A 3D GIF, or Graphics Interchange Format file, is a moving graphic image on a Web page that appears to "pop" off the screen or to transcend its two-dimensional field, rather like a three-dimensional movie image. Unlike 3D cinematic images, the optical effect of 3D GIFs is not contingent upon the observer wearing special 3D glasses.
GIFs are by definition animated. The animated component is partly what contributes to the illusion of a 3D GIF and does not necessarily imply a 3D GIF made with animated, or cartoon, images. GIFs become seemingly 3D when the moving graphic or two dimensional object oversteps a visible or invisible boundary between the background, midground and foreplane of the image.
White, black or invisible vertical lines may denote the boundaries between levels of depth within the context of a frame or field of vision. Painters incorporate the same principles to establish and conjure the illusion of depth within a two-dimensional image.
Most GIFs already possess all of the necessary visual depth that the insertion of vertical lines merely dramatizes, which causes the image to pop or appear to be 3D. The inserted lines effectively trick the viewer's brain, which perceives the lines to be closer than the image, or overlaid. The image remains the same; only the viewer's perspective changes in relation to the image.