mental ray is described as '® is a rendering software which generates images of outstanding quality and unsurpassed realism. The software combines the physically correct simulation of the behavior of light with full programmability for the creation of any imaginable visual effect'. There are more than 25 alternatives to mental ray for a variety of platforms, including Windows, Mac, Linux, Autodesk 3ds Max and Sketchup. The best alternative is Adobe Dimension. It's not free, so if you're looking for a free alternative, you could try LuxCoreRender or POV-Ray. Other great apps like mental ray are Keyshot (Paid), V-Ray (Paid), appleseed (Free, Open Source) and RenderMan (Free Personal).
Adobe Dimension (formerly Project Felix and then Adobe Dimension CC) is the 3D design tool made for graphic designers. With extensive integration with Adobe Stock, you can easily composite 2D and 3D assets to build product shots, scene visualizations, and abstract art.
LuxCoreRender is a physically based and unbiased rendering engine. Based on state of the art algorithms, LuxCoreRender simulates the flow of light according to physical equations, thus producing realistic images of photographic quality.
The Persistence of Vision Raytracer is a high-quality, totally free tool for creating stunning three-dimensional graphics. It is available in official versions for Windows, Mac OS/Mac OS X and i86 Linux. The source code is available for those wanting to do their own ports.
V-Ray 2.0 for 3ds Max is a complete rendering solution, which combines the power of the well-known V-Ray award-winning production rendering engine with the flexibility and exceptional speed of the interactive rendering system V-Ray RT and the even more impressive new V-Ray RT...
appleseed is a modern, open source, physically-based rendering engine designed to produce photorealistic images, animations and visual effects. It provides individuals and small studios with an efficient, reliable suite of tools built on robust foundations and open technologies.