There are two approaches to simulate scene moving with six degrees of freedom of own-ship in multi-channel visual system, using movable platform and graphics or to implement it exactly through graphics techniques. The first approach uses movable platform to present the roll, pitch and heaving of own-ship simulated, but the yawing, swaying and surging of own-ship are simulated through the image's change of visual system. This approach can provide operator a strong feeling of immersion, but movable platform is very expensive and is difficult to maintain. The second approach can save a lot of money and is easy to maintain. This approach can provide operator a good feeling of immersion too. When using movable platform, three parameters, roll (β), pitch (γ) and heave (z) are sent to control devices of movable platform for controlling the movement of it. Other three parameters of own-ship, yawing angle (α), swaying and surging movements (x,y) are sent to the image generating computers of multi-channel visual system. The second approach sends all of the six parameters (x, y, z, α, β, γ) to image generating computers by high speed Ethernet. Computers with high speed graphics accelerator will generate images dynamically according to own-ship's movement with six degrees of freedom.

　

3.1 Dynamically Setting of the Vewing Direction of Each Channel

　

 When own-ship moves, image generating computer of each channel must set transformation matrix correctly according to the movement parameters (x,y,z,α,β,γ)of own-ship.

　

 Suppose the rotation angles of own-ship around x, y, z axis are (α,β,γ). The viewing direction of each channel can be calculated along the following steps:

　

1) Calculate the initial rotation matrix Mt according to the channel number m

Rxi,Ryi,Rzi are determined by the initial rotation angles(α0,β0,γ0) of viewing direction around x, y, z axis in world coordinate system as:

 Here, n is the number of center channel, θ is the horizontal field of view of single channel, m is the number of other channels, m = ...n-4, n-3, n-2, n-1, n, n+1, n+2, n+3 or n+4...

2) When own-ship rotates around the x, y, z axis of it's local coordinate system, suppose the angles are (α,β,γ), the viewing direction should be rotated accordingly. Suppose the rotating matrix is M, then:

3) Viewing direction of each channel can be determined by Mc:

3.2 Dynamically Setting of Eye-point of Each Channel

　

 The eye-point of every channel in multi-channel visual system is same. It's position should be calculated dynamically according to the current position of own-ship when own-ship has moved.

　

 Suppose the eye-point is located at (xci,yci,zci) in the local coordinate system of own-ship, (x,y,z,α,β,γ) are the movement parameters calculated by mathematical model of own-ship. The translation matrix and rotation matrix are T, and MS respectively, the matrixes can be calculated as:

　

　

 Then, the position of eye-point (xci,yci,Zci) can be calculated by following formula:

　

　

 Although the eye-point of every channel is same, but the viewing direction in different channel is different. Consequently, the data of eye-point position, yaw, roll and pitch used for generating image in each channel must be synchronized strictly to guarantee the image 's stability and continuity of multi-channel visual system. There are three ways for synchronizing of each channel^[3][4] software synchronization, buffer swap synchronization and frame synchronization.

　

4.1 Software Synchronization

　

 Synchronization is realized by sending all the six movement parameters of own-ship through high speed Ethernet to each channel's computer. The speed of network communication, refreshing rate of image of each channel and the Dead-Reckoning algorithm are the essential factors to affect the implement of synchronization.

　

4.2 Buffer Swap Synchronization

　

 After one frame of image has been rendered, the image generating computer must wait for the signals indicating that other channels have finished an image 's rendering before sending buffer swap command to graphics hardware. Only when all channels have finished image rendering, the command can be sent for displaying the image on screen. With this method of synchronization, all channels will run at the lowest frame update rate of all channels.

　

4.3 Frame Synchronization

　

 With this method of synchronization, one channel is chosen as the video master channel, all other channels are slaves. The vertical synchronizing signal is connected between all the channels. Each time the master channel generates a vertical frame synchronization signal, the slave channels will reset their internal video logic as though itself had generated a vertical synchronization.

　

 Buffer swap synchronization and frame synchronization require that graphics system should provide signals for synchronizing, but not all of graphics systems can meet this requirement.

　

 It is necessary to point out that software synchronization is the basic synchronization method of any multi-channel visual system. Only on the basis of strict synchronization of eye-point position and yaw, roll, pitch data of own-ship, the advantages of buffer swap synchronization and frame synchronization can be implemented.

　

 The speed of network communication, frame update rate of image of each channel and the Dead-Reckoning algorithm can affect the effect of software synchronization. The first two factors are mainly determined by hardware. It is not reasonable and not necessary to send data each frame. Dead-Reckon algorithm is adopted by image generating computers in each channel to reduce the capacity of network communication according to the eye-point position and viewing direction. The following formulas are for Dead-Reckon algorithm:

　

Here, x, y, z are eye-point position, ψ is viewing direction, (x',y',z'), ψ' are eye-point position and viewing direction of last period. νx, νy, νz are velocity of own-ship, ω is rotating angle speed. t is the rendering time of last image. Here, suppose that the image changes a little in the period between two frames, the rendering time of adjoining frames is almost equal.

　

 Multi-channel visual system is adopted in simulation system in which several operators are working together such as bridge simulator. Normal projector without ability of geometry correction can be utilized in multi-channel visual system using flat screen or polygon screen, but its environment reality created by visual system is not better than system using cylindrical or spherical screen. Multi-channel visual system using cylindrical screen or spherical screen must adopt more expensive projectors with the ability of geometry correction.

　

 Besides screen and projector selection, the key technologies for configuring multi-channel visual system include how to set eye-point position and viewing direction dynamically according to the movement parameters of own-ship with 6 degrees of freedom and how to synchronize each channel within the system. The approach of setting eye-point position and viewing direction introduced in the paper can be applied to multi-channel visual system using cylindrical screens or polygonal screens for generating 3-D view with 120-360 degrees horizontal angle of field of view. Software synchronization is the basic synchronization method of any multi-channel visual system, the effect of software synchronization is determined by speed of network communication, update rate of frame and Dead-Reckoning algorithm. The key technologies for configuring multi-channel visual system described above can be used in another simulators such as flight simulator and vehicle simulators.

　

[1] Mou Xian-chen, Huang An-xiang etc. "The key technology and its realization of stereo simulation visual system [C]". In Chinese: The Proceedings of Chinese Simulation Conference 2001, pp.539〜544

[2] Ma Ji-feng, Peng Xiao-yuan, Feng Qin. "The research and realization of PC-based graphics and image system for flight simulator[J]". In Chinese: Journal of System Simulation. 2002, 14(3): pp.307〜309.

[3] Lei Xia-yong, Dai Shu-ling, Wang Xing-ren. "Real-time 3D image generating system based on PC[C]". In Chinese: The Proceedings of Chinese Simulation Conference 2001, pp.624〜627, 2001

[4] Quantum3D Inc . "Multi-channel synchronization in visual simulation system [R]". In English: 2001 .2

　

　

 Jin Yicheng was born in 1944. He is a professor of Dalian Maritime University and the director of Marine Dynamic Simulation & Control Lab., Dalian Maritime University. He has worked in the field of marine simulation for about 20 years. His research interests include marine simulation and computer graphics. E-mail: jycdmu@dlmu.edu.cn

　

　

 Yin Yong was born in 1969. He received his B.S. and M.S. degree in navigation technology from Dalian Maritime University in 1991 and 1994 respectively. In 2001, he received Ph. D in Transportation Information Engineering and Control from Dalian Maritime University. His research interests include simulation technology, virtual reality and computer graphics. E-mail: bushyin_dmu@263.net