William F Pokorny <ano### [at] anonymousorg> wrote:

> The top row a disc{} is being intersected with a sphere{}. In the second
> row a roughly equivalent polygon{} is intersected in the same fashion.
>
> I believe the disc{} results are correct only in the second and last
> columns. The polygon row of intersections I believe is OK(*).

So, I need to question the whole top row.
I refer to the disc {} in its form with a hole using the mathematically accurate
term "annulus".

It's interesting that the annulus gets rendered with what appears to be a
bounding box with the sphere texture included.  I'm assuming that's part of the
erroneous behaviour - only the annulus ought to be rendered.

1. If column 2 and 5 both show the sphere, then why doesn't column 1 show the
lower part of the sphere?

(bounding boxes and perhaps some method of showing the inside () results of the
disc might help. VRandInObject or whatever it's called, or media)

2. Confused about why inverse shows so much of the sphere.
Does the disc act like a half-infinite cylinder?
Does inverse now cause it to be  NOT(half-infinite cylinder)?
Because even that doesn't seem like the correct result to me.

3. I guess it's wrong, but consistent with render 1.

4. perhaps this might be correct, if we're considering the disc/annulus to
technically have an inside () ?  Though I would expect for there to not be
anything, since the annulus surface is outside/tangent to the sphere

5.  No idea why you think that's correct, since there's now no overlap, unless
we're somehow considering the annulus to be some form of plane / half-space
object.  But then that leaves me confused about the rest of the renders in that
row.

I am in full agreement about the bottom row.

Suggest:
0. show bounding boxes and insidedness tests for the annulus.
1. apply an inside_texture to the sphere
2. use a sphere with a negative radius
3. use sphere {inverse}
4. use both inverse
5. make the major and minor radii of the annulus equal.

6. refer to my Boolean operations render, and see if that inspires any
additional experiments - my subconscious tells me that using that as a guide to
construct formalisms for what we're doing with objects and CSG (and using very
accurate, formal terminology apart from what we use in SDL) might help us
unravel what our less rigorous discussions may be hiding from us.

https://news.povray.org/povray.documentation.inbuilt/thread/%3Cweb.68573b7934ec28cb1f9dae3025979125%40news.povray.org%3
E/

"Since POV-Ray does not implement regularized Boolean operations (i.e., union,
intersection and difference are done as conventional set operations)"
(bottom of page)
https://pages.mtu.edu/~shene/COURSES/cs3621/LAB/povray/csg.html

P.S. As I was letting Second Coffee percolate into my blood stream this morning,
I was mixing thoughts about scene experiments, and I recently tried to render a
cone (a real, mathematical "double-cone") as an isosurface and had an issue,
until I reformulated how I was doing it to avoid division-by-zero.

Could you (someone) also render a cone {} with one radius positive, and the
other radius the unary inverse?  Use inside_texture and repeat CSG experiments.

Thanks,

- BW

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