Vray bump map bug? When using single plane

[Timmeh]

Hi,

We ran into bit of an issue using bump map in vray.

I have attached few screenshots to better illustrate the problem.

MR-howItSHouldRender.jpg – illustrates how I assume bump map should render. Where face that’s not directly lit, bump should not be visible.

vray-shadowOff.jpg – illustrates how vray renders it. Using very simple vrayMTL, with checker bump map. As you can see, even the face that’s not lit somehow shows the bump.

vray-shadowOn.jpg – what’s even more interesting, on the face that’s not directly lit, if there is a shadow casting on its backside, bump is not calculated.

We have tested lot of different settings..

Such as

Standard material + bump – when rendered with vray, does the same things. Makes us assume it’s not vrayMTL issue, but rather how vray render engine.

If, the object was not a single faced but rather a solid (as in corresponding face on back) it will render fine (it’s as if because that back facing face is casting shadow on front facing face its render)

When using vray2SidedMtl, if we turn off “force single-sided sub-materials” it works to an extent. However, the material is brighter, and when we adjust the translucency, it will reintroduce the same issue, or sometimes creates even bizarre results.

I am sure there is a perfect explanation for the behaviour, but would greatly appreciate if someone could explain to us what it is.

Also would appreciate any workarounds, or fixes for this issue.

Again thank you very much in advance! Look forward to your replies.

[vlado]

It seems to work as expected - you will get the same results with the scanline renderer and a standard material; it is mental ray that is the odd one out (but even in mental ray, you get the same effect when using a Standard material; and even with an Arch&Design material you can notice the same effect if you enable hardware shading in the viewports).

This effect happens because the bump map changes the surface normal, so while the base normal points away from the light, the bumped normal points towards the light. Since there is nothing blocking the light from reaching the surface, you get to see the illumination.

The best way to avoid this is to make sure that the surfaces have some actual thickness.

Best regards,
Vlado