* Performance and Image Quality

The level of image compression in TurboVNC can be adjusted to balance the
(sometimes conflicting) goals of high image quality and high performance.
There are four options that control the manner in which TurboVNC compresses
images:

	Allow JPEG compression :: {:}
		If this option is enabled, then TurboVNC will use JPEG compression for
		subrectangles that have a high number of unique colors, and it will use
		indexed color subencoding for subrectangles that have a low number of
		unique colors.  If this option is disabled, then TurboVNC will select
		between indexed color or raw subencoding, depending on the size of the
		subrectangle and its color count.

	JPEG image quality :: {:}
		Lower quality levels produce grainier JPEG images with more noticeable
		compression artifacts, but lower quality levels also use less network
		bandwidth and CPU time.

	JPEG chrominance subsampling :: {:}
		When compressing an image using JPEG, the RGB pixels are first converted to
		the YCbCr colorspace, a colorspace in which each pixel is represented as a
		brightness (Y, or "luminance") value and a pair of color (Cb & Cr, or
		"chrominance") values.  After this colorspace conversion, chrominance
		subsampling can be used to discard some of the chrominance components in
		order to save bandwidth.  This works because the human eye is more
		sensitive to changes in brightness than to changes in color.  1X
		subsampling (the default in TurboVNC) retains the chrominance components
		for all pixels, and thus it provides the best image quality but also uses
		the most network bandwidth and CPU time.  2X subsampling retains the
		chrominance components for every other pixel, and 4X subsampling retains
		the chrominance components for every fourth pixel.  (This is typically
		implemented as 2X subsampling in both X and Y directions.)  Grayscale
		throws out all of the chrominance components, leaving only luminance.  2X
		and 4X subsampling typically produce noticeable blurring of lines and other
		sharp features, but with photographic or other "smooth" image content, it
		may be difficult to detect any difference between 1X, 2X, and 4X.

	Compression level :: {:}
		In TurboVNC, the compression level specifies:
		{list type=ol:
			{item: the level of zlib compression that will be used with indexed
				color, mono, and raw subrectangles}
			{item: the "palette threshold" (the minimum number of colors that a
				subrectangle must have before it is encoded as JPEG or raw instead of
				indexed color)}
			{item: whether or not [[#InterframeComparison][interframe comparison]]
				should be used}
		}
		See {ref prefix="Section ": AdvancedCompression} below for more details.

These parameters can be adjusted by accessing the TurboVNC Viewer Options
dialog box.  (Click on the "Options" button in the New TurboVNC Connection
dialog box or, after connecting to the server, click on the Connection Options
button in the toolbar.)

The TurboVNC Viewer provides five preset "encoding methods" corresponding to
the most useful combinations of the image compression options described above:

|| Encoding method || Allow JPEG || JPEG image quality || \
	JPEG chrominance subsampling || Compression level || Notes ||
| "Tight + Perceptually Lossless JPEG" | Yes | 95 | 1x | 1 | \
	This encoding method should be perceptually lossless (that is, any image \
	compression artifacts it produces should be imperceptible to human vision) \
	under most viewing conditions.  This encoding method requires a great \
	deal of network bandwidth, however, and is generally not recommended except \
	on 50 Megabit/second and faster networks. |
| "Tight + Medium-Quality JPEG" | Yes | 80 | 2x | 6 | \
	For subrectangles that have a high number of unique colors, this encoding \
	method produces some minor, but generally not very noticeable, image \
	compression artifacts.  All else being equal, this encoding method \
	typically uses about twice the network bandwidth of the \
	"Tight + Low-Quality JPEG" encoding method and about half the bandwidth of \
	the "Tight + Perceptually Lossless JPEG" encoding method, making it \
	appropriate for medium-speed networks such as 10 Megabit Ethernet. \
	Interframe comparison is enabled with this encoding method.  (Compression \
	Level 6 = Compression Level 1 + interframe comparison.) |
| "Tight + Low-Quality JPEG" | Yes | 30 | 4x | 7 | \
	For subrectangles that have a high number of unique colors, this encoding \
	method produces very noticeable image compression artifacts.  However, it \
	performs optimally on low-bandwidth connections.  If image quality is more \
	critical than performance, then use one of the other encoding methods or \
	take advantage of the [[#LR][Lossless Refresh feature]].  In addition to \
	reducing the JPEG quality to a "minimum usable" level, this encoding method \
	also enables interframe comparison and Compression Level 2.  (CL 7 = CL 2 + \
	interframe comparison.)  Compression Level 2 can reduce network usage for \
	low-color application workloads that are not good candidates for JPEG \
	compression. |
| "Lossless Tight" | No | N/A | N/A | 0 | \
	This encoding method uses indexed color subencoding for subrectangles that \
	have a low number of unique colors and raw subencoding for subrectangles \
	that have a high number of unique colors.  If the VNC viewer supports the \
	"Tight Encoding Without Zlib" RFB extension, then zlib is bypassed, and all \
	subrectangles are sent without compression.  Otherwise, all subrectangles \
	are "compressed" using zlib with zlib compression level 0.  (Zlib \
	compression level 0 maintains the zlib state but does not perform any \
	actual compression.  However, the overhead of maintaining the zlib state \
	reduces overall Tight encoding performance by 10-60% vs. bypassing zlib, \
	depending on the zlib implementation.)  Lossless Tight uses significantly \
	less CPU time than any of the JPEG-based encoding methods, but it is \
	suitable only for gigabit and faster networks. |
| "Lossless Tight + Zlib" | No | N/A | N/A | 6 | \
	This encoding method uses indexed color subencoding for subrectangles that \
	have a low number of unique colors and raw subencoding for subrectangles \
	that have a high number of unique colors.  It compresses all subrectangles \
	using zlib with zlib compression level 1.  For certain types of low-color \
	workloads (CAD applications, in particular), this encoding method may use \
	less network bandwidth than the "Tight + Perceptually Lossless JPEG" \
	encoding method, but it also uses significantly more CPU time than any of \
	the JPEG-based encoding methods.  Interframe comparison is enabled with \
	this encoding method.  (Compression Level 6 = Compression Level 1 + \
	interframe comparison.) |
#CAP: TurboVNC Encoding Methods

