Ramy Hanna, author of the Ramy’s Renderings blog on 3dsmaxrendering.blogspot.com is a superb source of 3ds Max tips and Passes. He’s recently come up with the Velocity Pass, designed to let you use Motion Blur at the compositing stage. We don’t think anyone could explain it as well as Remy himself, so below is his guide to how to create and use the Velocity Pass.
“Animations with Motion Blur (MB) look so much better than without them. Sometimes just adding the MB can be the difference in making an animation go from good to great. But rendering with MB turned on can become very expensive and slow on your renders. So here is yet one more pass to add to your arsenal of compositing passes: the Velocity Pass. The velocity pass will allow you to fake MB in compositing. I’m sure the velocity pass can be used in Photoshop, but it is really geared for using in packages such as After Effects and Composite (which comes with 3dsmax for free now).
With Blur using velocity map (exaggerated for effect)
So here is how to create the Velocity map: just go to your render settings window, select the Render Elements tab, and click Add. Choose the element Velocity. That’s pretty much it. Under the Velocity Element Parameters be sure to tweak your Maximum Velocity, otherwise you may never see any results. Often I will check Update, render, then un-check Update. Having Update turned on will change the Maximum Velocity based on your scene at render time. So if you have an object moving crazy fast, it will know what to set that value to. For this post’s example, I found that a setting of 150 worked well for my spinning torus-knot. Also be sure that Filtering is not checked. Much like the Z-pass, the stepping on the pixels must not be aliased or you will get strange results. So now when you render your image sequence, a velocity map sequence will render as well.
So you know how to make a velocity pass; now how to use it. For this example, I will show you how to do this in After Effects, but it can also be achieved with other compositing programs. Drop your beauty pass (regular render) sequence and your velocity pass sequence into the timeline. Be sure your beauty pass is the top layer so that you can see it in the preview window. I like to use the CC Vector Blur effect. With your main layer selected, right click and choose Effect->Blur & Sharpen-> CC Vector Blur. In the Effects Window you can now see all the settings for the CC Vector Blur effect. Under Vector Map choose your vector layer. You can now see the blurring effect on your render. You can adjust the Type, Amount, Angle, Smoothness, and Softness to control the look of your motion blur.
With lengthy animations with a lot of motion, this method will be far faster than blurring straight into your render. I’m always a proponent of post work, because you can adjust your settings and see the results in real time without having to re-render.”