Documentation/userspace-api/media/v4l/colorspaces-details.rst - linux - Git at Google

 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

 ********************************
 Detailed Colorspace Descriptions
 ********************************


 .. _col-smpte-170m:

 Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M)
 =================================================

 The :ref:`smpte170m` standard defines the colorspace used by NTSC and
 PAL and by SDTV in general. The default transfer function is
 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
 range. The chromaticities of the primary colors and the white reference
 are:

 .. flat-table:: SMPTE 170M Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.630
       - 0.340
     * - Green
       - 0.310
       - 0.595
     * - Blue
       - 0.155
       - 0.070
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 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.

 The transfer function defined for SMPTE 170M is the same as the one
 defined in Rec. 709.

 .. math::

     L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le-0.018

     L' = 4.5L \text{, for } -0.018 < L < 0.018

     L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018

 Inverse Transfer function:

 .. math::

     L = -\left( \frac{L' - 0.099}{-1.099} \right) ^{\frac{1}{0.45}} \text{, for } L' \le -0.081

     L = \frac{L'}{4.5} \text{, for } -0.081 < L' < 0.081

     L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_601`` encoding:

 .. math::

     Y' = 0.2990R' + 0.5870G' + 0.1140B'

     Cb = -0.1687R' - 0.3313G' + 0.5B'

     Cr = 0.5R' - 0.4187G' - 0.0813B'

 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 :ref:`itu601` standard and this colorspace is sometimes called
 BT.601 as well, even though BT.601 does not mention any color primaries.

 The default quantization is limited range, but full range is possible
 although rarely seen.


 .. _col-rec709:

 Colorspace Rec. 709 (V4L2_COLORSPACE_REC709)
 ============================================

 The :ref:`itu709` standard defines the colorspace used by HDTV in
 general. The default transfer function is ``V4L2_XFER_FUNC_709``. The
 default Y'CbCr encoding is ``V4L2_YCBCR_ENC_709``. The default Y'CbCr
 quantization is limited range. The chromaticities of the primary colors
 and the white reference are:

 .. flat-table:: Rec. 709 Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.640
       - 0.330
     * - Green
       - 0.300
       - 0.600
     * - Blue
       - 0.150
       - 0.060
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 The full name of this standard is Rec. ITU-R BT.709-5.

 Transfer function. Normally L is in the range [0…1], but for the
 extended gamut xvYCC encoding values outside that range are allowed.

 .. math::

     L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le -0.018

     L' = 4.5L \text{, for } -0.018 < L < 0.018

     L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018

 Inverse Transfer function:

 .. math::

     L = -\left( \frac{L' - 0.099}{-1.099} \right)^\frac{1}{0.45} \text{, for } L' \le -0.081

     L = \frac{L'}{4.5}\text{, for } -0.081 < L' < 0.081

     L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_709`` encoding:

 .. math::

     Y' = 0.2126R' + 0.7152G' + 0.0722B'

     Cb = -0.1146R' - 0.3854G' + 0.5B'

     Cr = 0.5R' - 0.4542G' - 0.0458B'

 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
 [-0.5…0.5].

 The default quantization is limited range, but full range is possible
 although rarely seen.

 The ``V4L2_YCBCR_ENC_709`` encoding described above is the default for
 this colorspace, but it can be overridden with ``V4L2_YCBCR_ENC_601``,
 in which case the BT.601 Y'CbCr encoding is used.

 Two additional extended gamut Y'CbCr encodings are also possible with
 this colorspace:

 The xvYCC 709 encoding (``V4L2_YCBCR_ENC_XV709``, :ref:`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 according to the limited range formula:

 .. math::

     Y' = \frac{219}{256} * (0.2126R' + 0.7152G' + 0.0722B') + \frac{16}{256}

     Cb = \frac{224}{256} * (-0.1146R' - 0.3854G' + 0.5B')

     Cr = \frac{224}{256} * (0.5R' - 0.4542G' - 0.0458B')

 The xvYCC 601 encoding (``V4L2_YCBCR_ENC_XV601``, :ref:`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 according to the limited range formula:

 .. math::

     Y' = \frac{219}{256} * (0.2990R' + 0.5870G' + 0.1140B') + \frac{16}{256}

     Cb = \frac{224}{256} * (-0.1687R' - 0.3313G' + 0.5B')

     Cr = \frac{224}{256} * (0.5R' - 0.4187G' - 0.0813B')

 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
 [-0.5…0.5] and quantized without further scaling or offsets.
 The non-standard xvYCC 709 or xvYCC 601 encodings can be
 used by selecting ``V4L2_YCBCR_ENC_XV709`` or ``V4L2_YCBCR_ENC_XV601``.
 As seen by the xvYCC formulas these encodings always use limited range quantization,
 there is no full range variant. The whole point of these extended gamut encodings
 is that values outside the limited range are still valid, although they
 map to R', G' and B' values outside the [0…1] range and are therefore outside
 the Rec. 709 colorspace gamut.


