Contact us now to discuss your next project!

info@AloeDesign.com (202) 556-3842

Watch a 3 minute promo on YouTube

“After years of training, the astronauts on the 2009 Hubble mission made it look easy, but repairing the Space Telescope involved five long, risky spacewalks and intricate repairs never before attempted in space.”

Animated to the tune “Honestly Sincere,” this video shows a chunk of Coal trying to clean up his image by covering himself with green paint, but everywhere he goes he wreaks havoc: literally leaving carbon footprints, exuding coal dust, dirtying streams, heating up the atmosphere, causing air pollution, and in the end, destroying wildlife, and decimating an entire mountain top. There is nothing about Coal that is clean- and the coal industries’ ads are anything but “Honestly Sincere.”

Find out more about coal pollution at NotCleanCoal.com

This is a four-part tutorial about adding motion blur to a Maya render by first creating a 2D motion vector sequence and then using it with ReelSmart Motion Blur in post production. I’ll be covering Maya 2009′s 2D motion vector pass as well as installation and use of the lm_2DMV shader for earlier Maya versions. Once this technique is part of your pipeline, it can be a tremendous time saver compared with creating (and waiting for) motion blur from within Maya. It also provides the flexibility of applying any amount of motion blur without having to re-render.

Part 1: Motion Vector Basics and Other Fine Tips

In these tutorials, I will be using After Effects CS4 as a compositor and Vista 64 as the OS. File paths for the Mac will be listed, but have not been tested. The ReelSmart Motion Blur plugin is available for most compositors, so you should still be able to follow along if you are using something other than After Effects.

Know, first of all, that ReelSmart Motion blur is intended to be used on one object at a time, not on the entire scene. It is a useful technique, for example, if you have a fast-moving character and are able to (1) make a separate render layer for its motion vectors and alpha and (2) then have the background on a separate render layer that you don’t intend to blur (or will blur separately). I have also used this technique with moderate success on a full-scene basis by simply adding a big blur to the motion vector sequence.

Additionally, if you have an animation sequence in which your object-to-be-blurred is rendered on a separate layer than the background, then ReelSmart is able to try to calculate the motion blur without using a motion vector pass. It tries to track the pixels automatically from one frame to the next. This is a less accurate method, but sometimes it is good enough.

Below is the final animation that I made to demo this technique. Click on the icon to open it up. You should be able to drag the time slider back and forth to see areas where the blur is happening more or less on the character. This was created with the 2D motion vector remap pass and the RSMB Plugin.

What Is a Motion Vector??

A 2D motion vector image describes the amount of movement that’s happening in a scene by using two luminance maps and combining them into one image. One luminance pass describes the X translation (horizontal movement) by laying down black and white values (50% gray means no movement) into the image’s red color channel. The other is the Y translation (vertical) and is described in the image by using the green color channel. The blue channel is normally not used. Visually the result looks like a dark yellow image in areas where there is no movement (the 50% gray of the red and green channels combine to make 50% luminance pure yellow), and so the movement areas shift into red or green hues.

When you tell the motion blur plugin to look at this in post, it evaluates this description of X and Y movement and uses that description to determine how much blur to apply.

There are two ways to render motion vector data in Maya that the plugin can understand, and both will give identical results. Which method you use is up to you. The first way is to install a shader in Maya that you can assign as a layer override and can render as a separate sequence. The second way is to use Maya 2009′s render pass system, which should produce a quicker result, because it will make the vector output at the same time as your main render. These two methods are covered in the next two parts (see below for an index).

Common Rules, No Matter Which Method Is Used

• You will need to use Mental Ray, not software render.
• Render output to a 16-bit image in order to give enough detail in the color data. I’ll be using .EXR format with a 16-bit float frame buffer. Do not use an 8-bit buffer.
• When rendering as a 16-bit file, you may not be able to visually see green/red gradations in your rendered image because they go beyond the range of what your screen can display. That doesn’t mean that your render didn’t work right. I tweaked the contrast on the sample above so the result would be visible.
• If you are rendering out a depth map as well because you intend to add lens blur in post, it’s going to get weird on top of the ReelSmart plugin. You can kind of get around this quirk by 2D motion blurring the foreground geometry depth pass in a precomp so that it matches the main motion blur.
• You’ll probably need to turn on Full motion blur with matching open/close shutter settings for every other (non-motion blurred) layer in your scene so that they will match temporally. (see below)

