<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, 0, 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>
- <variablelist>
- <varlistentry>
- <term>Gamma Correction</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>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Construction of luminance and color-difference
-signals</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>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Re-normalized color-difference signals</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>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term>Quantization</term>
- <listitem>
- <para>[to do]</para>
- <para>Y' = (Lum. Levels - 1) · E'<subscript>Y</subscript> + Lum. Offset</para>
- <para>C<subscript>B</subscript> = (Chrom. Levels - 1)
-· P<subscript>B</subscript> + Chrom. Offset</para>
- <para>C<subscript>R</subscript> = (Chrom. Levels - 1)
-· 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>
- </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 < 0) return 0;
- else if (r > 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 < 0) return 0;
- else if (r > 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>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…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, 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…1] describes all colors that humans can perceive, but the transform to
+another colorspace will produce colors that are outside the [0…1] range.
+Once clamped to the [0…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…1] is translated to integer values in the
+range of [0…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…255] range. Y'CbCr values on the other hand are limited
+range with Y' using [16…235] and Cb and Cr using [16…240].</para>
+
+ <para>Unfortunately, in some cases limited range RGB is also used
+where the components use the range [16…235]. And full range Y'CbCr also exists
+using the [0…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 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>
- </row>
- <row>
- <entry>Red</entry>
- <entry>Green</entry>
- <entry>Blue</entry>
- <entry>Y'</entry>
- <entry>Cb, Cr</entry>
+ <entry>Identifier</entry>
+ <entry>Details</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 = 0.630, y = 0.340</entry>
- <entry>x = 0.310, y = 0.595</entry>
- <entry>x = 0.155, y = 0.070</entry>
- <entry>x = 0.3127, y = 0.3290,
- Illuminant D<subscript>65</subscript></entry>
- <entry>E' = 4.5 I for I ≤0.018,
-1.099 I<superscript>0.45</superscript> - 0.099 for 0.018 < I</entry>
- <entry>0.299 E'<subscript>R</subscript>
-+ 0.587 E'<subscript>G</subscript>
-+ 0.114 E'<subscript>B</subscript></entry>
- <entry>219 E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128</entry>
+ <entry>See <xref linkend="col-smpte-170m" />.</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 = 0.630, y = 0.340</entry>
- <entry>x = 0.310, y = 0.595</entry>
- <entry>x = 0.155, y = 0.070</entry>
- <entry>x = 0.3127, y = 0.3290,
- Illuminant D<subscript>65</subscript></entry>
- <entry>E' = 4 I for I ≤0.0228,
-1.1115 I<superscript>0.45</superscript> - 0.1115 for 0.0228 < I</entry>
- <entry>0.212 E'<subscript>R</subscript>
-+ 0.701 E'<subscript>G</subscript>
-+ 0.087 E'<subscript>B</subscript></entry>
- <entry>219 E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128</entry>
+ <entry><constant>V4L2_COLORSPACE_REC709</constant></entry>
+ <entry>See <xref linkend="col-rec709" />.</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 = 0.640, y = 0.330</entry>
- <entry>x = 0.300, y = 0.600</entry>
- <entry>x = 0.150, y = 0.060</entry>
- <entry>x = 0.3127, y = 0.3290,
- Illuminant D<subscript>65</subscript></entry>
- <entry>E' = 4.5 I for I ≤0.018,
-1.099 I<superscript>0.45</superscript> - 0.099 for 0.018 < I</entry>
- <entry>0.2125 E'<subscript>R</subscript>
-+ 0.7154 E'<subscript>G</subscript>
-+ 0.0721 E'<subscript>B</subscript></entry>
- <entry>219 E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128</entry>
+ <entry><constant>V4L2_COLORSPACE_SRGB</constant></entry>
+ <entry>See <xref linkend="col-srgb" />.</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 … 253, unlike Rec. 601 Y' = 16 …
-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 E'<subscript>R</subscript>
-+ 0.587 E'<subscript>G</subscript>
-+ 0.114 E'<subscript>B</subscript></entry>
- <entry><emphasis>237</emphasis> E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128 (probably)</entry>
+ <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>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 = 0.67, y = 0.33</entry>
- <entry>x = 0.21, y = 0.71</entry>
