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Commit 1afed883 authored by Hans Verkuil's avatar Hans Verkuil Committed by Mauro Carvalho Chehab
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[media] DocBook media: rewrite the Colorspace chapter 

The colorspace chapter in the V4L2 Specification was always poorly
written. This patch rewrites it, documenting the new Y'CbCr encoding
and quantization defines and going into much more detail with respect
to how colorspaces are used and what it all means.
Signed-off-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@osg.samsung.com>
parent 736d96b5
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Documentation/DocBook/media/v4l/biblio.xml
View file @ 1afed883
......@@ -178,6 +178,75 @@ Signal - NTSC for Studio Applications"</title>
1125-Line High-Definition Production"</title>
</biblioentry>
<biblioentry id="srgb">
<abbrev>sRGB</abbrev>
<authorgroup>
<corpauthor>International Electrotechnical Commission
(<ulink url="http://www.iec.ch">http://www.iec.ch</ulink>)</corpauthor>
</authorgroup>
<title>IEC 61966-2-1 ed1.0 "Multimedia systems and equipment - Colour measurement
and management - Part 2-1: Colour management - Default RGB colour space - sRGB"</title>
</biblioentry>
<biblioentry id="sycc">
<abbrev>sYCC</abbrev>
<authorgroup>
<corpauthor>International Electrotechnical Commission
(<ulink url="http://www.iec.ch">http://www.iec.ch</ulink>)</corpauthor>
</authorgroup>
<title>IEC 61966-2-1-am1 ed1.0 "Amendment 1 - Multimedia systems and equipment - Colour measurement
and management - Part 2-1: Colour management - Default RGB colour space - sRGB"</title>
</biblioentry>
<biblioentry id="xvycc">
<abbrev>xvYCC</abbrev>
<authorgroup>
<corpauthor>International Electrotechnical Commission
(<ulink url="http://www.iec.ch">http://www.iec.ch</ulink>)</corpauthor>
</authorgroup>
<title>IEC 61966-2-4 ed1.0 "Multimedia systems and equipment - Colour measurement
and management - Part 2-4: Colour management - Extended-gamut YCC colour space for video
applications - xvYCC"</title>
</biblioentry>
<biblioentry id="adobergb">
<abbrev>AdobeRGB</abbrev>
<authorgroup>
<corpauthor>Adobe Systems Incorporated (<ulink url="http://www.adobe.com">http://www.adobe.com</ulink>)</corpauthor>
</authorgroup>
<title>Adobe&copy; RGB (1998) Color Image Encoding Version 2005-05</title>
</biblioentry>
<biblioentry id="oprgb">
<abbrev>opRGB</abbrev>
<authorgroup>
<corpauthor>International Electrotechnical Commission
(<ulink url="http://www.iec.ch">http://www.iec.ch</ulink>)</corpauthor>
</authorgroup>
<title>IEC 61966-2-5 "Multimedia systems and equipment - Colour measurement
and management - Part 2-5: Colour management - Optional RGB colour space - opRGB"</title>
</biblioentry>
<biblioentry id="itu2020">
<abbrev>ITU&nbsp;BT.2020</abbrev>
<authorgroup>
<corpauthor>International Telecommunication Union (<ulink
url="http://www.itu.ch">http://www.itu.ch</ulink>)</corpauthor>
</authorgroup>
<title>ITU-R Recommendation BT.2020 (08/2012) "Parameter values for ultra-high
definition television systems for production and international programme exchange"
</title>
</biblioentry>
<biblioentry id="tech3213">
<abbrev>EBU&nbsp;Tech&nbsp;3213</abbrev>
<authorgroup>
<corpauthor>European Broadcast Union (<ulink
url="http://www.ebu.ch">http://www.ebu.ch</ulink>)</corpauthor>
</authorgroup>
<title>E.B.U. Standard for Chromaticity Tolerances for Studio Monitors"</title>
</biblioentry>
<biblioentry id="iec62106">
<abbrev>IEC&nbsp;62106</abbrev>
<authorgroup>
......@@ -266,4 +335,20 @@ in the frequency range from 87,5 to 108,0 MHz</title>
<subtitle>Version 1, Revision 2</subtitle>
</biblioentry>
<biblioentry id="poynton">
<abbrev>poynton</abbrev>
<authorgroup>
<corpauthor>Charles Poynton</corpauthor>
</authorgroup>
<title>Digital Video and HDTV, Algorithms and Interfaces</title>
</biblioentry>
<biblioentry id="colimg">
<abbrev>colimg</abbrev>
<authorgroup>
<corpauthor>Erik Reinhard et al.</corpauthor>
</authorgroup>
<title>Color Imaging: Fundamentals and Applications</title>
</biblioentry>
</bibliography>
Documentation/DocBook/media/v4l/pixfmt.xml
View file @ 1afed883
......@@ -296,343 +296,1003 @@ in the 2-planar version or with each component in its own buffer in the
<section id="colorspaces">
<title>Colorspaces</title>
<para>[intro]</para>
<para>'Color' is a very complex concept and depends on physics, chemistry and
biology. Just because you have three numbers that describe the 'red', 'green'
and 'blue' components of the color of a pixel does not mean that you can accurately
display that color. A colorspace defines what it actually <emphasis>means</emphasis>
to have an RGB value of e.g. (255,&nbsp;0,&nbsp;0). That is, which color should be
reproduced on the screen in a perfectly calibrated environment.</para>
<!-- See proposal by Billy Biggs, video4linux-list@redhat.com
on 11 Oct 2002, subject: "Re: [V4L] Re: v4l2 api", and
http://vektor.theorem.ca/graphics/ycbcr/ and
http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html -->
<para>In order to do that we first need to have a good definition of
color, i.e. some way to uniquely and unambiguously define a color so that someone
else can reproduce it. Human color vision is trichromatic since the human eye has
color receptors that are sensitive to three different wavelengths of light. Hence
the need to use three numbers to describe color. Be glad you are not a mantis shrimp
as those are sensitive to 12 different wavelengths, so instead of RGB we would be
using the ABCDEFGHIJKL colorspace...</para>
<para>
<para>Color exists only in the eye and brain and is the result of how strongly
color receptors are stimulated. This is based on the Spectral
Power Distribution (SPD) which is a graph showing the intensity (radiant power)
of the light at wavelengths covering the visible spectrum as it enters the eye.
The science of colorimetry is about the relationship between the SPD and color as
perceived by the human brain.</para>
<para>Since the human eye has only three color receptors it is perfectly
possible that different SPDs will result in the same stimulation of those receptors
and are perceived as the same color, even though the SPD of the light is
different.</para>
<para>In the 1920s experiments were devised to determine the relationship
between SPDs and the perceived color and that resulted in the CIE 1931 standard
that defines spectral weighting functions that model the perception of color.
Specifically that standard defines functions that can take an SPD and calculate
the stimulus for each color receptor. After some further mathematical transforms
these stimuli are known as the <emphasis>CIE XYZ tristimulus</emphasis> values
and these X, Y and Z values describe a color as perceived by a human unambiguously.
