Autodesk Mental Ray Tutorials ,Rendering,Lighting,Shading,Texture Mapping etc

1.29 Diagnostic modes

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:56:00 +0000

1.29 Diagnostic modes

mental ray supports a number of diagnostic modes that help visualizing and optimizing the rendering process. They modify the output image to include grid lines or dot patterns that indicate coordinate spaces or sampling or photon densities. These graphs allow simple detection of insufficient or excessive sampling densities, and help to tune parameters such as numbers of photons or sampling and contrast limits. Grid Mode This mode renders a grid on top of all objects in the scene, in object, camera, or world space. It?s useful to get an idea of the scene scale and to enable rough estimates of distances and areas.

Photon DensityModeThis mode replaces shows a false color rendering of photon density on all materials. This is useful when tuning the number of photons to trace in a scene, and to select the optimum accuracy settings for estimation of global illumination or caustics.

It also works well in combination with the Grid Mode described above. SamplesmodeThis mode shows how spatial supersampleswere placed in the rendered image, by producing a grayscale image signifying sample density. This is useful when tuning the level and the contrast threshold for spatial supersampling.

Diagnostic modes are enabled with the -diagnostic option on the command line, or the diagnostic statement in the options block in the scene description file.

1.28 Global Illumination in Participating Media

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:55:00 +0000

1.28 Global Illumination in Participating Media

Global illumination in volumes2:1can be used to simulate for example:

Color bleeding from a colored wall onto gray smoke.

Multiple light scattering in clouds or other participating media.

Global illumination in participating media is computed much the same way as global illumination on surfaces, except that volume shaders and volume photon shaders are needed. To change the number of photons used to compute the local intensity of global illumination in volumes, specify a photonvol accuracy (and optionally a radius) in the options (similar to volume caustics).

1.27 Volume Caustics

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:54:00 +0000

1.27 Volume Caustics

Volume caustics are caused by light that has been specularly reflected or refracted by one or more surfaces and is then scattered by a participating medium in a volume. Examples are:

Sunlight refracted by a wavy water surface and then scattered by little silt particles in the water.

Car fog lamps: light emitted by a bulb filament, reflected by a parabolic reflector, and scattered by fog.

In order to create volume caustics, the same light sources, material shaders, photon shaders, and caustic tags as for caustics are needed. But in addition, volume shaders and volume photon shaders are needed. For example:

material "volsurf" opaque # material for surfaces of volume
"transmat" ()
shadow "transmat" ()
photon "transmat_photon" ()
volume "parti_volume" (
"scatter" 0.05 0.05 0.05,
"extinction" 0.05,
"lights" ["arealight-i"]
)
photonvol "parti_volume_photon" (
"scatter" 0.05 0.05 0.05,
"extinction" 0.05
)
end material

Photons that get scattered multiple times in the volume are stored in a volume photon map. During rendering, volume shaders can call the function mi compute volume irradiance to get irradiance from the photons stored in the volume photon map.

In order to fine-tune the volume caustic, it is possible to change the number of photons that is used to compute the indirect light in the volume caustic. This is done with a photonvol accuracy statement in the options. The default is 30 photons and a radius that depends on the scene extent.

1.26 Final Gathering

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:52:00 +0000

1.26 Final Gathering

For diffuse scenes, final gathering2:1can improve the quality of the global illumination solution. Without final gathering, the global illumination on a diffuse surface is computed by estimating the photon density (and energy) near that point.

With final gathering, many new rays are sent out to sample the hemisphere above the point to determine the incident illumination. Some of these rays hit diffuse surfaces, and the global illumination at those points is then computed by the material shaders at these point, using illumination from the globillum photon map if available and other material properties.

Other rays hit specular surfaces and do not contribute to the final gather color (since that type of light transport is a secondary caustic).

