The next thing I did was time remap the zombie layer inside After Effects. I wanted the zombie to move really twitchy and fast. You can enable time remapping simply by selecting the layer and going to Layer -> Time -> Enable Time Remapping. You can then create and drag around keyframes and compress or expand certain time durations on the clip
I created keyframes around the turning movement of the zombie as well as for the 3 running sections of the shot and compressed the space between them to speed those sections up.
This resulted in the zombie turning around in a flash and racing with super human speed from side to side. At each side I let the speed return back to normal so that the running seems even faster by contrast and it doesn't just look like the entire clip is sped up
Since I want the zombie to appear to have super human speed, I added additional motion blur into the scene by applying the CC Force Motion Blur effect to the layer.
Next, I wanted dark shape shadows to follow Celina as she is racing from side to side to emphasise the speed of the movement and add a ghostly horror effect
For this, I added the clean plate photo into the composition.
Because I only used this layer as input to the Difference Matte effect, we can disable the visibility on it.
Next, I duplicated the time remapped layer of the zombie run and applied the Difference Matte to it. You can find this effect by searching in the Effects & Presets panel in After Effects.Next, in the settings for the effect, I selected my clean plate as the 'Difference Layer' and set the View to 'Matte Only'.
The Difference Matte effect will detect differences in pixels between the layer it is applied to and the difference layer provided. So compared to our clean plate, we have the Celina zombie in the shot. With the 'matte only' option selected, we will then get a rugged black and white image of Celina's shape.
I also increased the matching tolerance and matching softness of the Difference Matte effect to 13 and 31% respectively to shoothen out the matte so the shape looks more like a ghostly shadow.
Because I want a dark shadow so I can multiply the layer over my footage, I applied the Invert effect to my zombie layer
This gives me a black shadow where Celina's body is. Note that there are some unclean artefacts in the image that we can remove with masks
I then set my shadow trail layer to multiply and shifted it by a single frame to the right (= delayed it by one frame). This causes a dark shadow to trail directly behind Celina as she dashes from side to side.
Repeating this process a few more times and delaying each new shadow trail layer by an additional frame adds a nice smooth, ghostly trail to the zombie
Here is the shot with 3 shadow trails on top of the zombie speed run base footage!
Note that the clean plate visibility is turned off because it is only used as input to the Difference Matte effect
Since I only want the trail to be visible while the zombie is running, I animated the opacity on all the shadow trail layers to fade in only during the zombie run and fade out when the zombie stopped
Finally, let's add some cool particles to complete our zombie run effect!