 .. _col-srgb:

 Colorspace sRGB (V4L2_COLORSPACE_SRGB)
 ======================================

 The :ref:`srgb` standard defines the colorspace used by most webcams
 and computer graphics. The default transfer function is
 ``V4L2_XFER_FUNC_SRGB``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited range.

 Note that the :ref:`sycc` standard specifies full range quantization,
 however all current capture hardware supported by the kernel convert
 R'G'B' to limited range Y'CbCr. So choosing full range as the default
 would break how applications interpret the quantization range.

 The chromaticities of the primary colors and the white reference are:

 .. flat-table:: sRGB Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.640
       - 0.330
     * - Green
       - 0.300
       - 0.600
     * - Blue
       - 0.150
       - 0.060
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 These chromaticities are identical to the Rec. 709 colorspace.

 Transfer function. Note that negative values for L are only used by the
 Y'CbCr conversion.

 .. math::

     L' = -1.055(-L)^{\frac{1}{2.4} } + 0.055\text{, for }L < -0.0031308

     L' = 12.92L\text{, for }-0.0031308 \le L \le 0.0031308

     L' = 1.055L ^{\frac{1}{2.4} } - 0.055\text{, for }0.0031308 < L \le 1

 Inverse Transfer function:

 .. math::

     L = -((-L' + 0.055) / 1.055) ^{2.4}\text{, for }L' < -0.04045

     L = L' / 12.92\text{, for }-0.04045 \le L' \le 0.04045

     L = ((L' + 0.055) / 1.055) ^{2.4}\text{, for }L' > 0.04045

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_601`` encoding as defined by :ref:`sycc`:

 .. math::

     Y' = 0.2990R' + 0.5870G' + 0.1140B'

     Cb = -0.1687R' - 0.3313G' + 0.5B'

     Cr = 0.5R' - 0.4187G' - 0.0813B'

 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.


 .. _col-oprgb:

 Colorspace opRGB (V4L2_COLORSPACE_OPRGB)
 ===============================================

 The :ref:`oprgb` standard defines the colorspace used by computer
 graphics that use the opRGB colorspace. The default transfer function is
 ``V4L2_XFER_FUNC_OPRGB``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
 range.

 Note that the :ref:`oprgb` standard specifies full range quantization,
 however all current capture hardware supported by the kernel convert
 R'G'B' to limited range Y'CbCr. So choosing full range as the default
 would break how applications interpret the quantization range.

 The chromaticities of the primary colors and the white reference are:

 .. flat-table:: opRGB Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.6400
       - 0.3300
     * - Green
       - 0.2100
       - 0.7100
     * - Blue
       - 0.1500
       - 0.0600
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 Transfer function:

 .. math::

     L' = L ^{\frac{1}{2.19921875}}

 Inverse Transfer function:

 .. math::

     L = L'^{(2.19921875)}

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_601`` encoding:

 .. math::

     Y' = 0.2990R' + 0.5870G' + 0.1140B'

     Cb = -0.1687R' - 0.3313G' + 0.5B'

     Cr = 0.5R' - 0.4187G' - 0.0813B'

 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.


 .. _col-bt2020:

 Colorspace BT.2020 (V4L2_COLORSPACE_BT2020)
 ===========================================

 The :ref:`itu2020` standard defines the colorspace used by Ultra-high
 definition television (UHDTV). The default transfer function is
 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_BT2020``. The default Y'CbCr quantization is limited range.
 The chromaticities of the primary colors and the white reference are:

 .. flat-table:: BT.2020 Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.708
       - 0.292
     * - Green
       - 0.170
       - 0.797
     * - Blue
       - 0.131
       - 0.046
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 Transfer function (same as Rec. 709):

 .. math::

     L' = 4.5L\text{, for }0 \le L < 0.018

     L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

 Inverse Transfer function:

 .. math::

     L = L' / 4.5\text{, for } L' < 0.081

     L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

 Please note that while Rec. 709 is defined as the default transfer function
 by the :ref:`itu2020` standard, in practice this colorspace is often used
 with the :ref:`xf-smpte-2084`. In particular Ultra HD Blu-ray discs use
 this combination.

