Open Autodesk Composite, create a new Composition. Use to Middle Mouse Button (MMB) to open the Pick List, through which you can use the Image Import node to import your Render Passes (e.g. the Color Pass, the Ambient Occlusion Pass) AND the Motion Vector Pass.
With the last import, you could probably have an issue: it comes out completely black. This is only a visualization problem (because it is not rendered in RGB), so you'll need to select the Image Import node with the Motion Vector Pass, go to "Options" and select in the Channel Views the Motion Vector Pass. Now you are able to see your Motion Vector Pass.
Now it's time to compose our passes: create a Math Operation node (from the Composition section) from your pick list and choose the Multiply option, then connect the output of your Color Pass and Occlusion Pass (or the passes you rendered out) into the inputs of the Math Ops node.
Since the Motion Vector Pass has not an Alpha Channel, we need to associate the Alpha Channel of one of the other layers to the Motion Vector Pass. So take the Set Alpha node from the Alpha Section, connect the output of the Motion Vector Pass into the A input of our Set Alpha node and the output of one of the other passes into the B input. We obtain a Vector Motion pass with an Alpha Channel correctly associated.
At this point grab the Blur node from the Filtering section, connect the output of Maths Ops into the Input Image port of the Blur Node, and the output of Set Alpha into the Forward Sections input.
Nothing is really changed, it seems but if you select the Blur node and activate in the Vectors tab the View Vectors option, you'll be able to see some yellow arrows that show the direction towards which specific pixels are moving.
Now the most important thing: you need to set up your blur effect, by playing with the values of the Length option (into the Vectors tab) and Max Radius option, as you can see in the image below.
With the last import, you could probably have an issue: it comes out completely black. This is only a visualization problem (because it is not rendered in RGB), so you'll need to select the Image Import node with the Motion Vector Pass, go to "Options" and select in the Channel Views the Motion Vector Pass. Now you are able to see your Motion Vector Pass.
Now it's time to compose our passes: create a Math Operation node (from the Composition section) from your pick list and choose the Multiply option, then connect the output of your Color Pass and Occlusion Pass (or the passes you rendered out) into the inputs of the Math Ops node.
Since the Motion Vector Pass has not an Alpha Channel, we need to associate the Alpha Channel of one of the other layers to the Motion Vector Pass. So take the Set Alpha node from the Alpha Section, connect the output of the Motion Vector Pass into the A input of our Set Alpha node and the output of one of the other passes into the B input. We obtain a Vector Motion pass with an Alpha Channel correctly associated.
At this point grab the Blur node from the Filtering section, connect the output of Maths Ops into the Input Image port of the Blur Node, and the output of Set Alpha into the Forward Sections input.
Nothing is really changed, it seems but if you select the Blur node and activate in the Vectors tab the View Vectors option, you'll be able to see some yellow arrows that show the direction towards which specific pixels are moving.
Now the most important thing: you need to set up your blur effect, by playing with the values of the Length option (into the Vectors tab) and Max Radius option, as you can see in the image below.
Et voilà! You have your Blur Effect, without directly rendering it into Maya, saving a lot of time.
Remember to attach the last node (the Blur one) to the Output node and render the result.
In the video, you'll see the Blur Effect obtained. The scene has been taken from a Digital Tutors (Digital-Tutors) Tutorial, "Top 10 Rendering Tips for Maya Artists".
Rendered from Autodesk Composite as a sequence of Tiff images. Composed as a MOV movie format by using Adobe After Effects 5.5.