x11vnc has a client-side caching protocol, which is described in:

http://www.karlrunge.com/x11vnc/faq.html#faq-client-caching

It is compatible to most vnc clients, but in order to make use of it, the client needs to be able to crop the vnc frame buffer to the real screen size.

This patch implements some heuristics to correctly crop screen size, 
  * if scaling is enabled, and
  * the height of the frame buffer is much larger than the width of the frame buffer.

  the number of ncache is found by finding the server-side aspect ratio minimizes the cost function:
  
  (h * AspectRatio) / w - (int) (h * AspectRatio / w)

Three aspect ratios are tried: 4 / 3, 16 / 9 and 16 / 10.

There should be a configuration flag to disable this, which is not included in this patch.

It was developed based on the 3.4.2 tag (current version in Fedora 17), but applies cleanly (compilation untested) to the master.