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View Full Version : Smoothing and Double Sided Artifacts



Surrealist.
12-28-2011, 06:44 AM
Something I have never noticed before. Some setting I forgot?

Seems I can not have double sided and smoothing on at the same time without creating an artifact.

prometheus
12-28-2011, 07:38 AM
Same here, It looks something related to the light rays, if you rotate your distance light you will have it vanish or appear on a different angle.

And if you switch to dome light it will be gone I think.

Also if you change the ray precision in the render globals panel to 1 it will be gone too.

This is on lw 10.0 havent checked on lw 11 yet.

Michael

Surrealist.
12-28-2011, 08:25 AM
OK cool. Thanks, I'll check that stuff out.

probiner
12-28-2011, 04:42 PM
You can also use Shadow Offset, it makes the shadow walk though. But I don't know what kind of repercussion could a 0.1 Ray Precision have, especially with refractions and reflections. Spheres have 1 and 2 meters of radius
http://i153.photobucket.com/albums/s202/animatics/Lightwave/shadow-double-sided.gif

Cheers

Surrealist.
12-28-2011, 05:04 PM
Interesting. Thanks for the info.

Sensei
12-28-2011, 05:26 PM
When ray hits some polygon spot, there are send rays to the all light sources to find out light influence. If polygon is single sided, there is no sense sending ray in direction which is behind it, because of course it will never be calculated. But if polygon is double sided ray is send, omitting the polygon which spot is evaluated, and is hitting inside of your sphere, and marking whole polygon as "in shadow".

You can check whether theory is true, by using Spot Info node and changing transparency if double side is hit.. Transparency is affecting light influence, so for 50% transparency, your wrong shaded polygon should be twice brighter than now. You can't directly set 50% transparency because it will affect both front and back side, and difference won't be visible. Change also shadow color to some ridiculous.

Sensei
12-28-2011, 05:34 PM
Yeah, simple adjusting shadow color to red, is showing that these wrong polygons are tinted with red slightly.

Sensei
12-28-2011, 05:56 PM
Check out this.. adjust shadow color and translucency.. then switch back and forth double side toggle..

probiner
12-28-2011, 06:20 PM
Ah nice sensei =) Thanks for taking it forward.

I searched some threads and apprently lowering ray precision is not also good for Radiosity.

Cheers

Surrealist.
12-28-2011, 09:00 PM
I should probably tell you the reason I was going this was because for some reason I was getting light leaks on the side of the face of my model. Something I have never had before as long as shadows where on.

But in this case the only thing that solved it was turning on double sided.

I have since then replaced this model with another version of the model that had not gone through a conversion from LightWave to Blender and back again by way of a.obj and then saved out as a.lwo

What got me testing this was the fact that the model that had gone through this conversion was not able to display or render smoothing with double sided on.

So I decided to do some tests.

This is when I noticed this artifact and made me wonder.

Now the issue is resolved as far as the object is concerned. (that particular object was undoubtedly corrupt) But I have no idea what Sensi is talking about and how or why this is a solution to the problem or it is simply a indication of a bug or a failure in LightWave implementation.

Sensei
12-28-2011, 09:31 PM
In #6 post I was explaining why probiner in his animgif is seeing these black polygons when double side is turned on.

Post #8 also showed another issue related to this effect. Translucency is red, because renderer thinks that spots are in shadow, casted by double sided polygons inside of sphere (which are in a way between spot and light source). Or reverse- if single side, it thinks nothing is between spot and light source, therefor not tinting red if Double Sided is off.

Surrealist.
12-29-2011, 12:15 AM
OK, But what does that mean? Is this a bug? A poor implementation? What is the solution?

Sensei
12-29-2011, 12:24 AM
Bug. Not expected by programmers behavior. No solution, maybe except duplication of geometry, then flipping and small shrinking (so front and back are not exactly in the same position), if double side is really needed..

probiner
12-29-2011, 12:47 AM
/\ Yup, exact same conclusion I got to. Still it will present bad results if you turn translucency on.

It's basicly what shadow offset does (I think the values that gives good results for both shadow offset, and duplicated/flipped/sized-down geometry offset from the original geometry is the same) but without messing with projected shadows. Thought I don't know about how the results would be near the borders of the object where with transparency you might see there are 2 objects.

Anyway, where are the big-render-boys? :p

Cheers

Surrealist.
12-29-2011, 04:11 AM
OK. Well if it is a bug then we should report it. I mean this is a basic basic thing. Double sided. I find it hard to believe that it had not been noticed which makes me suspect I don't fully understand what is happening. But if it is a bug, so be it.

probiner
12-29-2011, 05:21 AM
It had been noticed several times. Here's one: http://forums.newtek.com/showthread.php?t=86479

Cheers

Surrealist.
12-29-2011, 08:38 AM
Ah yes, this thread comes back to haunt me. Completely forgotten. Good research. Thanks.

The last word:


Yes--I'd always assumed this was one of those all-app problems that had to be endured, but after hearing Modo's renderer doesn't have this problem, I think the guys at NewTek really need to get on it.