- <entry>x = 0.14, y = 0.08</entry>
- <entry>x = 0.310, y = 0.316, Illuminant C</entry>
- <entry>?</entry>
- <entry>0.299 E'<subscript>R</subscript>
-+ 0.587 E'<subscript>G</subscript>
-+ 0.114 E'<subscript>B</subscript></entry>
- <entry>219 E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128</entry>
+ <entry>See <xref linkend="col-sysm" />.</entry>
</row>
<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 = 0.64, y = 0.33</entry>
- <entry>x = 0.29, y = 0.60</entry>
- <entry>x = 0.15, y = 0.06</entry>
- <entry>x = 0.313, y = 0.329,
-Illuminant D<subscript>65</subscript></entry>
- <entry>?</entry>
- <entry>0.299 E'<subscript>R</subscript>
-+ 0.587 E'<subscript>G</subscript>
-+ 0.114 E'<subscript>B</subscript></entry>
- <entry>219 E'<subscript>Y</subscript> + 16</entry>
- <entry>224 P<subscript>B,R</subscript> + 128</entry>
+ <entry>See <xref linkend="col-sysbg" />.</entry>
</row>
<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 E'<subscript>R</subscript>
-+ 0.587 E'<subscript>G</subscript>
-+ 0.114 E'<subscript>B</subscript></entry>
- <entry>256 E'<subscript>Y</subscript> + 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 P<subscript>B,R</subscript> + 128</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_COLORSPACE_SRGB</constant></entry>
- <entry>8</entry>
- <entry>[?]</entry>
- <entry>x = 0.640, y = 0.330</entry>
- <entry>x = 0.300, y = 0.600</entry>
- <entry>x = 0.150, y = 0.060</entry>
- <entry>x = 0.3127, y = 0.3290,
- Illuminant D<subscript>65</subscript></entry>
- <entry>E' = 4.5 I for I ≤0.018,
-1.099 I<superscript>0.45</superscript> - 0.099 for 0.018 < I</entry>
- <entry spanname="spam">n/a</entry>
+ <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…1]
+is mapped to [0…255] (with possible clipping to [1…254] to avoid the
+0x00 and 0xff values). Cb and Cr are mapped from [-0.5…0.5] to [0…255]
+(with possible clipping to [1…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…1]
+is mapped to [16…235]. Cb and Cr are mapped from [-0.5…0.5] to [16…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>The transfer function defined for SMPTE 170M is the same as the
+one defined in Rec. 709. Normally L is in the range [0…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> + 0.099 for L ≤ -0.018</para>
+ <para>L' = 4.5L for -0.018 < L < 0.018</para>
+ <para>L' = 1.099L<superscript>0.45</superscript> - 0.099 for L ≥ 0.018</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = -((L' - 0.099) / -1.099)<superscript>1/0.45</superscript> for L' ≤ -0.081</para>
+ <para>L = L' / 4.5 for -0.081 < L' < 0.081</para>
+ <para>L = ((L' + 0.099) / 1.099)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.299R' + 0.587G' + 0.114B'</para>
+ <para>Cb = -0.169R' - 0.331G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.419G' - 0.081B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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>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…1]. The resulting Y', Cb and Cr values are scaled and offset:</term>
+ <listitem>
+ <para>Y' = (219 / 255) * (0.299R' + 0.587G' + 0.114B') + (16 / 255)</para>
+ <para>Cb = (224 / 255) * (-0.169R' - 0.331G' + 0.5B')</para>
+ <para>Cr = (224 / 255) * (0.5R' - 0.419G' - 0.081B')</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are clamped
+to the range [-0.5…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>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>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…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> + 0.099 for L ≤ -0.018</para>
+ <para>L' = 4.5L for -0.018 < L < 0.018</para>
+ <para>L' = 1.099L<superscript>0.45</superscript> - 0.099 for L ≥ 0.018</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = -((L' - 0.099) / -1.099)<superscript>1/0.45</superscript> for L' ≤ -0.081</para>
+ <para>L = L' / 4.5 for -0.081 < L' < 0.081</para>
+ <para>L = ((L' + 0.099) / 1.099)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.2126R' + 0.7152G' + 0.0722B'</para>
+ <para>Cb = -0.1146R' - 0.3854G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.4542G' - 0.0458B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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…1]. The resulting Y', Cb and Cr values are scaled and offset:</term>
+ <listitem>
+ <para>Y' = (219 / 255) * (0.2126R' + 0.7152G' + 0.0722B') + (16 / 255)</para>
+ <para>Cb = (224 / 255) * (-0.1146R' - 0.3854G' + 0.5B')</para>
+ <para>Cr = (224 / 255) * (0.5R' - 0.4542G' - 0.0458B')</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are clamped
