Depth Perception

Why do people have stereo perception when they see the images? People have thought this problem long time ago. In order to understand the generation of the stereo image, we must define the meaning of depth perception first. In the year of 280, Euclid explained the fact that as to the same object, if our two eyes see the different images at the same time, we would have depth perception. In modern psychology, this is called binocular parallax and is

the important reason of causing depth perception. Besides, convergence and the ability of eye accommodation also play important roles on depth perception.

2.1.1 The Formation of Binocular Images

The scientists considered that left eye and right eye see the different images is the most important reason of causing depth perception. The image on the retinex of a single eye is a 2D image. In the basis, the image on the retinex of a single eye doesn’t generate depth perception. It needs another image generating from the other eye, and let the retinex receives the different 2D image. By transmission of the optic nerves to the brain and the decision made by brain of two images, there is the depth perception of the distance relation between near or far of the objects.

2.1.2 The Cause of Generating Depth Perception

People know the existence and shape of objects in 3D space by the reflection of light on the retinex. The fundamental faculty of retinex is like the principle of camera capturing.

There are many optical nerve cells distributed on the retinex. So we can know the positions of the observed objects. However, it is not enough to know the depth perception of the observed objects only by retinex. In the viewpoints of and Physiology and Psychology, there are some causes of the depth perception.

(a) Physiological factors:

(1) Binocular disparity:

The distance between left and right eye is about 6cm. By the difference of formation of images from two eyes, people can perceive and determine the depth of the observed objects. In Fig 2.1, the left and right eyes see the pyramid, and get

different image. It is binocular disparity. By the formation of images of the binocular disparity, left and right eye get the part of real line of the image shape. And the part of dot line of the image shape is the composition stereo shape of the pyramid.

Fig. 2-1 The formation of image generating from binocular disparity

(2) Convergence:

It means the angle of the lines of left and right eye vision. The angle changes according to the distance between the observer and objects. If the distance is nearer, convergence is larger. On the contrary, if the distance is farer, convergence is smaller.

The angle of two eyes needs to modify according to the distances of the objects. Thus, people can feel the depth of the objects. In near distance, the change of convergence has significant contributions to the depth perception, especially when it cooperates with eye accommodation. But when the distance is over 10m, because of the tiny change of convergence, people can’t perceive the depth of objects.

(3) Accommodation:

The lens of eye is relative to the focus lens of camera. It can raise lens to let the image project on the surface of retinex. According to the degree of raise, people can perceive the depth of objects. Generally, when the object is over 2 m, it is difficult to actually perceive the depth.

(b) Psychology factors:

(1) When the observer relatively moves to the objects, the binocular disparity is generated by the motion of near and far scene. We said that it is the disparity of a single eye motion. The disparity of a single eye motion may provide the perception of the depth of objects.

(2) By telling from the difference of the size of the formation of images on retina, even if they are the same objects, people also can distinguish the near distance from far distance.

(3) From the arrangement of objects in 3D space, we can compose perception of depth and stereo.

(4) When people see the parallel lines of a distant place, people also can perceive the depth by their eyes.

(5) Even if people see the uniform objects of a distant place, for example the terraced field and the gradient surface, people also can perceive the depth by the different density.

(6) When people see the same contrast ratio object of a far distance, people also can perceive the depth and distance of objects by the decrease of the contrast ratio.

(7) By the shading of objects generating from the light, people also can feel the stereo shape of objects.

(c) The formation of generating depth perception:

Basically, when people see the scene, they approximately can divide the scene into three levels (far, middle, near). Thus, our eyes will adjust to comfortable angles to observe the objects. When the two eyes see the different positions and angles, binocular disparity will exist naturally. And when the eyes see the far, middle, and near scene, the change of lines of visions can be classified as follow:

(1) Uncrossed-Parallax.

(2) Zero-Parallax.

(3) Crossed-Parallax.

In order to simulate the situation of seeing the scene, we will let the screen be a reference surface and individually present to left eye and right eye. In Fig. 2-2, we will explain the change of focusing visual.

Fig. 2-2 The formation of generating depth perception

(1) Uncrossed-Parallax:

Two eyes don’t focus in front of the screen (two eyes see the distant scene parallel), and the image will be shown in back of the screen.

(2) Zero-Parallax:

Two eyes focus on the screen, and the image will be shown on the screen.

(3) Crossed-Parallax:

Two eyes focus in front of the screen, and the image will be shown in front of the screen.

The three methods will generate different stereo effect. And when we know the physical characters of the formation of stereo images and the binocular disparity, it is helpful for 2D to 3D image conversion algorithm to simulate binocular vision.

(d) Monoscopic Depth cues:

There are some monoscopic depth cues:

(1) Interposition:

An object that occludes another is closer.

Parallel lines converge at a single point.

(5) Surface texture gradient: