An autostereogram, also referred to as a single-image stereogram, or SIS, is an image created using computer graphics that deceives the brain into perceiving a three-dimensional, or 3D, display when actually viewing a two-dimensional, or 2D, image. Common types of autostereograms that include random dot and wallpaper images. 2D optical illusions producing a depth effect can usually be seen by using specific focusing techniques. Instead of looking at the image itself, the viewer must use wall-eyed, or cross-eyed, focusing to detect the hidden image.
In a typical 2D picture, colors, lines, and overlapped images create the illusion of depth by using perspective. Lighter colors bring an object forward, and lines that converge at a single point appear to travel off into the distance. The eyes also perceive depth when objects are strategically placed in front of each other using a gradient of light to dark colors. A random dot or wallpaper autostereogram uses layering techniques to produce a 3D image.
A random dot autostereogram first appears as a spattering of millions of tiny dots or pixels. The dots might be black and white or highly colored. Layers of texturing might also be added. Hidden within these dots is a design, object, or word, initially produced in a greyscale gradient. This is generally a black and white image using lighting, shading, and rendering to form a multidimensional image.
Using a stereogram program, the artist combines the two images. Through camouflage, the greyscale image borders are slightly altered in color compared to the surrounding dots or pixels, but the colors throughout the image match the background.
Wallpaper autostereograms use a similar technique, except the 2D image generally contains repeated patterns, strategically placed either horizontally or vertically at varying distances. Artists combine the color display with the greyscale image, and the borders of the greyscale image differ slightly from the surrounding colors. Viewing the hidden image contained within a random dot or wallpaper autostereogram involves altering the visual line of focus.
Normally, when an individual looks at a picture, each eye views the image from a slightly different angle. The brain receives this information, creates a composite single picture, and transmits the information back to the eyes. Recognizing hidden images in autostereograms generally requires changing the point of focus from the image itself to a point past the stereogram. This type of altered focus is commonly referred to as wall-eyed viewing. Altering the focal point in this way is similar to looking through a window to view scenery on the other side, where, eventually, the brain detects and reveals the closer image.
Cross-eyed 3D imaging involves two like images placed side by side. For the brain to perceive depth, the viewer must change the focal point from the images to an area in front of the picture, which crosses the eyes. This focal point combines the first two images and forms a third, which creates the illusion of depth.