Shading Group attributes
         
         In the Shading
               Group’s Attribute Editor, the mental
            ray section contains the following options:
         
         Flags
            These are the options
               in the Flags section.
            
            - Opaque
- 
                  Turn this on if the material
                        is fully opaque. It should assist in speeding up mental ray rendering.
                      
- Transparency Threshold
- 
                  Tweak this transparency threshold so that
                        all surface points that are less opaque than specified by this attribute
                        will be considered non-existent. For example, a value of 0.05 removes
                        all surface points with a transparency of 99.5% or more.This allows
                        your object to appear cut out from the texture. An example is to
                        map an image of a tree to a flat plane and use opacity to cut away
                        the parts of the tree that are not there.
                      
Contours
            These are the options
               in the Contours section. Each shading
               group can have individual contour rendering attributes defined.
               Set the following options to achieve the desired look. See also 
                        Contours in
               the Rendering guide.
            
            - Enable Contour Shading
- 
                  Enables contour shading
                        for all objects associated with the shading group. 
                      
- Color
- 
                  The color of the contour
                        lines. This attribute can only be a color and cannot have a procedural
                        or texture map connected to it.
                      
- Alpha
- 
                  The alpha of the contour
                        lines. 
                      
- Width
- 
                  The width of the contour
                        lines. 
                      
- Relative Width
- 
                  If off, Width is
                        specified as (fractional) number of pixels. If on, Width is
                        specified in percents of the horizontal image resolution thus maintaining
                        the relative contour size if the resolution changes.
                      
- Absolute Width
- 
                  If on, Absolute
                           Width is specified in percents of the horizontal image
                        resolution set by the user.
                      
Custom Shaders
            These are the options
               in the Custom Shaders section.
            
            - Suppress all Maya Shaders
- 
                  Use this for backwards
                        compatibility as it disables automatic translation of Maya shaders. Only
                        the mental ray ports are evaluated and exported.
                      
- Export with Shading Engine/Export
                        with Volume Sampler
- 
                  When turned on, these
                        both insert additional helper shaders into the export stream. These shaders
                        are required if you want to write custom shaders that cooperate
                        with the Maya base shader package delivered with mental ray for Maya.
                        These options are for advanced mental ray for Maya users only.
                      
NoteThe Export
                     with Shading Engine attribute is available for backward compatibility.
                  It should not be used for tuning and should be disabled in most cases.