We determine whether something is closer to us than something else in a number of ways - this 'depth' perception is critical in many incidents of life and the ability we have been given to perceive depth (the 3rd dimension - height and width being the other 2 dimensions) is amazing. However as you read below you will SEE that not all of us are equally endowed regarding depth perception and that some persons actually completely lack stereo-depth perception. Hence, if someone tells you that they do not SEE anything DIFFERENT in a stereo 3d image from a normal image, it is possible, if you see the depth and others do as well, that there is nothing 'wrong' with the way you created the stereo photograph but something wrong with their eyesight. The percentage of the population that suffers from a 3d deficiency is not known but as 3d movies become more and more popular the percentage of those who do not go to 3d movies may be an indication of the percentage of the population that has stereo-deficient depth perception.

See: http://webvision.med.utah.edu/book/part-viii-gabac-receptors/perception-of-depth/

The above click explains the science of depth perception and the 'cues' we are provided by our eyes and brain as to whether something is closer or farther away. A portion of the above link is included below for your consideration but please do visit the website above from whence this information has been collected and placed below:

The Organization of the Retina and Visual System

Perception of Depth

Michael Kalloniatis and Charles Luu

Stereopsis

Stereopsis refers to our ability to appreciate depth, that is the ability to distinguish the relative distance of objects with an apparent physical displacement between the objects. It is possible to appreciate the relative location of objects using one eye (monocular cues). However, it is the lateral displacement of the eyes that provides two slightly different views of the same object (disparate images) and allow acute stereoscopic depth discrimination.

Monocular Cues

Several strong monocular cues allow relative distance and depth to be judged. These monocular cues include:

• 1. Relative size

  2. Interposition

  3. Linear perspective

  4. Aerial perspective

  5. Light and shade

  6. Monocular movement parallax

Relative Size: Retinal image size allow us to judge distance based on our past and present experience and familiarity with similar objects. As the car drives away, the retinal image becomes smaller and smaller. We interpret this as the car getting further and further away. This is referred to as size constancy. A retinal image of a small car is also interpreted as a distant car (figure 1).

Figure 1. Relative size. A retinal image of a small car is considered to be distant

Interposition: Interposition cues occur when there is overlapping of objects. The overlapped object is considered further away (figure 2).

Figure 2. Interposition. The blue circle is reported to be closer since it overlaps the red circle

Linear Perspective: When objects of known distance subtend a smaller and smaller angle, it is interpreted as being further away. Parallel lines converge with increasing distance such as roads, railway lines, electric wires, etc (figure 3).

Figure 3. Linear perspective. Parallel lines such as railway lines converge with increasing distance

Aerial Perspective: Relative colour of objects give us some clues to their distance. Due to the scattering of blue light in the atmosphere, creating “wall” of blue light, distance objects appear more blue (figure 4). Thus distant mountains appear blue. Contrast of objects also provide clues to their distance. When the scattering of light blurs the outlines of objects, the object is perceived as distant. Mountains are perceived to be closer when the atmosphere is clear.

Figure 4. Aerial perspective. Mountains in the distance appear more blue

Light And Shade: Highlights and shadows can provide information about an object’s dimensions and depth (figure 5). Because our visual system assumes the light comes from above, a totally different perception is obtained if the image is viewed upside down.

Figure 5. Highlights and shadows provide information about depth

Monocular Movement Parallax: When our heads move from side to side, objects at different distances move at a different relative velocity. Closer objects move “against” the direction of head movement and farther objects move “with” the direction of head movement.

Binocular Cues

Stereopsis is an important binocular cue to depth perception. Stereopsis cannot occur monocularly and is due to binocular retinal disparity within Panum’s fusional space. Stereopsis is the perception of depth produced by binocular retinal disparity. Therefore, two objects stimulates disparate (non-corresponding) retinal points within Panum’s fusional area.