The encoding method can be set in the TurboVNC Viewer Options dialog box.
(Click on the "Options" button in the New TurboVNC Connection dialog box or,
after connecting to the server, click on the Connection Options button in the
toolbar.)

** Interframe Comparison
{anchor: InterframeComparison}

Certain ill-behaved applications can sometimes draw the same thing over and
over again, and this can cause redundant framebuffer updates to be sent to the
VNC viewer.  Additionally, modern GUI toolkits often use image-based drawing
methods (the X Rendering Extension, for instance), which can result in an
entire window being redrawn even if only a few pixels in the window have
changed.  The TurboVNC Server can guard against this by maintaining a copy of
the remote framebuffer for each connected viewer, comparing each new
framebuffer update rectangle against the pixels in the framebuffer copy, and
discarding any redundant portions of the rectangle before they are sent to the
viewer.

Interframe comparison has some tradeoffs.  Perhaps the most important of these
is that it increases the memory usage of the TurboVNC Server by a factor of N,
where N is the number of connected viewers.  This can prove to be quite
significant if the remote desktop size is relatively large.  2D applications
are most often the ones that generate duplicate framebuffer updates, so using
interframe comparison with such applications can significantly reduce the
network usage and the host CPU usage (since fewer rectangles are actually being
encoded.)  However, with 3D applications, the benefits of interframe comparison
are less clear, since it is less common for those applications to generate
duplicate framebuffer updates.  Interframe comparison may benefit certain
classes of 3D applications, such as design applications that render a model
against a static background-- particularly when the model is not zoomed in
enough to fill the entire window.  In real-world tests, however, interframe
comparison rarely reduces the network usage for 3D applications by more than
5-10%.  Furthermore, with games and other immersive applications that modify
most of the pixels on the screen each time a frame is rendered, interframe
comparison can actually increase both CPU usage and network usage.
Furthermore, the effects of duplicate framebuffer updates are not typically
noticeable on high-speed networks, but an increase in host CPU usage might be.

For these reasons, interframe comparison is not enabled by default and should
not generally be enabled except on bandwidth-constrained networks and with
applications for which it can be shown to be beneficial.  Interframe comparison
can be enabled by passing an argument of ''-interframe'' to ''vncserver'' when
starting a TurboVNC session, by using ''tvncconfig'' to set the ''Interframe''
parameter for a running TurboVNC session, or by requesting a compression level
of 5 or higher from the viewer (see below.)

