I always assumed the ‘double-sided’ and reflect on ‘back side’ options within the Vraymat were similar to the 2-sided option on the scanline materials. I see know that it’s not the case.
What exactly are these switches do? When should they be used? Do they cause a rendertime hit?
And is there no “force 2-sided” on vray then?
Thank you.
if you check the “force 2 sided” in the common rollout it doesn’t do that for you?
Double-sided in VRayMtl does the same thing as 2-sided in Standard material and Force 2-sided in Common tab in the render dialog. “Reflect on back side” forces VRay to compute REFLECTIONS for back-side polygons. 
Then why does a material with double-sided turned off still renders 2-sided?
this is what the manual says:
Double-sided - if this is true, VRay will flip the normal for back-facing surfaces with this material. Otherwise, the lighting on the “outer” side of the material will be computed always. You can use this to achieve a fake translucent effect for thin objects like paper.
Then why does a material with double-sided turned off still renders 2-sided?
I don’t see that here:
Standard material with 2-sided option:
Standard material without 2-sided option:
VRayMtl with double-sided option:
VRayMtl without double-sided option:
As you can see images 1 and 3 are identical. VRay double-sided option does NOT work exactly the same way as 2-sided in max standard, but produces the same effect - it’s straight-forward. As for what is written in the help - it explains how double-sided works technically.
'Scuse my ignorance, but aren’t the renderings with 2 sided off supposed to NOT show the back side of that shape? Thats at least how I’m used to working with doublesided stuff versus singlesided.
If you want to force VRay to not render the backside at all you have to apply a double sided material with the backside material set to opacity 0.
Dieter
What you can see happening in images 2 and 4 is that the back sides are being rendered using the normal directions of the front sides (which makes them look wrong). The two-sided options that are being discussed create a new set of normals for the back sides - allowing them to be rendered correctly if they are visible.
This shouldn’t be confused with the traditional concept of “disable backface culling” that has been around since the dawn of polygon-based rendering. Images 2 and 4 are what you would traditionally get if you turned off backface culling - making the back sides visible, but not giving them new normals. With VRay, backface culling is disabled by default - hence, images 2 and 4. Our options of making a material double-sided or 2-sided are for normals (as in images 1 and 3), not visibility.
Bottom line: if you will see the back side and want it to render as you would expect it to look, turn on double-sided or two-sided. If you want to make the backside invisible (enable backface cullling) then follow Dieter’s recommendation and apply a double sided material with the backside material set to opacity 0.
It is my understanding, though, that backface culling is disabled by default so that the algorithms within VRay (i.e. refraction) will function correctly.
-Jeremy
That clears up a lot! thanks
No problem.
Hey, Antwerp is a nice place. I bought my wife a diamond necklace there a few years ago. That’s what you are supposed to do in Antwerp, right - buy diamonds?
-Jeremy