The case of the too-low OptiX version

In this case, when the user tried to render after enabling the OptiX denoiser, they got this error:

ERROR | [gpu] OptiX version 0.30.91 is lower than the minimum required version 5.0.0

This was with MtoA 3.0.1, which means Arnold

So, that means there is an incompatible version version of the OptiX library on the system, and it’s being loaded instead of the OptiX that ships with MtoA.

I probably could have just checked the PATH setting, but I used Process Monitor to absolutely sure, and I found this:


The problem is that another renderer added itself to the PATH. That’s a bad thing 😉

The solution? Remove that folder from PATH. Create a batch file or wrapper script to set the required environment when you start Maya to use the other software. Rather like the mtoa module file sets PATH when you start Maya.

NOTE In previous versions of MtoA, this would prevent MtoA from even loading.

kicking ass with debug shading

You can use kick to render with debug shading. Here’s the flags to use:

  • -is to ignore the shaders assigned to the shapes
    When you ignore shaders, a default utility shader is used to render the scene.
  • -sm sets the shade mode ( ndoteye lambert flat ambocc plastic metal)
  • -cm sets the color mode (color ng ns n bary uv u v dpdu dpdv p prims uniformid wire polywire obj edgelength floatgrid reflectline bad_uvs nlights id bumpdiff pixelerror)

Here’s some examples that show how to kick with different types of debug shading (I’ve used the Arnold Render View debug shading modes for these examples)


Debug shading kick flags
Basic: disable all shaders in the scene, switching to a gray ‘ndoteye’ shader; a very fast shading mode.
kick -is


kick -is -sm notdoteye
Lighting: renders the scene with a white lambert shader
kick -is -sm lambert
Occlusion: use ambient occlusion shading
kick -is -sm ambocc
Wireframe: displays geometry as a wireframe
 kick -is -cm polywire


kick -is -sm ndoteye -cm polywire
Normal: visualizes the normal vector (between 0 and 1, in tangent space)
 kick -is -sm flat -cm n
UV: displays the coordinates of the primary UV set (red=U, green=V)
 kick -is -sm flat -cm uv
Primitive ID: displays random colors based on the per-primitive (triangle, curve) index
 kick -is -sm flat -cm prims
Barycentric: displays intra-primitive parametric coordinates (barycentric for triangles, parametric length, and width for curve segments)
 kick -is -sm flat -cm bary
Object: displays random colors based on the per-object ID
kick -is -sm flat -cm obj

Using an operator to override parameters in an ass file

Arnold 5.1 adds operators, which among other things, allow you to override parameters in ass files loaded by procedural nodes.

Here’s a quick example using the brand new MtoA 3.0

I exported some particles from Softimage, loaded them into Maya with an aiStandin (aka an Arnold procedural), and then used a set_parameter operator to scale the radius by 0.5

Note that I connect my operator by setting the Target Operator in the Render Settings.
I can chain two set_parameter operators together, to set the mode and then scale the radius:

compiled against non-compatible Arnold

If you see something like this:

[Arnold]: MAXtoA_Shaders.dll was compiled against non-compatible Arnold

it usually means you have an older Arnold trying to load something compiled with  a newer Arnold. There’s two common reasons for this happening:

  • The system PATH includes the location of the older Arnold, so the older Arnold is loaded instead of the Arnold version included with the plugin.
  • The plugin install is messed up, and has the wrong Arnold version in it.


Buying Arnold



Annual subscriptions

You get annual subscriptions from Autodesk resellers

You can get Arnold 5-packs at 50% of the usual price, or you can get single Arnold licenses at full price:

Monthly subscriptions

  • You can subscribe for one, two, or three months.
  • Monthly Subscriptions are £60 / €70 / $65 Per Month
  • Monthly subscriptions are available online from

Permanent licenses

Until 30 April 2018, you can still get permanent Arnold licenses. Contact your local Autodesk reseller.

Arnold light_groups and shadow_groups

Light linking in Maya, include/exclude objects for lights in CINEMA 4D, light masks in Houdini, inclusive/exclusive lights in Softimage, they all are translated to light_groups and shadow_groups in Arnold.

Here’s a sphere on a plane, with two area lights (cyan and green).


If we add a light_group to the sphere polymesh:

 use_light_group on
 light_group "CyanAreaLight"

then we get this (see below). The sphere is lit only by the cyan area light: the light_group is the lights that affect the object.

Note that the sphere still casts shadows from the green light, even the sphere isn’t lit by the green light. That’s because there’s no shadow_group, so all lights cast shadows from the object.

If we add a shadow group, so that the light_group and shadow_group are both “CyanAreaLight”, then the sphere is lit by the cyan light only, and casts shadows from the cyan light only.

use_light_group on
 light_group "CyanAreaLight"
use_shadow_group on
 shadow_group "CyanAreaLight"


And finally, here’s the sphere when light groups are on, but there’s no lights in the light group:

use_light_group on
use_shadow_group on
 shadow_group "CyanAreaLight"