[Figure 2] The flying debris with no motion blur.

Velocity Blur
Ahhhhh, wonderful, fanciful, imaginative, save you in the end, Velocity Blur. How many times has this saved a project? By my count, countless times! And I hope that you understand that I can count to at least 89. Velocity Blur is the true unsung hero of Mantra. Well, you can start the singing and tell the tale! I think you may know what comes next. Yes, a bit of poetic prose:

Oh Velocity Blur,
Oh Velocity Blur,
What is it in your essence
That so readily makes my renders purr?
If at times I overlook your contribution.
Please take this, a poem, a song, as my ablution.

[Figure 3] The flying debris with velocity motion blur.

Velocity Blur works by taking a vector attribute on the points of the object being rendered and using that to blur the geometry at render time. A vector is simply a direction relative to the point. So, Mantra has all the information it needs to blur the geometry within a single frame! The really cool thing about Velocity Blur is that changing point counts do not matter so you do not have to worry about your geometry's point counts changing frame to frame. Also, you can create your own attribute and actually "motion blur" static geometry for very interesting effects. Before you can fly like a flamingo, you must slither like a snake.

1. Load the MotionBlurStarter.hip file. Make sure the Tree and Flying_Cube objects are not displayed.

2. Make sure you reset to frame one, as this next part uses particles.

3. Turn on the Debris object's Display flag and move to a frame where you can see the rocks. Frame 67 looks fine.

4. Render with fast_mantra, which will render the "rocks" with no motion blur, as shown in Figure 2.

5. In the Render tab of the Debris object, set the Motion Blur parameter to Velocity Attribute Blur. Render again and you now see motion blur as shown in Figure 3 So far, nothing too exciting as you have seen motion blur numerous times already. However, go into the SOP network for the Debris object and there are a couple of interesting things to note.

The first is that you are rendering particles and the number of particles changes per frame. All particle systems have the v (velocity) attribute on them by default. So as long as particles are rendered with Velocity Attribute Motion Blur, it is all well and good.

[Figure 4] The flying debris with a multiplied velocity motion blur.

The second is that there is a Copy SOP copying a polygonal sphere onto the particles. This is a pretty common thing to do (copy geometry onto particles) so it is important to highlight an extra step here. Look at the Copy SOP's parameters in the Attribute page. Use Template Point Attributes is turned on and the To Point is using v in the Set column. This is copying the v attribute from the particle to the sphere that is being copied. This lets the spheres render using velocity motion blur.

Velocity Motion Blur -- The Remix
Now that the basics are out of the way, you can proceed into the realm of sweet delights.

1. Still inside the Debris object, append a Point SOP to the end and turn on its Display and Render flags.

2. In the Particle tab, change Keep Velocity to Add Velocity. Multiple each of the variables by five to get $VX*5, $VY*5, $VZ*5.