This page provides information on the V-Ray GPU Render Devices Select rollout.
This rollout allows you to choose the engine type and devices for GPU Rendering. Selecting the GPU devices from this rollout is equivalent to using the external Select devices for V-Ray GPU Rendering utility that is included with V-Ray (ocldeviceselect.exe) but allows the devices to be changed without restarting 3ds Max.
||Render Setup window|| > Performance tab > V-Ray GPU Render Devices Select rollout
Engine type – Choose the rendering device. The render engine can be CUDA or RTX.
Choosing RTX GPU mode utilizes available RTX card(s).
Depending on what type of hardware is installed on your machine, V-Ray will attempt to set this option automatically for you. For example, if you only have NVIDIA hardware installed, the option will be set to CUDA automatically and disabled for further altering.
Selecting specific devices to render with can help if you have multiple GPUs and you want to leave one of them free for working on the user interface or you may want to combine your CPU and GPU together for Hybrid Rendering.