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_BT2020`` encoding:

 .. math::

     Y' = 0.2627R' + 0.6780G' + 0.0593B'

     Cb = -0.1396R' - 0.3604G' + 0.5B'

     Cr = 0.5R' - 0.4598G' - 0.0402B'

 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.

 There is also an alternate constant luminance R'G'B' to Yc'CbcCrc
 (``V4L2_YCBCR_ENC_BT2020_CONST_LUM``) encoding:

 Luma:

 .. math::
     :nowrap:

     \begin{align*}
     Yc' = (0.2627R + 0.6780G + 0.0593B)'& \\
     B' - Yc' \le 0:& \\
         &Cbc = (B' - Yc') / 1.9404 \\
     B' - Yc' > 0: & \\
         &Cbc = (B' - Yc') / 1.5816 \\
     R' - Yc' \le 0:& \\
         &Crc = (R' - Y') / 1.7184 \\
     R' - Yc' > 0:& \\
         &Crc = (R' - Y') / 0.9936
     \end{align*}

 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.


 .. _col-dcip3:

 Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3)
 ==========================================

 The :ref:`smpte431` standard defines the colorspace used by cinema
 projectors that use the DCI-P3 colorspace. The default transfer function
 is ``V4L2_XFER_FUNC_DCI_P3``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_709``. The default Y'CbCr quantization is limited range.

 .. note::

    Note that this colorspace standard does not specify a
    Y'CbCr encoding since it is not meant to be encoded to Y'CbCr. So this
    default Y'CbCr encoding was picked because it is the HDTV encoding.

 The chromaticities of the primary colors and the white reference are:


 .. flat-table:: DCI-P3 Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.6800
       - 0.3200
     * - Green
       - 0.2650
       - 0.6900
     * - Blue
       - 0.1500
       - 0.0600
     * - White Reference
       - 0.3140
       - 0.3510


 Transfer function:

 .. math::

     L' = L^{\frac{1}{2.6}}

 Inverse Transfer function:

 .. math::

     L = L'^{(2.6)}

 Y'CbCr encoding is not specified. V4L2 defaults to Rec. 709.


 .. _col-smpte-240m:

 Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M)
 =================================================

 The :ref:`smpte240m` standard was an interim standard used during the
 early days of HDTV (1988-1998). It has been superseded by Rec. 709. The
 default transfer function is ``V4L2_XFER_FUNC_SMPTE240M``. The default
 Y'CbCr encoding is ``V4L2_YCBCR_ENC_SMPTE240M``. The default Y'CbCr
 quantization is limited range. The chromaticities of the primary colors
 and the white reference are:


 .. flat-table:: SMPTE 240M Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.630
       - 0.340
     * - Green
       - 0.310
       - 0.595
     * - Blue
       - 0.155
       - 0.070
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 These chromaticities are identical to the SMPTE 170M colorspace.

 Transfer function:

 .. math::

     L' = 4L\text{, for } 0 \le L < 0.0228

     L' = 1.1115L ^{0.45} - 0.1115\text{, for } 0.0228 \le L \le 1

 Inverse Transfer function:

 .. math::

     L = \frac{L'}{4}\text{, for } 0 \le L' < 0.0913

     L = \left( \frac{L' + 0.1115}{1.1115}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.0913

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_SMPTE240M`` encoding:

 .. math::

     Y' = 0.2122R' + 0.7013G' + 0.0865B'

     Cb = -0.1161R' - 0.3839G' + 0.5B'

     Cr = 0.5R' - 0.4451G' - 0.0549B'

 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.


 .. _col-sysm:

 Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M)
 ===================================================

 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 transfer function is ``V4L2_XFER_FUNC_709``. The default Y'CbCr
 encoding is ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is
 limited range. The chromaticities of the primary colors and the white
 reference are:


 .. flat-table:: NTSC 1953 Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.67
       - 0.33
     * - Green
       - 0.21
       - 0.71
     * - Blue
       - 0.14
       - 0.08
     * - White Reference (C)
       - 0.310
       - 0.316


 .. note::

    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.

