Where hides Mental Ray in Maya? - Shaders

 

·  Shading Group
· Camera
· Light
· Geometry
· Contours

 

     

 

Introduction

In the design of Mental Ray for Maya it was an important concept to integrate it into the existing Maya architecture and logic as smoothly as possible. The result of this is that the menus, options and settings of Mental Ray are scattered in Maya: they can usually be found in the context of similar Maya settings. In these writings I'd like to collect the places and functions where Mental Ray appears. The first part deals with shaders.

When dealing with Mental Ray shaders are an essential concept. These are not just materials (we have to avoid this common technical confusion), they simply define the behaviour they are attached to. You find shaders in many places of the rendering pipeline and by using them we can affect the renderings in ways that are not possible using the Maya renderer.

There are many kinds of Mental Ray shader types and all of them has to be used in a specific context. Shaders can be connected, modified, animated the same way as any Maya node. Sadly none of the standard Mental Ray shaders have well designed user interface, only the short attribute names are displayed and not even default values are set. I find this pretty awkward so you can download some custom user interface scripts from this site to overcome these nuisances.


Shading Group

Shaders that are related to the look of materials can be found on the Shading Group of the material. A new pulldown menu appears on every shading group (at least after Mental Ray for Maya is started). The easiest way of finding this is to select a material, open the attribute editor and click on the small triangle on the top that points to the right. This way the Shading Group is displayed in the window and a new menu entitled Mental Ray appears.

This menu and the Mental Ray related attributes are only created when you actually open the Shading Group in the AE. So you can only make connections using the Connection Editor after you opened the SG at least once. This can cause some problems when writing scripts because sometimes you have to connect attributes that does not exist yet. In this case you should use the miCreateMaterialAttributes name_of_the_ShadingGroup; MEL command to create the needed attributes before connecting, without opening the AE.

You find here all the shader types (there are plenty of them) that define the look of a material. If there is anything connected to one of these channels than that shader automatically replaces the Maya shader when Mental Ray is rendering. The shaders are only responsible for the channel you connect them to, so you can easily connect a single photon shader to a standard Maya material. This way you override only the way the material reacts to photon calculations.

The Mental Ray material shaders (and some other shader types) deal with light in a particular manner: they don't care about Maya's light linking mechanism and every light that effects a shader has to be explicitly connected to that shader. This can be done by connecting the message attribute of the light's shape node to the shader's lights attribute. (Precisely into one of the elements of the lights array: Lights[0] or Lights[1]...) This is an annoying procedure that is greatly simplified by the UI scripts I was referring to earlier.
Beside the connection making some shaders' mode parameter affects lights too: when mode is set to 1 it means that only the connected lights illuminate the object. Mode 2 is the opposite: only unconnected lights illuminate it. So if you'd like to have a shader illuminated by each and every visible light of the scene you can easily set the mode attribute to 2 and forget about connections. (This is the only way to get Maya's default light feature to work.)
But this solution does not work with all shaders, for example the physically plausible material shaders (like dgs_material) don't have a mode parameter.


Suppress all Maya shaders

By turning this switch on you ask maya not to use the Maya material for any calculation when rendering with MR. This can be useful if you don't need a surface material just a volume shader. This way you don't have to connect anything the the MR Material Shader to disable the maya material.

Opaque

If you turn this on MR treats the material as fully opaque and does not do any transparency calculations. This can speed up the shading process a bit, but it only works when the material is really non-transparent.

 


Material Shader - This is probably the most often used shader: basically the material and color of the surface. You should connect here shaders that define shading models. (For example mib_illum_cooktor, mib_illum_ward…) Mental Ray's default material shaders consist of the basic shading models (lambert, phong, blinn, cook-torrence, ward) and some physically plausible materials (DGS_material, Dielectric_material).

Shadow Shader - This shader defines how a material's cast shadow looks like. For effects like the shadow color attribute in Maya you'd use a shadow shader in Mental Ray (per-material). The only custom shadow shader in MR is mib_shadow_transparency, that computes transparent shadows for transparent materials. (You don't really need it when using Maya materials.)

Volume Shader (Volumetric Materials) - Volume shaders define what happens inside the volume of the material. This is similar to Maya's volumetric material. If you'd like to create some kind of fog or volumetric light effect inside an object, than the volume shader is the way to go. (mib_volume, parti_volume)

Photon Shader (Photonic Materials) - In every calculation that involves photons (global illumination, caustics) this shader defines how the material affects the photons, how it reflects, absorbs, scatters the photons and what colors it uses. This can be useful because you can modify global illumination effects without changing the actual look of your material shaders. Some MR shaders have their own photon version (pl. DGS_material_photon, dielectric_material_photon), that you can connect to this channel when you use indirect illumination effects. If the photon materials parameters are not defined (their values are 0) than automatically the material shader's values are used.
Most material shaders work as photon shaders too so you don't have to explicitly connect them to the photon shader channel too. The physically plausible photon shaders are useful because they can be used to create realistic light effects while still using standard shading models.

