The Magic of Houdini: Motion Blur -- Part 2

In VFXWorld's last excerpt from The Magic of Houdini, Will Cunningham finishes his analysis of the various kinds of motion blur.

By Will Cunningham | Monday, February 19, 2007 at 12:00 am

Posted In | Magazines: VFXWorld

All images from The Magic of Houdini by Will Cunningham. Reprinted with permission.

This is the last in a series of excerpts from the Thomson Course Technology book The Magic of Houdini by Will Cunningham. This month, VFXWorld readers conclude learning about the basics of the dominant tool that has been used in the creation of some of the most awe-inspiring animation and cinematic effects ever made.

Deformation Blur
You have motion blur on objects that are moving around and that is simply super. What about if the object is changing shape? Well, that is where Deformation Blur flies in and says simply super is not enough, man. Remember this general rule of thumb. If the only animation on an object is in the Object's Transform tab, you can just use Transformation Blur and be done with it. However, if anything is animated inside the object at the SOP level, Deformation motion blur will be required. It is important to note that Deformation motion blur also includes Transformation blur, so if you have animation inside the object and animation of the object itself, use Deformation blur and you will get the correct results.

[Figure 1] The swinging tree with deformation blur.

It is even more important to remember the following fact. If your geometry changes point numbers from frame to frame, deformation blur will not work! Deformation blur works by comparing the point positions at the current frame to the point positions at the next frame. If the point numbers or point count change from frame to frame, your motion blur will freak out! Thankfully, Velocity Blur can save you and you will look at it in just a moment.

1. Load the MotionBlurStarter.hip file, if you don't already have it open.

2. Turn on the Display flag of the Tree object. Turn off the Display flag of the Flying_Cube and Debris, if they are not already off.

3. Play the animation back and you will that the tree is deforming. If you want, check out the SOP network for the tree, it's just a Twist SOP with a simple expression moving the tree back and forth.

4. Move to a frame where the tree is moving quickly. Frame 59 seems like a good choice.

5. Render with the fast_mantra ROP. OooOoo and there is no blur.

TIP: When testing motion blur, it is usually a good idea to render a frame without motion blur, and then another with motion blur to the same Mplay window. That way, you can quickly compare the two renders. Very often, a render with motion blur will be very subtle, making it hard to tell if there is any blur at all!

6. On the Tree object, set the Motion Blur parameter to Deformation Blur. On the fast_mantra ROP in the render commands dialog box, toggle off Render Quality (fine control) so you are again motion blurring raytraced shadows. Render the frame. Woowoo and there is now blur. Figure 1 shows this using the good_mantra output.

7. If you want, toggle back on the Render Quality (fine control) option so that the raytraced shadow is not motion blurred and generate an image sequence to see the blur across numerous frames. All the tips from the Transformation blur section apply here too.