* Overview
{anchor: Overview}

TurboVNC is a derivative of VNC (Virtual Network Computing) that is tuned to
provide peak performance for 3D and video workloads.  TurboVNC was originally
a fork of [[http://www.tightvnc.com][TightVNC]] 1.3.x, and on the surface, the
X server and Windows viewer still behave similarly to their parents.  However,
the current version of TurboVNC contains a much more modern X server code base
(based on X.org 7.7) and a variety of other features and fixes, including a
high-performance zero-install Java viewer.  TurboVNC compresses 3D and video
workloads significantly better than the "tightest" compression mode in TightVNC
1.3.x while using only typically 15-20% of the CPU time of the latter.  Using
non-default settings, TurboVNC can also match the best compression ratios
produced by TightVNC 1.3.x for 2D workloads (see
{ref prefix="Section ": AdvancedCompression}.)

All VNC implementations, including TurboVNC, use the RFB (remote framebuffer)
protocol to send "framebuffer updates" from the
VNC server to any connected "viewers."  Each framebuffer update can contain
multiple "rectangles" (regions that have changed since the last update.)  As
with TightVNC, TurboVNC analyzes each rectangle, splits it into multiple
"subrectangles", and attempts to encode each subrectangle using the
"subencoding type" that will provide the most efficient compression, given the
number of unique colors in the subrectangle.  The process by which TurboVNC
does this is referred to as an "encoding method."  A rectangle is first
analyzed to determine if any significant portion of it is solid,
and if so, that portion is encoded as a bounding box and a fill color ("Solid
subencoding.")  Of the remaining subrectangles, those with only two colors are
encoded as a 1-bit-per-pixel bitmap with a 2-color palette ("Mono
subencoding"), those with low numbers of unique colors are encoded as a color
palette and an 8-bit-per-pixel bitmap ("Indexed color subencoding"), and
subrectangles with high numbers of unique colors are encoded using either JPEG
or arrays of RGB pixels ("Raw subencoding"), depending on the encoding method.
zlib can optionally be used to compress the indexed color, mono and raw
subrectangles.

Part of TurboVNC's speedup comes from the use of libjpeg-turbo, the same
high-speed SIMD-optimized JPEG codec used by VirtualGL.  However, TurboVNC also
eliminates the CPU-hungry smoothness detection routines that TightVNC uses to
determine whether a subrectangle is a good candidate for JPEG compression, and
TurboVNC's encoding methods tend to favor the use of JPEG more, since it is now
generally the fastest subencoding type.  Furthermore, TurboVNC eliminates
buffer copies, it maximizes network efficiency by splitting framebuffer updates
into relatively large subrectangles, and it uses only the zlib compression
levels that can be shown to have a measurable performance benefit.

TurboVNC is the product of extensive research, in which many different
permutations of the TightVNC encoder were benchmarked at the low level against
a variety of captured RFB sessions that simulate real-world application
workloads, both 2D and 3D.  For more information on the research leading to
TurboVNC's encoder design, see
[[http://www.TurboVNC.org/pmwiki/uploads/About/tighttoturbo.pdf][this report]].

In addition to high performance, other notable features of TurboVNC include:

	* Fine-grained control over the JPEG image quality and the level of
		chrominance subsampling

	* Double buffering on the client side to reduce tearing artifacts in 3D and
		video applications

	* Flexible and configurable full-screen/multi-screen support

	* Full support for IPv6

	* Advanced flow control and continuous updates.  This allows viewers
		to receive framebuffer updates without specifically requesting them, which
		can improve performance dramatically on high-latency connections.

	* Authentication with one-time passwords or Unix login credentials.  Access
		control lists can be used to share VNC sessions with only certain users.

	* TurboVNC allows security/authentication policies to be set globally for a
		particular server machine.

	* Multithreaded Tight encoding

	* "Lossless refresh" allows a viewer to request a lossless copy of the
		current screen image.  This is useful in situations in which image quality
		is critical but the network is too slow to support sending a high-quality
		image for every frame.  Lossless refreshes can be performed manually when a
		certain hotkey is pressed, or the TurboVNC Server can be configured to send
		a lossless refresh automatically if the user stops interacting with the
		application for a certain period of time.

	* High-performance zero-install Java viewer, which can be launched via Java
		Web Start using TurboVNC's built-in web server or deployed on a dedicated
		web server.  By calling libjpeg-turbo through JNI, the Java TurboVNC Viewer
		achieves similar levels of performance to the native TurboVNC Viewer, and
		its performance is generally equal to or better than that of the TigerVNC
		native viewers.

	* The TurboVNC Server and Java TurboVNC Viewer can be used with an instance
		of the UltraVNC Repeater in Mode I or II.

TurboVNC, when used with VirtualGL, provides a highly performant and robust
solution for remotely displaying 3D applications over all types of networks.

On "modern" hardware, TurboVNC is capable of streaming 50+ Megapixels/second
over a 100 Megabit/second local area network with perceptually lossless image
quality.  TurboVNC can stream between 10 and 12 Megapixels/second over a 5
Megabit/second broadband connection at reduced (but usable) image quality.

TurboVNC is compatible with other VNC distributions.  See
{ref prefix="Chapter ": Compatibility} for more information.  The official
TurboVNC binaries can be installed onto the same system as other VNC
distributions without interference.