Tracing many rays (each with a photon map lookup) is very time-consuming, so it is only done when necessary ? in most cases, interpolation and extrapolation from previous nearby final gathers is sufficient. Final gathering is useful in scenes with slow variation in the indirect illumination. For example purely diffuse scenes.

For such scenes, final gathering eliminates photon map artifacts such as low frequency noise and dark corners. With final gathering, fewer photons are needed in the globillum photon map and lower globillum accuracy is sufficient since each final gather averages over many values of indirect illumination. Final gathering is off by default, but can be turned on in the options. To change the number of rays shot in each final gather (and optionally the max distance at which a final gathering result can be used for interpolation and the min distance at which is must be used), specify a finalgather accuracy in the options.

For example,

finalgather accuracy 1000 1.5 0.25

Increasing the number of rays reduces noise in scenes with complex illumination and geometry; the default number of rays is 1000. The default maximum distance depends on the scene extent; decreasing it will reduce noise but increase render time. The default minimum distance is 10%of the maximum distance.

1.25.3 Fine-tuning Global Illumination

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:51:00 +0000

1.25.3 Fine-tuning Global Illumination

To change the number of photons used to compute the local intensity of global illumination, specify a globillum accuracy (and optionally a maximum radius) in the options. For example,

globillum accuracy 300 2.0

The default number is 500; larger numbers make the global illumination smoother but increases render time. The default radius depends on the scene extent.

1.25.2 Objects

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:47:00 +0000

1.25.2 Objects

By default, all objects can participate in global illumination computations. This is necessary for simulation of real global illumination. However, sometimes one might just be interested in color bleeding from one object to another, and the rest of the scene does not need to participate in the global illumination simulation. To simulate global illumination more efficiently in such cases, objects can be flagged such that the photons are only emitted towards certain objects and stored only on selected objects.

Objects are then divided into globillum-casting (globillum 1 flag) and globillum-receiving (globillum 2 flag), or both (globillum 3 flag), or neither (globillum 0 flag). For example, color bleeding from a red diffuse table onto a diffuse white wall requires a globillum-casting table and a globillum-receiving wall. Objects can also be flagged globillum off, which means that globillum photons will not hit them at all (the objects will be ?invisible? to globillum photons), or flagged globillum on which is the same as globillum 3.

The globillum mode is an object attribute. Photons are emitted only in the direction of globillum-casting objects, and only stored on globillum-receiving objects. To use this optimization requires that the default object globillum flag (specified in the options) is set to something different than 3 (which is the default value, enabling all objects to cast and receive global illumination).

1.25.1 Light Sources

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:47:00 +0000

1.25.1 Light Sources

Each light source that should emit global illumination should have an energy statement (just as for caustics). Each light source can also optionally have a globillum photons statement to specify how many photons should be emitted (similar to caustic photons for caustics). The default value is 100000 globillum photons. For example:

light "globillum-light" "physical_light" (
"color" 700.0 700.0 700.0
)
origin 20.0 30.0 -40.0
energy 700 700 700
globillum photons 100000
end light

1.25 Global Illumination

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:46:00 +0000

1.25 Global Illumination

Global illumination is the simulation of all light interreflection effects in a scene (except caustics). This includes effects such as color bleeding: if a red table is next to awhitewall, thewhite wall gets a slightly pink tint. This effect is not possible with ordinary ray tracing algorithms.

But if the pink tint is lacking in an image, the image looks fake, even though it might be hard to point out precisely why. Global illumination effects are subtle but add realism to a scene. 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.

Visually pleasing lighting effects for applications in the entertainment industry. Here physical accuracy is not the most important aspect, the images just have to look believable.

The computation of global illumination requires photon tracing, just like computation of caustics. In fact, the same photon material shaders can be used. Since caustics are treated separately in mental ray, the global illumination simulation does not include caustics. So if all light interreflections should be simulated, both global illumination and caustics must be enabled.

The photons stored during global illumination simulation are stored in a separate photon map, the global illumination photon map. When the material shader calls mi compute irradiance, the irradiance from both the caustics photon map and the global illumination photon map are computed. To turn global illumination on, specify globillum on in the options or give command-line option -globillum on.

