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Monday, May 05, 2008

Autodesk Mental Ray functions


mental ray offers an optimal powerful implementation of all the features traditionally expected of photorealistic rendering software, together with unique functionality not found in any other rendering software. The following sections describe rendering features, color handling and shading, and geometry processing capabilities in mental ray.


Ray Tracing

The software is based on a ray tracing architecture, which allows for the flexible implementation of any

imaginable phenomena and lighting effects, including reflections, refractions, global illumination, and subsurface scattering.

It uses an advanced BSP tree (more precisely a kd-tree) algorithm to speed up the ray intersection calculations. This structure is built on demand and cached. It can handle very large data sets and supports scalable multithreading.


Besides ray tracing, Autodesk Mental Ray also offers other rendering methods for situations where desired results can be produced much more efficiently.

A rasterizer is available for efficient first-hit rendering of directly visible objects and transparency. By separating visibility sampling and shading, high quality anti-aliasing can be provided while performing fewer of the expensive shading calculations (e.g. once per pixel). Motion blur can be computed with a relatively small performance impact, by shading once in the motion interval and carry this result along the motion path. This method is well suited e.g. for high quality cinematographic rendering.

A second scanline first-hit rendering method is also available, which is sometimes faster on smaller scenes with relatively simple shading.

Global Illumination

Global illumination is the simulation of all light inter-reflection effects in a scene. This includes indirect illumination caused by the scattering of light, and effects such as caustics and color bleeding: if a red table is next to a white wall, the white wall gets a reddish tint under typical lighting conditions. Without this reddish tint the image would look fake, even though it might be hard to point out precisely why. Global illumination effects are subtle but essential to true photorealism.

Simulation of global illumination has at least two distinct uses:

_ Physically accurate simulation of the illumination in an environment, for example the light

distribution inside an office building .

mental ray offers two fundamental approaches to compute global illumination which can be used together.They differ in the way lighting information is traced through the scene, either starting from the light (photon mapping) or starting from the eye (final gathering).
The photon mapping technique emits small energy particles (photons) from light sources in a preprocess, traces their trajectories through the scene including reflections, refractions, and interactions with participating media, and stores the final energy distribution in a specialized 3d data structure called photon map.

In the final rendering phase this global illumination contribution is added by collecting the color intensities of nearby photons.
Photon mapping supports caustics (e.g. sun light patterns on the ground of a swimming pool), participating media (e.g. shafts of light in a dusty room), color bleeding, arbitrarily many light bounces, as well as advanced material properties like glossy reflections.
Photon maps can be pre-computed and saved on disk for later reuse in subsequent renderings. Photon merging allows to reduce memory consumption for photon storage in scenes where a high number of photons must be shot to provide sufficient illumination.
The nal gathering technique computes global illumination by tracing rays as usual from a shading point into the scene to catch light.

In contrast to collecting the contribution from a distinct number of light sources it sends out many more rays into all possible directions of the hemisphere to capture illumination from other objects that interact with light (indirect lighting) like diffuse reflections (diffuse materials spread incoming light into many directions). This exact nal gathering mode is physically accurate but also expensive to compute.
mental ray provides an efficient technique to reduce the render time dramatically while retaining the final image quality as much as possible. The full final gather tracing is performed only on distinct and well selected surface points. All other surface points quickly interpolate the global illumination contribution from nearby final gather points. The final gather points are selected according to the local surface and illumination variation and their impact on final image quality. Several final gathering modes are provided which are optimized for ease of use, fly-through animations through static environments, and full user control, respectively.
The final gather data can also be saved on disk, and refined and re-used for subsequent renderings. Final gathering gives particularly good results for diffuse surfaces with a single light scattering bounce only.

Further variations of the base algorithms provide simplified and therefore more efficient applications for global illumination effects like contact shadows (ambient occlusion), and importance-driven photon mapping (importons).

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