0001 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
0002
0003 ********************************
0004 Detailed Colorspace Descriptions
0005 ********************************
0006
0007
0008 .. _col-smpte-170m:
0009
0010 Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M)
0011 =================================================
0012
0013 The :ref:`smpte170m` standard defines the colorspace used by NTSC and
0014 PAL and by SDTV in general. The default transfer function is
0015 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
0016 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
0017 range. The chromaticities of the primary colors and the white reference
0018 are:
0019
0020 .. flat-table:: SMPTE 170M Chromaticities
0021 :header-rows: 1
0022 :stub-columns: 0
0023 :widths: 1 1 2
0024
0025 * - Color
0026 - x
0027 - y
0028 * - Red
0029 - 0.630
0030 - 0.340
0031 * - Green
0032 - 0.310
0033 - 0.595
0034 * - Blue
0035 - 0.155
0036 - 0.070
0037 * - White Reference (D65)
0038 - 0.3127
0039 - 0.3290
0040
0041
0042 The red, green and blue chromaticities are also often referred to as the
0043 SMPTE C set, so this colorspace is sometimes called SMPTE C as well.
0044
0045 The transfer function defined for SMPTE 170M is the same as the one
0046 defined in Rec. 709.
0047
0048 .. math::
0049
0050 L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le-0.018
0051
0052 L' = 4.5L \text{, for } -0.018 < L < 0.018
0053
0054 L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018
0055
0056 Inverse Transfer function:
0057
0058 .. math::
0059
0060 L = -\left( \frac{L' - 0.099}{-1.099} \right) ^{\frac{1}{0.45}} \text{, for } L' \le -0.081
0061
0062 L = \frac{L'}{4.5} \text{, for } -0.081 < L' < 0.081
0063
0064 L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081
0065
0066 The luminance (Y') and color difference (Cb and Cr) are obtained with
0067 the following ``V4L2_YCBCR_ENC_601`` encoding:
0068
0069 .. math::
0070
0071 Y' = 0.2990R' + 0.5870G' + 0.1140B'
0072
0073 Cb = -0.1687R' - 0.3313G' + 0.5B'
0074
0075 Cr = 0.5R' - 0.4187G' - 0.0813B'
0076
0077 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0078 [-0.5…0.5]. This conversion to Y'CbCr is identical to the one defined in
0079 the :ref:`itu601` standard and this colorspace is sometimes called
0080 BT.601 as well, even though BT.601 does not mention any color primaries.
0081
0082 The default quantization is limited range, but full range is possible
0083 although rarely seen.
0084
0085
0086 .. _col-rec709:
0087
0088 Colorspace Rec. 709 (V4L2_COLORSPACE_REC709)
0089 ============================================
0090
0091 The :ref:`itu709` standard defines the colorspace used by HDTV in
0092 general. The default transfer function is ``V4L2_XFER_FUNC_709``. The
0093 default Y'CbCr encoding is ``V4L2_YCBCR_ENC_709``. The default Y'CbCr
0094 quantization is limited range. The chromaticities of the primary colors
0095 and the white reference are:
0096
0097 .. flat-table:: Rec. 709 Chromaticities
0098 :header-rows: 1
0099 :stub-columns: 0
0100 :widths: 1 1 2
0101
0102 * - Color
0103 - x
0104 - y
0105 * - Red
0106 - 0.640
0107 - 0.330
0108 * - Green
0109 - 0.300
0110 - 0.600
0111 * - Blue
0112 - 0.150
0113 - 0.060
0114 * - White Reference (D65)
0115 - 0.3127
0116 - 0.3290
0117
0118
0119 The full name of this standard is Rec. ITU-R BT.709-5.
0120
0121 Transfer function. Normally L is in the range [0…1], but for the
0122 extended gamut xvYCC encoding values outside that range are allowed.
0123
0124 .. math::
0125
0126 L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le -0.018
0127
0128 L' = 4.5L \text{, for } -0.018 < L < 0.018
0129
0130 L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018
0131
0132 Inverse Transfer function:
0133
0134 .. math::
0135
0136 L = -\left( \frac{L' - 0.099}{-1.099} \right)^\frac{1}{0.45} \text{, for } L' \le -0.081
0137
0138 L = \frac{L'}{4.5}\text{, for } -0.081 < L' < 0.081
0139
0140 L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081
0141
0142 The luminance (Y') and color difference (Cb and Cr) are obtained with
0143 the following ``V4L2_YCBCR_ENC_709`` encoding:
0144
0145 .. math::
0146
0147 Y' = 0.2126R' + 0.7152G' + 0.0722B'
0148
0149 Cb = -0.1146R' - 0.3854G' + 0.5B'
0150
0151 Cr = 0.5R' - 0.4542G' - 0.0458B'
0152
0153 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0154 [-0.5…0.5].
