There are many visual cues to distance, including relative size, perspective, and motion parallax. All of these cues are available to one eye, and (except parallax) can be presented in a flat image. They are cues present in the scene. In this chapter, however, we focus on binocular cues, those cues that arise from the use of two eyes. Open and follow the instructions in its caption. This illustrates the two binocular cues, convergence and disparity. Each cue is limited in its useful range and is used along with scene cues.
We can use disparity to see depth in a flat scene. Try with red-blue glasses. The red lens goes over the left eye. (If you easily see “magic eye” images, you may want to choose the cross-eye or parallel-eye option instead of using the glasses.) As you move the disparity control, you should see the image move in and out of the screen. also shows the disparity in degrees and the depth in millimeters. If you were unable to see depth even when using the red-blue glasses, you may be one of the 2-10% of the population that is totally or partly stereoblind. Those who are stereoblind make do with other depth cues and are usually unaware of the condition until tested. See Barry (2009) for a fascinating account of learning to see in stereo after a lifetime of stereo blindness.
All methods of creating depth in flat scenes, such as the red-blue anaglyphs you just saw, work by presenting a slightly different image to each eye. See for an explanation of the main techniques.
If you calibrated your monitor with , the disparity and depth reported by will be accurate. If not, they should be close, assuming a viewing distance of 18″ and a monitor of average resolution.