Computer Vision (19)

Disparity is a must to perceive depth in a stereoimage pair and so our brain needs at least two separated points with disparity to extract the distance between them. Disparity at a later stage would use triangulation to perceive depth, but this triangle would depend on the separation between the images and not the depth of the actual object. The below image illustrates triangulation from disparity when a stereogram is cross viewed.

The red lines are traced when the eyes combine the rectangle and the green lines when they combine the circle. The point of intersection of the red lines gives the 3D location of the rectangle and the green lines that of the circle. As mentioned earlier the circle is in front of the rectangle when cross viewed. One of the points for the formation of the triangle comes from the point of intersection of either the red or the green lines and the other two points are the two eyes. The distance of the point of intersection of lines from the two eyes (d), depends on the separation between the images, so the absolute distance of the objects remains unknown in the stereo image pair. The relative depth of different objects from one another is obtained by corresponding objects form the two images, which moves the point of intersection of the lines according to the 3D placement of the objects (similar to red and green lines).

This is not the case when we extract depth from the actual 3D surrounding because our eye makes use of triangulation from the convergence of the eyes and not disparity. Our eyes assist to perceive the absolute depth of our surrounding while in stereograms we can only perceive the relative depth of one object from the other.