0155
0156 The default quantization is limited range, but full range is possible
0157 although rarely seen.
0158
0159 The ``V4L2_YCBCR_ENC_709`` encoding described above is the default for
0160 this colorspace, but it can be overridden with ``V4L2_YCBCR_ENC_601``,
0161 in which case the BT.601 Y'CbCr encoding is used.
0162
0163 Two additional extended gamut Y'CbCr encodings are also possible with
0164 this colorspace:
0165
0166 The xvYCC 709 encoding (``V4L2_YCBCR_ENC_XV709``, :ref:`xvycc`) is
0167 similar to the Rec. 709 encoding, but it allows for R', G' and B' values
0168 that are outside the range [0…1]. The resulting Y', Cb and Cr values are
0169 scaled and offset according to the limited range formula:
0170
0171 .. math::
0172
0173 Y' = \frac{219}{256} * (0.2126R' + 0.7152G' + 0.0722B') + \frac{16}{256}
0174
0175 Cb = \frac{224}{256} * (-0.1146R' - 0.3854G' + 0.5B')
0176
0177 Cr = \frac{224}{256} * (0.5R' - 0.4542G' - 0.0458B')
0178
0179 The xvYCC 601 encoding (``V4L2_YCBCR_ENC_XV601``, :ref:`xvycc`) is
0180 similar to the BT.601 encoding, but it allows for R', G' and B' values
0181 that are outside the range [0…1]. The resulting Y', Cb and Cr values are
0182 scaled and offset according to the limited range formula:
0183
0184 .. math::
0185
0186 Y' = \frac{219}{256} * (0.2990R' + 0.5870G' + 0.1140B') + \frac{16}{256}
0187
0188 Cb = \frac{224}{256} * (-0.1687R' - 0.3313G' + 0.5B')
0189
0190 Cr = \frac{224}{256} * (0.5R' - 0.4187G' - 0.0813B')
0191
0192 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0193 [-0.5…0.5] and quantized without further scaling or offsets.
0194 The non-standard xvYCC 709 or xvYCC 601 encodings can be
0195 used by selecting ``V4L2_YCBCR_ENC_XV709`` or ``V4L2_YCBCR_ENC_XV601``.
0196 As seen by the xvYCC formulas these encodings always use limited range quantization,
0197 there is no full range variant. The whole point of these extended gamut encodings
0198 is that values outside the limited range are still valid, although they
0199 map to R', G' and B' values outside the [0…1] range and are therefore outside
0200 the Rec. 709 colorspace gamut.
0201
0202
0203 .. _col-srgb:
0204
0205 Colorspace sRGB (V4L2_COLORSPACE_SRGB)
0206 ======================================
0207
0208 The :ref:`srgb` standard defines the colorspace used by most webcams
0209 and computer graphics. The default transfer function is
0210 ``V4L2_XFER_FUNC_SRGB``. The default Y'CbCr encoding is
0211 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited range.
0212
0213 Note that the :ref:`sycc` standard specifies full range quantization,
0214 however all current capture hardware supported by the kernel convert
0215 R'G'B' to limited range Y'CbCr. So choosing full range as the default
0216 would break how applications interpret the quantization range.
0217
0218 The chromaticities of the primary colors and the white reference are:
0219
0220 .. flat-table:: sRGB Chromaticities
0221 :header-rows: 1
0222 :stub-columns: 0
0223 :widths: 1 1 2
0224
0225 * - Color
0226 - x
0227 - y
0228 * - Red
0229 - 0.640
0230 - 0.330
0231 * - Green
0232 - 0.300
0233 - 0.600
0234 * - Blue
0235 - 0.150
0236 - 0.060
0237 * - White Reference (D65)
0238 - 0.3127
0239 - 0.3290
0240
0241
0242 These chromaticities are identical to the Rec. 709 colorspace.
0243
0244 Transfer function. Note that negative values for L are only used by the
0245 Y'CbCr conversion.
0246
0247 .. math::
0248
0249 L' = -1.055(-L)^{\frac{1}{2.4} } + 0.055\text{, for }L < -0.0031308
0250
0251 L' = 12.92L\text{, for }-0.0031308 \le L \le 0.0031308
0252
0253 L' = 1.055L ^{\frac{1}{2.4} } - 0.055\text{, for }0.0031308 < L \le 1
0254
0255 Inverse Transfer function:
0256
0257 .. math::
0258
0259 L = -((-L' + 0.055) / 1.055) ^{2.4}\text{, for }L' < -0.04045
0260
0261 L = L' / 12.92\text{, for }-0.04045 \le L' \le 0.04045
0262
0263 L = ((L' + 0.055) / 1.055) ^{2.4}\text{, for }L' > 0.04045
0264
0265 The luminance (Y') and color difference (Cb and Cr) are obtained with
0266 the following ``V4L2_YCBCR_ENC_601`` encoding as defined by :ref:`sycc`:
0267
0268 .. math::
0269
0270 Y' = 0.2990R' + 0.5870G' + 0.1140B'
0271
0272 Cb = -0.1687R' - 0.3313G' + 0.5B'
0273
0274 Cr = 0.5R' - 0.4187G' - 0.0813B'
0275
0276 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0277 [-0.5…0.5]. This transform is identical to one defined in SMPTE
0278 170M/BT.601. The Y'CbCr quantization is limited range.
0279
0280
0281 .. _col-oprgb:
0282
0283 Colorspace opRGB (V4L2_COLORSPACE_OPRGB)
0284 ===============================================
0285
0286 The :ref:`oprgb` standard defines the colorspace used by computer
0287 graphics that use the opRGB colorspace. The default transfer function is
0288 ``V4L2_XFER_FUNC_OPRGB``. The default Y'CbCr encoding is
0289 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
0290 range.
0291
0292 Note that the :ref:`oprgb` standard specifies full range quantization,
0293 however all current capture hardware supported by the kernel convert
0294 R'G'B' to limited range Y'CbCr. So choosing full range as the default
0295 would break how applications interpret the quantization range.
0296
0297 The chromaticities of the primary colors and the white reference are:
0298
0299 .. flat-table:: opRGB Chromaticities
0300 :header-rows: 1
0301 :stub-columns: 0
0302 :widths: 1 1 2
0303
0304 * - Color
0305 - x
0306 - y
0307 * - Red
0308 - 0.6400
0309 - 0.3300
0310 * - Green
0311 - 0.2100
0312 - 0.7100
0313 * - Blue
0314 - 0.1500
0315 - 0.0600
0316 * - White Reference (D65)
0317 - 0.3127
0318 - 0.3290
0319
0320
0321
0322 Transfer function:
0323
0324 .. math::
0325
0326 L' = L ^{\frac{1}{2.19921875}}
0327
0328 Inverse Transfer function:
0329
0330 .. math::
0331
0332 L = L'^{(2.19921875)}
0333
0334 The luminance (Y') and color difference (Cb and Cr) are obtained with
0335 the following ``V4L2_YCBCR_ENC_601`` encoding:
0336
0337 .. math::
0338
0339 Y' = 0.2990R' + 0.5870G' + 0.1140B'
0340
0341 Cb = -0.1687R' - 0.3313G' + 0.5B'
0342
0343 Cr = 0.5R' - 0.4187G' - 0.0813B'
0344
0345 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0346 [-0.5…0.5]. This transform is identical to one defined in SMPTE
0347 170M/BT.601. The Y'CbCr quantization is limited range.
0348
0349
0350 .. _col-bt2020:
0351
0352 Colorspace BT.2020 (V4L2_COLORSPACE_BT2020)
0353 ===========================================
0354
0355 The :ref:`itu2020` standard defines the colorspace used by Ultra-high
0356 definition television (UHDTV). The default transfer function is
0357 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
0358 ``V4L2_YCBCR_ENC_BT2020``. The default Y'CbCr quantization is limited range.
0359 The chromaticities of the primary colors and the white reference are:
0360
0361 .. flat-table:: BT.2020 Chromaticities
0362 :header-rows: 1
0363 :stub-columns: 0
0364 :widths: 1 1 2
0365
0366 * - Color
0367 - x
0368 - y
0369 * - Red
0370 - 0.708
0371 - 0.292
0372 * - Green
0373 - 0.170
0374 - 0.797
0375 * - Blue
0376 - 0.131
0377 - 0.046
0378 * - White Reference (D65)
0379 - 0.3127
0380 - 0.3290
0381
0382
0383
0384 Transfer function (same as Rec. 709):
0385
0386 .. math::
0387
0388 L' = 4.5L\text{, for }0 \le L < 0.018
0389
0390 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
0391
0392 Inverse Transfer function:
0393
0394 .. math::
0395
0396 L = L' / 4.5\text{, for } L' < 0.081
0397
0398 L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
0399
0400 Please note that while Rec. 709 is defined as the default transfer function
0401 by the :ref:`itu2020` standard, in practice this colorspace is often used
0402 with the :ref:`xf-smpte-2084`. In particular Ultra HD Blu-ray discs use
0403 this combination.
0404
0405 The luminance (Y') and color difference (Cb and Cr) are obtained with
0406 the following ``V4L2_YCBCR_ENC_BT2020`` encoding:
0407
0408 .. math::
0409
0410 Y' = 0.2627R' + 0.6780G' + 0.0593B'
0411
0412 Cb = -0.1396R' - 0.3604G' + 0.5B'
0413
0414 Cr = 0.5R' - 0.4598G' - 0.0402B'
0415
0416 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0417 [-0.5…0.5]. The Y'CbCr quantization is limited range.
0418
0419 There is also an alternate constant luminance R'G'B' to Yc'CbcCrc
0420 (``V4L2_YCBCR_ENC_BT2020_CONST_LUM``) encoding:
0421
0422 Luma:
0423
0424 .. math::
0425 :nowrap:
0426
0427 \begin{align*}
0428 Yc' = (0.2627R + 0.6780G + 0.0593B)'& \\
0429 B' - Yc' \le 0:& \\
0430 &Cbc = (B' - Yc') / 1.9404 \\
0431 B' - Yc' > 0: & \\
0432 &Cbc = (B' - Yc') / 1.5816 \\
0433 R' - Yc' \le 0:& \\
0434 &Crc = (R' - Y') / 1.7184 \\
0435 R' - Yc' > 0:& \\
0436 &Crc = (R' - Y') / 0.9936
0437 \end{align*}
0438
0439 Yc' is clamped to the range [0…1] and Cbc and Crc are clamped to the
0440 range [-0.5…0.5]. The Yc'CbcCrc quantization is limited range.
0441
0442
0443 .. _col-dcip3:
0444
0445 Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3)
0446 ==========================================
0447
0448 The :ref:`smpte431` standard defines the colorspace used by cinema
0449 projectors that use the DCI-P3 colorspace. The default transfer function
0450 is ``V4L2_XFER_FUNC_DCI_P3``. The default Y'CbCr encoding is
0451 ``V4L2_YCBCR_ENC_709``. The default Y'CbCr quantization is limited range.
0452
0453 .. note::
0454
0455 Note that this colorspace standard does not specify a
0456 Y'CbCr encoding since it is not meant to be encoded to Y'CbCr. So this
0457 default Y'CbCr encoding was picked because it is the HDTV encoding.
0458
0459 The chromaticities of the primary colors and the white reference are:
0460
0461
0462 .. flat-table:: DCI-P3 Chromaticities
0463 :header-rows: 1
0464 :stub-columns: 0
0465 :widths: 1 1 2
0466
0467 * - Color
0468 - x
0469 - y
0470 * - Red
0471 - 0.6800
0472 - 0.3200
0473 * - Green
0474 - 0.2650
0475 - 0.6900
0476 * - Blue
0477 - 0.1500
0478 - 0.0600
0479 * - White Reference
0480 - 0.3140
0481 - 0.3510
0482
0483
0484
0485 Transfer function:
0486
0487 .. math::
0488
0489 L' = L^{\frac{1}{2.6}}
0490
0491 Inverse Transfer function:
0492
0493 .. math::
0494
0495 L = L'^{(2.6)}
0496
0497 Y'CbCr encoding is not specified. V4L2 defaults to Rec. 709.
0498
0499
0500 .. _col-smpte-240m:
0501
0502 Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M)
0503 =================================================
0504
0505 The :ref:`smpte240m` standard was an interim standard used during the
0506 early days of HDTV (1988-1998). It has been superseded by Rec. 709. The
0507 default transfer function is ``V4L2_XFER_FUNC_SMPTE240M``. The default
0508 Y'CbCr encoding is ``V4L2_YCBCR_ENC_SMPTE240M``. The default Y'CbCr
0509 quantization is limited range. The chromaticities of the primary colors
0510 and the white reference are:
0511
0512
0513 .. flat-table:: SMPTE 240M Chromaticities
0514 :header-rows: 1
0515 :stub-columns: 0
0516 :widths: 1 1 2
0517
0518 * - Color
0519 - x
0520 - y
0521 * - Red
0522 - 0.630
0523 - 0.340
0524 * - Green
0525 - 0.310
0526 - 0.595
0527 * - Blue
0528 - 0.155
0529 - 0.070
0530 * - White Reference (D65)
