A Research On Image Transfer Scheme For Virtual Working Platform

The usual way of visiting applications on the heterogeneous platform is using remote desktop software to access the remote host and execute remote applications. But there are several inconveniences in the remote desktop software. Customers have to work under a completely unacquainted environment on the server. VirtualDesk system offers an easy way to access the remote applications just like accessing local application, and customers need not to worry about unfamiliar desktop environment.The VirtualDesk system accesses the remote application by transferring window image from server to client. An efficient scheme to grab and transferring the pixel map of the window is required by VirtualDesk. Most remote desktop software transfers the desktop image as a whole, and could not clip the image to window rectangle, which is not suitable for VirtualDesk.Because of the problems above, we develop a window image transferring scheme to support the VirtualDesk system. It works fine with heterogeneous machines, and it transfers the user input event as well to enable the client to control remote applications. VirtualDesk monitors the repaint region of windows on server side, and writes the refreshed window image to the frame buffer. VirtualDesk then sends the image in frame buffer to client. The server also receives the user inputs from client and generates input events on server side.VirtualDesk supports two operating systems: Windows and Linux. On Windows, VirtualDesk grabs window image by GDI interface, and monitors window events by Hook technology. On Linux, VirtualDesk makes use of Xlib interface to control the window, uses SHM extension to grab the window image and uses the XTest extension to generate input event.The VirtualDesk system applies a modified remote frame buffer protocol (RFB) to codec window image, and defines the logic to maintain session between VirtualDesk client and server. We add some new types of message into the standard RFB protocol to support more control of windows."SETWINDOW"message is added to support changing the controlled application window, and"RESIZEWINDOW"message is added to change the size of the window. We also applied some technology to improve the performance of the system, and minimize the network overload. We compare the new frame buffer to the buffer sent to client last time to find the region which has actually changed. VirtualDesk sends the image which has actual changed but not the window system tells it has been changed. The client of VirtualDesk is programmed in java, and the server of the system is programmed in C++, which has two versions, a Windows version and a Linux version. We have tested the system and the results show that VirtualDesk has completely realized the function of transfer window image and user input events. The average delay of the system is less that 100ms, and the average bandwidth is less than 100kbps. The average CPU utilization is less that 5%, and the memory consume is less that 20MB.