Known Limitations

Ambient Occlusion: There is currently no scene selection mechanism supported to exclude certain objects from ambient occlusion computation. Especially, the finalgather object and instance flags are not respected by ambient occlusion.; The ambient occlusion cache can not be stored in files on disks.
Assemblies: Assemblies may not update after incremental changes of their scene representation.
BRDF/BSDF 3.7: There is currently no support for measured BRDF data as input to a mental ray BRDF. The BRDF/BSDF which is attached to a material is currently not used to estimate lighting distribution in any illumination algorithm in mental ray to improve performance, but instead it is point-sampled like regular material 'black-box' shaders.
Catmull-Clark Meshes: The current implementation does not support automatic splitting of large input faces.; The current implementation does not support variable creases.
Frame Buffer Files: For cached frame buffers, the temporary disk files are saved in 'tiled' .map format which is limited to a file size of 2GB. Therefore, a single frame buffer may not exceed this 2GB size limit. For example, the resolution of a square 8-bit RGBA frame buffer is limited to about 23,000x23,000 pixels.
Irradiance Particles 3.7: Irradiance Particles cannot be used in combination with globillum photons and final gathering. mental ray will automatically adjust the rendering options on attempts to enable incompatible features, to allow existing scenes to be rendered with the new algorithm easily. However, Irradiance Particles are compatible with caustic photons.
MetaSL Support 3.7: The current support for MetaSL features via CIL back-end using .NET is incomplete. However, it may be used for demonstrational and experimental purposes. Typical shaders should work in the current version, but certain advanced effects may pose problems and specialized operators like swizzles are not yet fully supported.; The (temporary) DLL files generated by CIL backend uses the __stdcall linkage convention. Current version of mental ray does not support this convention for regular shader libraries. An attempt to load such a shader library in a different mental ray process would cause a crash.
Progressive Rendering 3.7: In this rendering mode just the main color framebuffer is computed. Although the traditional shading model is supported certain advanced features implemented in shaders may not work. Especially shaders which perform oversampling are generally not well suited for this rendering mode because sampling cannot be controlled and optimized by mental ray.
Rasterizer: The order of compositing of the samples from transparent objects is undetermined, like not using a depth-sorted ordering for example.; Visible area lights are not rendered by the rasterizer.
MetaSL Support: This feature is under development. Both the software and hardware rendering support for MetaSL shaders is limited. However, the automatic compilation of MetaSL source code and linking of generated binaries into mental ray is supported if capable C++ compiler and linker tools are available on the target system. Missing rendering features include advanced ray tracing effects like volume rendering, global illumination, and BRDF support.
Map Data 3.7: Map data are currently not considered for automatic splitting, memory flushing, and on-demand loading. Large point sets should be separated into smaller blocks in the scene description, and given to shaders as an array of maps. Material shaders are currently not able to edit a map during rendering but limited to read-only access to an existing map in the scene database, which has been generated by an application or a geometry shader before rendering starts. There are currently no support for map files in image tools, and no tools are available yet to manipulate map files, for example to merge several maps into one.