POV-Ray : Newsgroups : povray.binaries.images : Weird problem with transparent box : Re: Weird problem with transparent box Server Time
14 Aug 2022 15:14:19 EDT (-0400)
  Re: Weird problem with transparent box  
From: William F Pokorny
Date: 23 Jun 2022 19:19:48
Message: <62b4f514$1@news.povray.org>
On 6/23/22 15:48, Chris R wrote:
> I took my main scene and eliminated everything but the transparent box, and even
> added media to it and everything looked fine.  I added one scene element at a
> time back and it was all fine until I added in the drip of water I had modeled
> falling from the downspout.  As soon as I added that object back into the scene,
> the transparent box with media became a reflective mirror instead!
> The drip of water is modeled as a quartic.  It had a material to look like
> water, (so transparent, ior 1.33, reflective, etc.).  When I changed the
> material to just a simple gray pigment, the media box was no longer visible or
> reflective, but the media also disappeared.


What version of POV-Ray is being used?

Are the shapes involved part of the same union or merge?

The only obvious thing I can think of would be if the water material was 
defined by name and happened to have the same name as the one used for 
the media box - and the water material definition was seen last.

If you can get a simpler scene showing the issues I'll add it to my 
collection for debug, though no promises as to when I'd dig. I've been 
busy with other than POV-Ray of late.

--- Deeper and less likely possibility...

With some older shape types - due ill advised automatic polynomial order 
reduction code combined with the lack of tight automatic / manual 
bounding for some shapes - we sometimes end up with an additional root 
(and so another surface) being introduced(a) by the auto-order reduction 

Maybe something like this is going on but, that surface by order 
reduction would have to have a very particular position to see the 
results you posted.

A test for this fail would be to change your quartic to a simple sphere 
or similar and see if the apparent reflection propagation to the box 
problem persists?

(a) - One of the shipped scenes long has had a vertical line in the 
result due the auto-order reduction changing two roots (4th order eqn) 
to three roots (third order eqn) for one or more of the scene's shapes. 
I cannot remember which it is for sure - primitiv.pov maybe? The povr 
branch fixed this particular exposure(b) by ripping out the automatic 
order reduction code.

(b) - Order reduction introduces slight root/surface shifts in the 
intended roots even when it mostly works in a consistent way. The 
reduction code also sometimes flips 'sturm on' to fixed solvers, without 
notice due the order reduction, on a per ray->surface equation basis. 
The sturm vs fixed solver results for identical equations are often not 
themselves identical and this occasionally introduce at same surface, 
subtle, consistency issues - which are sometimes visible. And there's 
more 'solver noise' possible due the order reduction I won't detail - 
partly because I no longer remember all the possible implications!

Bill P.

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