How does VrayMTL translucency work...?

[LudvikKoutny]

Hi,

i can not wrap my head around how translucency in VrayMTL works. It seems to be rather SSS than translucency fuction, and there are many things that do not make sense. But so far, what confuses me most is this:

Refraction glossiness function affects behavior of translucency effect, so activating translucency is not really a switch between SSS and refraction, but some sort of a blend. But if i change refraction glossiness, then i pretty much corrupt correctness of scatter coefficient and forward/backward coefficient parameters. I really do not understand that i have to blend SSS settings with refraction settings, and get into situation where i have over 10 parameters that affect each other in a very unintuitive way. Fog parameter for example has some multiplier, instead of physical depth in world units. Then it's color works not only as a color but also as a switch to activate/deactivate translucency. Then there is a thickness parameter, which is in world units....

So i am pretty much lost on what controls what...

[svetlozar.draganov]

Hello Recon442

This is very interesting question - and unfortunately it requires some time to get used with it. The process is not straight-forward and you have to spend some time with it - but I'll try to explain everything.
The first and very important thing is to forget about all Translucency parameters: Thickness / Scatter Coeff / Fwd/Bck Coeff / Light Multiplier - they are not important at the beginning, you can use them at later stage for fine tweaking but at the very start they are not important at all.

1. Ok let's say that we have a model and we want to create Translucency material - the first thing is to assign default VrayMtl (diffuse value = 12 and add a light behind it - we want to illuminate the object behind in order to see the translucency effect. My model is a human in real-size-scale , and the light is placed directly behind it with Multiplier=50.

(click on images for larger-resolution)


2. For better understanding I'll change the color to blue - and I'll decrease a bit the value of the color = 50 , because I want to see nice gradient outlining my model.



3. At this point we only have diffuse component. In order to see through to model we need to add refraction.



4. Now we have some shading to the whole model - you can see that in the previous image we had only diffuse shading but now we have also some light inside the volume. The next step is to turn on Translucency and add some Back-Side color. In my example I have used Hard(wax) type and pure red color.


5. Now we have some translucency - and here is the most important step. Most of the people are trying to adjust material appearance with translucency settings but this is not possible because the translucency in Vray like in Nature working with refraction parameter. So please don't waste your time trying to achieve the effect you want with translucency settings - everything you need to adjust are the colors - Fog Color / Refraction color / Back-Side-Translucency color. In the next image I have changed Fog Color from its default color value = 255 to 180. Please pay attention to humans finger - you can see the Back-Side color in this area because it is the the thinnest area. Please pay attention that I will only use VALUE in Refraction and For colors , and I'll use Hue/Saturation/Value for Back-Side color - this is very important for expected result - in other words refraction and refraction color should be grayscaled.



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[svetlozar.draganov]

6. If we want to pass light through more thicker areas , the only thing we need to do is to increase fog-color value - in the next few image I'll change only this value.


7. The result now looks correct - but let's play some scenarios , if for example we want to create more contrast between the darker and brighter are - we could do it by adding negative Fog Bias.



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[svetlozar.draganov]

8. Once we got our material to this stage - it is now time to play around with Translucency parameters. As you can see everything so far was created only with colors and nothing else.
I'll remove some changes before to continue with the next changes > Fog Bias is set to default 0 and Fog multiplier is change to 235



9. The next step is to change Fwd/Bck Coeff from default 1 to 0 - by doing this we tell Vray to pass all entered in the volume translucency rays through the entire volume, there won't be any rays scattered backwards - every entered ray will continue to travel forward through the surface. You can see that in the thicker areas the color is now brighter - which is what is expected.



10. The next change is Scatter Coeff from 0 to 1 - with this change Vray won't change the direction of the scattered rays - they will travel with the same direction through the entire volume. The result is that the entire model is a little brighter - which is also expected because all rays are traveling parallel to each other.



11. And finally we can play with Thickness value from 1000 to 1 - with this change Vray will calculate only translucency in the thinnest areas of our model - and the result is exactly what is expected.

[sandykoufax]

This is very good tuto. Thanks!

[svetlozar.draganov]

Here is the model if someone wants to play with it.
translucency.zip

As a conclusion I would like to point out that the most important thing here is to choose the right colors for Diffuse/ Refraction / Fog Color and Translucency Back-Side Color- they are the main parameters - translucency parameters are only for fine tweaking. Also please hold in mind to use only gray scaled colors for Reflection and Fog colors - otherwise the result won't be expected.

Here is one more example with added GI / Sun / Environment / Reflection

[joconnell]

Svetlozar, the fog multiplier value was changed recently wasn't it so you don't need to change values to take into account scene units wasn't it?

[Dariusz Makowski (Dadal)]

Heya

I just tried it with Maya Vray but I get nothing... at point 4 all goes bad... Any chance to get some help with point 4+ on maya ?

Thanks, bye.

[Morbid Angel]

This thread should be a sticky. Great info. Year I think in maya its a bit more tricky..

[zeronove]

IMHO this infos should go straight to spot3d - tutorial section, as well

[sandykoufax]

Svetlozar, can I post your images and descriptions on my blog?

[Macker]

That is brilliant. I've always been "shooting in the dark" when it comes to the vraymtl translucency, just guessing at numbers and colours and hoping for a correct result.

Gonna link to this on my blog; can non-subscribed members see the forums at all?

[Dariusz Makowski (Dadal)]

Nonsubscribed members cant see forum.

And yea please Maya explanation. I get no results

Thanks, bye.

[3LP]

This awesome,

Thanks for this info, like Macker, I was also always shooting in the dark

This should be stickyed and straight to tutorial section and it's going straight to my bookmarks

Stan

[Macker]

I tried this, and got good results however I can still quite clearly see the vraylight behind my object (i'm creating a backlit marble screen in a reception area) so presumably this is where you'd use refraction glossiness to blur it out?