Vertices and the maximum valence limit

The valence of a vertex is the number of edges connected to that vertex. In Arnold, the maximum valence is 255 (that’s because of the data type we use to store the valence; we minimize the memory requirements since this is stored per-vertex).

If a mesh in the scene has a vertex with more than 255 edges, you’ll get a WARNING like this:

 [polymesh] example_mesh: mesh has at least a vertex with valence higher than 255, disabling subdivision

But if the adaptive subdivision results in a vertex with too many edges, you’ll get an ERROR:

 ERROR: [arnold] [subdiv] example_mesh: edge (144578,287741) in face 263433 has a vertex that exceeded the max valence limit of 255 

[MtoA] Unable to dynamically load mtoa.mll …The specified procedure could not be found.

If the MtoA plug-in  does not load and you get a “The specified procedure could not be found” error like this:

// Error: line 1: Unable to dynamically load : C:/solidangle/mtoadeploy/2016/plug-ins/mtoa.mll
The specified procedure could not be found.
// Error: line 1: The specified procedure could not be found.
 (mtoa) //

then there’s a few basic things you need to check first:

  • Does your processor support SSE4.2? As of Arnold (MtoA, support for SSE4.2 is required. You won’t be able to load MtoA or use Arnold if your processor doesn’t support SSE4.2.
  • Do you have the right MtoA version for your version of Maya? Major versions of Maya are not binary compatible, so for Maya 2016 you need “MtoA for Maya 2016”.
    And Maya 2016 Extension 2 is not binary compatible with Maya 2016 Extension 1, so if you have Extension 2, you need “MtoA for Maya 2016.5”

If your processor is good and you have the right version of MtoA, then it’s on to basic troubleshooting:

  • Check the environment in Maya. For example, does the PATH include any older versions of MtoA? If you’re on Windows, run the MEL command system(“set”) to get the environment. On OSX or Linux, run system(“printenv”).
  • Check for missing or conflicting libraries. On Windows, you can use Dependency Walker to  check for any conflicts or missing DLLs.

    Download and extract Dependency Walker. Then start it from inside Maya (to inherit the Maya environment). For example, I do something like this:
    system( “start C:/Users/stephen/Downloads//depends22_x64/depends.exe” );

    Then load mtoa.mll.

[MtoA] mtoa missing from Plug-in Manager

If mtoa.mll is not listed in the Plug-in Manager, that means that Maya did not find the MtoA module file (mtoa.mod). And if you try to manually load mtoa.mll, you’ll get errors like this:

// Error: file: C:/Program Files/Autodesk/Maya2016/scripts/others/pluginWin.mel line 781: Unable to dynamically load : C:/solidangle/mtoadeploy/2016/plug-ins/mtoa.mll
The specified module could not be found. // 
// Error: file: C:/Program Files/Autodesk/Maya2016/scripts/others/pluginWin.mel line 781: The specified module could not be found. (mtoa) //

To load MtoA, you need to make sure that Maya finds the MtoA module file.

By default, the MtoA installer puts the mtoa.mod file in the user’s modules folder. For example:


If you installed MtoA using one user account, and try to run Maya with a different user account, Maya will not find the module file.

The module file has to be in the MAYA_MODULE_PATH. For example, for the user account “StephenBlair”, here are the default places where Maya looks for modules:

  • C:/Program Files/Autodesk/Maya2016/modules
  • C:/Users/StephenBlair/Documents/maya/2016/modules
  • C:/Users/StephenBlair/Documents/maya/modules
  • C:/Program Files/Common Files/Alias Shared/Modules/maya/2016
  • C:/Program Files/Common Files/Alias Shared/Modules/maya
  • C:/Program Files/Common Files/Autodesk Shared/Modules/maya/2016

If you want MtoA to available to all users, then you could copy mtoa.mod to one of the common locations.

The case of the blue render view

Reason #35 why you should check the Arnold log

In this case, a scene that used to render yesterday, now just resulted in a blue render view, like this:


Anytime the render view doesn’t update when your render, you can be fairly sure there’s some ERROR in the Arnold log.