1.24.5 Fine-tuning Caustics

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:44:00 +0000

1.24.5 Fine-tuning Caustics

The number of photons used to estimate the caustic brightness can be changed with the global option caustic accuracy. The accuracy controls how many photons are considered during rendering. The default is 100; larger numbers make the caustic smoother.

There is also an optional radius parameter. The radius controls the maximum distance at which mental ray considers photons. For example, to specify that at most 200 photons should be used to compute the caustic brightness, and that only photons within 1 scene unit away should be used, specify:

caustic accuracy 200 1.0

in the options. (Similar accuracy parameters are available for global illumination, volume caustics and global illumination, and final gathering.) Accuracy parameters can be used to select two fundamentally different sampling policies.

If R is relatively large (as is the default) the overall limiting factor becomes N and R only catches runaway situations with very few photons. Since darker areas have fewer photons than brighter areas, the effective radius within which the N photons are found is larger in dark areas. The effect is that low intensity areas will have less detail than high intensity areas.

Also, increasing the number of photons in the scene will result in the effective radius becoming smaller, so if N is not adjusted to compensate the same amount of noise will be seen, only on a smaller scale. The other policy is to select R small, on the scale of the detail the user wishes to see. N is then kept high, or even set to 0 (unlimited). In this case, a constant radius is examined which results in the scale of the detail remaining constant between light and dark areas.

Increasing the number of photons in the map will have the effect of reducing the error noise. In practice, one will often use a combination of the two, with a small radius to get detail in dark areas, and N set at a moderate value to speed up rendering. Irrespective of the chosen policy, a large effective radius gives less noise, but a more blurred result. To decrease the noise without blurring detail, it is necessary to increase the number of photons in the photon map.

It is very instructive to explore the effect of setting these options with the aid of the diagnostic photon2:1 option since the false color image it generates shows the difference in estimated density more clearly. For fast previewing of caustics it can be useful to use N=20.

1.24.4 Shader Functions

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:43:00 +0000

1.24.4 Shader Functions

There are two functions that are especially important to writers of photon shaders. The shader interface function mi choose scatter type chooses a scatter type for a photon based on the probabilities for diffuse, glossy, and specular reflection and refraction.

It can also choose absorption, that is, that the photon should be traced no further. The function also ensures a correct energy level in the scene by altering the reflection coefficients according to the scatter choice.

During regular ray tracing, material shaders of caustic-receiving objects should call the mi compute - irradiance function to ?pick up? the illumination caused by photons reaching the object during preprocessing.

1.24.3.2 Physically Plausible Material Shaders

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:42:00 +0000

1.24.3.2 Physically Plausible Material Shaders

A pair of material shaders that emphasize physical accuracy are dgs material and dgs material - photon. They simulate three types of reflection: diffuse (Lambert?s cosine law), glossy (isotropic or anisotropic), specular (mirror), aswell as the corresponding types of refraction and translucency, and any combination of these.

Therefore, they can simulate mirrors, glossy paint or plastic, anisotropic glossy materials such as brushed metal, diffuse materials such as paper, translucent materials such as frosted glass, and any combination of these. Each material declaration using dgs material has to have a photon "dgs material photon" () statement. These two shaders should be used together for physical accuracy.

An example is:

material "mirror" opaque # ideal mirror material
"dgs_material" (
"specular" 1.0 1.0 1.0,
"lights" ["arealight-i"]
)
shadow "dgs_material" ()
photon "dgs_material_photon" ()
end material

Another pair of physically based shaders is dielectric material and dielectric material - photon. For further details on dgs material, dielectric material, and their photon shaders, see the documentation of the Physics Shader Library.

