Ok, here's a nifty little way to add motion blur to an already rendered CG element. Which helps save on some rendering time.
Import your rendered CG elements into shake and put them together using the correct layer nodes. I created a quick example below using a simple bouncing ball animation.
Alright now, create a rotoshape and make a shape similar to your CG element . Don't worry it doesn't have to be exact.
Now add a Mix node to the Rotoshape and CG element. This will help ensure that the two shapes are aligned during the animation process.
Add a Move2D node to the Rotoshape. Align the two shapes and key the xPan and yPan to have it follow the same movement as the CG element.
Turn the motionBlur to 1 and adjustd the shuterTiming and shutterOffset to get the right amount of blur you want.
Now we need to apply this blur to our actual CG element. Take the Move2D from the Rotoshape and apply it to the CG element. Scroll down the parameters of the Move2D and click the useReference button. This applies the motionBlur without moving the object.
Now play your animation and you'll notice it now has a motion blur.
