p_maya_shadingengine - surface shader (utility) for Maya only
This shader needed for "converting mental ray shader to maya native shader type", in other words change return type of shader from "RGBA"(mentalray) to "RGB, RGB, RGB, RGB"(maya native).
All mental ray shaders return 'RGBA', all maya native shaders return 4 colors (OutColor, OutTransparency, OutMatteOpacity and OutGlowColor) with 3 components every(RGB without A).
This conversion is very usefull if you are using maya's bump with p_MegaTK or any other mental ray shader.
Please view tutorial Bump with p_MegaTK for more details of usage of p_maya_shadingengine shader.
Some explanations...
Maya compute fine bump with filtering only for maya native shaders. And if you'll connect maya bump node to your mental ray shader you'll never get same result as with maya native shader, because bump not will be filtered. So for enable filtering we need to convert mental ray shader to "maya" type and enable 'Export with Shading Engine' option on shading group (this option inform mental ray that input will be maya's type "RGB, RGB, RGB, RGB", but not "RGBA")
Warning! You should always enable 'Export with Shading Engine' option on shading group when you are using p_maya_shadingengine shader(or any maya's shader), otherwise Maya will crash during render.
Additional feature of p_maya_shadingengine shader is that you may add maya's bump to any mental ray shader)
If you'll convert all your mental ray shader to "maya" type you may disable 'Pass Custom Alpha Channel' option in Render Setting since you don't need it anymore for get alpha channel.
Input Color used for connecting mental ray shader
Enable 'Input Normal' slot for using bump
This slot is used for connecting vector(in internal/world space) that returned by bump node (for example maya's bump2d or bump3d node). Please don't try connect texture directly, it's not correct. You may connect texture if texture is float point and contain final normal (normal map).
Control Matte Opacity Mode like in maya's shaders
Control Matte Opacity like in maya's shaders