1.24.3.1 Softimage Material Shaders

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:40:00 +0000

1.24.3.1 Softimage Material Shaders

The Softimage material shader can be used with caustics (even though it is not physically correct). This allows the user to have a creative attitude towards realism. The Softimage material shader computes two types of reflection: specular and Lambertian diffuse.

The specular reflection of a light source is modeled by Phong?s reflection model, while specular reflection of light from other parts of the environment is modeled by mirror reflection. For the Softimage material, the photon material shader is called soft material photon.

Each material declaration in the scene has to have a photon "soft material photon" () statement. It is possible to use other photon material shaders with soft material, but this is not recommended as their parameters may be different or have different meanings.

Using these Softimage material shaders makes it possible to design a scene without caustics, and then add the caustics as the ?final touch? without the whole image changing drastically and without having to redesign all materials in the scene.

1.24.3 Material Shaders and Photon Shaders for Caustics

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:39:00 +0000

1.24.3 Material Shaders and Photon Shaders for Caustics

mental ray comes with threematerial shaders that support caustics (and global illumination2:1) soft material, dgs material and dielectric material. Their photon shader equivalents are soft material photon, dgs - material photon and dielectric material photon. When defining a material it is necessary to specify both the regular material shader and the photon shader. Most often, however, the photon shader can inherit the parameter setting from the regular material shader. In addition to these six shaders, users can write new material and photon material shaders.

1.24.2 Objects

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:38:00 +0000

1.24.2 Objects

By default, all objects can cast and receive caustics, that is, photons are emitted in all directions from a point light source (and with all possible origins for a directional light source). For some scenes, this is fine ? for example if a point light source is surrounded by specular surfaces.

But for some scenes, it is very inefficient ? for example a point light far away from a single small specular object.To generate caustics more efficiently in such scenes, objects can be flagged such that the photons are only emitted towards certain objects and stored only on selected objects. Objects are then divided into causticcasting (caustic 1 flag) and caustic-receiving (caustic 2 flag), or both (caustic 3 flag), or neither (caustic 0 flag).

For example, caustics on the bottom of a swimming pool require a caustic-casting water surface and a caustic-receiving pool bottom. Objects can also be flagged caustic off, which means that caustic photons will not hit them at all (the objects will be ?invisible? to caustic photons), or flagged caustic on which is the same as caustic 3.

The caustic mode is an object attribute. Photons are emitted only in the direction of caustic-casting objects, and only stored on caustic-receiving objects. To use this optimization requires that the default object caustic flag (specified in the options) is set to something different than 3 (which is the default value, enabling all objects to cast and receive caustics). For example, the options can contain

caustic on

caustic 0

The definition of a caustic-casting object can for example begin as object "revol4" caustic 1 visible shadow trace tag 13 The material of a caustic-casting object has to be mainly specular (little or no diffuse reflection), and for Softimage materials, the sum of reflection and transparency has to be close to or larger than 1. For caustics generated by refraction, the index of refraction has to be different from 1. For example, the index of refraction for water is 1.33, for glass 1.5 to 1.7, and for diamond 2.42.

1.24.1 Light Sources

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:36:00 +0000

1.24.1 Light Sources

Photons are emitted from the light sources in the scene. It is necessary to attach some extra information to each light source to control the energy being distributed into the scene (and optionally the number of photons emitted). To generate caustics from a particular light source, one must specify the energy emitted by the light source. This is given by the energy keyword. The energy is the flux distributed by the light source and it will be distributed into the scene by each photon which will carry a fraction of the light source energy. If the energy is zero (the default), no photons will be emitted. Another important factor is the number of photons to be generated by this light source.

This can optionally be specified using the caustic photons keyword (10000 photons is the default). This will be the number of photons stored in the photon map and thus a good indication of the quality of the generated caustics. It is also a direct indication of the memory usage which will be proportional to the number of photons in the photon map. For quick, low-quality caustics, caustic photons 10000 is adequate, formedium quality 100000 is typically needed, and for highly accurate effects, caustic photons 1000000 can be necessary.

