Ignoring self-occulsion

Question: how do you set up the Arnold ambient_occlusion shader so that it ignores self-occlusion?

Let’s start with the default ambient occlusion. here’s a sphere and a plane. Each has it’s own Ambient Occlusion shader. For the sphere, Ambient Occlusion.Black is Red.

image (1)

Now, let’s disable Receives Shadows on the sphere.
That means no occlusion based on the plane (shadow rays from the sphere ignore the plane); but there’s still self-occlusion.

image (2)

Next, Cast Shadows disabled. No self-occlusion, but no ambient occlusion on the plane either

image (3)

And finally, Self Shadows disabled. Now, there’s no self-occlusion. The ambient occlusion on the sphere is coming solely from the planeimage (4)

The case of the missing Yeti fur

In this case, a client complained that Arnold wasn’t rendering Yeti fur. I asked him to try with a simple sphere, and to send me the log (at verbosity level Warnings + Info).

This what I was looking for:

00:00:00  1222MB         | there are 1 light and 2 objects:
00:00:00  1222MB         |       1 persp_camera
00:00:00  1222MB         |       1 skydome_light
00:00:00  1222MB         |       1 utility
00:00:00  1222MB         |       1 lambert
00:00:00  1222MB         |       1 driver_exr
00:00:00  1222MB         |       1 gaussian_filter
00:00:00  1222MB         |       1 polymesh
00:00:00  1222MB         |       1 list_aggregate
00:00:00  1222MB         |       1 MayaShadingEngine
00:00:00  1222MB         |       1 renderview_display

So, what’s there? Well, the important thing is not what’s there, but what’s not there.

There’s no procedural. If Yeti was installed properly, there would a procedural node. The Yeti extension exports a procedural node when MtoA translates the scene.

That means the PATH or MTOA_EXTENSIONS_PATH wasn’t set up properly. I always use the Yeti module file for that.

The case of the noisy shadows

In this case, a client reported a lot of noise in the shadows of his forest of opacity-mapped trees. In a progressive render, he saw lots of fireflies at the low AA levels, so it seemed to be something out of the ordinary, since he’d never had this problem before in other, similar scenes.

He managed the get rid of the noise by cranking up the light samples, the AA, and the transparency depth, at the cost of extremely long render times.

But sampling wasn’t the problem, or the solution, in this case. The problem was a large  “sky” sphere that had the skydome HDR mapped to it. This sphere was just there to make the sky visible, but this sphere was visible to all ray types. The solution was to make the sphere visible to camera rays only.

Unlike a Skydome light, a textured sphere isn’t importance-sampled intelligently by Arnold. So you’ll get noise and fireflies from the random diffuse rays that happen to hit a super bright pixel in the sky texture.

hat tip: TI

[MtoA] Warning: Renderer “arnold” does not provide batch rendered options

Not all renderers have batch render options, so if you click the box beside the Batch Render menu command:


then you’ll get a warning:

// Warning: file: C:/Program Files/Autodesk/Maya2017/scripts/others/batchRenderOptions.mel line 19: Renderer "arnold" does not provide batch rendered options. // 
// Warning: file: C:/Program Files/Autodesk/Maya2017/scripts/others/batchRenderOptions.mel line 19: Renderer "mayaHardware" does not provide batch rendered options. // 
// Warning: file: C:/Program Files/Autodesk/Maya2017/scripts/others/batchRenderOptions.mel line 19: Renderer "mayaHardware2" does not provide batch rendered options. // 
// Warning: file: C:/Program Files/Autodesk/Maya2017/scripts/others/batchRenderOptions.mel line 19: Renderer "mayaVector" does not provide batch rendered options. //

All the rendering options for Arnold are in the Arnold Render Settings.

To start a batch render, click the Batch Render text in the Render menu:

[MtoA] The case of the machine that was unable to dynamically load mtoa.mll

In this case, a Windows 7 machine did support SSE4.2, but Maya 2017 still couldn’t load mtoa.mll.

I didn’t get a full Process Monitor log from the client, but I did get a Dependency Walker log, and this case, that was enough.

When you first open a Dependency Walker (dwi) file, it’s easy to focus on the wrong thing. In this case, the missing MSVCR90.DLL (Visual Studio 2008 redistributable) might catch your eye.


But you can ignore that, because if you take a closer look, you’ll see that MSVCR90.DLL is indeed found and loaded.


Likewise, you can ignore all these. You’ll almost always see most of those in a Dependency Walker log for Windows 7 and up.


What’s important in this depends log is the warning for AI.DLL.


That warning means that there’s missing functions: MtoA (MTOA.MLL) expects to use certain functions provided by Arnold (AI.DLL), but those functions aren’t there. For example:


And finally, if we click View > Full Paths, we see the reason for the problem:


There’s some older version of Arnold on the system, and that old version is being loaded by MtoA.mll. Most likely, the system PATH includes this location.

With a Process Monitor log, we would have seen right away that ai.dll was being loaded from a non-standard location.


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 

Sampling after the first bounce

With the default Diffuse = 2 sampling, you’ll get four diffuse rays for each camera ray. Those four diffuse rays are the first bounce after the camera ray “hits” a shape.


After the first bounce, Arnold sets all the sampling settings back to 1, so for each of those four diffuse rays, you get one second-bounce diffuse ray. This prevents an exponential explosion of rays as secondary rays like diffuse rays bounce around a scene.


The same thing is true for the other secondary rays such as glossy, refraction, and shadow rays.

So, in summary:

  • For a camera ray you can get multiple secondary rays (for example, multiple diffuse rays or multiple glossy rays, and for light sampling, multiple shadow rays)
  • But for a secondary ray, you’ll get just one ray. For example: one diffuse ray, one glossy ray (if any), one shadow ray, and so on.