Matte Shadow is an option in the material settings that can be used to cast Ambient Occlusion on an invisible floor when compositing an object into your scene.
Add an object to your scene and set the anchor to bottom. Leave this object on Group 1. You can download this iPhone model here
and the stock footage here
Add the plane object from the primitives and set this object to Group 2.
Hit OK at the top right and you can scale the particle size of the plane to fill a large area. Use the Group 2 settings to scale the object. You can use the Camera Options
to position the scene.
In the Element Render settings Tab there is a drop-down called Ambient Occlusion. Check the Enable AO box and you will see some shading appear at the base of the iphone and plane model.
Then go back in to scene setup and select the material that is one the Plane. Near the bottom of the material settings there is an option called Matte Shadow. Check the box and you will see your object disappear.
Hit OK to get back to the After Effects Comp and you will see that the plane is invisible but is still catching the Ambient Occlusion shading on the ground.
You can also use the Matte Shadow to obsure objects like the are inside the ground. Just position the Group 1 object like you are just pushing it down through the plane like normal. Depending on how far you are pushing it down, you may need to scale your plane up so that you don't see the other object going though the other side.
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