Phantogram

Phantograms, also known as Phantaglyphs, Op-Ups, free-standing anaglyphs, levitated images, and book anaglyphs, are a form of optical illusion. Phantograms use perspectival anamorphosis to produce a 2D image that is distorted in a particular way so as to appear, to a viewer at a particular vantage point, three-dimensional, standing above or recessed into a flat surface. The illusion of depth and perspective is heightened by stereoscopy techniques; a combination of two images, most typically but not necessarily an anaglyph (color filtered stereo image). With common (red–cyan) 3D glasses, the viewer's vision is segregated so that each eye sees a different image.

Phantograms can be created using drawn images, photographs, or computer-generated images. Phantograms are usually placed horizontally and are intended to be viewed standing back from the image, though they can also be placed vertically and viewed at an angle from above or below.

Phantograms work by presenting the viewer with a pair of flat images precisely distorted to mimic the anticipated perspective of a three-dimensional object viewed from the phantogram's intended vantage point. As with other forms of stereoscopy, the illusion reproduces many of the visual cues associated with binocular depth perception, fooling the viewer's vision into perceiving the two-dimensional images as having actual depth. The illusion is limited, however; phantograms lack some cues for depth perception such as convincing parallax, so the viewer must be stationary at the illusion's "sweet spot", a specific point at which the phantogram is designed to be most convincing.

The anamorphic distortion of the source image crucial to the illusion can be understood by likening the images to projections of a 3D object onto a plane (e.g. a sheet of paper) originating from the location of the viewer's eyes. The base of the object meets the plane where the object stands, while the tip of the object is "projected" to a more distant point on the plane. Two projections, one for each eye, are made to produce a pair of images suitable for any dual-image form of stereoscopy (usually anaglyph imaging viewed through colored filter glasses). When the viewer is presented with flat images distorted in this way, the position of points on the image plane matches the points the actual object occupied, producing the illusion.

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