Glass fog colour bug?

[Shopper2]

Hi guys,

so i have a question.

i have 3 part which make up a gas mask for a hospital. They are made of transparent acrylic.

I create a standard glass from the standard library. I turn on fog shadow and alpha and turn on double sided. I have added a slight colour to the fog colour to get that little blue in the platic.

2 parts are small and the 3rd is the full mask. They are all just as thick though (2mm)

i have a problem with the large part. It seems to render bright green or bright red or black depending on what section of the RGB field i take. Its not nice and transparent like the other parts. The part works fine if i take the fog colour to white. It also seems to become slightly lighter if i set the fog intensity to 0.005....but that seems a bit low

Ive had this problem before with a washing machine door.


Any ideas? have you seen this problem before?

Justin

[Gijs]

Re: Glass fog colour bug?

Hi Justin,

check and make sure that none of the rgb values is at 255, because vray doesn't like that in the fog color. In the Vray manual are some examples that show the difference between using a dark saturated color and a lighter, unsaturated color. It's worth taking a look at those.

[dalomar_cg]

Re: Glass fog colour bug?

I'm not sure if I can explain what you're seeing, but here are a few things about fog that might help...

1. Fog is a volumetric shader. So I could have the same material on a thick object and a thin object and get 2 entirely different results...This is something that people tend to forget from time to time.

2. The multiplier is tied into your scene units, and through the multiplier you can control the appearance of the fog based on the thickness of your object. Since a multiplier of 1 is always related to 1 scene unit, if your in mm and your object is 1 mm thick, then a multiplier of 1 will result in fog that is very close (because it depends on your viewing angle) to the fog color that you input. The thicker your object, the lower that multiplier needs to be in order to get close to that fog color. The thinner the object, the higher that multiplier needs to be in order to get that color. Luckily this is consistent enough to actually be able to predict its reaction based on the thickness of your object...the formula is as below.

fogMultiplier = 1/(thickness of your object in scene units)

A simple inverse relationship, but this will help you be able to set a good multiplier and get better fog results.

3. Fog is always dependant on how the object is viewed and its own refractions. Fog is volumetric, meaning that it isn't looking at whats happening on the surface, but how much its traveling through a solid object. Basically, the more of that object it travels through, the darker the result will be.

Lets take a sheet of glass for instance, as something that is very thin in one dimension, but usually considerably longer in its other dimensions...Looking at that piece of glass straight on is going to reveal very little shading due to fog simply because it doesn't have very much of the object to travel through and typically there aren't too many internal refractions when viewing glass at this angle. Now lets look at the glass through its edge. It has a much different (darker) fog effect because you're looking through more of the glass. Not only that, but looking through the edge of glass will tend to have a lot of internal refractions, so this contributes to the darkening as well. So although this is an extreme example, it still holds true for looking at objects at different angles...one angle may catch just a thin part of the object, another may catch it at its thickest...keep that in mind when evaluating the results.