 The transfer function was never properly defined for NTSC 1953. The Rec.
 709 transfer function is recommended in the literature:

 .. math::

     L' = 4.5L\text{, for } 0 \le L < 0.018

     L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

 Inverse Transfer function:

 .. math::

     L = \frac{L'}{4.5} \text{, for } L' < 0.081

     L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_601`` encoding:

 .. math::

     Y' = 0.2990R' + 0.5870G' + 0.1140B'

     Cb = -0.1687R' - 0.3313G' + 0.5B'

     Cr = 0.5R' - 0.4187G' - 0.0813B'

 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.


 .. _col-sysbg:

 Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG)
 =========================================================

 The :ref:`tech3213` standard defines the colorspace used by PAL/SECAM
 in 1975. Note that this colorspace is not supported by the HDMI interface.
 Instead :ref:`tech3321` recommends that Rec. 709 is used instead for HDMI.
 The default transfer function is
 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
 range. The chromaticities of the primary colors and the white reference
 are:


 .. flat-table:: EBU Tech. 3213 Chromaticities
     :header-rows:  1
     :stub-columns: 0
     :widths:       1 1 2

     * - Color
       - x
       - y
     * - Red
       - 0.64
       - 0.33
     * - Green
       - 0.29
       - 0.60
     * - Blue
       - 0.15
       - 0.06
     * - White Reference (D65)
       - 0.3127
       - 0.3290


 The transfer function was never properly defined for this colorspace.
 The Rec. 709 transfer function is recommended in the literature:

 .. math::

     L' = 4.5L\text{, for } 0 \le L < 0.018

     L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

 Inverse Transfer function:

 .. math::

     L = \frac{L'}{4.5} \text{, for } L' < 0.081

     L = \left(\frac{L' + 0.099}{1.099} \right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_601`` encoding:

 .. math::

     Y' = 0.2990R' + 0.5870G' + 0.1140B'

     Cb = -0.1687R' - 0.3313G' + 0.5B'

     Cr = 0.5R' - 0.4187G' - 0.0813B'

 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.


 .. _col-jpeg:

 Colorspace JPEG (V4L2_COLORSPACE_JPEG)
 ======================================

 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 transfer function use is
 ``V4L2_XFER_FUNC_SRGB``. The Y'CbCr encoding is ``V4L2_YCBCR_ENC_601``
 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].

 .. note::

    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
    ``V4L2_COLORSPACE_JPEG`` can be considered to be an abbreviation for
    ``V4L2_COLORSPACE_SRGB``, ``V4L2_XFER_FUNC_SRGB``, ``V4L2_YCBCR_ENC_601``
    and ``V4L2_QUANTIZATION_FULL_RANGE``.

 ***************************************
 Detailed Transfer Function Descriptions
 ***************************************

 .. _xf-smpte-2084:

 Transfer Function SMPTE 2084 (V4L2_XFER_FUNC_SMPTE2084)
 =======================================================

 The :ref:`smpte2084` standard defines the transfer function used by
 High Dynamic Range content.

 Constants:
     m1 = (2610 / 4096) / 4

     m2 = (2523 / 4096) * 128

     c1 = 3424 / 4096

     c2 = (2413 / 4096) * 32

     c3 = (2392 / 4096) * 32

 Transfer function:
     L' = ((c1 + c2 * L\ :sup:`m1`) / (1 + c3 * L\ :sup:`m1`))\ :sup:`m2`

 Inverse Transfer function:
     L = (max(L':sup:`1/m2` - c1, 0) / (c2 - c3 *
     L'\ :sup:`1/m2`))\ :sup:`1/m1`

 Take care when converting between this transfer function and non-HDR transfer
 functions: the linear RGB values [0…1] of HDR content map to a luminance range
 of 0 to 10000 cd/m\ :sup:`2` whereas the linear RGB values of non-HDR (aka
 Standard Dynamic Range or SDR) map to a luminance range of 0 to 100 cd/m\ :sup:`2`.

 To go from SDR to HDR you will have to divide L by 100 first. To go in the other
 direction you will have to multiply L by 100. Of course, this clamps all
 luminance values over 100 cd/m\ :sup:`2` to 100 cd/m\ :sup:`2`.

