Visible/Invisible shapes

fs
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Visible/Invisible shapes Dec 30, 2018 22:09:10 GMT Jamis likes this

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Post by fs on Dec 30, 2018 22:09:10 GMT

I was playing with a reconstruction of the CSG chapter's banner image, and decided to investigate the "dark" core visible in the sphere with the wedges removed (it's in the book image, and I had reproduced it).

Analogous to the extra feature to allow shapes to opt out of casting shadows (mentioned at end of chapter 11), I added an option to opt out of visibility too.
I have two versions of hit(intersections); one for visibility checks, and one for shadow checks - they skip invisible shapes, and shapes that don't cast shadows respectively.

Here's the reconstruction, when the wedges are visible, which is broadly in agreement with the book image:

Here's the reconstruction where the wedges are invisible:

Jamis Administrator Posts: 310	Visible/Invisible shapes Dec 30, 2018 22:15:54 GMT Quote Select Post Deselect Post Link to Post Member Give Gift Back to Top Post by Jamis on Dec 30, 2018 22:15:54 GMT That looks great!

ascotti Junior Member Posts: 56	Visible/Invisible shapes Dec 30, 2018 23:52:48 GMT Quote Select Post Deselect Post Link to Post Member Give Gift Back to Top Post by ascotti on Dec 30, 2018 23:52:48 GMT Wow, it looks fantastic! :-o I have just implemented CSG but haven't played with it much yet. How is the sphere done? Could you get the "invisible" effect by difference with another smaller sphere inside?

fs
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Visible/Invisible shapes Dec 31, 2018 2:08:52 GMT

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Post by fs on Dec 31, 2018 2:08:52 GMT

Wow, it looks fantastic! :-o

Thanks, but Jamis put together the scene

How is the sphere done?

I just took Jamis' hint - in the last paragraph just before the Putting it Together section at the end of Chapter 16; rotate and squash/scale a cube to make a wedge, make several copies rotated around the Y axis in a group, and CSG difference that from a sphere. Here's the wedges I'm removing:

Could you get the "invisible" effect by difference with another smaller sphere inside?

Yes, with a small caveat; the sphere shell needs to have some remaining thickness instead of being an "infinitely thin skin". Probably a minimum thickness of at least 2 EPSILON to avoid unexpected interactions with the over_point in the prepared comps.

Here's an example where I used that approach instead of the invisibility option:

csg(difference, csg(difference, sphere, wedges), sphere scaled 95%)

IMHO it looks better this way - and there's no need for "opt out of shadows" support either (actually the shadows will look wrong with this approach if that option is used).
However, I tried the invisibility approach because I wanted to try to get the thin skin look as per the book image, and also to later experiment with invisible shapes (eg can't be seen but cast shadows).

ascotti Junior Member Posts: 56	Visible/Invisible shapes Dec 31, 2018 9:50:26 GMT Quote Select Post Deselect Post Link to Post Member Give Gift Back to Top Post by ascotti on Dec 31, 2018 9:50:26 GMT HI fs, thanks for the explanation! I think the shape invisibility is a cool idea and the carved sphere looks great in both versions!

Post by fs on Dec 30, 2018 22:09:10 GMT

Post by Jamis on Dec 30, 2018 22:15:54 GMT

Post by ascotti on Dec 30, 2018 23:52:48 GMT

Post by fs on Dec 31, 2018 2:08:52 GMT

Post by ascotti on Dec 31, 2018 9:50:26 GMT