POV-Ray : Newsgroups : povray.bugreports : sphere slicing problem : Re: sphere slicing problem Server Time
19 Oct 2021 11:36:58 EDT (-0400)
  Re: sphere slicing problem  
From: William F Pokorny
Date: 15 Oct 2019 07:35:40
Message: <5da5af0c$1@news.povray.org>
On 10/15/19 5:52 AM, jr wrote:
> "jr" <cre### [at] gmailcom> wrote:
>> "Bald Eagle" <cre### [at] netscapenet> wrote:
>>> Nope.  It's putting textures in too early.  The clear texture is what eliminates
>>> the surface[s] of the slice.
>> ...
> 
> just for the record, after trying suggested variations (clipped_by, bounded_by),
> I also tried with 'no_image' for both object and "slice", separate and together.
>   still the same looking output: of 200 frames only the first (and last) twenty
> or so result in complete rings, then progressive "break up", to nothing shown at
> all for circa ten frames around the middle.
> 

Looks like generally your are looking to do a scan of any object to find 
the edges? Though the use a of transparent box to do this is inspired - 
it won't work reliably for the reasons I suggested in my earlier post.
Expect scanning a box works even less well?

Guessing at what might be confusing, when POV-Ray does CSG, it isn't 
creating intermediate shapes or surfaces. It's describing the treatment 
for the ray surface intersections found along the ray. This means for 
you, you'll have both coincident (sometimes wrongly ordered) 
intersections and ones less and less likely to align with the 
orthographic camera rays as the sampled surface becomes parallel to the 
ray. Such rays, if they hit, would be near tangents to the surface - the 
shortest of these tends to get filtered in many objects too for a few 
reasons.

If you want to try something that might be better with your current set 
up, the new v38 +am3 anti-alias mode might help(1). It can be slow. 
Here, if it works better, it would sort of be picking up 'enough' noise 
(enough intersections in the chance right way) to resolve the edges. I 
still expect it to struggle on the middle most rings no matter and 
perhaps not help much when scanning a box say with whole sides parallel 
to the camera rays.

(1) - And perhaps help more with more aggressive options (more than +am3).

There is edge detection of Bald_Eagle's solid shapes which can be run in 
gimp, photoshop, command line packages like netpbm and such. You can 
emulate edge detection methods in POV-Ray too folks have done it, but 
using something canned for 2d I expect easier/faster.

Your after the textures (and lighting... +AM3 best bet for that) at the 
edges - others have warned in posts about the complications thereof. 
What eval_pigment does is what can somewhat simply and reliably be done. 
For the rest angles of rays camera, shadow, slightly adjacent rays, etc 
play a part in the resultant pixel color.

If your looking to create meshes only for a few of the simpler shapes 
like spheres and boxes you can create meshes from the HF macros in 
shapes.inc.

What are you really trying to do - and for what inputs and outputs? 
Knowing that maybe some method to get you edges can be worked out. Maybe 
someone remembers a macro or two that already does what you need.

Bill P.


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