 There are better methods, see e.g. :ref:`colimg` for more in-depth information
 about this.
	.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

	********************************
	Detailed Colorspace Descriptions
	********************************


	.. _col-smpte-170m:

	Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M)
	=================================================

	The :ref:`smpte170m` standard defines the colorspace used by NTSC and
	PAL and by SDTV in general. The default transfer function is
	``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
	range. The chromaticities of the primary colors and the white reference
	are:

	.. flat-table:: SMPTE 170M Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.630
	- 0.340
	* - Green
	- 0.310
	- 0.595
	* - Blue
	- 0.155
	- 0.070
	* - White Reference (D65)
	- 0.3127
	- 0.3290


	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.

	The transfer function defined for SMPTE 170M is the same as the one
	defined in Rec. 709.

	.. math::

	L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le-0.018

	L' = 4.5L \text{, for } -0.018 < L < 0.018

	L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018

	Inverse Transfer function:

	.. math::

	L = -\left( \frac{L' - 0.099}{-1.099} \right) ^{\frac{1}{0.45}} \text{, for } L' \le -0.081

	L = \frac{L'}{4.5} \text{, for } -0.081 < L' < 0.081

	L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_601`` encoding:

	.. math::

	Y' = 0.2990R' + 0.5870G' + 0.1140B'

	Cb = -0.1687R' - 0.3313G' + 0.5B'

	Cr = 0.5R' - 0.4187G' - 0.0813B'

	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 :ref:`itu601` standard and this colorspace is sometimes called
	BT.601 as well, even though BT.601 does not mention any color primaries.

	The default quantization is limited range, but full range is possible
	although rarely seen.


	.. _col-rec709:

	Colorspace Rec. 709 (V4L2_COLORSPACE_REC709)
	============================================

	The :ref:`itu709` standard defines the colorspace used by HDTV in
	general. The default transfer function is ``V4L2_XFER_FUNC_709``. The
	default Y'CbCr encoding is ``V4L2_YCBCR_ENC_709``. The default Y'CbCr
	quantization is limited range. The chromaticities of the primary colors
	and the white reference are:

	.. flat-table:: Rec. 709 Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.640
	- 0.330
	* - Green
	- 0.300
	- 0.600
	* - Blue
	- 0.150
	- 0.060
	* - White Reference (D65)
	- 0.3127
	- 0.3290


	The full name of this standard is Rec. ITU-R BT.709-5.

	Transfer function. Normally L is in the range [0…1], but for the
	extended gamut xvYCC encoding values outside that range are allowed.

	.. math::

	L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le -0.018

	L' = 4.5L \text{, for } -0.018 < L < 0.018

	L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018

	Inverse Transfer function:

	.. math::

	L = -\left( \frac{L' - 0.099}{-1.099} \right)^\frac{1}{0.45} \text{, for } L' \le -0.081

	L = \frac{L'}{4.5}\text{, for } -0.081 < L' < 0.081

	L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_709`` encoding:

	.. math::

	Y' = 0.2126R' + 0.7152G' + 0.0722B'

	Cb = -0.1146R' - 0.3854G' + 0.5B'

	Cr = 0.5R' - 0.4542G' - 0.0458B'

	Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
	[-0.5…0.5].

	The default quantization is limited range, but full range is possible
	although rarely seen.

	The ``V4L2_YCBCR_ENC_709`` encoding described above is the default for
	this colorspace, but it can be overridden with ``V4L2_YCBCR_ENC_601``,
	in which case the BT.601 Y'CbCr encoding is used.

	Two additional extended gamut Y'CbCr encodings are also possible with
	this colorspace:

	The xvYCC 709 encoding (``V4L2_YCBCR_ENC_XV709``, :ref:`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 according to the limited range formula:

	.. math::

	Y' = \frac{219}{256} * (0.2126R' + 0.7152G' + 0.0722B') + \frac{16}{256}

	Cb = \frac{224}{256} * (-0.1146R' - 0.3854G' + 0.5B')

	Cr = \frac{224}{256} * (0.5R' - 0.4542G' - 0.0458B')

	The xvYCC 601 encoding (``V4L2_YCBCR_ENC_XV601``, :ref:`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 according to the limited range formula:

	.. math::

	Y' = \frac{219}{256} * (0.2990R' + 0.5870G' + 0.1140B') + \frac{16}{256}

	Cb = \frac{224}{256} * (-0.1687R' - 0.3313G' + 0.5B')

	Cr = \frac{224}{256} * (0.5R' - 0.4187G' - 0.0813B')

	Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
	[-0.5…0.5] and quantized without further scaling or offsets.
	The non-standard xvYCC 709 or xvYCC 601 encodings can be
	used by selecting ``V4L2_YCBCR_ENC_XV709`` or ``V4L2_YCBCR_ENC_XV601``.
	As seen by the xvYCC formulas these encodings always use limited range quantization,
	there is no full range variant. The whole point of these extended gamut encodings
	is that values outside the limited range are still valid, although they
	map to R', G' and B' values outside the [0…1] range and are therefore outside
	the Rec. 709 colorspace gamut.


