Introduction
The iray rendering engine uses a fixed set of BRDF materials and shading effects. Many existing shaders are automatically mapped to this internal shading model. If a shader is not supported by the shader translation mechanism then the material or effect is skipped, which typically renders this effect black or invisible.
A Maya scene can be tuned to use only the subset of shaders and effects which is supported by iray. Different versions of iray handle certain shaders differently. For example, the Shading Engine (also known as Shading Group) may be properly detected and skipped by iray, or, otherwise, it has to be disabled in the Attribute Editor of the Shading Engine.
Prepare Scene for iray
The iray shader mapping supports a list of shaders. Currently suggested shaders are:
mia_material, mia_material_x as material surface shader, disable Multiple Outputs
Maya file node, or mental ray base shaders, for texture image lookup
mia_exposure_photographic as camera lens shader
mia_physical_sky as camera environment shader
physical_lens_dof as camera lens shader
Avoid shading tricks and non-physical approximations
The iray rendering engine is simulating laws of physics for lighting. It delivers superior photo-realistic results when the whole scene description is based on physically plausible models, like
light sources with size and shape,
surface models with thickness, like no single-sided glass,
surface materials based on BRDF descriptions, or mia_material shaders which are emulated,
surface structure that is explicitly modeled, including displacement but no bump or normal mapping.
Note, that most of these traditional workflows are supported by iray as much as possible, but this may break physical correctness thus introduce artifacts like improper surface shading or wrong illumination.
How to achieve certain effects
Camera Lens Effects
use phyiscal_lens_dof on Maya camera mental ray Lens slot
Tone Mapping / Film Exposure
use mia_exposure_photographic on Maya camera mental ray Lens slot
use mia_exposure_simple on Maya camera mental ray Lens slot
Common Light Sources (ambient, directional, point, spot)
Maya light sources with high intensity values
Maya light sources with custom light shader physical_light
Area (Shape) Lights
Maya Area Light with mental ray Use Light Shape enabled, all shapes supported except user
Environment Lighting
use Maya camera Environment / Background Color
use environment shader on Maya camera mental ray Environment slot
for IBL from HDR image, use mib_lookup_sperical
for Sky Light, use mia_physicalsky
Light Emitting Objects
mia_material_x with additional color on any shape
mia_light_surface on any shape
Surface Structure (Shaded)
Maya bump2d or bump3d node on mia_material overall bump
Surface Structure (Tessellated)
Maya displacement, will be performed by mental ray CPU and final tessellation passed to iray
Single Surface Material
use mia_material and mia_material_x, start with the presets
Layered Material / Coating
not supported as shading effect, model the layer as a thin offset surface
Decal / Textured Material
use Maya file node on any input of surface shader
use mib_texture_lookup on any input of surface shader
Volumetric Shading
not supported as shading effect, model volume as grid of surfaces
Subsurface Scattering
not supported as shading effect, model subsurface layer as surface with low gloss and translucency
