<P> The simplest type of autostereogram consists of horizontally repeating patterns (often separate images) and is known as a wallpaper autostereogram . When viewed with proper convergence, the repeating patterns appear to float above or below the background . The well - known Magic Eye books feature another type of autostereogram called a random dot autostereogram . One such autostereogram is illustrated above right . In this type of autostereogram, every pixel in the image is computed from a pattern strip and a depth map . A hidden 3D scene emerges when the image is viewed with the correct convergence . </P> <P> Autostereograms are similar to normal stereograms except they are viewed without a stereoscope . A stereoscope presents 2D images of the same object from slightly different angles to the left eye and the right eye, allowing us to reconstruct the original object via binocular disparity . When viewed with the proper vergence, an autostereogram does the same, the binocular disparity existing in adjacent parts of the repeating 2D patterns . </P> <P> There are two ways an autostereogram can be viewed: wall - eyed and cross-eyed . Most autostereograms (including those in this article) are designed to be viewed in only one way, which is usually wall - eyed . Wall - eyed viewing requires that the two eyes adopt a relatively parallel angle, while cross-eyed viewing requires a relatively convergent angle . An image designed for wall - eyed viewing if viewed correctly will appear to pop out of the background, while if viewed cross-eyed it will instead appear as a cut - out behind the background and may be difficult to bring entirely into focus . </P> <P> In 1838, the British scientist Charles Wheatstone published an explanation of stereopsis (binocular depth perception) arising from differences in the horizontal positions of images in the two eyes . He supported his explanation by showing pictures with such horizontal differences, stereograms, separately to the left and right eyes through a stereoscope he invented based on mirrors . When people looked at these flat, two - dimensional pictures, they experienced the illusion of three - dimensional depth . </P>

Pictures that you have to look at cross eyed