	.. _col-srgb:

	Colorspace sRGB (V4L2_COLORSPACE_SRGB)
	======================================

	The :ref:`srgb` standard defines the colorspace used by most webcams
	and computer graphics. The default transfer function is
	``V4L2_XFER_FUNC_SRGB``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited range.

	Note that the :ref:`sycc` standard specifies full range quantization,
	however all current capture hardware supported by the kernel convert
	R'G'B' to limited range Y'CbCr. So choosing full range as the default
	would break how applications interpret the quantization range.

	The chromaticities of the primary colors and the white reference are:

	.. flat-table:: sRGB Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.640
	- 0.330
	* - Green
	- 0.300
	- 0.600
	* - Blue
	- 0.150
	- 0.060
	* - White Reference (D65)
	- 0.3127
	- 0.3290


	These chromaticities are identical to the Rec. 709 colorspace.

	Transfer function. Note that negative values for L are only used by the
	Y'CbCr conversion.

	.. math::

	L' = -1.055(-L)^{\frac{1}{2.4} } + 0.055\text{, for }L < -0.0031308

	L' = 12.92L\text{, for }-0.0031308 \le L \le 0.0031308

	L' = 1.055L ^{\frac{1}{2.4} } - 0.055\text{, for }0.0031308 < L \le 1

	Inverse Transfer function:

	.. math::

	L = -((-L' + 0.055) / 1.055) ^{2.4}\text{, for }L' < -0.04045

	L = L' / 12.92\text{, for }-0.04045 \le L' \le 0.04045

	L = ((L' + 0.055) / 1.055) ^{2.4}\text{, for }L' > 0.04045

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_601`` encoding as defined by :ref:`sycc`:

	.. math::

	Y' = 0.2990R' + 0.5870G' + 0.1140B'

	Cb = -0.1687R' - 0.3313G' + 0.5B'

	Cr = 0.5R' - 0.4187G' - 0.0813B'

	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.


	.. _col-oprgb:

	Colorspace opRGB (V4L2_COLORSPACE_OPRGB)
	===============================================

	The :ref:`oprgb` standard defines the colorspace used by computer
	graphics that use the opRGB colorspace. The default transfer function is
	``V4L2_XFER_FUNC_OPRGB``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
	range.

	Note that the :ref:`oprgb` standard specifies full range quantization,
	however all current capture hardware supported by the kernel convert
	R'G'B' to limited range Y'CbCr. So choosing full range as the default
	would break how applications interpret the quantization range.

	The chromaticities of the primary colors and the white reference are:

	.. flat-table:: opRGB Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.6400
	- 0.3300
	* - Green
	- 0.2100
	- 0.7100
	* - Blue
	- 0.1500
	- 0.0600
	* - White Reference (D65)
	- 0.3127
	- 0.3290



	Transfer function:

	.. math::

	L' = L ^{\frac{1}{2.19921875}}

	Inverse Transfer function:

	.. math::

	L = L'^{(2.19921875)}

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_601`` encoding:

	.. math::

	Y' = 0.2990R' + 0.5870G' + 0.1140B'

	Cb = -0.1687R' - 0.3313G' + 0.5B'

	Cr = 0.5R' - 0.4187G' - 0.0813B'

	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.


	.. _col-bt2020:

	Colorspace BT.2020 (V4L2_COLORSPACE_BT2020)
	===========================================

	The :ref:`itu2020` standard defines the colorspace used by Ultra-high
	definition television (UHDTV). The default transfer function is
	``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_BT2020``. The default Y'CbCr quantization is limited range.
	The chromaticities of the primary colors and the white reference are:

	.. flat-table:: BT.2020 Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.708
	- 0.292
	* - Green
	- 0.170
	- 0.797
	* - Blue
	- 0.131
	- 0.046
	* - White Reference (D65)
	- 0.3127
	- 0.3290



	Transfer function (same as Rec. 709):

	.. math::

	L' = 4.5L\text{, for }0 \le L < 0.018

	L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

	Inverse Transfer function:

	.. math::

	L = L' / 4.5\text{, for } L' < 0.081

	L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

	Please note that while Rec. 709 is defined as the default transfer function
	by the :ref:`itu2020` standard, in practice this colorspace is often used
	with the :ref:`xf-smpte-2084`. In particular Ultra HD Blu-ray discs use
	this combination.

