Why does VRay2SidedMtl go black when translucency goes towards 100%?

[snivlem]

I have been meaning to ask this question for a while.

I was just wondering why does it go towards diffuse black the more translucent an object gets? If the 2sided material was designed for paper and leaves then I'm wondering how to achieve that rather than solid 100% black materials.



Here you can see in this draft render these "cutout" people have a standard VRayMtl shader applied to them with a diffuse value of .9

However if I then apply a 2SidedMtl to this with 90% translucency they become almost pure black



Just wondering why this is?

Even if you do 50% translucency it seems that the material loses 50% diffuse. I know the laws of energy conservation but wondering how one can actually achieve a real life paper effect with VRay2SidedMtl?



Scene is lit by a HDRI and a few vray lights around in the interior.

[Algehov]

This is something that has always bugged me abit aswell. Also that if you tint it with a color, it will get the inverted color on the other side.

I belive the reason behind it is that the translucency color/slider decides how much percentage of the light hitting the front side will get transfered over to the backside. So if its pure white, 100% of the light will be on the backside, resulting in no light on the front side, hence black.
Maybe thats the realistic way it works aswell, but it makes it very hard to create for example a leaf shader that you can "art direct" the way you want. Right now im faking this problem by multiplying the diffuse by the translucency color (only works on gray colors). So if I have 50% translucency, the diffuse will then be boosted by 50% in the shader to not have it appear to dark in the front side.


It would be nice to have a fake 2sided mtl shader that works similiar to how Arnold does their translucency effect. There the front side is never affected, and the backside gets all the light the front side gets, multiplied by the translucency color/texture. And ofcours the other way around, the front side gets all the light the backside gets, multiplied by the same texture. It feels like vray multiply it with the recieved light, (which is dark on the backside) and making the lit side very dark, rather than "add" all the backside light on to the front side, masked by the translucency.

Im not sure if thats totaly unrealistic, but would be so much easier to get a 2 sided shader to behave like the effect that you are after.
You can probably code an OSL-shader to behave like this, but I dont have the knowledge how to.

[snivlem]

Thanks for the post, there are a few good ideas in there. Yes - I also struggle with foliage which is mainly where I use this material. I usually have highlighter green diffuse and by the time 2SidedMtl is done with it, it goes back to a darker green. I haven't tried Arnold but with Corona for Maya in beta, it is easy to get great looking foliage without messing around with exaggerated diffuse.

I've read the tutorial with V-Ray too;
http://help.chaosgroup.com/vray/imag...leaf_tutorial/

I understand it has to be physically plausible in that it shouldn't give off more light than it is absorbing but I do not understand how the diffuse can get to 0%. How does one replicate really thin materials, for instance like tissues, without the material going dark or spring leafs or white flower petals?

[Morbid Angel]

I had to do a balloon fabric material. The only real way I could get that translucent effect was to use vray fast sss2 material in raytrace mode and plug that into 2sided mtl.

When you set the 2sided mtl to either full black or full white it just returns 100% of that shader color.

[snivlem]

Thanks MA, I'll have a go at that idea.