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 posts 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.
With lengthy animations with a lot of motion, this method will by far be 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.