These X, Y and Z values are all in the range [0&hellip;1].</para>
<para>The Y value in the CIE XYZ colorspace corresponds to luminance. Often
the CIE XYZ colorspace is transformed to the normalized CIE xyY colorspace:</para>
<para>x = X / (X + Y + Z)</para>
<para>y = Y / (X + Y + Z)</para>
<para>The x and y values are the chromaticity coordinates and can be used to
define a color without the luminance component Y. It is very confusing to
have such similar names for these colorspaces. Just be aware that if colors
are specified with lower case 'x' and 'y', then the CIE xyY colorspace is
used. Upper case 'X' and 'Y' refer to the CIE XYZ colorspace. Also, y has nothing
to do with luminance. Together x and y specify a color, and Y the luminance.
That is really all you need to remember from a practical point of view. At
the end of this section you will find reading resources that go into much more
detail if you are interested.
</para>
<para>A monitor or TV will reproduce colors by emitting light at three
different wavelengths, the combination of which will stimulate the color receptors
in the eye and thus cause the perception of color. Historically these wavelengths
were defined by the red, green and blue phosphors used in the displays. These
<emphasis>color primaries</emphasis> are part of what defines a colorspace.</para>
<para>Different display devices will have different primaries and some
primaries are more suitable for some display technologies than others. This has
resulted in a variety of colorspaces that are used for different display
technologies or uses. To define a colorspace you need to define the three
color primaries (these are typically defined as x,&nbsp;y chromaticity coordinates
from the CIE xyY colorspace) but also the white reference: that is the color obtained
when all three primaries are at maximum power. This determines the relative power
or energy of the primaries. This is usually chosen to be close to daylight which has
been defined as the CIE D65 Illuminant.</para>
<para>To recapitulate: the CIE XYZ colorspace uniquely identifies colors.
Other colorspaces are defined by three chromaticity coordinates defined in the
CIE xyY colorspace. Based on those a 3x3 matrix can be constructed that
transforms CIE XYZ colors to colors in the new colorspace.
</para>
<para>Both the CIE XYZ and the RGB colorspace that are derived from the
specific chromaticity primaries are linear colorspaces. But neither the eye,
nor display technology is linear. Doubling the values of all components in
the linear colorspace will not be perceived as twice the intensity of the color.
So each colorspace also defines a transfer function that takes a linear color
component value and transforms it to the non-linear component value, which is a
closer match to the non-linear performance of both the eye and displays. Linear
component values are denoted RGB, non-linear are denoted as R'G'B'. In general
colors used in graphics are all R'G'B', except in openGL which uses linear RGB.
Special care should be taken when dealing with openGL to provide linear RGB colors
or to use the built-in openGL support to apply the inverse transfer function.</para>
<para>The final piece that defines a colorspace is a function that
transforms non-linear R'G'B' to non-linear Y'CbCr. This function is determined
by the so-called luma coefficients. There may be multiple possible Y'CbCr
encodings allowed for the same colorspace. Many encodings of color
prefer to use luma (Y') and chroma (CbCr) instead of R'G'B'. Since the human
eye is more sensitive to differences in luminance than in color this encoding
allows one to reduce the amount of color information compared to the luma
data. Note that the luma (Y') is unrelated to the Y in the CIE XYZ colorspace.
Also note that Y'CbCr is often called YCbCr or YUV even though these are
strictly speaking wrong.</para>
<para>Sometimes people confuse Y'CbCr as being a colorspace. This is not
correct, it is just an encoding of an R'G'B' color into luma and chroma
values. The underlying colorspace that is associated with the R'G'B' color
is also associated with the Y'CbCr color.</para>
<para>The final step is how the RGB, R'G'B' or Y'CbCr values are
quantized. The CIE XYZ colorspace where X, Y and Z are in the range
[0&hellip;1] describes all colors that humans can perceive, but the transform to
another colorspace will produce colors that are outside the [0&hellip;1] range.
Once clamped to the [0&hellip;1] range those colors can no longer be reproduced
in that colorspace. This clamping is what reduces the extent or gamut of the
colorspace. How the range of [0&hellip;1] is translated to integer values in the
range of [0&hellip;255] (or higher, depending on the color depth) is called the
quantization. This is <emphasis>not</emphasis> part of the colorspace
definition. In practice RGB or R'G'B' values are full range, i.e. they
use the full [0&hellip;255] range. Y'CbCr values on the other hand are limited
range with Y' using [16&hellip;235] and Cb and Cr using [16&hellip;240].</para>
<para>Unfortunately, in some cases limited range RGB is also used
where the components use the range [16&hellip;235]. And full range Y'CbCr also exists
using the [0&hellip;255] range.</para>
<para>In order to correctly interpret a color you need to know the
quantization range, whether it is R'G'B' or Y'CbCr, the used Y'CbCr encoding
and the colorspace.
From that information you can calculate the corresponding CIE XYZ color
and map that again to whatever colorspace your display device uses.</para>
<para>The colorspace definition itself consists of the three
chromaticity primaries, the white reference chromaticity, a transfer
function and the luma coefficients needed to transform R'G'B' to Y'CbCr. While
some colorspace standards correctly define all four, quite often the colorspace
standard only defines some, and you have to rely on other standards for
the missing pieces. The fact that colorspaces are often a mix of different
standards also led to very confusing naming conventions where the name of
a standard was used to name a colorspace when in fact that standard was
part of various other colorspaces as well.</para>
<para>If you want to read more about colors and colorspaces, then the
following resources are useful: <xref linkend="poynton" /> is a good practical
book for video engineers, <xref linkend="colimg" /> has a much broader scope and
describes many more aspects of color (physics, chemistry, biology, etc.).