Fusion describes the neural process that brings the retinal images in the two eyes to form one single image. Fusion occurs to allow single binocular vision. Fusion takes place when the objects are the same. When the objects are different, suppression, superimposition or binocular (“retinal”) rivalry may occurs. Suppression occurs to eliminate one image to prevent confusion. Superimposition results in one image presented on top of the other image. Binocular rivalry describes alternating suppression of the two eyes resulting in alternating perception of the two images. This usually occurs when lines are presented to the two eyes differ in orientation, length or thickness. An example of binocular rivalry occurs when one eye is presented with a horizontal line and the other eye is presented with a vertical line. Binocular rivalry occurs at the intersection of the lines and some suppression also exists (figure 6)

Figure 6. (a) Binocular rivalry can be demonstrated by placing a pen between yourself and the screen. Keep you eye on the tip of the pen and notice the two bars merge. You may need to slowly move the pen from the screen towards you. (b) Result of (a)

Panum’s fusional area is the region of binocular single vision. Outside Panum’s fusional area, physiological diplopia occurs. Using the haplopic method of determining the horopter, Panum’s area can be determined (figure 7).

Figure 7. Haplopic method of determining the horopter involves locating the region of single binocular vision at a distance of 40cm. Panum’s fusional area lies between the outer and inner limits of the region of single binocular vision

Retinal disparity: Retinal disparate points are retinal points that give rise to different principal visual direction and diplopia. However, retinal disparity within Panum’s fusional area (zone of single binocular vision) can be fused resulting in single vision. Retinal disparity is essential for stereoscopic depth perception as stereoscopic depth perception results from fusion of slightly dissimilar images. Due to the lateral displacement of our eyes, slightly dissimilar retinal images result from the different perception of the same object from each eye.

Clinical Tests used to measure Stereopsis

There are two groups of clinical tests used to measure stereopsis. These are the contour stereotests and the random-dot stereotest. Random-dot stereograms were first used by Julesz (1960) to eliminate monocular cues. As there are no contours, depth perception (stereopsis) can only be appreciated when binocular fusion occurs. Two process of stereopsis are used and these are local and global stereopsis. Local stereopsis exists to evaluate the two horizontally disparate stimuli. This process is sufficient for contour stereotests. Global stereopsis is required in random-dot stereogram when the evaluation and correlation of corresponding points and disparate points are needed over a large retinal area.

An example of a contour stereotest used in the clinic is the Titmus Fly Stereotest. In the Titmus Fly Stereotest, horizontal disparity is presented via the vectographic technique (Fricke and Siderov, 1997). When tested a 40 cm the fly has a disparity of 3,600 sec of arc; the disparity of the animals range from 400 – 100 sec of arc and the disparity of the Wirt rings range from 800 – 40 sec of arc (figure 8).

Figure 8. Titmus Fly Stereotest

Examples of random-dot stereotests used in the clinic are the Frisby Stereotest, the Randot Stereotest, the Random-dot E Stereotest and the Lang Stereotest. The Frisby Stereotest (figure 9) uses real depth to determine stereoacuity. Three perspex of different thicknesses are used. Four squares of geometric shapes are painted on one side of the perspex. In one of the squares, a circle of these geometric shape is painted on the other side of the perspex. Both the Randot (figure 10) and the Random-dot E uses crossed polarised filters. Disparity is also constructed vectographically. The Randot Stereotest uses modified animals and ring designs with random dot backgrounds to eliminate monocular cues. The Lang Stereotest uses a panographic technique (Fricke and Siderov, 1997) to present disparity, therefore, no filters are required. Patients are required to identify pictures on the Lang Stereotest. The Lang II Stereotest has a monocularly visible shape on it (figure 11).

Figure 11. The Lang II

All the tests provides a measure of stereoacuity by asking the patient to identify the correct target that has stereoscoptic depth (target with disparity). The working distance and interpupillary distance will need to be taken into consideration when calculating stereoacuity. Patients with disturbed binocular vision or different refractive error in one eye, will perform poorly on depth discrimination tests.

