I have a scene where reflections in mirrors come out darker compared to CPU. What’s causing this? Please check the screenshots below. The mirror is 100% reflective (no fresnel). The only workaround I’ve found so far is disabling Max Ray Intensity, but this is not feasible since it causes tons of fireflies and render time shoots up a lot.
Hard to say without the scene. Could you strip down the scene and attach it here (or send it via the contact form)?
check Trace depth. GPU defaults to 5.
I increased trace depth to 15, but it didn’t seem to make a difference
Sure. I’ve just submitted a request via the contact form containing a download link for the scene. Ticket number is #39113. BTW, I forgot to mention that I remember having this problem with Vray GPU (reflections appearing darker than they should) a few years back too - https://forums.chaos.com/forum/v-ray…-chrome-object And here’s another thread of mine that also seems to be related to these issues with Vray GPU: Light emitters behind a glass don't generate lens effects until I hide the glass object - Vantage :: Issues - Chaos Forum
Do you have any form of bump / normal map on the surface showing differences in the two rendering engines ?
I don’t know if it’s relevant but we had major differences between GPU and CPU around the issue of bumps maps (especially if they have quite high bumps values) resulting in darker surfaces in CPU than in GPU (wich is totally oposite of your result but we never know if it’s helping)
Best,
Nicolas
Yes, there are bump/normal maps on quite a few of the objects in the scene, but they are not too strong.
I found some new things. It’s not just reflections, but lights also seem to work incorrectly. I have tried to make this as clear as possible with the images below and with this video I recorded (download expires in 7 days). Can someone explain why having more lights turned on results in missing lights and darker reflections? BTW, the mirror is 100% reflective (255 RGB) and fresnel is unchecked.
Hi your models for the lamps have VRay2SidedMtl - this is supported partially in V-Ray GPU - https://docs.chaos.com/display/VMAX/…orted+Features
Your lamp model in the example above is a closed sphere without shell or thickness - for V-Ray this means a solid object.
Please try to edit the model and the material. I did a test in your scene that you have sent trough the ticketing system where I added a shell modifier to the lamp’s “balloon” and also replaced the 2sided material with a regular VRayMtl with refraction and a little refraction glossiness.
Also your VRayLight that is inside the lamp model is visible for specular - this will make the spherical VRayLight visible in the mirror.
Here is a test with all lights off according to your video:
Here is a test with all lights on according to your video:
Hi Vladimir,
Thanks for the images. I’m not using any unsupported features and materials by GPU. According to the help docs, only “multiply front by diffuse” is not supported which I have not used in this scene.
I did another test with a simple refractive VrayMtl and adding a Shell modifier to the lamp shade, as you suggested. I have also enabled “visible” and “affect reflections” for the light inside the lamp. The results? Equally unrealistic and incorrect. Please see the screenshots below. The only conclusion I can gather from this is that Vray GPU just doesn’t work correctly with refractive materials when glossiness is below 1.0 nor with Vray2SidedMtl materials. I am not sure why you keep denying that there are any issues with my scene and Vray GPU. I am sure that other users will agree with me that these images look very bad and completely incorrect with GPU.
I am sad to say that using Vray GPU has been a complete nightmare and a waste of my time (hence money) in this project and costed me enough time and money to troubleshoot all the bugs. And then I have to waste even more time converting the whole scene to the CPU engine in order to render it correctly and realistically. All of this has caused me an immense amount of stress because the deadline for delivering the project is literally in 2 days and I can’t deliver subpar images to this client (they are very strict about realism). These clearly are bugs because I have none of these issues with CPU - refractive materials with blurriness and 2sided materials both render realistically and as expected.
I don’t know what else to say except that I’m really disappointed in Vray GPU and with the quality of the support I’m receiving which is completely the opposite experience I’ve had with Vray and Vray support over the years up until now.
Hi @Alex_M I am sorry to hear that you felt like we are not doing our best to help you.
I tested your 2sided material and the simple vray refractive material in a simple scene.
This is the simple refractive material
This is the 2sided material merged from your scene - very slight difference here
In your scene however there is something really weird going on.
It seems that one group of vraydisc lights and another single sphere light are causing the issue.
We will continue looking for now I haven’t found a reason for this to happen.
Have you merged lights or can you remember something suspicions about these lights from the video above?
Hi @vladimir_krastev ,
You are correct. All these problems appear when there are objects in the include list of the ceiling lights. To me this is no doubt a bug in the GPU engine. I replaced the ceiling lights with completely new plane lights to rule out problems with the merged asset, but the bug still persists.
I will update the support ticket with a download link to the scene from the screenshots below. It’s set up and ready to render so you can see this for yourself.
Yes that will be great thank you.
I also noticed the include/exclude has something to do with it and testing here but still hasn’t got a solid conclusion that I can share with the forum.
Two things (the include list does not matter):
1. There’s a known issue (internal bug-tracker id: VGPU-883) with very small and very strong VRayLights producing artifacts on GPU. Increase the radius to resolve it.
2. VRayMtl’s reflection/refraction cutoff threshold is not supported on the GPU, which causes the major differences.
Thanks for the info, @hermit.crab . Are you saying that the problem will go away if I increase the radius of the ceiling lights? I changed it to 20cm and reduced their brightness, but the problem still persists. 
Hi
After testing it was confirmed in a simple scene that
If we have instanced VRay IES Lights
and we have a Physical Camera Exposure Control
and we have an additional VRayLight with include/exclude list set to include
and we have a refractive object with a VRayLight inside it and refraction glossiness lower than 1
the refraction of the refractive object does not render correctly in V-Ray GPU
In the video above I am switching on and off the single VRayLight that has include/exclude list but does not contribute to the lighting.
I logged this under VMAX-12222 for developer review.
THANK YOU! I’m not going insane after all! 
Is there anything we can do about this problem until a fix is available? Any workaround? I’ve already converted my project to CPU and managed to deliver it (thank god!), but it would be good to know if I can still use and rely on Vray GPU for future projects without it breaking like this if a workaround is available.
wow. among all the easy and straightforward ways to break vray gpu you’ve just found the most elaborate one I know.. hats off, sir.
Hi @vladimir_krastev ,
Unfortunately, I’m still encountering this bug in another scene. I was very careful not to use any IES lights, include/exclude lists in the lights, and any Exposure controls, as suggested by you in the list above. For the bulb I’m also using an extremely simple refractive VrayMtl material (no 2-sided materials). So a very very simple lighting and material setup. Yet, the bug still happens. 
I’m frankly afraid to continue using the GPU engine for any remotely serious work from now on and I’ll probably be doing all of my future project on CPU instead. Even though GPU is faster in a lot of cases, I’m finding myself actually losing time and money with the GPU engine because I’m constantly fighting bugs like this one and other strange behaviors. I can’t be the only person using GPU for production work, can I? I’m dumbfounded that no one else has noticed this bug before.
what do you mean by that? what do you change going from gpu to cpu?
If I try a similar setup (assuming absolute simplicity) then the results are practically identical, so possibly something else is causing this.
Post the lamp/light and maybe we can discover the culprit.
Alternatively, yes CPU will just do what you need without having to convert anything. It’s really only the other way around that it can be
a little tiresome 