Phoenix, smoke, and GI. What is the intended use?

[drosselmir]

Hey everyone,

Long time Phoenix user. Want to throw out this general question asking about using GI with Phoenix. I'll make a simple smoke simulation, light it with either simple lights or sun/sky, and then in rendering I'll set the smoke color scattering to ENABLED. I'll then render with and without GI and the results are basically the same sans a decrease in light intensity.

The renders are basically identical. What is the intentional use of GI with smoke? I have assumed it was to get a volumetric lighting effect, almost like subsurface scatter. However the only way I have gotten close to this effect is by using the heat haze render method. Though my render times go way way up and results are inconsistent.

Can someone pass along some guidance? Thanks!

Dave

[Ivaylo Katev]

in general GI is very rarely used with volumetric effects, because the volume shaders decrease the ray tracing alot and the GI techniques require huge amount of rays to work, you can compare the numbers in the vray console.
so reading your post i decided that we have broken GI and nobody noticed this, but my tests showed that it's not true, the GI is working as expected.
see the images below
full GI rendering took 300 sec
GI with disabled scattering (analytic GI) took 25 sec
no GI with and without analytic correction took 2 sec each

you can see the full GI image has fade out of the lighting in the light-shadow transition, because the smoke in the shadowed area is lit by the near bright smoke.

i made this experiment not using natural smoke, but i used a cube shaped smoke put in a shadow that covers it particularly. why i did the test like that? because in natural smoke is very hard to observe the GI effect. that's why the FX artists use GI very rarely.

[Paul Oblomov]

I wonder in mesh mode + vray volume scatter wouldn't it be the fastest route ?

Here's the pic. Works fine with progressive. With retrace off - u get lightning fast results, enough for preview. Here's domelight + vraylight + GI with retrace to low. Of course, in mesh mode, you lack opacity at the borders... But for now it is the fastest route.

[drosselmir]

Thank you guys for the explanations. I suppose I need to be more specific about what I'm hoping to achieve. Ivaylo thanks for the info on the GI theory. I will digest that info and come up with a more specific example of the effect I'm searching for.

Paul, I love the test image you posted. That is the type of internal lighting I was searching for... I will play with the settings you posted about.

Also BTW here is a reference image that I was trying to mimic. What do you guys think about recreating this image?

http://inapcache.boston.com/universa...032111/bp1.jpg

[Paul Oblomov]

I think u need to mix two approaches
Also use RGB channel
http://www.youtube.com/watch?v=IUgHmoP6XkQ

P.S. This kind of shading is exctly what vrayfastSSS2 with preset "milk whole" is like!

[drosselmir]

I LOVE using RGB! the Mixing of colors is fantastic with Phoenix.

Yes I agree. It looks just like milk in water.

[Svetlin.Nikolov]

Oh yes - to get that kind of look on the smoke, we need to add varying absorption for the R, G, and B.

[Paul Oblomov]

What you think of vray scatter volume ? Looks like vlado did all the math for you

[Svetlin.Nikolov]

Well, the smoke must have some transparency too sometimes.