The image processing of the stereoscopic images is done based on the stereovision technique to obtain the depth or distance of the region of view from the point of vision.
An artificial stereo vision system uses two cameras at two known positions. Both cameras take a picture of the scene at the same time. Using the geometry of the cameras, the geometry of the environment can be computed. As in the biological system, the closer the object is to the cameras, the greater its difference in position in the two pictures taken with those cameras. The measure of that distance is called the disparity. Fig 2.3 illustrates the geometry of stereo vision. In this example, the optical axes of the cameras are aligned parallel and the camera optical centres are separated by a baseline of distance, b.
Fig 2.3: Goemetry of stereovision
A coordinate system is attached in which the x-axis is parallel to the baseline and the z-axis is parallel to the optical axes. The points labeled “Left Camera” and “Right Camera” are the optical centres of two cameras. The distance f is the perpendicular distance from each optical centre to its corresponding image plane and it is the focal length. Point P is some point in space which appears in the images taken by these cameras. Point P has coordinates (x, y, z) measured with respect to a reference frame that is fixed to the two cameras and whose origin is at the midpoint of the line connecting the optical centres. Plane passing through the optical centres and a point in the scene is the epipolar plane and the intersection of the epipolar plane with the image plane is the epipolar line. Point in the scene that is visible to both cameras (binocularly visible) will be projected to a pair of points in the two images and they make the conjugate pair. The projection of point P is shown as Pr in the right image and Pl in the left image and the coordinates of these points are written as (xr, yr) and (xl, yl) in terms of the image plane coordinate systems shown in the figure. Note that the disparity defined above is xl -xr. Using simple geometry,
These equations can be rearranged to solve for the coordinates (x, y, z) of Point P.
The above equations show that distance is inversely proportional to disparity and that disparity is directly proportional to the baseline. When cameras are aligned horizontally, each image shows a horizontal difference, x lx r, in the location of Pr and Pl, but no vertical difference. Each epipolar line in one image has a corresponding epipolar line in the other image. The image processing is done in order to arrange the epipolar lines in a particular horizontal line. These two matching lines have the same pixels, with a disparity in the location of the pixels. The process of stereo correlation finds the matching pixels so that the disparity of each point can be known. Note that objects at a great distance will appear to have no disparity. Since disparity and baseline are proportional, increasing the baseline will make it possible to detect a disparity in objects that are farther away. However, it is not always advantageous to increase the baseline because objects that are closer will disappear from the view of one or both cameras. Thus, the disparity of all points in the image is measured and a disparity map is also calculated.
Stereorecording and display
The HD stereoscopic image recording uses the general rule of the 3D format because stereoscopic images are limited by memory capacity, processing time, synchronization problem between left and right video signals, and processing time for the en/decoding. The devices for the general rule of the 3D format are generally called 3D Mux (multiplexer) and 3D DeMux (demultiplexer). The 3D Mux has the function of compressing two channel video signals into one channel and the 3D DeMux has the reverse function. There are two types of 3D Mux/DeMux of the frame sequential and spatial compression (side by side) methods. Fig 2.4 shows 3D Mux/DeMux of the two types of 3D formatter.
Fig 2.4: Schematic diagram of 3D Mux/DeMux for two types of the 3D format.
For the stereoscopic display system for a microscope in medical applications, the most important conditions are precision to make accurate observations and stability for long time use and minimized watching fatigue. Table 2.1 shows an explanation of the typical stereoscopic display methods. There are other stereoscopic display devices which have advantages as well as disadvantages. The combination of cathode ray tube monitors (or LCD) with shutter glasses is a widely proven method. Its main drawback is the need for synchronization between the display and the glasses, either by a cable or by an infrared link. Autostereoscopic displays avoid the necessity to wear glasses and the Fig 2.5 shows the different autostereoscopic displays. However, the range of movement is extremely limited as there are only a few defined positions in front of the screen to obtain a proper stereoscopic image as shown in Fig 2.6.