Hello,
I hope I am posting this to the right thread, if not please move it wherever it seems more fitting.
I have been trying to set up a scene to make a finicky render. I have a dome light with a plain light grey colour as HDRI. I have a plane set to shadow catcher as ground and another simple default Vray Light lighting my object in the scene(plane). I am trying to light this object with these two lights as follows: I need the object to be lighted by the dome light alone. And I need the other light to cast the only shadow in the scene.
It does not matter what I try I can’t get this to work in Vray GPU. Using the latest version. I always get artefacts from the dome light. Exclude and include objects does not work. I tried to simply turn off cast shadows. Did not work. I tried to play with the shadow catcher options on the ground to no avail. I even tried disabling all the affect diffuse, specular and reflection options…somehow I still get flickers on the metallic bolts in the scene. Is there a way to prevent my dome light to cast shadows at all but simultaneously light my object in the scene so that I can light it with a “spotlight” to control the shadow of that object?
Of course to add to this problem I need the ground to be invisible as a shadow catcher as I would only need the shadow and the object. I have attached a couple of screenshots and explanations that hopefully can help with understanding my problem (I saved my .png renders on a white jpg background as the issue stems from the shadow being visible at the edges of the render).
What am I doing wrong here? I can’t seem to control the dome light at all, no matter what option I try to turn on or off.
Any help would be appreciated. Thank you.
Simply disabling the VRayDome light’s cast shadows and the VRay light’s affect diffuse, reflection, specular does the job. Check the attached scene and see if this is indeed the desired effect. Otherwise, do attach the scene so we can take a look.
It doesn’t work using gpu, which is why I never use it for such things. There are many posts about it and no fix as yet, unfortunately 
Works fine with cpu of course, so you can do a pass with that for the matte stuff, if that helps.
The light from the bolts is from GI, so you can turn off ‘Generate GI’ in props to solve that.
Thank you for your comment Fixeighted. This would explain a lot of things. I have been trying to get this to work for a couple of days now and none of the options seemed to satisfactorily do what I wanted them to do. At least this way I can try to work around it somehow. Maybe there is a different, better approach under the Vray GPU version?
Sadly there just isn’t any way for it to work like we want or expect with gpu.
If a light is ‘on’ then it will affect GI on all surfaces regardless of settings that suggest it can be turned off.
It is my thinking that gpu should have at least an entirely different set of UI parameters, so
that something which will not work is simply unavailable as an option, or there be some way of flagging the inoperability, as the problems arise from having
e.g. a tick box which suggests that it will do something, when in reality it will not, resulting in us thinking
we have either done something wrong or that something else is preventing the expected behaviour.
In this case, a separate UI would definitely be beneficial. Thank you for clarifying.
Sorry for the late reply. Regarding the dome light shadow, it seems to be fixed in the latest nightly builds and it will be implemented in stable builds soon enough. I cannot seem to get the light leak around the bolts in your scene, though, it seems to render fine with or without the “affect” options. Regarding the longer than the set render time issue, it most likely happens because the time limit is in effect after the GI calculation (thing is, in GPU, there is always a form of a GI active).
