Could someone please explain to me how I can make a 3D environment, where I have a floor and a back "wall" that seamlessley merge so that the join isn't obvious, even when a AE light/shadow is cast onto it?
I'm kind of looking to recreate a background like the one in this image:
You'll need to fake the effect of the light on the floor and wall layers with the Ramp effect (or a something similar). Set these layers to NOT Accept Lights but to Accept Shadows.
Apply Effect>Generate>Ramp to the wall layer and set your start and end colors and points. Then apply Effect>Generate>Ramp to the floor layer. Match the appropriate start/end points and colors to achieve the desired effect.
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AE can provide many options, especially if you include 3rd party options.
To get the shadow you could set a floor layer with a mask that feathers to the far side in z space. This will make it blend in whatever background layer you have, 3d or 2d.
For a wall/floor combo with a seamless transition you can bend a layer in 3d space using FreeForm Pro. You'll have to fake the floor shadow though.