Simple Shadow
Probably the cheapest and easiest way to add a “shadow” to an object is by creating a shadow mesh that acts as the shadow for the object. This was a common technique in older 3D games such as Pokemon Stadium and Super Mario 64 both on the Nintendo 64.
One way to create a fake shadow is to draw one to a canvas texture then use that texture as the color map for a plane mesh. The plane mesh is placed below the object but above the ground. Note that it’s important to offset the shadow mesh from the ground slightly to avoid z-fighting.
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Another advantage to using this kind of shadow is that since it’s a mesh it’s very easy to translate, scale, or rotate it as needed.