Differences Between lm_2DMV and Mental Ray’s 2D Motion Vector (Remapped) Pass

• With lm_2DMV shader, you have to turn on Mental Ray motion blur and then set the shutter open and shutter close to the same value. The manufacturer recommends 0.5.
• With Mental Ray motion vectors, you do not need to turn on motion blur, but be aware that it turns on motion blur internally at render time. It sets the shutter open and close values to the same value for any render layer for which you have a motion vector pass applied. The value that it sets is close to .00001.
• lm_2DMV produces output image channels called R, G, and B.
• Mental Ray produces motion vector output image channels called X, Y, and Z.
• The largest displacement clamping value (normalize) available on lm_2DMV is 512.
• The largest displacement clamping value (max disp.) available on Mental Ray motion vectors is 1024. This value may be better than lm_2DMV’s if you are rendering HD scenes with a very fast object, but 512 will be more than enough for most situations.

Tutorial Sections

• Intro: You’re lookin’ at it. General items that pertain to both workflows and other tips.

• Part 2: Installation and Use of the lm_2DMV Mental Ray Shader to Create a Motion Vector Sequence

• Part 3: Rendering a 2D Motion Vector Sequence Using Maya 2009′s Built-in Render Pass

• Part 4: Use of the plugin ReelSmart Motion Blur to Apply the Motion Vectors to the Image

Step 1. Understanding The Problem

There seems to be a bug with the color clamping in most Maya image output file formats when using render passes. Even with most of the file types that support higher bit depths there seems to be a tendency to clamp down to an 8 bit range, and also do it incorrectly (for compositing needs). The shadow pass is the first area where I noticed this. Instead of a “normal” shadow pass that only involves black and white values, this Maya Mental Ray shadow pass now includes hues from the scene and is also luminance inverted.

This is the shadow pass image as it comes out of Maya.

The correct and normal method to composite this type of shadow pass would be to combine or merge it on top of the diffuse color pass using a “difference” apply mode. This almost works, but for some reason (improper clamping) it causes inverted colors to appear in the deeper shadows.

This is how the shadow composite should not look. Notice the incorrect green hues in the deeper shadow areas.

This is the Beauty Pass. This is what the shadows are supposed to look like.

This clamping issue causes inconsistency between the different color layers that need to fit together just so in your compositor. This bug also affects other render pass types. Direct Irradiance pass doesn’t composite properly with diffuse material color pass. depth maps do not create proper values over the entire scene area, instead only the front part of the scene. A whole mess of problems emerge. So now you are stuck using a 32 bit file format to get those render passes to work right. Well that’s fine.. but then on top of that it turns out that only one of the possible image file format works properly, and the rest clamp down to 8 bit even though they may be theoretically compatible with higher bit depths. EXR is our savior. For some reason there seems to be a bug with many other file types such as targa, tiff, even iff, do not work right, even if the frame buffer is set to 32 bit. Until Autodesk fixes this issue, we need to work around it.

Step 2. So Now What??

Here are the steps to output 32 bit EXR sequences out of Maya.

1. Maya-> Render Settings-> Common tab-> Image Format-> OpenEXR

2. Maya-> Render Settings-> Quality tab-> Data Type-> RGBA [float] 4×32 Bit

Now you should end up with a sequence of EXR files, either a bunch of different passes in separate files or a ton of channels all crammed into the same file. Here’s how to deal with them inside of After Effects.

1. After Effects->Import->select your EXR sequence passes

2. Lay your passes onto the timeline. If you’ve made one EXR with all your passes inside of it, then make a duplicate layer on the timeline for each pass type you made.

3. Select each layer and apply Effect-> 3D Channel-> EXtractoR

This plugin is now included with After Effect CS4 . If you have an earlier version then you can still download it for free from this site. Also, I find that version 1.3 is much more stable than version 1.2 (that shipped with CS4), so you may want to update in any case.

4. Click the EXtratoR Channel Info Box in the Effect Controls tab. Here you will need to pick the R, G, and B layers that are appropriate to the pass you are trying to display in each layer. If you have separate EXR pass files then there will probably only be three choices in each, but if you have a composite EXR with all your passes then you may have a very long list. You can see how I chose the RGB shadow pass channels for my shadow layer in the image above.

5. Select the appropriate apply modes for all your layers. Shadow should be on Difference, Specular should be on Add, Direct Irradiance should be on Multiply.

At this point you should still be seeing a messed up looking composite that does not match your beauty pass- but we’re almost there! We need to set the entire project to a 32 bit color space.

6. After Effects-> File -> Project Settings -> Color Settings -> Depth -> 32 bits per channel (float)

Now at last you should have a 3D composite that matches your beauty pass!