Such an installation isn't complex, but require some knowledge, especially to code something that fits your needs.
I think there are 2 steps.
# For the graphics
You could code a mini Processing application (see attached files for inspiration) or create a effect from this Shadertoy code : https://www.shadertoy.com/view/XsKGzK (see this tutorial to know more about custom effects in Millumin).
# For the human detection
A simple method is using a camera, filming from the top, and making the difference between the image with humans and without. I attached a Millumin project showing the process and using a blob-tracking.
I hope it'd help you. If you're not confortable with all this, be sure to find someone that is.
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