Photon Volumetric Shader - Affects how photons behave right inside a material. (scattering / absorption.) Very similar to photon shader.

Displacement Shader - This is basically like Maya's displacement.

Environment Shader - If a ray cast by the material shader (for example for computing reflection) does not hit any objects in the scene, or there is no raytrace calculation at all, or there is raytracing but the raytrace limit is reached than this shader is called to define what should it display (instead of black). So if you connect an image here the shader will display it in the reflection and refraction.

Light Map Shader - This shader bakes light calculations into file textures. Since maya has the detailed Convert to File Texture mechanism, this shader is rarely used manually. But it is there for completeness` sake, and for hardcore Mental Ray users.

Contour Shader - It is used to render cartoon like contours and lines over the object. These shaders define how to display the contours (standard / varying line width, colour, texture, randomness) based on the information that is stored while rendering. To render contours you have to use some other shaders in conjunction. More about this topic later.

 

 

Camera

When selecting a camera you see that there is a Mental Ray menu here too. Four different shader types can be connected here:

Output Shader - The output shaders make calculations after the sampling of the image is finished. (Like Maya's 2d motion blur, depth of field or Paint Effects) Although similar modifications can be done using a 2d composite software after the rendering, there is much information at this state that is not available in the final rendered image. (For example motion vectors, contours, normal vectors, pixel coverage) You can explicitly set what kind of information to pass on to the output shader, in the Framebuffer parameters.
The standard Mental Ray output shaders are just for displaying contour lines. (contour_composite, contour_only, contour_ps)

Volume Shader - This is similar to the material's volume shader, but it effects the whole scene and not just one object. So if you'd like to render a fog that fills the entire scene this is the way to go.

Lens Shader - Lens shaders define how and where to shoot rays from the camera. They are mostly used for lens distortion effects and correct depth of field calculations. (Lens shader calculations work automatically in raytrace mode because they modify the eye ray direction.)
By default only the physical_lens_dof end oversampling_lens are present: physical_lens_dof calculates depth of field, and oversampling_lens oversamples every rendered pixel by a number of rays.

Environment Shader - Similar to a material's environment shader, but it effects every material in the scene that has no environment shader of its own. Using this you can quickly set an environment that every material „sees”.

The current version of Mental Ray for Maya (in contrast with the standalone Mental Ray) only uses one lens and output shader. In the future this will probably change.
The MEL command that creates the camera attributes without opening the AE is:
miCreateOutputAttributes name_of_the_camera_shape;

 

 

Light

Light Shader - You can connect different light types here and they override the behaviour of this light. Only the position and the direction of the light is used, every other parameter mustbe set on the light shader node.

Photon Emitter - This shader affects how a light emits photons for global illumination calculations. This is a very specialized effect and Mental Ray does not even have any custom Photon Emitter shaders beside the default ones that are used by the standard light types.

The MEL command that creates the camera attributes without opening the AE is:
miCreateLightExportAttributes name_of_the_light_shape;




Geometry

Geometry shaders can be connected to maya transform nodes. They copy or create a different object in the place of the original one. This way you can swap a simple animatable model with a highly detailed one for rendering or create insane amount of procedural geometry without polluting the scene you work with.

At the moment Geometry Shaders can be rendered only with Mental Ray shaders (by default they inherit the object's material you connect them to). The standard geometry shaders are basic primitives (cone, box, cylinder, sphere, plane, torus) and two shaders that creates instance copies of previously exported mi geometry files. (mib_geo_instance, mib_geo_instance_mlist) As far as I know these two are pretty unusable right now through the current Maya user interface.


Contours

Contour shaders can be used in the Render Globals/Contours menu. Because of some obscured reason these shaders have no specified shader type, they are not separated to a specific tab so you have to rummage among the Miscellaneous tab to find them.

Contrast Shader - This shader type defines what kind of geometrical features should Mental Ray interpret as contours. The contour_contrast_function_simple is a very simple process and it only deals with outlines. Contour_contrast_function_levels is more robust and it even has some parameters to tune.

Store Shader - Store shaders do nothing else but store the lines defined by the contour_contrast shader. The contour_store_function_simple deals with outlines only so basically you use the two simple versions together. The contour_store_function is the one that works for everything else.

And although it has no parameters the UI still has an ugly bug: when you create the shader through the Render Globals window it does not connect it by default (as for example the contrast works). You have to manually connect it using the connection editor or MEL scripting to get it working. So connect the contour_store_function's normalGeom output to the miDefaultOptions contourStore input channel.

If you close the connection editor by mistake the easiest way of finding the miDefaultOptions node, is turning off the Show DAG Object Only flag in the outliner, select the node, and press the Reload Right button in the Connection Editor. Or use this mel command after you created the store shader:
connectAttr -force contour_store_function1.normal_geom miDefaultOptions.contourStore;