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diff --git a/Documentation/media/uapi/v4l/colorspaces-details.rst b/Documentation/media/uapi/v4l/colorspaces-details.rst new file mode 100644 index 000000000000..b5d551b9cc8f --- /dev/null +++ b/Documentation/media/uapi/v4l/colorspaces-details.rst @@ -0,0 +1,807 @@ +.. -*- coding: utf-8; mode: rst -*- + +******************************** +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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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-adobergb: + +Colorspace Adobe RGB (V4L2_COLORSPACE_ADOBERGB) +=============================================== + +The :ref:`adobergb` standard defines the colorspace used by computer +graphics that use the AdobeRGB colorspace. This is also known as the +:ref:`oprgb` standard. The default transfer function is +``V4L2_XFER_FUNC_ADOBERGB``. 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: + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. flat-table:: Adobe RGB 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 R'G'B' quantization is limited +range (!), and so is the default Y'CbCr quantization. The chromaticities +of the primary colors and the white reference are: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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. 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: + + + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| + +.. 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_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. |