page_title: Van Hare Effect
section_title: Technique
hierarchical_section_title: Van Hare Effect / Technique
context_page_description: The Van Hare Effect is a 3D stereoscopic viewing technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision using psychophysical percepts. The Van Hare Effect creates the illusion of dimensionality, rather than actual dimensionality in the subject being viewed. The Van Hare Effect is achieved by employing the stereoscopic cross-eyed viewing technique on a pair of identical images placed side-by-side. In doing so, it artificially tricks the human brain and optical center into seeing depth in what is actually a two-dimensional, non-stereoscopic image. The illusion of depth is interesting in that even if the image pair is not itself originally stereoscopic, the brain perceives it as if it is.
  No special viewing apparatus is required as the brain is tricked into creating a false perception of a three-dimensional still image or video simply by viewing it with the cross-eyed technique. In videos or films viewed this way, the artificial depth perception derived can to be diminished for some viewers, perhaps due to the fast changing visual picture, which appears to occupy more of the brain.
context_section_description: A regular, two-dimensional image is all that is needed.  The viewer prepares two prints of the identical image and these are placed side-by-side, horizontally, and close together.  The pair of identical images are then viewed with the cross-eyed viewing method, the same as one would use in interpreting stereoscopic image pairs.  The cross-eyed viewing method has its roots in aerial photography and aerial reconnaissance image interpretation.
  No special glasses, filters, colored lens or other apparatus are required to employ the Van Hare Effect.  Simply by utilizing the cross-eyed viewing technique, the resulting pair of identical images are seen in three-dimensions, even if there are no stereoscopic differences based on the interstitial distance included in the image being viewed.
  The cross-eyed viewing method, in traditional stereoscopy, swaps the left and right eye images so that they will be correctly seen cross-eyed, the left eye viewing the image on the right and vice versa.  A fused three-dimensional image thus appears to the eye, though it also appears to be smaller and closer than the actual images, so that large objects and scenes appear miniaturized.
  For videos with the artificial depth implied by the Van Hare Effect, the viewer plays the same video side-by-side, as above, and views it with the cross-eyed viewing technique.
  As an aid for those wishing to learn how to use the cross-eyed viewing method and thereby create image pair fusion, a fingertip can be placed just below the division between the two images, then slowly brought straight toward the viewer's eyes, keeping the eyes directed at the fingertip; at a certain distance, a fused three-dimensional image should seem to be hovering just above the finger.
  A second aid, alternatively, involves using a piece of paper with a small opening cut into it.  This can be used in a similar manner so that when the paper is correctly positioned between the image pair and the viewer's eyes, which can be achieved by moving the paper forward and backward until the two images come into full view, whereupon the hole in the paper will seem to frame a small three-dimensional image.
  A third aid is that the viewer goes cross-eyed, and when this is done, the two images appear as three images, due to the lines of sight when cross-eyed.  If the viewer focuses on the middle of the three images, it will come into focus as stereoscopic, without any viewing apparatus or use of a finger to get the eyes correctly focused.
caption: View of Boston, c. 1860; an early stereoscopic card for viewing a scene from nature, where actual stereoscopic image data is included.
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