+to the range [-0.5…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>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>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>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> + 0.055 for L < -0.0031308</para>
+ <para>L' = 12.92L for -0.0031308 ≤ L ≤ 0.0031308</para>
+ <para>L' = 1.055L<superscript>1/2.4</superscript> - 0.055 for 0.0031308 < L ≤ 1</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = -((-L' + 0.055) / 1.055)<superscript>2.4</superscript> for L' < -0.04045</para>
+ <para>L = L' / 12.92 for -0.04045 ≤ L' ≤ 0.04045</para>
+ <para>L = ((L' + 0.055) / 1.055)<superscript>2.4</superscript> for L' > 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' = 0.2990R' + 0.5870G' + 0.1140B'</para>
+ <para>Cb = -0.1687R' - 0.3313G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.4187G' - 0.0813B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are clamped
+to the range [-0.5…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>Color</entry>
+ <entry>x</entry>
+ <entry>y</entry>
+ </row>
+ </thead>
+ <tbody valign="top">
+ <row>
+ <entry>Red</entry>
+ <entry>0.6400</entry>
+ <entry>0.3300</entry>
+ </row>
+ <row>
+ <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' = 0.299R' + 0.587G' + 0.114B'</para>
+ <para>Cb = -0.169R' - 0.331G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.419G' - 0.081B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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 for 0 ≤ L < 0.018</para>
+ <para>L' = 1.099L<superscript>0.45</superscript> - 0.099 for 0.018 ≤ L ≤ 1</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = L' / 4.5 for L' < 0.081</para>
+ <para>L = ((L' + 0.099) / 1.099)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.2627R' + 0.6789G' + 0.0593B'</para>
+ <para>Cb = -0.1396R' - 0.3604G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.4598G' - 0.0402B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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' = (0.2627R + 0.6789G + 0.0593B)'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>B' - Yc' ≤ 0:</term>
+ <listitem>
+ <para>Cbc = (B' - Y') / 1.9404</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>B' - Yc' > 0:</term>
+ <listitem>
+ <para>Cbc = (B' - Y') / 1.5816</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>R' - Yc' ≤ 0:</term>
+ <listitem>
+ <para>Crc = (R' - Y') / 1.7184</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <variablelist>
+ <varlistentry>
+ <term>R' - Yc' > 0:</term>
+ <listitem>
+ <para>Crc = (R' - Y') / 0.9936</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Yc' is clamped to the range [0…1] and Cbc and Crc are
+clamped to the range [-0.5…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 for 0 ≤ L < 0.0228</para>
+ <para>L' = 1.1115L<superscript>0.45</superscript> - 0.1115 for 0.0228 ≤ L ≤ 1</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = L' / 4 for 0 ≤ L' < 0.0913</para>
+ <para>L = ((L' + 0.1115) / 1.1115)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.2122R' + 0.7013G' + 0.0865B'</para>
+ <para>Cb = -0.1161R' - 0.3839G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.4451G' - 0.0549B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Yc' is clamped to the range [0…1] and Cbc and Crc are
+clamped to the range [-0.5…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 for 0 ≤ L < 0.018</para>
+ <para>L' = 1.099L<superscript>0.45</superscript> - 0.099 for 0.018 ≤ L ≤ 1</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = L' / 4.5 for L' < 0.081</para>
+ <para>L = ((L' + 0.099) / 1.099)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.299R' + 0.587G' + 0.114B'</para>
+ <para>Cb = -0.169R' - 0.331G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.419G' - 0.081B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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 for 0 ≤ L < 0.018</para>
+ <para>L' = 1.099L<superscript>0.45</superscript> - 0.099 for 0.018 ≤ L ≤ 1</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>Inverse Transfer function:</term>
+ <listitem>
+ <para>L = L' / 4.5 for L' < 0.081</para>
+ <para>L = ((L' + 0.099) / 1.099)<superscript>1/0.45</superscript> for L' ≥ 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' = 0.299R' + 0.587G' + 0.114B'</para>
+ <para>Cb = -0.169R' - 0.331G' + 0.5B'</para>
+ <para>Cr = 0.5R' - 0.419G' - 0.081B'</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>Y' is clamped to the range [0…1] and Cb and Cr are
+clamped to the range [-0.5…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…255] and Cb/Cr are scaled to [-128…128] and
+then clipped to [-128…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">
projects / linux-beck.git / commitdiff