The <ulink url="http://www.brucelindbloom.com">http://www.brucelindbloom.com</ulink>
website is an excellent resource, especially with respect to the mathematics behind
colorspace conversions. The wikipedia <ulink url="http://en.wikipedia.org/wiki/CIE_1931_color_space#CIE_xy_chromaticity_diagram_and_the_CIE_xyY_color_space">CIE 1931 colorspace</ulink> article
is also very useful.</para>
</section>
<section>
<title>Defining Colorspaces in V4L2</title>
<para>In V4L2 colorspaces are defined by three values. The first is the colorspace
identifier (&v4l2-colorspace;) which defines the chromaticities, the transfer
function, the default Y'CbCr encoding and the default quantization method. The second
is the Y'CbCr encoding identifier (&v4l2-ycbcr-encoding;) to specify non-standard
Y'CbCr encodings and the third is the quantization identifier (&v4l2-quantization;)
to specify non-standard quantization methods. Most of the time only the colorspace
field of &v4l2-pix-format; or &v4l2-pix-format-mplane; needs to be filled in. Note
that the default R'G'B' quantization is always full range for all colorspaces,
so this won't be mentioned explicitly for each colorspace description.</para>
<table pgwide="1" frame="none" id="v4l2-colorspace">
<title>V4L2 Colorspaces</title>
<tgroup cols="2" align="left">
&cs-def;
<thead>
<row>
<entry>Identifier</entry>
<entry>Details</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry><constant>V4L2_COLORSPACE_SMPTE170M</constant></entry>
<entry>See <xref linkend="col-smpte-170m" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_REC709</constant></entry>
<entry>See <xref linkend="col-rec709" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_SRGB</constant></entry>
<entry>See <xref linkend="col-srgb" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_ADOBERGB</constant></entry>
<entry>See <xref linkend="col-adobergb" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_BT2020</constant></entry>
<entry>See <xref linkend="col-bt2020" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_SMPTE240M</constant></entry>
<entry>See <xref linkend="col-smpte-240m" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_470_SYSTEM_M</constant></entry>
<entry>See <xref linkend="col-sysm" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_470_SYSTEM_BG</constant></entry>
<entry>See <xref linkend="col-sysbg" />.</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_JPEG</constant></entry>
<entry>See <xref linkend="col-jpeg" />.</entry>
</row>
</tbody>
</tgroup>
</table>
<table pgwide="1" frame="none" id="v4l2-ycbcr-encoding">
<title>V4L2 Y'CbCr Encodings</title>
<tgroup cols="2" align="left">
&cs-def;
<thead>
<row>
<entry>Identifier</entry>
<entry>Details</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry><constant>V4L2_YCBCR_ENC_DEFAULT</constant></entry>
<entry>Use the default Y'CbCr encoding as defined by the colorspace.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_601</constant></entry>
<entry>Use the BT.601 Y'CbCr encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_709</constant></entry>
<entry>Use the Rec. 709 Y'CbCr encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_XV601</constant></entry>
<entry>Use the extended gamut xvYCC BT.601 encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_XV709</constant></entry>
<entry>Use the extended gamut xvYCC Rec. 709 encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_SYCC</constant></entry>
<entry>Use the extended gamut sYCC encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_BT2020</constant></entry>
<entry>Use the default non-constant luminance BT.2020 Y'CbCr encoding.</entry>
</row>
<row>
<entry><constant>V4L2_YCBCR_ENC_BT2020_CONST_LUM</constant></entry>
<entry>Use the constant luminance BT.2020 Yc'CbcCrc encoding.</entry>
</row>
</tbody>
</tgroup>
</table>
<table pgwide="1" frame="none" id="v4l2-quantization">
<title>V4L2 Quantization Methods</title>
<tgroup cols="2" align="left">
&cs-def;
<thead>
<row>
<entry>Identifier</entry>
<entry>Details</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry><constant>V4L2_QUANTIZATION_DEFAULT</constant></entry>
<entry>Use the default quantization encoding as defined by the colorspace.
This is always full range for R'G'B' and usually limited range for Y'CbCr.</entry>
</row>
<row>
<entry><constant>V4L2_QUANTIZATION_FULL_RANGE</constant></entry>
<entry>Use the full range quantization encoding. I.e. the range [0&hellip;1]
is mapped to [0&hellip;255] (with possible clipping to [1&hellip;254] to avoid the
0x00 and 0xff values). Cb and Cr are mapped from [-0.5&hellip;0.5] to [0&hellip;255]
(with possible clipping to [1&hellip;254] to avoid the 0x00 and 0xff values).</entry>
</row>
<row>
<entry><constant>V4L2_QUANTIZATION_LIM_RANGE</constant></entry>
<entry>Use the limited range quantization encoding. I.e. the range [0&hellip;1]
is mapped to [16&hellip;235]. Cb and Cr are mapped from [-0.5&hellip;0.5] to [16&hellip;240].
</entry>
</row>
</tbody>
</tgroup>
</table>
</section>
<section>
<title>Detailed Colorspace Descriptions</title>
<section>
<title id="col-smpte-170m">Colorspace SMPTE 170M (<constant>V4L2_COLORSPACE_SMPTE170M</constant>)</title>
<para>The <xref linkend="smpte170m" /> standard defines the colorspace used by NTSC and PAL and by SDTV
in general. The default Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_601</constant>.
The default Y'CbCr quantization is limited range. The chromaticities of the primary colors and
the white reference are:</para>
<table frame="none">
<title>SMPTE 170M Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.630</entry>
<entry>0.340</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.310</entry>
<entry>0.595</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.155</entry>
<entry>0.070</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<para>The red, green and blue chromaticities are also often referred to
as the SMPTE C set, so this colorspace is sometimes called SMPTE C as well.</para>