** Advanced Compression Options
{anchor: AdvancedCompression}

One of the underlying principles of TurboVNC's design is to expose only the
options that have proven to be useful (that is, the options that have proven to
have good performance tradeoffs.)  Thus, the TurboVNC Viewer Options dialog
normally only allows you to select Compression Levels 1-2 if JPEG subencoding
is enabled (6-7 if interframe comparison is also enabled) or Compression Levels
0-1 if JPEG subencoding is disabled (5-6 if interframe comparison is enabled.)
Other compression levels can, however, be specified using the TurboVNC Viewer's
''CompressLevel'' parameter, and doing so will enable a compatibility mode in
the TurboVNC Viewer Options dialog that allows any compression level from 0 to
9 to be requested.

When connected to a TurboVNC session, requesting a particular compression level
has the following effect:

|| Compression level || Zlib compression level (non-JPEG subrectangles) \
	|| Palette threshold || Interframe comparison || Notes ||
| 0 | 1 | 24 | No | \
	Same as Compression Level 1.  Bypassing zlib when JPEG is enabled would \
	only reduce the CPU usage for non-JPEG subrectangles, which is of limited \
	usefulness.  Furthermore, bypassing zlib requires an RFB protocol extension \
	that is not supported by non-TurboVNC viewers (as of this writing.)  It is \
	presumed that, if one wants to reduce the CPU usage, then one wants to do \
	so for all subrectangles, so CL 0 without JPEG (AKA "Lossless Tight") \
	should be used. |
| 1 | 1 | 24 | No | \
	See the description of the "Tight + JPEG" encoding methods above. |
| 2 | 3 | 96 | No | \
	A higher palette threshold causes indexed color subencoding to be used more \
	often than with CL 1, and indexed color subrectangles are compressed using \
	a higher zlib compression level.  This can provide typically 20-40% better \
	compression than CL 1 (with a commensurate increase in CPU usage) for \
	workloads that have a low number of unique colors.  However, Compression \
	Level 2 can increase the CPU usage for some high-color workloads without \
	providing significantly better compression. |
| 3-4 | 3 | 96 | No | \
	Same as Compression Level 2 (reserved for future expansion) |
| 5-6 | 1 | 24 | Yes | \
	Same as Compression Level 1 but with interframe comparison enabled |
| 7-8 | 3 | 96 | Yes | \
	Same as Compression Level 2 but with interframe comparison enabled |
| 9 | 7 | 256 | Yes | \
	This mode is included only for backward compatibility with TightVNC.  It \
	provides approximately the same level of compression for 2D applications as \
	Compression Level 9 in TightVNC 1.3.x while using much less CPU time.  It \
	also provides much better compression than TightVNC for 3D and video \
	applications.  However, relative to Compression Level 2, this mode uses \
	approximately twice as much CPU time and only achieves about 10-20% better \
	average compression for 2D applications (and has no noticeable benefit for \
	3D and video applications.)  Thus, its usefulness is generally very \
	limited. |
#CAP: Compression Levels Supported by the TurboVNC Server (JPEG Enabled)

{:}

|| Compression level || Zlib compression level (indexed color subrectangles) \
	|| Zlib compression level (raw subrectangles) || Palette threshold \
	|| Interframe comparison || Notes ||
| 0 | None | None | Subrectangle size / 4 | No | \
	See the description of the "Lossless Tight" encoding method above. |
| 1 | 1 | 1 | Subrectangle size / 96 | No | \
	See the description of the "Lossless Tight + Zlib" encoding method above. |
| 2-4 | 1 | 1 | Subrectangle size / 96 | No | \
	Same as Compression Level 1 (reserved for future expansion) |
| 5 | None | None | Subrectangle size / 4 | Yes | \
	Same as Compression Level 0 but with interframe comparison enabled |
| 6-8 | 1 | 1 | Subrectangle size / 96 | Yes | \
	Same as Compression Level 1 but with interframe comparison enabled |
| 9 | 7 | 5 | Subrectangle size / 96 | Yes | \
	This mode is included only for backward compatibility with TightVNC.  It \
	provides approximately the same level of compression for 2D applications as \
	Compression Level 9 in TightVNC 1.3.x, while using much less CPU time.  It \
	also provides much better compression than TightVNC for 3D and video \
	applications.  However, relative to Compression Level 1, this mode uses \
	approximately twice as much CPU time and only achieves about 10% better \
	average compression for 2D applications (and has no noticeable benefit for \
	3D and video applications.)  Thus, its usefulness is generally very \
	limited. |
#CAP: Compression Levels Supported by the TurboVNC Server (JPEG Disabled)

** Lossless Refresh
{anchor: LR}

Since both of TurboVNC's mathematically lossless encoding methods have
performance drawbacks, another option for image-quality-critical applications
is the Lossless Refresh feature.  When a lossless refresh is requested by a
TurboVNC viewer, the VNC server will send a mathematically lossless image of
the remote desktop to the requesting viewer.  A user could, for instance, use
the "Tight + Low-Quality JPEG" encoding method on a low-bandwidth network to
improve the performance of rotating/panning/zooming an object in a 3D
application, then the user could request a lossless refresh when they are ready
to interpret or analyze the object.

