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diff --git a/Documentation/DocBook/media/v4l/pixfmt.xml b/Documentation/DocBook/media/v4l/pixfmt.xml index df5b23d46552..ccf6053c1ae4 100644 --- a/Documentation/DocBook/media/v4l/pixfmt.xml +++ b/Documentation/DocBook/media/v4l/pixfmt.xml @@ -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, 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"> |