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I have seen two modes in 2.81 other than CUDA in the system settings - OptiX and OpenCL. What are they and how do I use them?

When I click on it, it's giving a message:

No compatible GPU found for path tracing. Cycles will render on the CPU.

For both (OptiX and OpenCL), I have a graphic which is enabled in CUDA. How do I enable and use it?

Any suggestion, video tutorial, or help.

Enter image description here

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OptiX™ is for RTX series graphic cards that include a ray trace (RT) core. For example, RTX 2080Ti.

Nvidia provides some API to let a program best use this GPU hardware for best results and also benefit from the original paper published in 2010. If your GPU is not Nvidia or it is not RTX (for example, GTX series), OptiX won't work. There is a Blender development blog post to explain it more.

And for OpenCL, it is for an AMD GPU to utilize the hardware since AMD doesn't support CUDA at all.

It is all listed in the user manual:

CUDA

If the system has a compatible NVIDIA CUDA device, it will show up an option for rendering with Cycles.

OpenCL

If the system has a compatible AMD OpenCL device, it will show up an option for rendering with Cycles.

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If you have a Nvidia GPU you can use either CUDA or OptiX. OptiX is specifically built for ray tracing and is probably faster than CUDA, which is built for general compute. Blender OptiX option uses Nvidia RT tech, which is only available on the newer Nvidia cards (NVIDIA GeForce, Quadro and Tesla products with Maxwell and newer generation GPUs.)

Here are the compatible cards for Nvidia ray tracing: https://www.nvidia.com/en-us/geforce/news/geforce-gtx-dxr-ray-tracing-available-now/ GTX 1660 seems to be the absolute minimum.

If you didn't have a Nvidia GPU you could only choose between CPU and OpenCL. OpenCL is a Khronos standard for open computing for multiple different platforms.

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