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_BT2020`` encoding:

	.. math::

	Y' = 0.2627R' + 0.6780G' + 0.0593B'

	Cb = -0.1396R' - 0.3604G' + 0.5B'

	Cr = 0.5R' - 0.4598G' - 0.0402B'

	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.

	There is also an alternate constant luminance R'G'B' to Yc'CbcCrc
	(``V4L2_YCBCR_ENC_BT2020_CONST_LUM``) encoding:

	Luma:

	.. math::
	:nowrap:

	\begin{align*}
	Yc' = (0.2627R + 0.6780G + 0.0593B)'& \\
	B' - Yc' \le 0:& \\
	&Cbc = (B' - Yc') / 1.9404 \\
	B' - Yc' > 0: & \\
	&Cbc = (B' - Yc') / 1.5816 \\
	R' - Yc' \le 0:& \\
	&Crc = (R' - Y') / 1.7184 \\
	R' - Yc' > 0:& \\
	&Crc = (R' - Y') / 0.9936
	\end{align*}

	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.


	.. _col-dcip3:

	Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3)
	==========================================

	The :ref:`smpte431` standard defines the colorspace used by cinema
	projectors that use the DCI-P3 colorspace. The default transfer function
	is ``V4L2_XFER_FUNC_DCI_P3``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_709``. The default Y'CbCr quantization is limited range.

	.. note::

	Note that this colorspace standard does not specify a
	Y'CbCr encoding since it is not meant to be encoded to Y'CbCr. So this
	default Y'CbCr encoding was picked because it is the HDTV encoding.

	The chromaticities of the primary colors and the white reference are:


	.. flat-table:: DCI-P3 Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.6800
	- 0.3200
	* - Green
	- 0.2650
	- 0.6900
	* - Blue
	- 0.1500
	- 0.0600
	* - White Reference
	- 0.3140
	- 0.3510



	Transfer function:

	.. math::

	L' = L^{\frac{1}{2.6}}

	Inverse Transfer function:

	.. math::

	L = L'^{(2.6)}

	Y'CbCr encoding is not specified. V4L2 defaults to Rec. 709.


	.. _col-smpte-240m:

	Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M)
	=================================================

	The :ref:`smpte240m` standard was an interim standard used during the
	early days of HDTV (1988-1998). It has been superseded by Rec. 709. The
	default transfer function is ``V4L2_XFER_FUNC_SMPTE240M``. The default
	Y'CbCr encoding is ``V4L2_YCBCR_ENC_SMPTE240M``. The default Y'CbCr
	quantization is limited range. The chromaticities of the primary colors
	and the white reference are:


	.. flat-table:: SMPTE 240M Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.630
	- 0.340
	* - Green
	- 0.310
	- 0.595
	* - Blue
	- 0.155
	- 0.070
	* - White Reference (D65)
	- 0.3127
	- 0.3290


	These chromaticities are identical to the SMPTE 170M colorspace.

	Transfer function:

	.. math::

	L' = 4L\text{, for } 0 \le L < 0.0228

	L' = 1.1115L ^{0.45} - 0.1115\text{, for } 0.0228 \le L \le 1

	Inverse Transfer function:

	.. math::

	L = \frac{L'}{4}\text{, for } 0 \le L' < 0.0913

	L = \left( \frac{L' + 0.1115}{1.1115}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.0913

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_SMPTE240M`` encoding:

	.. math::

	Y' = 0.2122R' + 0.7013G' + 0.0865B'

	Cb = -0.1161R' - 0.3839G' + 0.5B'

	Cr = 0.5R' - 0.4451G' - 0.0549B'

	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.


	.. _col-sysm:

	Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M)
	===================================================

	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 transfer function is ``V4L2_XFER_FUNC_709``. The default Y'CbCr
	encoding is ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is
	limited range. The chromaticities of the primary colors and the white
	reference are:


	.. flat-table:: NTSC 1953 Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.67
	- 0.33
	* - Green
	- 0.21
	- 0.71
	* - Blue
	- 0.14
	- 0.08
	* - White Reference (C)
	- 0.310
	- 0.316


	.. note::

	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.

