Is there any other open-source thin client system ?

[lizhong]

Hi all,
    I have spent several months in learning the source code of vnc free edition. Now I'm trying to get codes of some other thin client systems.I know VNC is a partly open source project, and X is also open source. But X is over complex and may cost me much time in learning it. 
    Is there any other choice? I just want to know the details in the protocols. VNC is a pixel-based system. It delivers pixel in different encoding forms through the network.There are also some other protocols which send drawing commands to the client, such as X, SunRay.
    Is there any suggestion?
    Thank you for any help!
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lizhong
lizhong "at" ncic.ac.cn