<variablelist>
<varlistentry>
<term>Gamma Correction</term>
<term>The transfer function defined for SMPTE 170M is the same as the
one defined in Rec. 709. Normally L is in the range [0&hellip;1], but for the extended
gamut xvYCC encoding values outside that range are allowed.</term>
<listitem>
<para>[to do]</para>
<para>E'<subscript>R</subscript> = f(R)</para>
<para>E'<subscript>G</subscript> = f(G)</para>
<para>E'<subscript>B</subscript> = f(B)</para>
<para>L' = -1.099(-L)<superscript>0.45</superscript>&nbsp;+&nbsp;0.099&nbsp;for&nbsp;L&nbsp;&le;&nbsp;-0.018</para>
<para>L' = 4.5L&nbsp;for&nbsp;-0.018&nbsp;&lt;&nbsp;L&nbsp;&lt;&nbsp;0.018</para>
<para>L' = 1.099L<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;L&nbsp;&ge;&nbsp;0.018</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>Construction of luminance and color-difference
signals</term>
<term>Inverse Transfer function:</term>
<listitem>
<para>[to do]</para>
<para>E'<subscript>Y</subscript> =
Coeff<subscript>R</subscript> E'<subscript>R</subscript>
+ Coeff<subscript>G</subscript> E'<subscript>G</subscript>
+ Coeff<subscript>B</subscript> E'<subscript>B</subscript></para>
<para>(E'<subscript>R</subscript> - E'<subscript>Y</subscript>) = E'<subscript>R</subscript>
- Coeff<subscript>R</subscript> E'<subscript>R</subscript>
- Coeff<subscript>G</subscript> E'<subscript>G</subscript>
- Coeff<subscript>B</subscript> E'<subscript>B</subscript></para>
<para>(E'<subscript>B</subscript> - E'<subscript>Y</subscript>) = E'<subscript>B</subscript>
- Coeff<subscript>R</subscript> E'<subscript>R</subscript>
- Coeff<subscript>G</subscript> E'<subscript>G</subscript>
- Coeff<subscript>B</subscript> E'<subscript>B</subscript></para>
<para>L = -((L'&nbsp;-&nbsp;0.099)&nbsp;/&nbsp;-1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&le;&nbsp;-0.081</para>
<para>L = L'&nbsp;/&nbsp;4.5&nbsp;for&nbsp;-0.081&nbsp;&lt;&nbsp;L'&nbsp;&lt;&nbsp;0.081</para>
<para>L = ((L'&nbsp;+&nbsp;0.099)&nbsp;/&nbsp;1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.081</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>Re-normalized color-difference signals</term>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with
the following <constant>V4L2_YCBCR_ENC_601</constant> encoding:</term>
<listitem>
<para>The color-difference signals are scaled back to unity
range [-0.5;+0.5]:</para>
<para>K<subscript>B</subscript> = 0.5 / (1 - Coeff<subscript>B</subscript>)</para>
<para>K<subscript>R</subscript> = 0.5 / (1 - Coeff<subscript>R</subscript>)</para>
<para>P<subscript>B</subscript> =
K<subscript>B</subscript> (E'<subscript>B</subscript> - E'<subscript>Y</subscript>) =
0.5 (Coeff<subscript>R</subscript> / Coeff<subscript>B</subscript>) E'<subscript>R</subscript>
+ 0.5 (Coeff<subscript>G</subscript> / Coeff<subscript>B</subscript>) E'<subscript>G</subscript>
+ 0.5 E'<subscript>B</subscript></para>
<para>P<subscript>R</subscript> =
K<subscript>R</subscript> (E'<subscript>R</subscript> - E'<subscript>Y</subscript>) =
0.5 E'<subscript>R</subscript>
+ 0.5 (Coeff<subscript>G</subscript> / Coeff<subscript>R</subscript>) E'<subscript>G</subscript>
+ 0.5 (Coeff<subscript>B</subscript> / Coeff<subscript>R</subscript>) E'<subscript>B</subscript></para>
<para>Y'&nbsp;=&nbsp;0.299R'&nbsp;+&nbsp;0.587G'&nbsp;+&nbsp;0.114B'</para>
<para>Cb&nbsp;=&nbsp;-0.169R'&nbsp;-&nbsp;0.331G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.419G'&nbsp;-&nbsp;0.081B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5]. This conversion to Y'CbCr is identical to the one
defined in the <xref linkend="itu601" /> standard and this colorspace is sometimes called BT.601 as well, even
though BT.601 does not mention any color primaries.</para>
<para>The default quantization is limited range, but full range is possible although
rarely seen.</para>
<para>The <constant>V4L2_YCBCR_ENC_601</constant> encoding as described above is the
default for this colorspace, but it can be overridden with <constant>V4L2_YCBCR_ENC_709</constant>,
in which case the Rec. 709 Y'CbCr encoding is used.</para>
<variablelist>
<varlistentry>
<term>Quantization</term>
<term>The xvYCC 601 encoding (<constant>V4L2_YCBCR_ENC_XV601</constant>, <xref linkend="xvycc" />) is similar
to the BT.601 encoding, but it allows for R', G' and B' values that are outside the range
[0&hellip;1]. The resulting Y', Cb and Cr values are scaled and offset:</term>
<listitem>
<para>[to do]</para>
<para>Y' = (Lum. Levels - 1) &middot; E'<subscript>Y</subscript> + Lum. Offset</para>
<para>C<subscript>B</subscript> = (Chrom. Levels - 1)
&middot; P<subscript>B</subscript> + Chrom. Offset</para>
<para>C<subscript>R</subscript> = (Chrom. Levels - 1)
&middot; P<subscript>R</subscript> + Chrom. Offset</para>
<para>Rounding to the nearest integer and clamping to the range
[0;255] finally yields the digital color components Y'CbCr
stored in YUV images.</para>
<para>Y'&nbsp;=&nbsp;(219&nbsp;/&nbsp;255)&nbsp;*&nbsp;(0.299R'&nbsp;+&nbsp;0.587G'&nbsp;+&nbsp;0.114B')&nbsp;+&nbsp;(16&nbsp;/&nbsp;255)</para>
<para>Cb&nbsp;=&nbsp;(224&nbsp;/&nbsp;255)&nbsp;*&nbsp;(-0.169R'&nbsp;-&nbsp;0.331G'&nbsp;+&nbsp;0.5B')</para>
<para>Cr&nbsp;=&nbsp;(224&nbsp;/&nbsp;255)&nbsp;*&nbsp;(0.5R'&nbsp;-&nbsp;0.419G'&nbsp;-&nbsp;0.081B')</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<example>
<title>ITU-R Rec. BT.601 color conversion</title>
<para>Forward Transformation</para>
<programlisting>
int ER, EG, EB; /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r &lt; 0) return 0;
else if (r &gt; 255) return 255;
else return r;
}
r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0;
y1 = 0.299 * r + 0.587 * g + 0.114 * b;
pb = -0.169 * r - 0.331 * g + 0.5 * b;
pr = 0.5 * r - 0.419 * g - 0.081 * b;
Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128);
/* or shorter */
y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;
Y1 = clamp ( (219 / 255.0) * y1 + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);
</programlisting>