To perform a lossless refresh, press CTRL-ALT-SHIFT-L or click on the Lossless
Refresh toolbar icon.

** Automatic Lossless Refresh
{anchor: ALR}

Passing an argument of {pcode: -alr __timeout__} to ''vncserver'' (or using
''tvncconfig'' to set the ''ALR'' parameter for a running TurboVNC session)
enables the Automatic Lossless Refresh (ALR) feature for the TurboVNC
session.  ALR monitors all of the VNC viewer connections, and if more than
__''timeout''__ seconds have elapsed since the last framebuffer update was sent
to a given viewer, then the TurboVNC Server sends that viewer a mathematically
lossless copy of any "ALR-eligible" screen regions that have been affected by
lossy compression.  You can also pass arguments of ''-alrqual'' and
''-alrsamp'' to ''vncserver'' (or use ''tvncconfig'' to set the ''ALRQual'' and
''ALRSamp'' parameters for a running TurboVNC session) to specify that
automatic lossless refreshes should be sent using JPEG instead.  (See the
''Xvnc'' man page for details.)

The ALR feature is designed mainly for use with interactive visualization
applications.  The idea is that, on a low-bandwidth connection, low-quality
JPEG can be used while the 3D scene is rotated/panned/zoomed, but when the
motion stops, a fully lossless copy of the 3D image is sent and can be studied
in detail.

The default is for any regions drawn with ''X[Shm]PutImage()'' to be
ALR-eligible-- as well as any regions drawn with CopyRect, if the source of the
CopyRect operation was affected by lossy compression.  (CopyRect is an RFB
encoding that allows the VNC server to request that a VNC viewer move a
rectangle of pixels from one location to another.)  When used with VirtualGL,
this means that ALRs will mainly just be sent for the OpenGL-rendered regions
of the remote desktop.  That should be fine for most 3D applications, since the
OpenGL-rendered regions are the ones that are quality-critical.  The default
ALR behavior also prevents what might best be called the "blinking cursor
dilemma."  Certain programs have a blinking cursor that may update more
frequently than the ALR timeout.  Since an ALR is triggered based on a period
of inactivity relative to the last framebuffer update, these frequent updates
prevent an ALR from ever being sent.  Fortunately, blinking cursors are not
typically drawn using ''X[Shm]PutImage()'', so the problem is effectively
worked around by limiting the ALR-eligible regions to just the subset of
regions that were drawn using ''X[Shm]PutImage()'' and CopyRect.

	!!! NOTE: Ill-behaved applications that continuously render the same image
	will cause a variation of the "blinking cursor dilemma" and thus defeat ALR
	unless [[#InterframeComparison][interframe comparison]] is enabled.

You can override the default ALR behavior, thus making all screen regions
eligible for ALR, by setting the ''TVNC_ALRALL'' environment variable to ''1''
on the TurboVNC host prior to starting a TurboVNC session or by using
''tvncconfig'' to set the ''ALRAll'' parameter for a running TurboVNC session.

** Multithreading
{anchor: Multithreading}

By default, the TurboVNC Server uses multiple threads to perform image encoding
and compression, thus allowing it to take advantage of multi-core or
multi-processor systems.  The server splits the screen vertically into N tiles,
where N is the number of threads, and assigns each tile to a separate thread.
The scalability of this algorithm is nearly linear when used with demanding 3D
or video applications that fill most of the screen.  However, whether or not
multithreading improves the overall performance of TurboVNC depends largely on
the performance of the viewer and the network.  If either the viewer or the
network is the primary performance bottleneck, then enabling multithreading in
the server will not help.  Multithreading is also not currently implemented
with non-Tight encoding types.

To disable server-side multithreading, set the ''TVNC_MT'' environment variable
to ''0'' on the host prior to starting ''vncserver'', or pass an argument of
''-nomt'' to ''vncserver''.  The default behavior is to use as many threads as
there are cores on the TurboVNC host (up to a maximum of 4), but you can set
the ''TVNC_NTHREADS'' environment variable or pass an argument of ''-nthreads''
to ''vncserver'' to override this.
