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1.. -*- coding: utf-8; mode: rst -*-
2
3********************************
4Detailed Colorspace Descriptions
5********************************
6
7
8.. _col-smpte-170m:
9
10Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M)
11=================================================
12
13The :ref:`smpte170m` standard defines the colorspace used by NTSC and
14PAL and by SDTV in general. The default transfer function is
15``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
16``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
17range. The chromaticities of the primary colors and the white reference
18are:
19
20
21
22.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
23
24.. flat-table:: SMPTE 170M Chromaticities
25 :header-rows: 1
26 :stub-columns: 0
27 :widths: 1 1 2
28
29 * - Color
30 - x
31 - y
32 * - Red
33 - 0.630
34 - 0.340
35 * - Green
36 - 0.310
37 - 0.595
38 * - Blue
39 - 0.155
40 - 0.070
41 * - White Reference (D65)
42 - 0.3127
43 - 0.3290
44
45
46The red, green and blue chromaticities are also often referred to as the
47SMPTE C set, so this colorspace is sometimes called SMPTE C as well.
48
49The transfer function defined for SMPTE 170M is the same as the one
50defined in Rec. 709.
51
52.. math::
53
54 L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le-0.018
55
56 L' = 4.5L \text{, for } -0.018 < L < 0.018
57
58 L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018
59
60Inverse Transfer function:
61
62.. math::
63
64 L = -\left( \frac{L' - 0.099}{-1.099} \right) ^{\frac{1}{0.45}} \text{, for } L' \le -0.081
65
66 L = \frac{L'}{4.5} \text{, for } -0.081 < L' < 0.081
67
68 L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081
69
70The luminance (Y') and color difference (Cb and Cr) are obtained with
71the following ``V4L2_YCBCR_ENC_601`` encoding:
72
73.. math::
74
75 Y' = 0.2990R' + 0.5870G' + 0.1140B'
76
77 Cb = -0.1687R' - 0.3313G' + 0.5B'
78
79 Cr = 0.5R' - 0.4187G' - 0.0813B'
80
81Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
82[-0.5…0.5]. This conversion to Y'CbCr is identical to the one defined in
83the :ref:`itu601` standard and this colorspace is sometimes called
84BT.601 as well, even though BT.601 does not mention any color primaries.
85
86The default quantization is limited range, but full range is possible
87although rarely seen.
88
89
90.. _col-rec709:
91
92Colorspace Rec. 709 (V4L2_COLORSPACE_REC709)
93============================================
94
95The :ref:`itu709` standard defines the colorspace used by HDTV in
96general. The default transfer function is ``V4L2_XFER_FUNC_709``. The
97default Y'CbCr encoding is ``V4L2_YCBCR_ENC_709``. The default Y'CbCr
98quantization is limited range. The chromaticities of the primary colors
99and the white reference are:
100
101
102
103.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
104
105.. flat-table:: Rec. 709 Chromaticities
106 :header-rows: 1
107 :stub-columns: 0
108 :widths: 1 1 2
109
110 * - Color
111 - x
112 - y
113 * - Red
114 - 0.640
115 - 0.330
116 * - Green
117 - 0.300
118 - 0.600
119 * - Blue
120 - 0.150
121 - 0.060
122 * - White Reference (D65)
123 - 0.3127
124 - 0.3290
125
126
127The full name of this standard is Rec. ITU-R BT.709-5.
128
129Transfer function. Normally L is in the range [0…1], but for the
130extended gamut xvYCC encoding values outside that range are allowed.
131
132.. math::
133
134 L' = -1.099(-L)^{0.45} + 0.099 \text{, for } L \le -0.018
135
136 L' = 4.5L \text{, for } -0.018 < L < 0.018
137
138 L' = 1.099L^{0.45} - 0.099 \text{, for } L \ge 0.018
139
140Inverse Transfer function:
141
142.. math::
143
144 L = -\left( \frac{L' - 0.099}{-1.099} \right)^\frac{1}{0.45} \text{, for } L' \le -0.081
145
146 L = \frac{L'}{4.5}\text{, for } -0.081 < L' < 0.081
147
148 L = \left(\frac{L' + 0.099}{1.099}\right)^{\frac{1}{0.45} } \text{, for } L' \ge 0.081
149
150The luminance (Y') and color difference (Cb and Cr) are obtained with
151the following ``V4L2_YCBCR_ENC_709`` encoding:
152
153.. math::
154
155 Y' = 0.2126R' + 0.7152G' + 0.0722B'
156
157 Cb = -0.1146R' - 0.3854G' + 0.5B'
158
159 Cr = 0.5R' - 0.4542G' - 0.0458B'
160
161Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
162[-0.5…0.5].
