Known Limitations

Ambient Occlusion: There is no additional scene selection mechanism supported to exclude certain objects from native ambient occlusion computation. The regular shadow object and instance flags are respected, but the finalgather flags are not taken into account. If this is desired then final gathering may be used to calculate just the ambient occlusion information, even if it takes longer to finish due to calling of shaders.; The native ambient occlusion cache can not be stored in files on disks.; 3.13 The ambient occlusion pass can take advantage of a CUDA-capable GPU, and may switch to CPU computation automatically if the GPU does not meet the requirements, like if the GPU generation is too old, or GPU memory is not sufficient. The GPU memory detection mechanism may be incorrect or fail in certain setups, but render options in the scene or on the command line can be used to override the behavior and chose GPU or CPU explicitly.
Assemblies: Assemblies may not update after incremental changes of their scene representation.
BRDF/BSDF: 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 used to estimate lighting distribution for direct illumination and IBL only, to improve performance and quality. It is not used similarly in any indirect illumination algorithm in mental ray, 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. If this is needed the regular hierarchical subdivision surfaces should be used.
Detail Shadowmaps: Detail shadow maps cannot be used in segmented shadow mode. They do not support shadowmap merging.
Emissive Objects 3.13: The object light "marker" shader can turn a single object into a light, but not more than one. If multiple objects should behave as light sources then each of these objects should get another instance of the "marker" assigned, even if the parameters are identical and suggest to share a single instance of the same shader.
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 23,000 x 23,000 pixels.
Global Illumination Engine (Prototype) 3.13: This engine does only work on the CPU at the moment. It does not take advantage of a GPU. On the other hand, and in contrast to the previous GI GPU mode, it works with all the mental ray rendering effects and most custom shaders seamlessly, with few exceptions (some volume shaders).; This engine is supposed to cover all previous GI modes, like final gathering, GI photons, irradiance particles, with a single solution. If this mode is enabled then the settings of the older features are ignored.
GI GPU 3.13: This engine is going to be superseded by the New Global Illumination Engine.; Motion blur is not fully supported. A scene with motion blur will render but use just a static GI GPU solution, which may show artifacts if the movement is large or the lighting will change noticeably in the animation. Ray perturbing lens, volume, and scattering shaders are not supported. Hair and particle scene primitives are not supported.
Hair: A single motion vector is supported for hair but not multiple, leading to a linear blur effect only.
Importons: When tracing importons lens shaders are currently ignored.
Irradiance Particles: Irradiance Particles cannot be used in combination with globillum photons. 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 and final gathering.
iray Rendering Mode: The iray rendering mode is functional but may be limited on certain platforms. If the installed GPU hardware, graphics driver software, or CUDA software is not capable of running iray then mental ray will execute a CPU version of the algorithm instead, which delivers identical results but typically requires much more time to finish.
Light Importance Sampling 3.13: Moving lights and user area lights are currently not supported. For lights with IES profiles, especially those with "blind spots", pre-sampling may estimate intensity incorrectly leading to illumination artifacts or slow convergence rate.
Map Data: There is currently no support for visualization of map files in the native image tools.; Using map data in distributed network rendering is not fully supported yet. Although material shaders are allowed to manipulate a map at the same time it is used during rendering, this mode currently requires special attention in custom shaders to synchronize the map content properly. This is not necessary in more common use cases of generating the map content prior to rendering, like in a pre-process, a previous rendering, or by an external application.
MetaSL Support: The current support for MetaSL features is not completed. Two back-ends are currently supported for software rendering on all platforms: the C/C++ back-end using an external compiler, and the LLVM back-end not depending on external tools, and replacing the previously offered .NET back-end running on Windows only. Most MetaSL shaders should work in the current version, but certain advanced effects may show problems until more MetaSL features are fully functional.
MDL Support: The MDL materials are considered a closed entity in mental ray currently. That means, they can not be connected to any other shaders, including other MDL-based nodes. The published parameters of MDL materials can be controlled by setting explicit values, but they can not be driven by outputs of other nodes. However, MDL materials and traditional shaders can co-exist within a scene and should render without problems with mental ray.; The MDL materials are intended to render a similar result when using mental ray, or iray - GPU or CPU. The quality and certain features of MDL effects may differ though, dependent on the capabilities of the render mode chosen.
Multiple Importance Sampling: The same limitations apply as for light importance sampling.
Progressive Rendering: 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. The progressive rendering performance may suffer noticeably in such cases.
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.
Stereoscopic: The only algorithms affected by stereoscopic rendering are the "first hit" renderers: scanline, rasterizer and tracing eye rays. Other view dependent algorithms like final gathering, importons, or tessellation, operate as if a single camera was used, that is they work from the "center eye". Similarly, shaders can not determine which of the two eyes is being rendered. In particular, state->camera refers to the "center eye", not the one actually being rendered.