It is also possible to specify a second integer, which is the maximum number2:1of photons to be emitted from the light source.

By default there is no upper limit (indicated by the value 0), in which case emission will continue until the specified number of photons have been stored. Notice that the emitted number of photons and the preprocessing time is most often proportional to the number of photons generated in the photon map. For most light sources, the distribution of energy using photons will give the natural inverse square falloff of the energy. This might be an unwanted effect since some light shaders implement a linear fall-off. It can be avoided by using the exponent keyword. If the exponent is p, the fall-off is 1=rp, where r is the distance to the light source.

Exponent values between 1 and 2 make the indirect light less dependent on distance. Exponents of less than 1 is not advisable, as it often gives visible noise. Exponent 2 is the default. The following is an example of a light that uses the soft point light shader and is capable of generating caustics: light "caustic-light1" "soft_point" ( "color" 1.0 1.0 0.95, "shadow" on, "factor" 0.6, "atten" on, "start" 16.0, "stop" 200.0 ) origin 20.0 30.0 -40.0 energy 700 700 700 caustic photons 100000 exponent 1.5 end light An example of a light source which uses the inverse square fall-off to compute illumination (and the default 10000 photons) is: light "point1" "physical_light" ( "color" 700.0 700.0 700.0 ) origin 20.0 30.0 -40.0 energy 700 700 700 end light It is important to note the difference between color and energy. color is the power of the direct illumination, while energy will be the power of the caustic. It is therefore possible to tune the brightness of caustics to make them more or less visible.

If area light source information, such as a rectangle statement, is added to the light source definition, both the direct and global illumination will be emitted from an area light source. This tends to make caustics more fuzzy. To emphasize caustics, the energy of the light sources can be higher than their colors (that determine the direct illumination).

If, for whatever reason, the user wants to have the sources of caustics to be at different positions than the sources of direct illumination, this is possible too. It might also be that a single light source is sufficient for the caustics, while several light sources are needed to fine-tune the direct illumination.

1.24 Caustics

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 08:14:00 +0000

1.24 Caustics

Caustics are light patterns that are created when light from a light source illuminates a diffuse surface via one or more specular reflections or transmissions. Examples are: The light patterns created on the bottom of a swimming pool as light is refracted by the water surface and reflected by the diffuse pool bottom.

Light being focused by a glass of water onto a diffuse table cloth.

The light emanating from the headlights of a car: the light is emitted by the filament of a light bulb, reflected by a parabolic mirror reflector (thereby being focused in the forward direction), and reflected by the diffuse road surface.

Caustics cannot be simulated efficiently using standard ray tracing since predicting the potential specular paths to a light source from any given surface is a difficult (and in many situations impossible) task. To overcome this problem mental ray uses a photon map.4 The photon map is generated in a preprocessing step in which photons are emitted from the light sources and traced through the scene using photon tracing.

The emission of photons is controlled using either one of the standard photon emitters for point lights, spot lights, directional lights, and area lights, or by using a user defined photon emitting shader. A photon leaving the light source can be reflected or transmitted specularly by objects.

The photon is traced through the scene until it either hits a diffuse surface or until it has been reflected or transmitted a maximum number of times as indicated by the photon trace depth. When a caustic photon hits a diffuse object it is stored in a caustic photon map and not traced any further.

To control the behavior of photons as they hit objects in the scene, it is necessary to attach photon material shaders to these objects. Photon material shaders are similar to normal material shaders with the main difference being that they trace the light in the opposite direction.

Also, a photon shader distributes energy (flux) instead of collecting a color (radiance). Another important difference is the fact that photon material shaders do not need to send light rays to sample the contribution from the light sources in the scene.

In order to use the photon shaders, it is necessary to include the physics.mi file which contains the declarations of all the physics-based material shaders and photon shaders ? or the softimage.mi file which contains the Softimage material shader and photon shader. To turn caustics on, specify caustic on in the options or use the command-line option -caustic on.