<para>Inverse Transformation</para>
<programlisting>
int Y1, Cb, Cr; /* gamma pre-corrected input [0;255] */
int ER, EG, EB; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r &lt; 0) return 0;
else if (r &gt; 255) return 255;
else return r;
}
y1 = (Y1 - 16) / 219.0;
pb = (Cb - 128) / 224.0;
pr = (Cr - 128) / 224.0;
r = 1.0 * y1 + 0 * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0 * pr;
ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);
</programlisting>
</example>
<table pgwide="1" id="v4l2-colorspace" orient="land">
<title>enum v4l2_colorspace</title>
<tgroup cols="11" align="center">
<colspec align="left" />
<colspec align="center" />
<colspec align="left" />
<colspec colname="cr" />
<colspec colname="cg" />
<colspec colname="cb" />
<colspec colname="wp" />
<colspec colname="gc" />
<colspec colname="lum" />
<colspec colname="qy" />
<colspec colname="qc" />
<spanspec namest="cr" nameend="cb" spanname="chrom" />
<spanspec namest="qy" nameend="qc" spanname="quant" />
<spanspec namest="lum" nameend="qc" spanname="spam" />
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are clamped
to the range [-0.5&hellip;0.5]. The non-standard xvYCC 709 encoding can also be used by selecting
<constant>V4L2_YCBCR_ENC_XV709</constant>. The xvYCC encodings always use full range
quantization.</para>
</section>
<section>
<title id="col-rec709">Colorspace Rec. 709 (<constant>V4L2_COLORSPACE_REC709</constant>)</title>
<para>The <xref linkend="itu709" /> standard defines the colorspace used by HDTV in general. The default
Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_709</constant>. The default Y'CbCr quantization is
limited range. The chromaticities of the primary colors and the white reference are:</para>
<table frame="none">
<title>Rec. 709 Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry morerows="1">Identifier</entry>
<entry morerows="1">Value</entry>
<entry morerows="1">Description</entry>
<entry spanname="chrom">Chromaticities<footnote>
<para>The coordinates of the color primaries are
given in the CIE system (1931)</para>
</footnote></entry>
<entry morerows="1">White Point</entry>
<entry morerows="1">Gamma Correction</entry>
<entry morerows="1">Luminance E'<subscript>Y</subscript></entry>
<entry spanname="quant">Quantization</entry>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.640</entry>
<entry>0.330</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.300</entry>
<entry>0.600</entry>
</row>
<row>
<entry>Blue</entry>
<entry>Y'</entry>
<entry>Cb, Cr</entry>
<entry>0.150</entry>
<entry>0.060</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<para>The full name of this standard is Rec. ITU-R BT.709-5.</para>
<variablelist>
<varlistentry>
<term>Transfer function. Normally L is in the range [0&hellip;1], but for the extended
gamut xvYCC encoding values outside that range are allowed.</term>
<listitem>
<para>L' = -1.099(-L)<superscript>0.45</superscript>&nbsp;+&nbsp;0.099&nbsp;for&nbsp;L&nbsp;&le;&nbsp;-0.018</para>
<para>L' = 4.5L&nbsp;for&nbsp;-0.018&nbsp;&lt;&nbsp;L&nbsp;&lt;&nbsp;0.018</para>
<para>L' = 1.099L<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;L&nbsp;&ge;&nbsp;0.018</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = -((L'&nbsp;-&nbsp;0.099)&nbsp;/&nbsp;-1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&le;&nbsp;-0.081</para>
<para>L = L'&nbsp;/&nbsp;4.5&nbsp;for&nbsp;-0.081&nbsp;&lt;&nbsp;L'&nbsp;&lt;&nbsp;0.081</para>
<para>L = ((L'&nbsp;+&nbsp;0.099)&nbsp;/&nbsp;1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.081</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the following
<constant>V4L2_YCBCR_ENC_709</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.2126R'&nbsp;+&nbsp;0.7152G'&nbsp;+&nbsp;0.0722B'</para>
<para>Cb&nbsp;=&nbsp;-0.1146R'&nbsp;-&nbsp;0.3854G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.4542G'&nbsp;-&nbsp;0.0458B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5].</para>
<para>The default quantization is limited range, but full range is possible although
rarely seen.</para>
<para>The <constant>V4L2_YCBCR_ENC_709</constant> encoding described above is the default
for this colorspace, but it can be overridden with <constant>V4L2_YCBCR_ENC_601</constant>, in which
case the BT.601 Y'CbCr encoding is used.</para>
<variablelist>
<varlistentry>
<term>The xvYCC 709 encoding (<constant>V4L2_YCBCR_ENC_XV709</constant>, <xref linkend="xvycc" />)
is similar to the Rec. 709 encoding, but it allows for R', G' and B' values that are outside the range
[0&hellip;1]. The resulting Y', Cb and Cr values are scaled and offset:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;(219&nbsp;/&nbsp;255)&nbsp;*&nbsp;(0.2126R'&nbsp;+&nbsp;0.7152G'&nbsp;+&nbsp;0.0722B')&nbsp;+&nbsp;(16&nbsp;/&nbsp;255)</para>
<para>Cb&nbsp;=&nbsp;(224&nbsp;/&nbsp;255)&nbsp;*&nbsp;(-0.1146R'&nbsp;-&nbsp;0.3854G'&nbsp;+&nbsp;0.5B')</para>
<para>Cr&nbsp;=&nbsp;(224&nbsp;/&nbsp;255)&nbsp;*&nbsp;(0.5R'&nbsp;-&nbsp;0.4542G'&nbsp;-&nbsp;0.0458B')</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are clamped
to the range [-0.5&hellip;0.5]. The non-standard xvYCC 601 encoding can also be used by
selecting <constant>V4L2_YCBCR_ENC_XV601</constant>. The xvYCC encodings always use full
range quantization.</para>
</section>
<section>
<title id="col-srgb">Colorspace sRGB (<constant>V4L2_COLORSPACE_SRGB</constant>)</title>
<para>The <xref linkend="srgb" /> standard defines the colorspace used by most webcams and computer graphics. The
default Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_SYCC</constant>. The default Y'CbCr quantization
is full range. The chromaticities of the primary colors and the white reference are:</para>
<table frame="none">
<title>sRGB Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry><constant>V4L2_COLORSPACE_SMPTE170M</constant></entry>
<entry>1</entry>
<entry>NTSC/PAL according to <xref linkend="smpte170m" />,
<xref linkend="itu601" /></entry>
<entry>x&nbsp;=&nbsp;0.630, y&nbsp;=&nbsp;0.340</entry>