0531 - 0.3127
0532 - 0.3290
0533
0534
0535 These chromaticities are identical to the SMPTE 170M colorspace.
0536
0537 Transfer function:
0538
0539 .. math::
0540
0541 L' = 4L\text{, for } 0 \le L < 0.0228
0542
0543 L' = 1.1115L ^{0.45} - 0.1115\text{, for } 0.0228 \le L \le 1
0544
0545 Inverse Transfer function:
0546
0547 .. math::
0548
0549 L = \frac{L'}{4}\text{, for } 0 \le L' < 0.0913
0550
0551 L = \left( \frac{L' + 0.1115}{1.1115}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.0913
0552
0553 The luminance (Y') and color difference (Cb and Cr) are obtained with
0554 the following ``V4L2_YCBCR_ENC_SMPTE240M`` encoding:
0555
0556 .. math::
0557
0558 Y' = 0.2122R' + 0.7013G' + 0.0865B'
0559
0560 Cb = -0.1161R' - 0.3839G' + 0.5B'
0561
0562 Cr = 0.5R' - 0.4451G' - 0.0549B'
0563
0564 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the
0565 range [-0.5…0.5]. The Y'CbCr quantization is limited range.
0566
0567
0568 .. _col-sysm:
0569
0570 Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M)
0571 ===================================================
0572
0573 This standard defines the colorspace used by NTSC in 1953. In practice
0574 this colorspace is obsolete and SMPTE 170M should be used instead. The
0575 default transfer function is ``V4L2_XFER_FUNC_709``. The default Y'CbCr
0576 encoding is ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is
0577 limited range. The chromaticities of the primary colors and the white
0578 reference are:
0579
0580
0581 .. flat-table:: NTSC 1953 Chromaticities
0582 :header-rows: 1
0583 :stub-columns: 0
0584 :widths: 1 1 2
0585
0586 * - Color
0587 - x
0588 - y
0589 * - Red
0590 - 0.67
0591 - 0.33
0592 * - Green
0593 - 0.21
0594 - 0.71
0595 * - Blue
0596 - 0.14
0597 - 0.08
0598 * - White Reference (C)
0599 - 0.310
0600 - 0.316
0601
0602
0603 .. note::
0604
0605 This colorspace uses Illuminant C instead of D65 as the white
0606 reference. To correctly convert an image in this colorspace to another
0607 that uses D65 you need to apply a chromatic adaptation algorithm such as
0608 the Bradford method.
0609
0610 The transfer function was never properly defined for NTSC 1953. The Rec.
0611 709 transfer function is recommended in the literature:
0612
0613 .. math::
0614
0615 L' = 4.5L\text{, for } 0 \le L < 0.018
0616
0617 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
0618
0619 Inverse Transfer function:
0620
0621 .. math::
0622
0623 L = \frac{L'}{4.5} \text{, for } L' < 0.081
0624
0625 L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
0626
0627 The luminance (Y') and color difference (Cb and Cr) are obtained with
0628 the following ``V4L2_YCBCR_ENC_601`` encoding:
0629
0630 .. math::
0631
0632 Y' = 0.2990R' + 0.5870G' + 0.1140B'
0633
0634 Cb = -0.1687R' - 0.3313G' + 0.5B'
0635
0636 Cr = 0.5R' - 0.4187G' - 0.0813B'
0637
0638 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0639 [-0.5…0.5]. The Y'CbCr quantization is limited range. This transform is
0640 identical to one defined in SMPTE 170M/BT.601.
0641
0642
0643 .. _col-sysbg:
0644
0645 Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG)
0646 =========================================================
0647
0648 The :ref:`tech3213` standard defines the colorspace used by PAL/SECAM
0649 in 1975. Note that this colorspace is not supported by the HDMI interface.