1.23 Memory-mapped Textures

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:47:00 +0000

1.23 Memory-mapped Textures

mental ray supports memory mapping of textures in UNIX environments. Memory mapping means that the texture is not loaded into memory, but is accessed directly from disk when a shader accesses it. There is no special keyword or option for this; if a texture is memory-mappable, mental ray will recognize it and memory-map it automatically. Only the map image file format (extension .map) can be mapped. See the Output Shaders chapter for a list of supported file formats.

Note that memory mapping is based on the concept that the image data on disk does not require decoding or data type conversion, but is available in the exact format thatmental ray uses internally during rendering.

Normally mental ray will attempt to auto-convert image data formats; for example if a color image file is given in a scalar texture construct, mental ray will silently convert the color pixels to scalars as the texture is read in. Most data types are auto-converted to most other data types. This does not work for memory-mapped textures.

Memory mapping requires several preparation steps:

The texturemust be converted to .map format using a utility likemental images? imf copy. The scene file must be changed to reference this texture. Note that mental ray recognizes .map textures even if they have an extension other than .map; this can be exploited by continuing to use the old file name with the ?wrong? extension.

Memory-mapped textures are automatically considered local by mental ray, as if the local keyword had been used in the scene file. This means that if the scene is rendered on multiple hosts, each will try to access the given path instead of transferring the texture across the network, which would defeat memory mapping. The given path must be valid on every host participating in the render.

The texture should not be on an NFS-mounted file system (one that is imported across the network from another host). Although this simplifies the requirement that the texture must exist on all hosts, the necessary network transfers reduce the effectiveness and can easilymakememory-mapping slower than regular textures.

Memory-mappingworks best if there are extremely large textures containing many tens ofmegabytes that are sampled infrequently because then most of the large texture file is never loaded into memory. If the textures and the scene are so large that they do not fit into physical memory, loading a texture is equivalent to loading the file into memory, decompressing it, and copying it out to swap.

(The swap is a disk partition that acts as a low-speed extension of the physical memory that exists as RAM chips in the computer). From then on, accessing a texture means accessing the swap. Memory mapping eliminates the read-decompress-write step and accesses the texture from the file system instead of from swap. This has the side effect that less swap space is needed. If the texture and scene are not large and fit into memory, and if the texture is accessed frequently, memory-mapped textures are slower than regular textures because the swap would not have been used.

1.22 Contours

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:45:00 +0000

1.22 Contours

Contour lines can be an important visual cue to distinguish objects and accentuate their forms and spatial relationship. Contour lines are especially useful for cartoon animation production. Contours can be placed at discontinuities of depth or surface orientation, between different materials, or where the color contrast is high. 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. 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. It is not possible to render contours in a scene with motion blur.

Contour shaders are called while the normal color image is created. Contours are computed using information stored by a contour store shader. The contour store shader is called once for each intersection of a ray with a material. The position of contours are determined by a contour contrast shader. It compares the two sets of information for a pair of points, and decides whether there should be a contour between the points. The color and thickness of the contours are determined by contour shaders.

1.21 Output Shaders

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:44:00 +0000

1.21 Output Shaders

mental ray can generate more than one type of image. There are up to five main frame buffers: for RGBA, depth, normal vectors, motion vectors, and labels. The depth, normal vector, motion vector, and label frame buffers store the Z coordinate, the normal vector, the motion vector, and the label of the frontmost object at each sample of the image. If multiple samples are taken for a pixel, the frame buffer value for that pixel may be either any one sample value, or a blend of all samples.

The number and type of frame buffers to be rendered is controlled by output statements. Output statements specify what is to be done with each frame buffer. If a frame buffer is not listed by any output statement, it is not rendered (except for RGBA, which always exists).

There are two types of output statements, those specifying output shaders and those specifying files to write. There are also up to eight user-defined frame buffers2:1that can be defined with any data type, using a frame buffer statement in the options block.