<entry>x&nbsp;=&nbsp;0.310, y&nbsp;=&nbsp;0.595</entry>
<entry>x&nbsp;=&nbsp;0.155, y&nbsp;=&nbsp;0.070</entry>
<entry>x&nbsp;=&nbsp;0.3127, y&nbsp;=&nbsp;0.3290,
Illuminant D<subscript>65</subscript></entry>
<entry>E' = 4.5&nbsp;I&nbsp;for&nbsp;I&nbsp;&le;0.018,
1.099&nbsp;I<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&lt;&nbsp;I</entry>
<entry>0.299&nbsp;E'<subscript>R</subscript>
+&nbsp;0.587&nbsp;E'<subscript>G</subscript>
+&nbsp;0.114&nbsp;E'<subscript>B</subscript></entry>
<entry>219&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
<entry>Red</entry>
<entry>0.640</entry>
<entry>0.330</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_SMPTE240M</constant></entry>
<entry>2</entry>
<entry>1125-Line (US) HDTV, see <xref
linkend="smpte240m" /></entry>
<entry>x&nbsp;=&nbsp;0.630, y&nbsp;=&nbsp;0.340</entry>
<entry>x&nbsp;=&nbsp;0.310, y&nbsp;=&nbsp;0.595</entry>
<entry>x&nbsp;=&nbsp;0.155, y&nbsp;=&nbsp;0.070</entry>
<entry>x&nbsp;=&nbsp;0.3127, y&nbsp;=&nbsp;0.3290,
Illuminant D<subscript>65</subscript></entry>
<entry>E' = 4&nbsp;I&nbsp;for&nbsp;I&nbsp;&le;0.0228,
1.1115&nbsp;I<superscript>0.45</superscript>&nbsp;-&nbsp;0.1115&nbsp;for&nbsp;0.0228&nbsp;&lt;&nbsp;I</entry>
<entry>0.212&nbsp;E'<subscript>R</subscript>
+&nbsp;0.701&nbsp;E'<subscript>G</subscript>
+&nbsp;0.087&nbsp;E'<subscript>B</subscript></entry>
<entry>219&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
<entry>Green</entry>
<entry>0.300</entry>
<entry>0.600</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_REC709</constant></entry>
<entry>3</entry>
<entry>HDTV and modern devices, see <xref
linkend="itu709" /></entry>
<entry>x&nbsp;=&nbsp;0.640, y&nbsp;=&nbsp;0.330</entry>
<entry>x&nbsp;=&nbsp;0.300, y&nbsp;=&nbsp;0.600</entry>
<entry>x&nbsp;=&nbsp;0.150, y&nbsp;=&nbsp;0.060</entry>
<entry>x&nbsp;=&nbsp;0.3127, y&nbsp;=&nbsp;0.3290,
Illuminant D<subscript>65</subscript></entry>
<entry>E' = 4.5&nbsp;I&nbsp;for&nbsp;I&nbsp;&le;0.018,
1.099&nbsp;I<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&lt;&nbsp;I</entry>
<entry>0.2125&nbsp;E'<subscript>R</subscript>
+&nbsp;0.7154&nbsp;E'<subscript>G</subscript>
+&nbsp;0.0721&nbsp;E'<subscript>B</subscript></entry>
<entry>219&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_BT878</constant></entry>
<entry>4</entry>
<entry>Broken Bt878 extents<footnote>
<para>The ubiquitous Bt878 video capture chip
quantizes E'<subscript>Y</subscript> to 238 levels, yielding a range
of Y' = 16 &hellip; 253, unlike Rec. 601 Y' = 16 &hellip;
235. This is not a typo in the Bt878 documentation, it has been
implemented in silicon. The chroma extents are unclear.</para>
</footnote>, <xref linkend="itu601" /></entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>0.299&nbsp;E'<subscript>R</subscript>
+&nbsp;0.587&nbsp;E'<subscript>G</subscript>
+&nbsp;0.114&nbsp;E'<subscript>B</subscript></entry>
<entry><emphasis>237</emphasis>&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128 (probably)</entry>
<entry>Blue</entry>
<entry>0.150</entry>
<entry>0.060</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_470_SYSTEM_M</constant></entry>
<entry>5</entry>
<entry>M/NTSC<footnote>
<para>No identifier exists for M/PAL which uses
the chromaticities of M/NTSC, the remaining parameters are equal to B and
G/PAL.</para>
</footnote> according to <xref linkend="itu470" />, <xref
linkend="itu601" /></entry>
<entry>x&nbsp;=&nbsp;0.67, y&nbsp;=&nbsp;0.33</entry>
<entry>x&nbsp;=&nbsp;0.21, y&nbsp;=&nbsp;0.71</entry>
<entry>x&nbsp;=&nbsp;0.14, y&nbsp;=&nbsp;0.08</entry>
<entry>x&nbsp;=&nbsp;0.310, y&nbsp;=&nbsp;0.316, Illuminant C</entry>
<entry>?</entry>
<entry>0.299&nbsp;E'<subscript>R</subscript>
+&nbsp;0.587&nbsp;E'<subscript>G</subscript>
+&nbsp;0.114&nbsp;E'<subscript>B</subscript></entry>
<entry>219&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<para>These chromaticities are identical to the Rec. 709 colorspace.</para>
<variablelist>
<varlistentry>
<term>Transfer function. Note that negative values for L are only used by the Y'CbCr conversion.</term>
<listitem>
<para>L' = -1.055(-L)<superscript>1/2.4</superscript>&nbsp;+&nbsp;0.055&nbsp;for&nbsp;L&nbsp;&lt;&nbsp;-0.0031308</para>
<para>L' = 12.92L&nbsp;for&nbsp;-0.0031308&nbsp;&le;&nbsp;L&nbsp;&le;&nbsp;0.0031308</para>
<para>L' = 1.055L<superscript>1/2.4</superscript>&nbsp;-&nbsp;0.055&nbsp;for&nbsp;0.0031308&nbsp;&lt;&nbsp;L&nbsp;&le;&nbsp;1</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = -((-L'&nbsp;+&nbsp;0.055)&nbsp;/&nbsp;1.055)<superscript>2.4</superscript>&nbsp;for&nbsp;L'&nbsp;&lt;&nbsp;-0.04045</para>
<para>L = L'&nbsp;/&nbsp;12.92&nbsp;for&nbsp;-0.04045&nbsp;&le;&nbsp;L'&nbsp;&le;&nbsp;0.04045</para>
<para>L = ((L'&nbsp;+&nbsp;0.055)&nbsp;/&nbsp;1.055)<superscript>2.4</superscript>&nbsp;for&nbsp;L'&nbsp;&gt;&nbsp;0.04045</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the following
<constant>V4L2_YCBCR_ENC_SYCC</constant> encoding as defined by <xref linkend="sycc" />:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.2990R'&nbsp;+&nbsp;0.5870G'&nbsp;+&nbsp;0.1140B'</para>
<para>Cb&nbsp;=&nbsp;-0.1687R'&nbsp;-&nbsp;0.3313G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.4187G'&nbsp;-&nbsp;0.0813B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are clamped
to the range [-0.5&hellip;0.5]. The <constant>V4L2_YCBCR_ENC_SYCC</constant> quantization is always
full range. Although this Y'CbCr encoding looks very similar to the <constant>V4L2_YCBCR_ENC_XV601</constant>
encoding, it is not. The <constant>V4L2_YCBCR_ENC_XV601</constant> scales and offsets the Y'CbCr
values before quantization, but this encoding does not do that.</para>
</section>
<section>
<title id="col-adobergb">Colorspace Adobe RGB (<constant>V4L2_COLORSPACE_ADOBERGB</constant>)</title>
<para>The <xref linkend="adobergb" /> standard defines the colorspace used by computer graphics
that use the AdobeRGB colorspace. This is also known as the <xref linkend="oprgb" /> standard.