163
164The default quantization is limited range, but full range is possible
165although rarely seen.
166
167The ``V4L2_YCBCR_ENC_709`` encoding described above is the default for
168this colorspace, but it can be overridden with ``V4L2_YCBCR_ENC_601``,
169in which case the BT.601 Y'CbCr encoding is used.
170
171Two additional extended gamut Y'CbCr encodings are also possible with
172this colorspace:
173
174The xvYCC 709 encoding (``V4L2_YCBCR_ENC_XV709``, :ref:`xvycc`) is
175similar to the Rec. 709 encoding, but it allows for R', G' and B' values
176that are outside the range [0…1]. The resulting Y', Cb and Cr values are
177scaled and offset according to the limited range formula:
178
179.. math::
180
181 Y' = \frac{219}{256} * (0.2126R' + 0.7152G' + 0.0722B') + \frac{16}{256}
182
183 Cb = \frac{224}{256} * (-0.1146R' - 0.3854G' + 0.5B')
184
185 Cr = \frac{224}{256} * (0.5R' - 0.4542G' - 0.0458B')
186
187The xvYCC 601 encoding (``V4L2_YCBCR_ENC_XV601``, :ref:`xvycc`) is
188similar to the BT.601 encoding, but it allows for R', G' and B' values
189that are outside the range [0…1]. The resulting Y', Cb and Cr values are
190scaled and offset according to the limited range formula:
191
192.. math::
193
194 Y' = \frac{219}{256} * (0.2990R' + 0.5870G' + 0.1140B') + \frac{16}{256}
195
196 Cb = \frac{224}{256} * (-0.1687R' - 0.3313G' + 0.5B')
197
198 Cr = \frac{224}{256} * (0.5R' - 0.4187G' - 0.0813B')
199
200Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
201[-0.5…0.5] and quantized without further scaling or offsets.
202The non-standard xvYCC 709 or xvYCC 601 encodings can be
203used by selecting ``V4L2_YCBCR_ENC_XV709`` or ``V4L2_YCBCR_ENC_XV601``.
204As seen by the xvYCC formulas these encodings always use limited range quantization,
205there is no full range variant. The whole point of these extended gamut encodings
206is that values outside the limited range are still valid, although they
207map to R', G' and B' values outside the [0…1] range and are therefore outside
208the Rec. 709 colorspace gamut.
209
210
211.. _col-srgb:
212
213Colorspace sRGB (V4L2_COLORSPACE_SRGB)
214======================================
215
216The :ref:`srgb` standard defines the colorspace used by most webcams
217and computer graphics. The default transfer function is
218``V4L2_XFER_FUNC_SRGB``. The default Y'CbCr encoding is
219``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited range.
220
221Note that the :ref:`sycc` standard specifies full range quantization,
222however all current capture hardware supported by the kernel convert
223R'G'B' to limited range Y'CbCr. So choosing full range as the default
224would break how applications interpret the quantization range.
225
226The chromaticities of the primary colors and the white reference are:
227
228
229
230.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
231
232.. flat-table:: sRGB Chromaticities
233 :header-rows: 1
234 :stub-columns: 0
235 :widths: 1 1 2
236
237 * - Color
238 - x
239 - y
240 * - Red
241 - 0.640
242 - 0.330
243 * - Green
244 - 0.300
245 - 0.600
246 * - Blue
247 - 0.150
248 - 0.060
249 * - White Reference (D65)
250 - 0.3127
251 - 0.3290
252
253
254These chromaticities are identical to the Rec. 709 colorspace.
255
256Transfer function. Note that negative values for L are only used by the
257Y'CbCr conversion.
258
259.. math::
260
261 L' = -1.055(-L)^{\frac{1}{2.4} } + 0.055\text{, for }L < -0.0031308
262
263 L' = 12.92L\text{, for }-0.0031308 \le L \le 0.0031308
264
265 L' = 1.055L ^{\frac{1}{2.4} } - 0.055\text{, for }0.0031308 < L \le 1
266
267Inverse Transfer function:
268
269.. math::
270
271 L = -((-L' + 0.055) / 1.055) ^{2.4}\text{, for }L' < -0.04045
272
273 L = L' / 12.92\text{, for }-0.04045 \le L' \le 0.04045
274
275 L = ((L' + 0.055) / 1.055) ^{2.4}\text{, for }L' > 0.04045
276
277The luminance (Y') and color difference (Cb and Cr) are obtained with
278the following ``V4L2_YCBCR_ENC_601`` encoding as defined by :ref:`sycc`:
279
280.. math::
281
282 Y' = 0.2990R' + 0.5870G' + 0.1140B'
283
284 Cb = -0.1687R' - 0.3313G' + 0.5B'
285
286 Cr = 0.5R' - 0.4187G' - 0.0813B'
287
288Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
289[-0.5…0.5]. This transform is identical to one defined in SMPTE
290170M/BT.601. The Y'CbCr quantization is limited range.
