Client-side Caching
The RFB (VNC) protocol is odd when compared to other network graphics protocols, such as X11 and RDP, in that there is no provision for viewer-side caching of pixel data. While this makes the client easier to implement, there is a price to pay in terms of interactive response. For example, every re-exposure of a window or background region needs to have its (compressed) pixel data resent over the network. This effect is particularly noticeable for windows with complex or photo regions (such as a web browser window) that gets iconified and deiconified or re-exposed often.
x11vnc has an experimental and somewhat brute-force implementation of client-side caching. It is enabled via the -ncache option. When creating the RFB frame buffer in this mode, x11vnc allocates a very large scratch region below the top portion used for the actual (on-screen) pixel data. x11vnc can then use the RFB CopyRect command to instruct the viewer to move rectangles of pixel data into and out of the scratch region. These moves are done locally on the viewer side. In this way x11vnc can manage the scratch region to store and retrieve pixel data without having to resend it over the network.
x11vnc's client-side caching mode can give noticeable interactive response improvements for many activities. Since it uses the existing RFB CopyRect command, the scheme will work with any (i.e. unmodified) VNC viewer. There are some disadvantages, however. The first is that it consumes a large amount of memory. For good performance a scratch region 10 to 20 times larger than the actual screen should be used. So instead using 5 MB for a 1280x1024 truecolor frame buffer, closer to 100 MB will be used (on both the VNC client and server sides.) This is not so much of an issue on modern computers, but would not be possible on a low memory device. Second, the VNC viewer may treat the scratch region in ways that confuse the user, for example displaying it to the user or automatically panning down into it if the mouse reaches the bottom of the real screen. The Unix VNC viewer in SSVNC automatically hides the scratch region. Finally, x11vnc's heuristics for caching and reusing window pixel data are not perfect and can lead to unexpected flashing of a window's contents and other undesired effects.
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