Note: CUDA is not required to use the radiation model. The radiation model supports a Vulkan compute backend that works on all platforms, including macOS and systems without NVIDIA GPUs. CUDA is only needed if you want to use the OptiX backend (NVIDIA systems only) or to build the Aerial LiDAR plugin.

Picking the right version of the NVIDIA CUDA driver can be the most difficult part of the CUDA install. If you choose the wrong version, the resulting error messages may not always be clear. This guide will help you to choose the right CUDA version for your system. It may also be necessary to change your OptiX version if using an old GPU along with the radiation model. Fortunately, this is easy and will also be described below.

CUDA Version

There are three main considerations when choosing your CUDA version:

GPU compute capability: The compute capability of your GPU is essentially an identifier corresponding to the architecture generation. Older GPUs will have a lower compute capability. You can look up your GPU's compute capability here. You need a compute capability of 3.5 or higher to build Helios. If your compute capability is 3.5, you need to enable the OPTIX_VERSION_LEGACY option as described below.
C++ compiler version: Only a certain range of compiler versions are supported for a given CUDA version. The procedure for looking up your compiler version is different for different operating systems, and is described below.

The system requirements for any CUDA version can be found in the release notes, which is both packaged with the CUDA toolkit and can be found on the page where you download CUDA.

The table below gives recommendations for Windows and Linux Systems based on your compute capability.

Operating System	Compute capability	Recomm. Compiler	Recomm. CUDA	Use legacy OptiX?
Windows	3.5	Visual Studio 2019	10.2	Yes
Windows	5.0+	Visual Studio 2022	Latest	No
Linux	3.5	gcc/g++ 6.0-8.0	10.2	Yes
Linux	5.0+	gcc/g++ 7.0-11.0	Latest	No

OptiX Version

The NVIDIA OptiX library is used for ray-tracing calculations in the radiation model. Helios bundles three versions of OptiX and selects among them automatically based on your NVIDIA driver version:

Driver version	OptiX version used	CUDA requirement
≥ 560	OptiX 8.1 (default, recommended)	CUDA 12.0+
< 560	OptiX 6.5 (legacy)	CUDA 9.0+
Any (or no NVIDIA GPU)	Vulkan compute backend	None

Note: NVIDIA drivers 590.x and later dropped support for OptiX 6.5. If you have a modern GPU and driver, Helios automatically uses OptiX 8.1, which supports all current and future NVIDIA drivers. The driver 590+ incompatibility that existed in earlier Helios versions is fully resolved.

Legacy GPUs (compute capability 3.5): If you are using a GPU with compute capability < 5.0, you can force the OptiX 6.5 legacy backend by setting -DOPTIX_VERSION_LEGACY=ON. There are two ways to do this:

Pass option to CMake: Set the following CMake option in CLion: "-DOPTIX_VERSION_LEGACY=ON". Go to CLion settings, then "build, execution, deployment -> CMake", and type "-DOPTIX_VERSION_LEGACY=ON" into the CMake Options box. You can also pass this option to CMake if building from the command line.
Add the variable directly in your project CMakeLists.txt file: Add the line set( OPTIX_VERSION_LEGACY ON ) to your project CMakeLists.txt file.

	1.3.66