291
292
293.. _col-adobergb:
294
295Colorspace Adobe RGB (V4L2_COLORSPACE_ADOBERGB)
296===============================================
297
298The :ref:`adobergb` standard defines the colorspace used by computer
299graphics that use the AdobeRGB colorspace. This is also known as the
300:ref:`oprgb` standard. The default transfer function is
301``V4L2_XFER_FUNC_ADOBERGB``. The default Y'CbCr encoding is
302``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
303range.
304
305Note that the :ref:`oprgb` standard specifies full range quantization,
306however all current capture hardware supported by the kernel convert
307R'G'B' to limited range Y'CbCr. So choosing full range as the default
308would break how applications interpret the quantization range.
309
310The chromaticities of the primary colors and the white reference are:
311
312
313.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
314
315.. flat-table:: Adobe RGB Chromaticities
316 :header-rows: 1
317 :stub-columns: 0
318 :widths: 1 1 2
319
320 * - Color
321 - x
322 - y
323 * - Red
324 - 0.6400
325 - 0.3300
326 * - Green
327 - 0.2100
328 - 0.7100
329 * - Blue
330 - 0.1500
331 - 0.0600
332 * - White Reference (D65)
333 - 0.3127
334 - 0.3290
335
336
337
338Transfer function:
339
340.. math::
341
342 L' = L ^{\frac{1}{2.19921875}}
343
344Inverse Transfer function:
345
346.. math::
347
348 L = L'^{(2.19921875)}
349
350The luminance (Y') and color difference (Cb and Cr) are obtained with
351the following ``V4L2_YCBCR_ENC_601`` encoding:
352
353.. math::
354
355 Y' = 0.2990R' + 0.5870G' + 0.1140B'
356
357 Cb = -0.1687R' - 0.3313G' + 0.5B'
358
359 Cr = 0.5R' - 0.4187G' - 0.0813B'
360
361Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
362[-0.5…0.5]. This transform is identical to one defined in SMPTE
363170M/BT.601. The Y'CbCr quantization is limited range.
364
365
366.. _col-bt2020:
367
368Colorspace BT.2020 (V4L2_COLORSPACE_BT2020)
369===========================================
370
371The :ref:`itu2020` standard defines the colorspace used by Ultra-high
372definition television (UHDTV). The default transfer function is
373``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
374``V4L2_YCBCR_ENC_BT2020``. The default R'G'B' quantization is limited
375range (!), and so is the default Y'CbCr quantization. The chromaticities
376of the primary colors and the white reference are:
377
378
379
380.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
381
382.. flat-table:: BT.2020 Chromaticities
383 :header-rows: 1
384 :stub-columns: 0
385 :widths: 1 1 2
386
387 * - Color
388 - x
389 - y
390 * - Red
391 - 0.708
392 - 0.292
393 * - Green
394 - 0.170
395 - 0.797
396 * - Blue
397 - 0.131
398 - 0.046
399 * - White Reference (D65)
400 - 0.3127
401 - 0.3290
402
403
404
405Transfer function (same as Rec. 709):
406
407.. math::
408
409 L' = 4.5L\text{, for }0 \le L < 0.018
410
411 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
412
413Inverse Transfer function:
414
415.. math::
416
417 L = L' / 4.5\text{, for } L' < 0.081
418
419 L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
420
421Please note that while Rec. 709 is defined as the default transfer function
422by the :ref:`itu2020` standard, in practice this colorspace is often used
423with the :ref:`xf-smpte-2084`. In particular Ultra HD Blu-ray discs use
424this combination.
425
426The luminance (Y') and color difference (Cb and Cr) are obtained with
427the following ``V4L2_YCBCR_ENC_BT2020`` encoding:
428
429.. math::
430
431 Y' = 0.2627R' + 0.6780G' + 0.0593B'
432
433 Cb = -0.1396R' - 0.3604G' + 0.5B'
434
435 Cr = 0.5R' - 0.4598G' - 0.0402B'
436
437Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
438[-0.5…0.5]. The Y'CbCr quantization is limited range.