Acknowlegements

We like to thank Tim Fricke for providing Figures 8-11.

References.

Fricke TR and Siderov J (1997) Stereopsis, stereotest and their relation to vision screening and clinical practice. Clin Exp Optom. 80: 165-172.

Julesz B. Binocular depth perception of computer generated patterns. Bell Syst Tech J. 1960;39:1125–1162.2.

Moses RA and Hart WM (1987) Adler’s Physiology of the eye, Clinical Application, 8^th ed. St. Louis: The C. V. Mosby Company.

Ogle KN (1950) Researches in Binocular Vision. London: Saunders. 1950

Schwartz SH (1999)Visual Perception, 2^nd ed. Connecticut: Appleton and Lange.

Last Update: June 6, 2007.

The author

Michael Kalloniatis was born in Athens Greece in 1958. He received his optometry degree and Master’s degree from the University of Melbourne. His PhD was awarded from the University of Houston, College of Optometry, for studies investigating colour vision processing in the monkey visual system. Post-doctoral training continued at the University of Texas in Houston with Dr Robert Marc. It was during this period that he developed a keen interest in retinal neurochemistry, but he also maintains an active research laboratory in visual psychophysics focussing on colour vision and visual adaptation. He was a faculty member of the Department of Optometry and Vision Sciences at the University of Melbourne until his recent move to New Zealand. Dr. Kalloniatis is now the Robert G. Leitl Professor of Optometry, Department of Optometry and Vision Science, University of Auckland. e-mail: m.kalloniatis@unsw.edu.au

http://adcnj3d.wordpress.com/the-science-of-anaglyph-3d/

Anaglyph is the name for color filtered 3D images. Anaglyph comes in a few flavors including amber/blue, red/green, red/blue and red/cyan. Red/cyan is the most popular and the one we use on this site. More technical information about it can be found onWikipedia and other sources.

The basic idea is that the two images which make up the stereo view are filtered by these colors so that only one of the two images can be seen by each eye to create the 3D viewing effect. Because color is used to filter the images, accurate color in the image is hard to achieve. For example, with red/cyan glasses reds and blues are hard to show correctly. Red is the most troublesome color to render with red/cyan.

The advantage to anaglyph images is they can be shown on any standard screen; no special 3D TV or monitor is required. They can also be printed with standard techniques on any color printer. In addition, the glasses are inexpensive and easy to distribute. These facts keep anaglyph as a serious option for showing 3D today and in the future, especially when doing it in print.

A skilled stereophotographer can plan and prepare for anaglyph to provide a very satisfying result. This can be done manually in programs such as Photoshop or automatically with software made just for 3D. StereoPhoto Maker is one of the best programs for preparing images and it is free and available online here.

One of the people who have advanced the potential of anaglyph is Eric Dubois. He has created a version of preparing anaglyph images that gives very good results. Info on his work can be found here.

Enjoy Eric’s out-of-this-world anaglyph imagery below.

About Eric Dubois:

Eric Dubois is an electrical engineer and professor at the School of Information Technology and Engineering at the University of Ottawa in Ottawa, Canada. He became interested in stereoscopic imagery in the late 1990′s when he worked on a research project in collaboration with IMAX. During this project, he attempted to characterize the anaglyph process, taking into account the spectral characteristics of the display devices, the colored filters in the anaglyph glasses, and the cones in the human eye. Based on this characterization, he developed a least-squares algorithm to generate an optimized anaglyph from the stereo pair. The red-cyan version has been incorporated into many popular stereoscopic viewing programs including StereoPhoto Maker, but the algorithm also works well for green-magenta and amber-blue anaglyphs.

He continues to carry out research on stereoscopic imaging, including stereoscopic omnidirectional panoramas, as well as on color image processing. He has done a lot of work on color interpolation for digital cameras and has written a book entitled, “The Structure and Properties of Color Spaces and the Representation of Color Images” published by Morgan and Claypool in Oct. 2009.