Motion Blur in Houdini

Motion Blur in Houdini

So I just finished a project with the use of heavy motion blur. A stylistic piece, sort of like a Picasso but not really. Actually, not even close. Anyways, a change came down the pipe. I know, shocking. One of the animated objects in the scene had way too much motion blur and another object should not have motion blur at all.  ¯\_(ツ)_/¯

I figured, well, I’ll just turn down the shutter time on that one object and turn off the motion blur for that other object…

Motion Blur in Houdini 101

Enabling motion blur in Houdini is as simple as toggling the Allow Motion Blur toggle in the Mantra node Rendering tab. This will reveal three additional parameters: Xform Time Samples, Geo Time Samples, and Shutter Offset.

Motion Blur Option
Motion Blur Option

Xform Time Samples controls the number of transformation motion blur samples. Think quality. The more complex the motion, the higher you have to crank these values. Side Effects recommends a value of 2 for most scenarios.

Something important to keep in mind is that Transformation Blur refers to objects being transformed at the object level. In other words, this will not work for deforming objects. So for example, if your geometry is deforming and turning into a hot dog, it will not work and you will have a hot dog without motion blur. For deforming objects, you need to look at the Geo Time Samples parameter.

Geo Time Samples controls the number of deformation motion blur samples. As with Xform Time Samples, the more complex the deformation of the geometry or if an object is moving really fast, you will have to crank up these values. However, Unlike Xform Time Samples, this is an expensive operation as Houdini has to keep a copy of the geometry in memory for each additional sample.

Shutter Offset just determines the segment of time by which motion blur will be generated. So to generate motion blur at the current position of your object and the position of the object on the next frame, you would use a value of 1. If you want Houdini to generate from the current position to a position on the previous frame, you would use a value of -1. Finally, a value of 0 will generate motion blur using an interval halfway between the previous and next frame.

Per Object Motion Blur

As an example, witness the following cube and torus spinning wildly in the wind. Both have the same amount of rotation. The task is to eliminate or reduce the motion blur on the cube while not affecting the motion blur on the torus.

Houdini Motion Blur On
Houdini Motion Blur On

Enabling Motion blur is a global setting on the Mantra node. Makes sense but here is the pickle. What happens if you want to turn off the motion blur for certain objects or reduce the samples for objects that really don’t matter much in a scene to reduce the memory footprint? You might think to yourself, “Well, I’ll just turn off motion blur or try reducing my Shutter Time on that particular object. Or how about reducing Xform Time samples to a value of 1 and that will essentially disable motion blur for the object.” Sounds reasonable.

However, if you start poking around the object’s node parameters, you won’t find Xform or Geo Time sample control properties on individual objects. So where are they? You need to add the properties yourself!

Properties

I won’t go into properties but if you have done the whole “Edit Parameter Interface…” dance, it’s the same thing. Head on over to the object’s gear icon on the top, right-hand side of the parameter window and select Edit Rendering Parameters.... You can select either one. They’re just tabs so you can switch once the window opens up.

Edit Rendering Parameters
Edit Rendering Parameters

Now, you can hunt through all the folders looking for the properties in question but we don’t have time for that. We’ll use the handy Filter option at the bottom of the window. Assuming you know the name of the property you are looking for, just type it in and Houdini will get you in the vicinity. All you have to do now is drag the property over to the right hand side into the area of your choice. You don’t have to touch any of the default Parameters.

Search Rendering Properties
Search Rendering Properties

As a side note, if you do have the time, going through all the folders is a great way to learn about all the properties available to you. Also keep in mind a great deal of the properties are already exposed in the node’s parameter interface. You will know this because the property will be grayed out if it’s in use.

It’s Shutter Time

So how about adjusting the Geo Time Samples or Shutter Offset for an object? It’s the same deal. Just find your properties and add them to the node’s parameter interface. You really want to add all three properties to your objects if you go about using this method.

Per Object Motion Blur Properties
Per Object Motion Blur Properties

Here I’m essentially turning off motion blur for the cube by making the Xform Time Samples 1. The global Xform Time Samples in the Mantra node is still 2. Compare the box, which has its own motion blur properties, to the torus which is still using the global motion blur property values in the Mantra ROP.

Xform Time Samples
Xform Time Samples
Motion Blur Properties
Motion Blur Properties

There you have it! Couldn’t be any easier! Time for a wheatgrass smoothie!

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990adjustments

990adjustments

I am a motion designer & developer based out of South Florida. When not designing or animating pixels, I wrangle some code. If all else fails, I watch Twilight Zone, I Love Lucy, or Three's Company reruns.

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