Introduction to mental ray

mental ray is a general-purpose renderer which creates images of exceptional quality and achieves high performance through the exploitation of parallelism on both multiprocessor machines and across networks of machines.

The software uses advanced rendering acceleration techniques such as a scanline rendering algorithms for fast primary visible surface determination and BSP (binary space partitioning) algorithms for

efficient rendering of ray tracing effects. These algorithms can be fine-tuned by optional user-settable parameters to achieve even higher performance than normally achieved by the built-in automatic scene cost analysis.

mental ray supports caustics and global illumination simulation using the Photon Map method. Caustics caused by multiple reflections and/or refractions, caustics that are themselves reflected or refracted, and volume caustics are supported. Complete, physically correct simulation of general global illumination is also supported: any combination of diffuse, glossy, and specular reflection and transmission can be simulated, such as color bleeding caused by diffuse interreflections, and multiple volume scattering.

mental ray has been designed to take full advantage of parallel hardware, including both thread parallelism on a single machine, and process level parallelism across networks of machines, and on massively parallel distributed-memory systems. mental ray takes advantage of thread parallelism automatically; the use of other machines on the network as render slaves may be configured by the user. The renderer balances the computational load among the available processors using a distributed shared database that distributes parts of the scene in an optimal way based on demand.

mental ray can be combined with any suitable modeling and/or animation system via the .mi file format, or by integrating the library version into the modeling and animation system, or by combining the library with a translator that reads the modeling system's native file format and converts it directly to mental ray scene description. Finally, mental ray is available as a stand-alone program for batch-mode rendering.

In the standard standalone version, input is via a scene file in ASCII format. The .mi format is the native scene description format of mental ray. Supported geometric primitives include polygon and triangle meshes, trimmed free-form surfaces, subdivision surfaces, and hair. All geometry may be procedurally displaced.

Free-form surfaces may be input in non-uniform rational B-spline (NURB), Bézier, Taylor, or cardinal form, or through the use of basis matrices. Free-form surfaces may be of arbitrary degree. The geometry of free-form surfaces may be further modified by the application of trimming curves and displacement maps. Trimming curves need not have the same representation as the surface. Surfaces are triangulated internally using a variety of available approximation techniques and quality criteria like distance of the surface to the camera.

Connectivity information between free-form surfaces can be given which will stitch surfaces together and close gaps. If the connectivity is unknown, it can be automatically determined at run-time.

mental ray supports efficient rendering of animations using incremental changes to the scene database. Only the parts of the scene that change from one frame to the next need to be redefined. This feature allows mental ray to optimize scene tessellation, preparation, acceleration data structure management, and network transfers, taking advantage of the time coherency of the animation.

The functionality of mental ray may be extended through runtime linking of user-supplied C or C++ subroutines, called shaders. Shaders can be used to create geometric elements at runtime of the renderer, procedural textures, including bump and displacement maps, materials, atmosphere and other volume rendering effects, environments, camera lenses, and light sources. The user has access to a convenient environment of supporting functions and macros for use in writing shaders. The parameters of a user-provided shader can be freely chosen with name and type; user-defined shaders are not restricted to a list of predefined parameters. Available parameter types include integers, scalars, vectors, colors, textures, light sources, arrays, and nested structures. When a user-defined shader is called, mental ray will provide parameter values according to standard C calling conventions.

Standard material shader libraries provide a rich variety of parameters for describing material properties, including ambient color, diffuse color, specular color, transmission and shadow colors, a specular exponent, reflectivity, and transparency coefficients, and an index of refraction. The standard physics shader library adds global illumination effects and other physically correct simulations such as indirect illumination, translucency, glossy reflections, true diffuse light transport (a sub-function of global illumination that is often called "radiosity"), and color bleeding. Bump mapping and displacement mapping are supported as well. The material parameters are interpreted by the shader specified for the material. All parameters may be mapped with one or more textures or other shaders, by connecting shader inputs with other shaders outputs resulting in complex shader graphs.

The illumination effects in mental ray are not hard-coded features but they are all defined by shaders. New illumination features, or variations of existing features, can be added simply by writing new or modify existing shaders.

Furthermore, light which is passing through the free space surrounding objects, as well as light passing through solid objects, can be modified according to volume shaders. This allows the creation of fog and non-homogeneous transparency effects as well as visible light beams. In addition to standard material environment maps, a global environment map can be specified that provides a solid background for rays leaving the scene.

The standard base, physics, and contour shader libraries are available as source code


mental ray can generate a variety of output formats, including common picture file formats and special-purpose formats for depth maps and label channels. Alpha channels are supported. Bit depths of 8 and 16 bits per component are supported, as well as a 32-bit floating-point component mode and RGBE HDR (high dynamic range) images. User-supplied functions can be applied to the rendered image before it is written to disk.

Contour lines can be rendered with mental ray. They can be placed at discontinuities of depth or surface orientation, between different materials, or where the color contrast is high, simply by using an appropriate contour shader. The contour lines are anti-aliased, and there can be several levels of contours created by reflection or seen through semitransparent materials. The contours can be different for each material, and some materials can have no contours at all. The color and thickness of the contours can depend on geometry, position, illumination, material, frame number, and various other parameters as determined by the shader. The resulting image may be output as a pure contour image, a contour image composited onto the regular image (in raster form in any of the supported formats), or as a PostScript file.

Phenomena consist of one or

more cooperating shaders or shader trees (actually, shader DAGs; a DAG is a directed acyclic graph). A phenomenon consists of an "interface node" that looks exactly like a regular shader to the outside, and in fact may be a regular shader, but generally it will contain a link to a shader DAG. mental ray takes care of integrating all aspects of the phenomenon into the scene, which may include the introduction or modification of geometry, introduction of lenses, environments, and compile options, and other shaders and parameters.

The Phenomenon concept is conceived to unify - by packaging and hiding complexity - all those seemingly disparate approaches, techniques, and tricks, most notably (but not limited to) the concept of a shader, which are characteristic for today's state of the art in high-end 3D animation and in digital visual effects production. The aim is to provide a comprehensive, coherent, and consistent foundation for the reproduction of all visual phenomena by means of rendering. The Phenomenon concept provides the missing framework for the completion of the definition of a scene for the purpose of rendering in a unified manner.

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