The Fast Blur blurs footage. And it renders fast. The Blurriness value increases the blur amount. Blur Dimensions controls which axis the blur will be applied. The default is Horizontal and Vertical, but you can set to blur in only Horizontal, or only Vertical. The Fast Blur is nearly identical to the Gaussian Blur, except it adds the vital “Repeat Edge Pixels” feature. This alone makes the Gaussian Blur obsolete, in my opinion.
With that out of the way, let’s make a lightsaber. We have our footage, or in this case an image. Create a new white solid. Turn off the solid’s visibility by hitting the eye next to the layer. Select the pen tool or hit G on your keyboard to activate it. Draw a box around the stick or whatever placeholder you have used in your footage. Once that’s done, you can turn the visibility back on. When using footage, you need to go frame by frame and mask out your placeholder.
Now we apply a Fast Blur. To apply a Fast Blur, select your solid and choose Effect, Blur & Sharpen, Fast Blur. The core of the lightsaber only needs a small amount of blur. The value you choose will be dependent on the resolution of your footage, but ultimately this is an aesthetic choice. Do what looks good to you.
Next, duplicate your white solid. Set the layer’s transfer mode to Screen. Change its color by choosing Effect, Generate, Fill, and select a color. Go into your colored solid’s effect settings and increase the blur to whatever looks good to you. That’s the general idea behind a very simple lightsaber. Duplicate your layer again.
Here is a quick workflow tip: you can parent all of your colored layers to be controlled by one Fill effect. Alt click on the stopwatch next to the Fill color to open up expressions. Drag the pickwhip to the Fill color of your original colored layer. Now you only have to change that one Fill color instead of having to open each individually if you decide to become a Sith instead of a Jedi. Anyway, with that done, increase the blur of your new layer. Continue to duplicate the colored layer and adjust the blurriness until you are happy with the results. I wind up with three to four colored layers depending on the footage.