The default Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_601</constant>. The default Y'CbCr
quantization is limited range. The chromaticities of the primary colors and the white reference
are:</para>
<table frame="none">
<title>Adobe RGB Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry><constant>V4L2_COLORSPACE_470_SYSTEM_BG</constant></entry>
<entry>6</entry>
<entry>625-line PAL and SECAM systems according to <xref
linkend="itu470" />, <xref linkend="itu601" /></entry>
<entry>x&nbsp;=&nbsp;0.64, y&nbsp;=&nbsp;0.33</entry>
<entry>x&nbsp;=&nbsp;0.29, y&nbsp;=&nbsp;0.60</entry>
<entry>x&nbsp;=&nbsp;0.15, y&nbsp;=&nbsp;0.06</entry>
<entry>x&nbsp;=&nbsp;0.313, y&nbsp;=&nbsp;0.329,
Illuminant D<subscript>65</subscript></entry>
<entry>?</entry>
<entry>0.299&nbsp;E'<subscript>R</subscript>
+&nbsp;0.587&nbsp;E'<subscript>G</subscript>
+&nbsp;0.114&nbsp;E'<subscript>B</subscript></entry>
<entry>219&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16</entry>
<entry>224&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry><constant>V4L2_COLORSPACE_JPEG</constant></entry>
<entry>7</entry>
<entry>JPEG Y'CbCr, see <xref linkend="jfif" />, <xref linkend="itu601" /></entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>?</entry>
<entry>0.299&nbsp;E'<subscript>R</subscript>
+&nbsp;0.587&nbsp;E'<subscript>G</subscript>
+&nbsp;0.114&nbsp;E'<subscript>B</subscript></entry>
<entry>256&nbsp;E'<subscript>Y</subscript>&nbsp;+&nbsp;16<footnote>
<para>Note JFIF quantizes
Y'P<subscript>B</subscript>P<subscript>R</subscript> in range [0;+1] and
[-0.5;+0.5] to <emphasis>257</emphasis> levels, however Y'CbCr signals
are still clamped to [0;255].</para>
</footnote></entry>
<entry>256&nbsp;P<subscript>B,R</subscript>&nbsp;+&nbsp;128</entry>
<entry>Red</entry>
<entry>0.6400</entry>
<entry>0.3300</entry>
</row>
<row>
<entry><constant>V4L2_COLORSPACE_SRGB</constant></entry>
<entry>8</entry>
<entry>[?]</entry>
<entry>x&nbsp;=&nbsp;0.640, y&nbsp;=&nbsp;0.330</entry>
<entry>x&nbsp;=&nbsp;0.300, y&nbsp;=&nbsp;0.600</entry>
<entry>x&nbsp;=&nbsp;0.150, y&nbsp;=&nbsp;0.060</entry>
<entry>x&nbsp;=&nbsp;0.3127, y&nbsp;=&nbsp;0.3290,
Illuminant D<subscript>65</subscript></entry>
<entry>E' = 4.5&nbsp;I&nbsp;for&nbsp;I&nbsp;&le;0.018,
1.099&nbsp;I<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&lt;&nbsp;I</entry>
<entry spanname="spam">n/a</entry>
<entry>Green</entry>
<entry>0.2100</entry>
<entry>0.7100</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.1500</entry>
<entry>0.0600</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<variablelist>
<varlistentry>
<term>Transfer function:</term>
<listitem>
<para>L' = L<superscript>1/2.19921875</superscript></para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = L'<superscript>2.19921875</superscript></para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the
following <constant>V4L2_YCBCR_ENC_601</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.299R'&nbsp;+&nbsp;0.587G'&nbsp;+&nbsp;0.114B'</para>
<para>Cb&nbsp;=&nbsp;-0.169R'&nbsp;-&nbsp;0.331G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.419G'&nbsp;-&nbsp;0.081B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5]. This transform is identical to one defined in
SMPTE 170M/BT.601. The Y'CbCr quantization is limited range.</para>
</section>
<section>
<title id="col-bt2020">Colorspace BT.2020 (<constant>V4L2_COLORSPACE_BT2020</constant>)</title>
<para>The <xref linkend="itu2020" /> standard defines the colorspace used by Ultra-high definition
television (UHDTV). The default Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_BT2020</constant>.
The default Y'CbCr quantization is limited range. The chromaticities of the primary colors and
the white reference are:</para>
<table frame="none">
<title>BT.2020 Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.708</entry>
<entry>0.292</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.170</entry>
<entry>0.797</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.131</entry>
<entry>0.046</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<variablelist>
<varlistentry>
<term>Transfer function (same as Rec. 709):</term>
<listitem>
<para>L' = 4.5L&nbsp;for&nbsp;0&nbsp;&le;&nbsp;L&nbsp;&lt;&nbsp;0.018</para>
<para>L' = 1.099L<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&le;&nbsp;L&nbsp;&le;&nbsp;1</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = L'&nbsp;/&nbsp;4.5&nbsp;for&nbsp;L'&nbsp;&lt;&nbsp;0.081</para>
<para>L = ((L'&nbsp;+&nbsp;0.099)&nbsp;/&nbsp;1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.081</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the
following <constant>V4L2_YCBCR_ENC_BT2020</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.2627R'&nbsp;+&nbsp;0.6789G'&nbsp;+&nbsp;0.0593B'</para>
<para>Cb&nbsp;=&nbsp;-0.1396R'&nbsp;-&nbsp;0.3604G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.4598G'&nbsp;-&nbsp;0.0402B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5]. The Y'CbCr quantization is limited range.</para>
<para>There is also an alternate constant luminance R'G'B' to Yc'CbcCrc
(<constant>V4L2_YCBCR_ENC_BT2020_CONST_LUM</constant>) encoding:</para>
<variablelist>
<varlistentry>
<term>Luma:</term>
<listitem>
<para>Yc'&nbsp;=&nbsp;(0.2627R&nbsp;+&nbsp;0.6789G&nbsp;+&nbsp;0.0593B)'</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>B'&nbsp;-&nbsp;Yc'&nbsp;&le;&nbsp;0:</term>
<listitem>
<para>Cbc&nbsp;=&nbsp;(B'&nbsp;-&nbsp;Y')&nbsp;/&nbsp;1.9404</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>B'&nbsp;-&nbsp;Yc'&nbsp;&gt;&nbsp;0:</term>
<listitem>
<para>Cbc&nbsp;=&nbsp;(B'&nbsp;-&nbsp;Y')&nbsp;/&nbsp;1.5816</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>R'&nbsp;-&nbsp;Yc'&nbsp;&le;&nbsp;0:</term>
<listitem>
<para>Crc&nbsp;=&nbsp;(R'&nbsp;-&nbsp;Y')&nbsp;/&nbsp;1.7184</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>R'&nbsp;-&nbsp;Yc'&nbsp;&gt;&nbsp;0:</term>
<listitem>
<para>Crc&nbsp;=&nbsp;(R'&nbsp;-&nbsp;Y')&nbsp;/&nbsp;0.9936</para>
</listitem>
</varlistentry>
</variablelist>
<para>Yc' is clamped to the range [0&hellip;1] and Cbc and Crc are
clamped to the range [-0.5&hellip;0.5]. The Yc'CbcCrc quantization is limited range.</para>
</section>
<section>
<title id="col-smpte-240m">Colorspace SMPTE 240M (<constant>V4L2_COLORSPACE_SMPTE240M</constant>)</title>
<para>The <xref linkend="smpte240m" /> standard was an interim standard used during the early days of HDTV (1988-1998).
It has been superseded by Rec. 709. The default Y'CbCr encoding is <constant>V4L2_YCBCR_ENC_SMPTE240M</constant>.
The default Y'CbCr quantization is limited range. The chromaticities of the primary colors and the
white reference are:</para>
<table frame="none">
<title>SMPTE 240M Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.630</entry>
<entry>0.340</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.310</entry>
<entry>0.595</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.155</entry>
<entry>0.070</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<para>These chromaticities are identical to the SMPTE 170M colorspace.</para>
<variablelist>
<varlistentry>
<term>Transfer function:</term>
<listitem>
<para>L' = 4L&nbsp;for&nbsp;0&nbsp;&le;&nbsp;L&nbsp;&lt;&nbsp;0.0228</para>
<para>L' = 1.1115L<superscript>0.45</superscript>&nbsp;-&nbsp;0.1115&nbsp;for&nbsp;0.0228&nbsp;&le;&nbsp;L&nbsp;&le;&nbsp;1</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = L'&nbsp;/&nbsp;4&nbsp;for&nbsp;0&nbsp;&le;&nbsp;L'&nbsp;&lt;&nbsp;0.0913</para>
<para>L = ((L'&nbsp;+&nbsp;0.1115)&nbsp;/&nbsp;1.1115)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.0913</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the
following <constant>V4L2_YCBCR_ENC_SMPTE240M</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.2122R'&nbsp;+&nbsp;0.7013G'&nbsp;+&nbsp;0.0865B'</para>
<para>Cb&nbsp;=&nbsp;-0.1161R'&nbsp;-&nbsp;0.3839G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.4451G'&nbsp;-&nbsp;0.0549B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Yc' is clamped to the range [0&hellip;1] and Cbc and Crc are
clamped to the range [-0.5&hellip;0.5]. The Y'CbCr quantization is limited range.</para>
</section>
<section>
<title id="col-sysm">Colorspace NTSC 1953 (<constant>V4L2_COLORSPACE_470_SYSTEM_M</constant>)</title>
<para>This standard defines the colorspace used by NTSC in 1953. In practice this
colorspace is obsolete and SMPTE 170M should be used instead. The default Y'CbCr encoding
is <constant>V4L2_YCBCR_ENC_601</constant>. The default Y'CbCr quantization is limited range.