Output shaders are user-written functions that can be linked at runtime that have access to every pixel in all available frame buffers after rendering. They can be used to perform operations like post-filtering or compositing. Files to write are specified with data type, file format and file name. If the data type is omitted a default data type is used that is assumed to be the ?best? type for the given image format.

The data type implies the frame buffer type. There are special file formats for depth, vector, and label files, in addition to a variety of standard color file formats. By listing the appropriate number and type of output statements, it is possible to writemultiple files. For example, both a filtered file and the unfiltered version can be written to separate files by listing three output statements: one to write the unfiltered image, one that runs an output shader that does the filtering, and finally another one to write the filtered image. Output statements are executed in sequence.

The following file formats are supported:

Each of these file formats implies a particular default data type (the first entry in column ?Supported data types?); for example, "pic" implies 8-bit RGBA, and "zt" implies Z. The default data type may be overridden by explicitly specifying another data type, such as a 16-bit type, in the output statement, as long as it is supported and appears in the above table. mental ray will adjust its frame buffer list to compute the requested types. For example, the standard RGBA frame buffer stores 8 bits per component by default, but if any output statement references a 16-bit type, the RGBA frame buffer also switches to 16 bits.

The available data types are:

The difference between "vta" and "vts", and between n and m, is significant only when automatic conversions are done. The file contents are identical except for the magic number in the file header. The floating-point RGBA data type "rgba fp" allows color and alpha values outside the normal range (0; : : : 1), and no dithering is applied even if explicitly enabled.

In contrast, any conversion to the 8-bit or 16-bit formats will clamp values outside this interval. Note that dithering reduces the effectivity of RLE compression. Allmental images file formats contain a header followed by simple uncompressed image data, pixel by pixel beginning in the lower left corner.

Each pixel consists of one to four 8-bit, 16-bit, or 32-bit component values, in RGBA, XYZ, or UV order. The header consists of a magic number byte identifying the format, a null byte, width and height as unsigned shorts, and two unused null bytes reserved for future use. All shorts, integers, and floats are big-endian (most significant byte first).mental ray can combine samples within a pixel in different ways. The combination of existing samples can also pad the frame buffers to ?bridge? unsampled pixels. Interpolation of colors, depths, normals, and motion vectors means that they are averaged, while interpolation of the labels means that the maximum label is used (taking the average label is not a good idea).

Interpolation of depths only takes the average of non-infinite depths, and interpolation of normals and motion vectors only takes the average of vectors different from the null vector. Interpolation is turned on by writing a ?+? in the beginning of the output type and turned off by writing a ??? there.

For example, to interpolate the depth samples, write ?+z? in the output statement. If interpolation is turned off for a frame buffer, the last sample value (color, normal, motion vector, or label) within each pixel is stored, and pixels without samples get a copy from one of the neighbor pixels.

Interpolation off for depth images is an exception: rather than using the last sample depth, the min depth is used?this can be useful for compositing. Interpolation is on by default for color frame buffers (including alpha and intensity frame buffers) and off by default for depth, normal, motion vector, and label frame buffers.

1.20 Color Calculations

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:40:00 +0000

1.20 Color Calculations

All colors in mental ray are given in the RGBA color space and all internal calculations are performed in RGBA. The alpha channel is used to determine transparency; 0 means fully transparent and 1 means fully opaque. mental ray uses premultiplied colors, which means that the R, G, and B components are scaled by A and may not exceed A.

Optionally, RGBA colors may be stored in the output image in non- 3OpenGL is a registered trademark of Silicon Graphics, Inc. 20 1 Functionality premultiplied form to increase the precision of highly transparent pixels, but internally mental ray and all shaders work with premultiplied colors.

Premultiplication is used to simplify compositing operations. Internally, colors are not restricted to the unit cube in RGB space. As a final step before output, colors are clipped using one of two methods. By default, the red, green and blue values are simply truncated.

