Problem with Shadows on ground with wrapper material

[didier_hoffman]

Hello,
I am trying to get a simple scene rendered with a wrapper material to get the shadows of a car on the ground.

Here are the steps I use :
1. I create the scene with the car, I just use a dome light with an HDRI to light the scene and to get the image of the road. The result of the render is the 01 picture attached.
2. I create an inifinite plan under the car, and put a wrapper material in this plane. The parameters I change in the Wrapper are these : a new Generic material in the base material, Alpha Contribution to "Black Alpha (-1), Enable "Matte" and enable "Shadows" (picture 02 attached)
3; When I render the scene, I cannot get anything else than the result in picture 03 ! I made all the possible combinations with the render engines (CPU, CUDA, RTX) : nothing changes.

I am getting a bit nervous ! Because I am almost sure I used exactly the same method with VRAY 4, and it worked... I am now in VRAY 5. Could the problem be there ? Or am I really getting old and I forgot something ?

Can somebody help me ???

Thank you in advance for your help.

[nikoleta.garkova]

Hi didier_hoffman,

Make sure to replace the Environment Background texture slot with the desired HDRI in order to get it working.
Please find this screen recording for better visual understanding - hope it will be of help.

Alternatively, you can find the following tutorial on Wrapper material here - https://www.youtube.com/watch?v=nUVh...UpTutorialTips

[didier_hoffman]

Hello nikoleta.garkova

Thank you very much...
So the bad news is I am getting old ! I forgot the environment !
See the picture attached : it is working perfectly.

By the way, meaby a next problem for you : the render you see was made with CPU rendering. When I try to render with CUDA or RTX, no shadows on the ground !
Is it normal ? Because with CPU the rendering is very slow...

[nikoleta.garkova]

Hey Didier,

Can't notice a difference, can you share some screenshots/recordings along with steps to reproduce? Thanks!

[didier_hoffman]

Hello nikoleta.garkova
I made a small video. How can I send it to you ? (Size is small : 4 Mo, but still too large for the limit of 2Mo)

[nikoleta.garkova]

You can get in touch via our contact form, just make sure to mention this thread (or my name) in the email. Thanks!

[didier_hoffman]

I just submitted the contact form.

[didier_hoffman]

Hello nikoleta.garkova
I just submitted a packed project via the contact form.

[nikoleta.garkova]

Hi Didier,

I assumed you were keeping the original setup as described with a Dome Light, but looks like it is omitted in this particular file.

Wrapper material has only partial support in GPU, so a light source will be required for it to work. That being said, if you add a Dome Light and make it invisible, it should suffice as a workaround. See if that helps.

[didier_hoffman]

Hello nikoleta.garkova
Effectively, I didn't put a Dome light in the project I sent you.
And that leads me to a question for you : is there a difference between the result of a render with a HDRI put in the environment, and a HDRI put in a dome light ?
In fact, in the project of the parking I sent you, I made the render with the two options, and it seems that the results are the same. That's why I didn't put a dome light !

I can imagine just one difference : if there is sun in the scene (from the HDRI put in the Dome light), there must be a dome to get the shadows of the sun. Otherwise you just get the ambient occlusion without shadows from the sun.
But this means that in a rendering in a parking like the last one, where there is no sun, the results are the same with the two rendering options.
Am I right ???

[konstantin_chaos]

Hi didier_hoffman,

There is a difference between a regular environment and a Dome.

Environment:
When you use an environment texture you are relying on the global illumination rays to sample the texture and illuminate the set.
This works well when the environment is relatively ambient and there is no intense bright spot in the HDR image.
If however there is a sun light or any other small bright light source in the image, the GI sampling will have hard time producing sharp shadow (expected in this case).

Dome Light:
In theory the result produced by the Dome light source should be the same as the previous one.
The main difference though is that the Dome uses an advanced importance sampling algorithm to sample the map.
Small bright spots in the texture will be sampled better resulting in much more accurate lighting.
If for example there is a sun light visible the HDR it'll produce sharp shadows on the ground.

I can imagine just one difference : if there is sun in the scene (from the HDRI put in the Dome light), there must be a dome to get the shadows of the sun. Otherwise you just get the ambient occlusion without shadows from the sun.

It is safe to assume that, yes.

But this means that in a rendering in a parking like the last one, where there is no sun, the results are the same with the two rendering options.

Not necessarily.
If there are some very bright small light sources in the parking image - the dome will still work better.

CPU/GPU differences:
The main limitation of the GPU engine is that it does not calculate GI for Matte surfaces.
This means that if you use an environment setup (no Dome), there will be no environment light in the shadows (the CPU will render a different, more correct result).
If on the other hand you use a Dome light - the results will be more or less identical.
This is because the Dome light does not require GI rays to sample the environment - the Dome is sampled as a direct light source.

Hope that clarifies things to some extent.

Regards,
Konstantin

[didier_hoffman]

Hello konstantin_chaos

Thank you very much for this explanation...

Things are more clear now !