above about 0.5. You have only triangles
> there - one surface - and when the normal inverts the surfaces turns
> inside out(1).
>
> Attached an image. All without the light blocking box. In the top left
> your original scene. In the upper right the bump size set at 0.5. In the
> lower left a v3.7 render showing the existing v3.7 / v3.8 quilted normal
> inverting with a bump size of 5. In the lower right the povr branch's
> fixed quilted normal - which does extra work to prevent the
> primary/major normal direction from inverting - still with a bump size
> of 5.
>
> If you had instead used small boxes with some thickness > the min
> intersection depth the shadow test code would likely pick up the
> "second" box surface correctly and see the intersection at the first
> surface as being in shadow.
>
> (1) - And this touches a POV-Ray secrete with respect to shadows... As
> often as not the surface appears to be in shadow because the normal
> points away from the light source and so it appears unlit when in fact
> POV-Ray considers it lit. The actual shadow detection code usually fails
> to correctly pick up the at surface intersection on the shadow test. ;-)
>
> Aside: My guess is that you needed the extreme normals for radiosity to
> reasonably light the wall texture? If so, an alternative is to lower the
> finish brilliance to something toward 0, so the shading model gives less
> than the usual importance to the incidence of the light rays hitting the
> wall. This is the purpose of brilliance(2).
>
> (2) - I have in my head there is a brilliance setting in the global
> radiosity block with which Christoph was playing in v3.8. I cannot
> remember the state of it at the moment. It was, I think, a boolean - if
> so, probably as to whether to use the textures finish brilliance setting
> if other than 1.0. But..? Not sure what if any implications for
> radiosity and finish brilliance with v3.7.
>
> Bill P.

An excellent tip - I'll test it immediately.

Norbert

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