Hair rendered using XGen (image courtesy of Jonas Tornqvist).
MtoA supports Maya's XGen, a powerful geometry instancing tool. There are some current limitations when using XGen with MtoA. These are as follows:
The Primitive Type 'Card' is not supported.
Ensure that the MtoA plugin is loaded prior to the XGen plugin otherwise, Arnold will not appear in the Renderer list.
If XGen splines do match the viewport in the Arnold RenderView window, make sure that you save the scene first.
To show the Arnold output settings:
Live allows XGen to query patches and guide animation directly from Maya. This does not support Motion Blur and will try to reuse information the preview generated.
Batch Render mode will always read geometry and animation from files. Batch render mode is always on when doing a batch render. You must first export the patches and guide animation to caches. Use this mode to make sure your caches are properly exported before doing a batch render.
Apply default Arnold XGen Hair Shader to the current Description. The shader reacts to root_color, tip_color and back_color XGen custom shader parameters.
Min Pixel Width - Arnold's minimum pixel width for curves. More information here.
Mode - Curve Primitives can be rendered as ribbon or thick. To get 'oriented' facing splines that do not face the camera, you must disable Face Camera in the XGen Primitives tab.
Choose between Global Settings (uses the MtoA motion blur Render Settings), On or Off.
Motion Blur and Splines
It is important that you use the following steps when rendering motion blur with XGen splines.
When using 'export patches for batch rendering', XGen will create an .abc file that is stored in the same folder as the scene. 'Use Aux Render Patch' enables you to explicitly choose the file that you want to use, for example, a previously exported .abc file.
Enable/disable Auxiliary Render Patch.
File path used for the exported .abc file.
To change the Opacity of XGen splines, you must first select the XGen description and then disable Ai Opaque within the Extra Attributes of the Attribute Editor.
To obtain color information from XGen splines, such as those found in the XGen Presets, you will need to add the following steps.
A scene that demonstrates this process can be downloaded here.
2. Assign a shader to the XGen description.
3. Connect an aiUserDataColor shader to the color attribute of the shader that you want to use (e.g. root color).
aiUserDataColor shader connected to standard_hair attributes
4. Add the name of the attribute to the Color Attr Name attribute of the aiUserDataColor shader. In this case it is 'root_color'.
XGen hair color rendered using aiUserDataColor
Implicit UVs on curves (root-to-tip) can be enabled in the Ramp's Extra attributes.
UVs are supported in "XGen splines". This is an XGen shape (available in Maya 2017), found under Generate->Create interactive groom splines.