Not sure how expensive it could be but maybe you could try creating the inside of the portal in 3D and masking it by the plane shape using a stencil buffer.
This post should give you an insight on how it look’s
Not sure how expensive it could be but maybe you could try creating the inside of the portal in 3D and masking it by the plane shape using a stencil buffer.
This post should give you an insight on how it look’s