The chromaticities of the primary colors and the white reference are:</para>
<table frame="none">
<title>NTSC 1953 Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.67</entry>
<entry>0.33</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.21</entry>
<entry>0.71</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.14</entry>
<entry>0.08</entry>
</row>
<row>
<entry>White Reference (C)</entry>
<entry>0.310</entry>
<entry>0.316</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Note that this colorspace uses Illuminant C instead of D65 as the
white reference. To correctly convert an image in this colorspace to another
that uses D65 you need to apply a chromatic adaptation algorithm such as the
Bradford method.</para>
<variablelist>
<varlistentry>
<term>The transfer function was never properly defined for NTSC 1953. The
Rec. 709 transfer function is recommended in the literature:</term>
<listitem>
<para>L' = 4.5L&nbsp;for&nbsp;0&nbsp;&le;&nbsp;L&nbsp;&lt;&nbsp;0.018</para>
<para>L' = 1.099L<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&le;&nbsp;L&nbsp;&le;&nbsp;1</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = L'&nbsp;/&nbsp;4.5&nbsp;for&nbsp;L'&nbsp;&lt;&nbsp;0.081</para>
<para>L = ((L'&nbsp;+&nbsp;0.099)&nbsp;/&nbsp;1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.081</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the
following <constant>V4L2_YCBCR_ENC_601</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.299R'&nbsp;+&nbsp;0.587G'&nbsp;+&nbsp;0.114B'</para>
<para>Cb&nbsp;=&nbsp;-0.169R'&nbsp;-&nbsp;0.331G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.419G'&nbsp;-&nbsp;0.081B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5]. The Y'CbCr quantization is limited range.
This transform is identical to one defined in SMPTE 170M/BT.601.</para>
</section>
<section>
<title id="col-sysbg">Colorspace EBU Tech. 3213 (<constant>V4L2_COLORSPACE_470_SYSTEM_BG</constant>)</title>
<para>The <xref linkend="tech3213" /> standard defines the colorspace used by PAL/SECAM in 1975. In practice this
colorspace is obsolete and SMPTE 170M should be used instead. The default Y'CbCr encoding
is <constant>V4L2_YCBCR_ENC_601</constant>. The default Y'CbCr quantization is limited range.
The chromaticities of the primary colors and the white reference are:</para>
<table frame="none">
<title>EBU Tech. 3213 Chromaticities</title>
<tgroup cols="3" align="left">
&cs-str;
<thead>
<row>
<entry>Color</entry>
<entry>x</entry>
<entry>y</entry>
</row>
</thead>
<tbody valign="top">
<row>
<entry>Red</entry>
<entry>0.64</entry>
<entry>0.33</entry>
</row>
<row>
<entry>Green</entry>
<entry>0.29</entry>
<entry>0.60</entry>
</row>
<row>
<entry>Blue</entry>
<entry>0.15</entry>
<entry>0.06</entry>
</row>
<row>
<entry>White Reference (D65)</entry>
<entry>0.3127</entry>
<entry>0.3290</entry>
</row>
</tbody>
</tgroup>
</table>
<variablelist>
<varlistentry>
<term>The transfer function was never properly defined for this colorspace.
The Rec. 709 transfer function is recommended in the literature:</term>
<listitem>
<para>L' = 4.5L&nbsp;for&nbsp;0&nbsp;&le;&nbsp;L&nbsp;&lt;&nbsp;0.018</para>
<para>L' = 1.099L<superscript>0.45</superscript>&nbsp;-&nbsp;0.099&nbsp;for&nbsp;0.018&nbsp;&le;&nbsp;L&nbsp;&le;&nbsp;1</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Inverse Transfer function:</term>
<listitem>
<para>L = L'&nbsp;/&nbsp;4.5&nbsp;for&nbsp;L'&nbsp;&lt;&nbsp;0.081</para>
<para>L = ((L'&nbsp;+&nbsp;0.099)&nbsp;/&nbsp;1.099)<superscript>1/0.45</superscript>&nbsp;for&nbsp;L'&nbsp;&ge;&nbsp;0.081</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>The luminance (Y') and color difference (Cb and Cr) are obtained with the
following <constant>V4L2_YCBCR_ENC_601</constant> encoding:</term>
<listitem>
<para>Y'&nbsp;=&nbsp;0.299R'&nbsp;+&nbsp;0.587G'&nbsp;+&nbsp;0.114B'</para>
<para>Cb&nbsp;=&nbsp;-0.169R'&nbsp;-&nbsp;0.331G'&nbsp;+&nbsp;0.5B'</para>
<para>Cr&nbsp;=&nbsp;0.5R'&nbsp;-&nbsp;0.419G'&nbsp;-&nbsp;0.081B'</para>
</listitem>
</varlistentry>
</variablelist>
<para>Y' is clamped to the range [0&hellip;1] and Cb and Cr are
clamped to the range [-0.5&hellip;0.5]. The Y'CbCr quantization is limited range.
This transform is identical to one defined in SMPTE 170M/BT.601.</para>
</section>
<section>
<title id="col-jpeg">Colorspace JPEG (<constant>V4L2_COLORSPACE_JPEG</constant>)</title>
<para>This colorspace defines the colorspace used by most (Motion-)JPEG formats. The chromaticities
of the primary colors and the white reference are identical to sRGB. The Y'CbCr encoding is
<constant>V4L2_YCBCR_ENC_601</constant> with full range quantization where
Y' is scaled to [0&hellip;255] and Cb/Cr are scaled to [-128&hellip;128] and
then clipped to [-128&hellip;127].</para>
<para>Note that the JPEG standard does not actually store colorspace information.
So if something other than sRGB is used, then the driver will have to set that information
explicitly. Effectively <constant>V4L2_COLORSPACE_JPEG</constant> can be considered to be
an abbreviation for <constant>V4L2_COLORSPACE_SRGB</constant>, <constant>V4L2_YCBCR_ENC_601</constant>
and <constant>V4L2_QUANTIZATION_FULL_RANGE</constant>.</para>
</section>
</section>
<section id="pixfmt-indexed">
......
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