0650 Instead :ref:`tech3321` recommends that Rec. 709 is used instead for HDMI.
0651 The default transfer function is
0652 ``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
0653 ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
0654 range. The chromaticities of the primary colors and the white reference
0655 are:
0656
0657
0658 .. flat-table:: EBU Tech. 3213 Chromaticities
0659 :header-rows: 1
0660 :stub-columns: 0
0661 :widths: 1 1 2
0662
0663 * - Color
0664 - x
0665 - y
0666 * - Red
0667 - 0.64
0668 - 0.33
0669 * - Green
0670 - 0.29
0671 - 0.60
0672 * - Blue
0673 - 0.15
0674 - 0.06
0675 * - White Reference (D65)
0676 - 0.3127
0677 - 0.3290
0678
0679
0680
0681 The transfer function was never properly defined for this colorspace.
0682 The Rec. 709 transfer function is recommended in the literature:
0683
0684 .. math::
0685
0686 L' = 4.5L\text{, for } 0 \le L < 0.018
0687
0688 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
0689
0690 Inverse Transfer function:
0691
0692 .. math::
0693
0694 L = \frac{L'}{4.5} \text{, for } L' < 0.081
0695
0696 L = \left(\frac{L' + 0.099}{1.099} \right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
0697
0698 The luminance (Y') and color difference (Cb and Cr) are obtained with
0699 the following ``V4L2_YCBCR_ENC_601`` encoding:
0700
0701 .. math::
0702
0703 Y' = 0.2990R' + 0.5870G' + 0.1140B'
0704
0705 Cb = -0.1687R' - 0.3313G' + 0.5B'
0706
0707 Cr = 0.5R' - 0.4187G' - 0.0813B'
0708
0709 Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
0710 [-0.5…0.5]. The Y'CbCr quantization is limited range. This transform is
0711 identical to one defined in SMPTE 170M/BT.601.
0712
0713
0714 .. _col-jpeg:
0715
0716 Colorspace JPEG (V4L2_COLORSPACE_JPEG)
0717 ======================================
0718
0719 This colorspace defines the colorspace used by most (Motion-)JPEG
0720 formats. The chromaticities of the primary colors and the white
0721 reference are identical to sRGB. The transfer function use is
0722 ``V4L2_XFER_FUNC_SRGB``. The Y'CbCr encoding is ``V4L2_YCBCR_ENC_601``
0723 with full range quantization where Y' is scaled to [0…255] and Cb/Cr are
0724 scaled to [-128…128] and then clipped to [-128…127].
0725
0726 .. note::
0727
0728 The JPEG standard does not actually store colorspace
0729 information. So if something other than sRGB is used, then the driver
0730 will have to set that information explicitly. Effectively
0731 ``V4L2_COLORSPACE_JPEG`` can be considered to be an abbreviation for
0732 ``V4L2_COLORSPACE_SRGB``, ``V4L2_XFER_FUNC_SRGB``, ``V4L2_YCBCR_ENC_601``
0733 and ``V4L2_QUANTIZATION_FULL_RANGE``.
0734
0735 ***************************************
0736 Detailed Transfer Function Descriptions
0737 ***************************************
0738
0739 .. _xf-smpte-2084:
0740
0741 Transfer Function SMPTE 2084 (V4L2_XFER_FUNC_SMPTE2084)
0742 =======================================================
0743
0744 The :ref:`smpte2084` standard defines the transfer function used by
0745 High Dynamic Range content.
0746
0747 Constants:
0748 m1 = (2610 / 4096) / 4
0749
0750 m2 = (2523 / 4096) * 128
0751
0752 c1 = 3424 / 4096
0753
0754 c2 = (2413 / 4096) * 32
0755
0756 c3 = (2392 / 4096) * 32
0757
0758 Transfer function:
0759 L' = ((c1 + c2 * L\ :sup:`m1`) / (1 + c3 * L\ :sup:`m1`))\ :sup:`m2`
0760
0761 Inverse Transfer function:
0762 L = (max(L':sup:`1/m2` - c1, 0) / (c2 - c3 *
0763 L'\ :sup:`1/m2`))\ :sup:`1/m1`
0764
0765 Take care when converting between this transfer function and non-HDR transfer
0766 functions: the linear RGB values [0…1] of HDR content map to a luminance range
0767 of 0 to 10000 cd/m\ :sup:`2` whereas the linear RGB values of non-HDR (aka
0768 Standard Dynamic Range or SDR) map to a luminance range of 0 to 100 cd/m\ :sup:`2`.
0769
0770 To go from SDR to HDR you will have to divide L by 100 first. To go in the other
0771 direction you will have to multiply L by 100. Of course, this clamps all
0772 luminance values over 100 cd/m\ :sup:`2` to 100 cd/m\ :sup:`2`.
0773
0774 There are better methods, see e.g. :ref:`colimg` for more in-depth information
0775 about this.
