Editing X11vnc (section)

== Polling algorithm ==

x11vnc keeps a copy of the X server's frame buffer in [[RAM]]. The [[X11]] programming interface <code>XShmGetImage</code> is used to retrieve the frame buffer [[pixel]] data.  x11vnc compares the X server's frame buffer against its copy to see which [[pixel]] regions have changed (and hence need to be sent to the VNC viewers.)  Reading pixel data from the physical [[frame buffer]] can be much slower than writing to it (because graphics devices are not optimized for reading) and so a sequential pixel by pixel check would often be too slow.

To improve the situation, x11vnc reads in full rows of pixels separated by 32 pixels vertically.  Once it gets to the bottom of the screen it starts again near the top with a slightly different offset. After 32 passes like this it has covered the entire screen. This method enables x11vnc to detect changes on the screen roughly 32 times more quickly than a sequential check would (unless the changes are very small, say only 1 pixel tall.) If the [[X11]] DAMAGE extension is present, x11vnc uses it to provide hints where to focus its polling, thereby finding changes even more quickly and also lowering the system load.