[Figure 2] The zooming cube with lower quality transformation blur.

4. Render using the good_mantra ROP, and the blur looks satisfyingly suave as shown in Figure 3.

This exercise illustrates the simplest use of motion blur in Mantra. Transformation blur simply means any transforms on the Object will be blurred. In this case, there is only a simple move in the X direction. If you want, add to the translate animation and keyframe rotations or scaling on the cube. These will also motion blur correctly when using Transformation motion blur.

Transformation blur is the fastest type of motion blur, as Mantra has to do the least amount of work to get the blur working. The primary influence on speed when rendering Transformation motion blur is the Super Sample setting in the ROP. The fast_mantra ROP has a Super Sample setting of 2×2 whereas the good_mantra ROP has a Super Sample setting of 5×5. The Super Sample setting will directly impact both the speed of the render and the quality of the motion blur. However, remember that there is another factor at play here that is important to be aware of. By default, Mantra renders all raytracing with motion blur! This significantly slows down rendering and is often not needed.

[Figure 3] The zooming cube with higher quality transformation blur.

5. In the fast_mantra ROP, click the + button next to Render Command. On the standard tab, turn on Render Quality (fine control) and set it to No ray-traced motion blur which turns off raytraced motion blur. Click Accept to close the dialog box. Render the cube again with fast_mantra and notice the render is even faster. However, the shadow is not motion blurred as shown in Figure 13.4. If you do need motion-blurred shadows, reflections or refractions, you probably only want to render them when doing final renders to save time. Generally, a test ROP will have the No ray-traced motion blur option on, while a final render ROP will not have this turned on. Even when rendering with ray-traced motion blur turned on, use the Decouple Ray Sampling to lower the sampling on the ray tracing, which will speed up the render and usually does not make a huge impact on the quality of the blurred shadows.

Another point to keep in mind is to view this stuff in motion! A still frame highlights problems with blur, such as a noisy appearance when sampling is too low. In motion, your eye will often not see these problems, which means you can actually render with lower super-sampling than you would expect from viewing the still images.

[Figure 4] The zooming cube with no raytraced motion blurred shadows.

Find out more about how to apply each of Houdini's features to your projects as you take on modeling, character animation, particle effects animation, dynamic simulation animation, shading, digital asset creation and rendering. The Magic of Houdini by Will Cunninham. Boston, MA: Thomson Course Technology, 2006. 355 pages with illustrations. ISBN: 1-59863-082-2 ($49.95). Check back to VFXWorld frequently to read new excerpts.

Will Cunningham began his trek by studying both traditional art subjects and 3D computer software at the Academy of Entertainment and Technology. After his studies, he was hired as a Houdini technical intern by Side Effects, the developers of the Houdini software package. Eager to create effects for the big screen, he then jumped into production with BlackBox Digital on the feature, The Prince and Me. Shortly thereafter, he also began teaching Introduction to Houdini at the Academy and has since taught both the introductory and intermediate Houdini courses. In the fall of 2004, he was awarded a fellowship grant by Santa Monica College to support his efforts in creating this book. Over the years, he has worked for a number of production studios on a variety of projects, including The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, Open Season and Ghost Rider. Currently, he is enjoying effects challenges and learning opportunities at Sony Pictures Imageworks.