In this case, the problem was a broken path to the IES file used by a photometric light. In the Arnold log, there was this ERROR:

00:00:04 870MB ERROR | [photometric] can't open C:/Assets/IES/example.ies

It’s easy to say “check the log”, but where do you find the log? It’s not like there’s a nice, color-coded button (red for error) on the render view UI that will pop up the log for you.

  • On Windows, you should see the Arnold log in the Output Windows. However, I find that this happens only when I start Maya from the command line. If I start Maya from the Windows Start menu, the Arnold log messages don’t show up in the Output Window. So I have to enable file logging, and check the log file.
  • On OSX and Linux, you’ll also have to enable file logging, unless you start Maya from a terminal.

Update: For Google:

  • Arnold renders a black screen
  • Arnold renders a blue screen


[MtoA] MayaFile node uses a default color for missing textures

Here’s something important to remember when you’re debugging a scene…

By default, the MayaFile node uses a default color if a texture is missing.


This means the render won’t abort because of missing textures, and you won’t see ERRORs like these in the Arnold log for missing texture files:

ERROR   |   [texturesys] OpenImageIO could not open "sourceimages/noicon.tx" as tx: Could not open file "sourceimages/noicon.tx"
ERROR   |   [texturesys] Invalid image file "sourceimages/noicon.tx"

So missing textures can easily go unnoticed.

If you need to disable Use Default Color for testing, an easy way is to export an ASS file and then render it with kick -set MayaFile.useDefaultColor false.

I suppose you could also modify the Maya scene file directly:

import maya.cmds as cmds
import os

def set_useDefaultColor(b):"file")
    for item in filenodes:
        cmds.setAttr( "%s.aiUseDefaultColor" % item, b )

set_useDefaultColor( False )

And finally, you can change the default value of the Use Default Color parameter by adding this to shaders/mtoa_shaders.mtd:

[node MayaFile]
	[attr useDefaultColor]
		default		BOOL	false

ERROR | [mtoa] [xgenTranslator] Could not find xgen_procedural in search path $ARNOLD_PLUGIN_PATH

This error

ERROR   | [mtoa] [xgenTranslator] Could not find xgen_procedural in search path $ARNOLD_PLUGIN_PATH

happens only when MtoA is first loaded, and it can be safely ignored. It doesn’t cause any problems when you render, and MtoA loads the XGen procedural from the MtoA procedurals folder.

The error is a consequence of how MtoA searches folders when it loads extensions. If you really need to get rid of the error, you can try copying the xgen_procedural library to the MtoA shaders folder.

[Linux] Missing .sog files?

Sometimes on Linux you may get “unable to load dynamic library” errors for .sog files. Like this:

00:00:00     0MB WARNING |  unable to load dynamic library: /home/stephen/solidangle/mtoa/2014/xgen_procedural.sog: cannot open shared object file: No such file or directory 

or this:

00:00:00     0MB WARNING |  unable to load dynamic library: /usr/apps/houdini/houdini-13.0.343/htoa/arnold_plugins/driver_houdini.sog: cannot open shared object file: No such file or directory 

Don’t worry about the “.sog” part. In most cases, Arnold isn’t actually looking for a .sog file. Arnold is trying to load a .so file, and either the .so itself or a dependency is missing. The “.sog” is printed by mistake in the log message (and this is fixed in the next Arnold version).

So (heh) focus your investigation on why Arnold couldn’t load the .so file (try running ldd on it).

In the two examples above:

  • couldn’t be loaded because LD_LIBRARY_PATH didn’t include the Maya lib folder, so all the tbb-related libraries were missing.
  • was missing some Houdini dependencies, but in this case it was being loaded into MtoA, so driver_houdini wasn’t needed and the warning could be safely ignored.

What’s a .sog?
If a plugin has a .sog extension, AiLoadPlugins() will load the plugin with RTLD_GLOBAL, which means the symbols from the plugin will be globally exposed and available to other plugins.

.sog is a Maya naming convention for an .so to be dlopen’ed with RLD_GLOBAL