Optionally, colors may be clipped using a desaturation method which maintains intensity (if possible), but shifts the hue towards the white axis of the cube. Desaturation color clipping may be selected with either the -desaturate on option on the command line or desaturate on in the options block in the scene. The alpha channel is always truncated.

1.19 OpenGL Acceleration

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:39:00 +0000

1.19 OpenGL Acceleration

By default, mental ray uses a scanline rendering algorithm for primary rays when no lens shaders are present that modify the ray direction. This algorithm can cope with both static and motion blurred scenes. In addition, mental ray supports OpenGLRc3 hardware to further accelerate scanline rendering of static scenes (without motion blurring). If the client host provides OpenGL acceleration with sufficient resolution, mental ray can use this to generate acceleration data that is subsequently used during regular rendering.

This combines the speed of a hardware OpenGL accelerator with the full shading capabilities of mental ray, and is particularly effective for scenes with high polygon counts. Shadow maps can also be rendering withOpenGL acceleration. This is particularly effective since shadow maps need no shading.

Most of the computation involved is then performed by the OpenGL system, which greatly improves performance. However, the accuracy of OpenGL acceleration is generally lower than that of the standard software scanline rendering algorithm.

1.18 Sampling Algorithms

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:38:00 +0000

1.18 Sampling Algorithms

Jittering, motion blurring, area light sources, and the depth-of-field lens shader are based on multiple sampling that is based on varying the sample locations in time, 2D or 3D space. mental ray offers a proprietary implementation of the Quasi-Monte Carlo method for achieving these variations. Sample locations in time, 2D and 3D space are deterministically chosen on fixed points that ensure optimal coverage of the sample space.

The algorithm is similar to fixed-raster algorithms, but avoids the regular lattice appearance of such algorithms. The resulting images are identical if the scene is re-rendered with the same options due to the deterministic nature of the algorithm. Quasi-Monte Carlo methods can be succinctly described as strictly deterministic sampling methods. Determinism enters in two ways, namely, by working with deterministic points rather than random samples and by the availability of deterministic error bounds ([Niederreiter 92]).

1.17 Motion Blur

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:38:00 +0000

1.17 Motion Blur

There are two different motion blur algorithms. One is completely general and computes motion blur of highlights, textures, shadows, reflections, refractions, transparency, and intersecting objects. The other algorithm is much faster, but cannot handle reflections and refractions (and shadows have to be approximated with shadow maps). However, motion blur of highlights, textures, transparency, and intersecting objects still work with the faster algorithm.

The faster algorithm is used for scanline samples (first-generation non-raytraced). Themovement of objects is specified by associating linear motion vectorswith polygon vertices and surface control points. These vectors give the direction and distance that the vertex or control point moves during one time unit.

If a motion vector is not specified, the vertex is assumed to be stationary. Motion blurring computations may be expensive, but note that these computations are only done for those polygons in a scene which include motion information.

A shutter speed may be given for the camera with the -shutter option on the command line or shutter in the options statement, with the default speed of zero turning motion blurring off. The shutter opens instantaneously at time zero and closes after the shutter speed time has elapsed.

1.16 Animation

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:36:00 +0000

1.16 Animation

The input file consists of a list of frames, each of which is complete and self-contained. Animation is accomplished by specifying geometry, light sources and materials which change incrementally from one frame to the next.

1.15 Depth of Field

noreply@blogger.com (sharath) — Sun, 07 Jun 2009 07:36:00 +0000

1.15 Depth of Field

Depth of field is an effect that simulates a plane of maximum sharpness, and blurs objects closer or more distant than this plane. There are two methods for implementing depth of field: a lens shader can be used that takes multiple samples using different paths to reach the same point on the focus plane to interpolate the depth effect; or a combination of a volume shader and an output shader that collect depth information during rendering and then apply a blurring filter as a postprocessing step over the finished image using this depth information. Both methods are supported by standard shaders supplied with mental ray.