it's hard to understand EXACTLY what do you want to achieve from your explanation. if it's a scene you already setup in Ae, it would be helpful to show it, is it video on green screen, full frame video? what are we talking about here? make sure you show the composition window and the timeline. a video file will be even better. try as best as you can - the more accurate you illustrate your problem/situation - the more chance to get an optimal solution
Totally understand, let me put this in a simplified term.
I am filming a man walking in the ground, a light source is behind my camera, so the man's shadow is falling 90 degree flat on the floor.
Now, I don't want the man to be there in my output but I just want his shadow alone moving. How can we do this?
Is this possible to make a man walking front of the green screen and darken him and lower opacity and bring that keyed layer on the ground layer and turn the keyed layer flat like 90 degree?
now it's much more clear. you need VFX advice before your shoot. you need to have a shadow without the man being there. I think you can go 2 ways with this: shoot on location twice or go the green screen approach. each with it's own complexities and advantages and disadvantages. I will describe the basics:
On Location Approach
if you want to try to do this on location you can:
1. shoot your scene with the man and the shadow
2. shoot the same scene without the man and the shadow
3. mask out the man with rotoscoping and tracking
this could work if you have no movement in your scene. or if you have motion control rig that will shoot the same movement exactly twice.
Green Screen Approach
to your question - if you can key a person out and make him a shadow flat on the ground - YES. this is the way I would try to do this - it's called a Shadow Catcher - a layer that will only accept your shadow. this could work with or without movement in the camera.
let's say you have this guy on green screen
and you want just his shadow on this background
these are the steps:
1. make the guy (keyed layer) 3D and add a floor corresponding to the floor* in the actual shot.
floor - you add a solid, make it 3D, add the grid effect and position it accordingly with adjusting a 2 node camera like this
2. add a point light layer and set it to cast shadow, set the keyed footage to cast shadow , and set the floor layer to accept shadow - only, and adjust the light
now you got this:
3. check that everything is o.k, now set the keyed footage to cast shadow - only
now you got just the shadow. you can adjust it to taste with shadow darkness and diffusion in the spot light properties
Brilliant, I can do this... Sincere thanks,
You are the best.
Great! run some tests what ever approach you choose. and if you can, post the results good luck.