Before Google Cardboard come to the idea of replacing expensive glasses which filtered quickly-alternating images (or low-cost glasses which filtered different color images) with simply one static image for each eye, that method was used widely on the decades that span between 1890 to 1940.
I wonder if there is some application to convert such old stereoscopic images (those postcards with one image for each eye, intended to be viewed with a special viewer) to cardboard. I guess that it should be simple, because you already have a different image for each eye and just need to change image centers to match those of Google Cardboard.
