I hope you enjoy this tutorial
One of the most frustrating things about creating visual effects for bullet hits is to ensure all the blood/debris/sparks appear behind your actors or objects in the scene. Imagine masking them out frame by frame and then you decide to move them or replace them with different footage. You end up having to go through the whole frame by frame masking nightmare againTo avoid this, I have decided to go another way and it has worked out pretty well so far
Instead of masking out the elements to sit behind my actors or objects, I rotoscope out the actors or objects and create an overlay layer. I then place all blood/spark/debris effects on top of my base footage layer without masking and then place the overlay layer on top of that. Instantly, all bullet hit effects appear to be sitting behind the elements in the overlay layer
And the best thing about this solution: it's flexible! I can move or replace all bullet hit elements without any extra work required.
Starting with the base footage, we first rotoscope out our actor for the duration of the bullet hit to create the overlay layer. In After Effects CS5 and up, I highly recommend using the rotobrush tool to help you with this task as it can be a little tedious. The rotobrush tool is not perfect, but it does make your job a lot easier
Be sure to enable the 'refine matte' option on the rotobrush effect, it will make the edges of your overlay look cleaner.
Because the rotobrush effect is really slow to render, we render out our overlay layer. To maintain the transparency of the video, I usually export as AVI and set the output channels to RGB+Alpha.
Once we've done that, we can reimport the clip into your scene and place it on top of your base footage. Don't forget to disable the rotobrush effect on your base footage as we no longer need it. I usually don't like to delete it just in case I need to make adjustments and re-export the overlay
Now let's create the actual bullet hit effect!