	The transfer function was never properly defined for NTSC 1953. The Rec.
	709 transfer function is recommended in the literature:

	.. math::

	L' = 4.5L\text{, for } 0 \le L < 0.018

	L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

	Inverse Transfer function:

	.. math::

	L = \frac{L'}{4.5} \text{, for } L' < 0.081

	L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_601`` encoding:

	.. math::

	Y' = 0.2990R' + 0.5870G' + 0.1140B'

	Cb = -0.1687R' - 0.3313G' + 0.5B'

	Cr = 0.5R' - 0.4187G' - 0.0813B'

	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.


	.. _col-sysbg:

	Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG)
	=========================================================

	The :ref:`tech3213` standard defines the colorspace used by PAL/SECAM
	in 1975. Note that this colorspace is not supported by the HDMI interface.
	Instead :ref:`tech3321` recommends that Rec. 709 is used instead for HDMI.
	The default transfer function is
	``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
	``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
	range. The chromaticities of the primary colors and the white reference
	are:


	.. flat-table:: EBU Tech. 3213 Chromaticities
	:header-rows: 1
	:stub-columns: 0
	:widths: 1 1 2

	* - Color
	- x
	- y
	* - Red
	- 0.64
	- 0.33
	* - Green
	- 0.29
	- 0.60
	* - Blue
	- 0.15
	- 0.06
	* - White Reference (D65)
	- 0.3127
	- 0.3290



	The transfer function was never properly defined for this colorspace.
	The Rec. 709 transfer function is recommended in the literature:

	.. math::

	L' = 4.5L\text{, for } 0 \le L < 0.018

	L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1

	Inverse Transfer function:

	.. math::

	L = \frac{L'}{4.5} \text{, for } L' < 0.081

	L = \left(\frac{L' + 0.099}{1.099} \right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081

	The luminance (Y') and color difference (Cb and Cr) are obtained with
	the following ``V4L2_YCBCR_ENC_601`` encoding:

	.. math::

	Y' = 0.2990R' + 0.5870G' + 0.1140B'

	Cb = -0.1687R' - 0.3313G' + 0.5B'

	Cr = 0.5R' - 0.4187G' - 0.0813B'

	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.


	.. _col-jpeg:

	Colorspace JPEG (V4L2_COLORSPACE_JPEG)
	======================================

	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 transfer function use is
	``V4L2_XFER_FUNC_SRGB``. The Y'CbCr encoding is ``V4L2_YCBCR_ENC_601``
	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].

	.. note::

	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
	``V4L2_COLORSPACE_JPEG`` can be considered to be an abbreviation for
	``V4L2_COLORSPACE_SRGB``, ``V4L2_XFER_FUNC_SRGB``, ``V4L2_YCBCR_ENC_601``
	and ``V4L2_QUANTIZATION_FULL_RANGE``.

	***************************************
	Detailed Transfer Function Descriptions
	***************************************

	.. _xf-smpte-2084:

	Transfer Function SMPTE 2084 (V4L2_XFER_FUNC_SMPTE2084)
	=======================================================

	The :ref:`smpte2084` standard defines the transfer function used by
	High Dynamic Range content.

	Constants:
	m1 = (2610 / 4096) / 4

	m2 = (2523 / 4096) * 128

	c1 = 3424 / 4096

	c2 = (2413 / 4096) * 32

	c3 = (2392 / 4096) * 32

	Transfer function:
	L' = ((c1 + c2 * L\ :sup:`m1`) / (1 + c3 * L\ :sup:`m1`))\ :sup:`m2`

	Inverse Transfer function:
	L = (max(L':sup:`1/m2` - c1, 0) / (c2 - c3 *
	L'\ :sup:`1/m2`))\ :sup:`1/m1`

	Take care when converting between this transfer function and non-HDR transfer
	functions: the linear RGB values [0…1] of HDR content map to a luminance range
	of 0 to 10000 cd/m\ :sup:`2` whereas the linear RGB values of non-HDR (aka
	Standard Dynamic Range or SDR) map to a luminance range of 0 to 100 cd/m\ :sup:`2`.

	To go from SDR to HDR you will have to divide L by 100 first. To go in the other
	direction you will have to multiply L by 100. Of course, this clamps all
	luminance values over 100 cd/m\ :sup:`2` to 100 cd/m\ :sup:`2`.

	There are better methods, see e.g. :ref:`colimg` for more in-depth information
	about this.