439
440There is also an alternate constant luminance R'G'B' to Yc'CbcCrc
441(``V4L2_YCBCR_ENC_BT2020_CONST_LUM``) encoding:
442
443Luma:
444
445.. math::
446 :nowrap:
447
448 \begin{align*}
449 Yc' = (0.2627R + 0.6780G + 0.0593B)'& \\
450 B' - Yc' \le 0:& \\
451 &Cbc = (B' - Yc') / 1.9404 \\
452 B' - Yc' > 0: & \\
453 &Cbc = (B' - Yc') / 1.5816 \\
454 R' - Yc' \le 0:& \\
455 &Crc = (R' - Y') / 1.7184 \\
456 R' - Yc' > 0:& \\
457 &Crc = (R' - Y') / 0.9936
458 \end{align*}
459
460Yc' is clamped to the range [0…1] and Cbc and Crc are clamped to the
461range [-0.5…0.5]. The Yc'CbcCrc quantization is limited range.
462
463
464.. _col-dcip3:
465
466Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3)
467==========================================
468
469The :ref:`smpte431` standard defines the colorspace used by cinema
470projectors that use the DCI-P3 colorspace. The default transfer function
471is ``V4L2_XFER_FUNC_DCI_P3``. The default Y'CbCr encoding is
472``V4L2_YCBCR_ENC_709``. The default Y'CbCr quantization is limited range.
473
474.. note::
475
476 Note that this colorspace standard does not specify a
477 Y'CbCr encoding since it is not meant to be encoded to Y'CbCr. So this
478 default Y'CbCr encoding was picked because it is the HDTV encoding.
479
480The chromaticities of the primary colors and the white reference are:
481
482
483
484.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
485
486.. flat-table:: DCI-P3 Chromaticities
487 :header-rows: 1
488 :stub-columns: 0
489 :widths: 1 1 2
490
491 * - Color
492 - x
493 - y
494 * - Red
495 - 0.6800
496 - 0.3200
497 * - Green
498 - 0.2650
499 - 0.6900
500 * - Blue
501 - 0.1500
502 - 0.0600
503 * - White Reference
504 - 0.3140
505 - 0.3510
506
507
508
509Transfer function:
510
511.. math::
512
513 L' = L^{\frac{1}{2.6}}
514
515Inverse Transfer function:
516
517.. math::
518
519 L = L'^{(2.6)}
520
521Y'CbCr encoding is not specified. V4L2 defaults to Rec. 709.
522
523
524.. _col-smpte-240m:
525
526Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M)
527=================================================
528
529The :ref:`smpte240m` standard was an interim standard used during the
530early days of HDTV (1988-1998). It has been superseded by Rec. 709. The
531default transfer function is ``V4L2_XFER_FUNC_SMPTE240M``. The default
532Y'CbCr encoding is ``V4L2_YCBCR_ENC_SMPTE240M``. The default Y'CbCr
533quantization is limited range. The chromaticities of the primary colors
534and the white reference are:
535
536
537
538.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
539
540.. flat-table:: SMPTE 240M Chromaticities
541 :header-rows: 1
542 :stub-columns: 0
543 :widths: 1 1 2
544
545 * - Color
546 - x
547 - y
548 * - Red
549 - 0.630
550 - 0.340
551 * - Green
552 - 0.310
553 - 0.595
554 * - Blue
555 - 0.155
556 - 0.070
557 * - White Reference (D65)
558 - 0.3127
559 - 0.3290
560
561
562These chromaticities are identical to the SMPTE 170M colorspace.
563
564Transfer function:
565
566.. math::
567
568 L' = 4L\text{, for } 0 \le L < 0.0228
569
570 L' = 1.1115L ^{0.45} - 0.1115\text{, for } 0.0228 \le L \le 1
571
572Inverse Transfer function:
573
574.. math::
575
576 L = \frac{L'}{4}\text{, for } 0 \le L' < 0.0913
577
578 L = \left( \frac{L' + 0.1115}{1.1115}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.0913
579
580The luminance (Y') and color difference (Cb and Cr) are obtained with
581the following ``V4L2_YCBCR_ENC_SMPTE240M`` encoding:
582
583.. math::
584
585 Y' = 0.2122R' + 0.7013G' + 0.0865B'
586
587 Cb = -0.1161R' - 0.3839G' + 0.5B'
588
589 Cr = 0.5R' - 0.4451G' - 0.0549B'
590
591Y' is clamped to the range [0…1] and Cb and Cr are clamped to the
592range [-0.5…0.5]. The Y'CbCr quantization is limited range.
593
594
595.. _col-sysm:
596
597Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M)
598===================================================
599
600This standard defines the colorspace used by NTSC in 1953. In practice
601this colorspace is obsolete and SMPTE 170M should be used instead. The
602default transfer function is ``V4L2_XFER_FUNC_709``. The default Y'CbCr
603encoding is ``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is
604limited range. The chromaticities of the primary colors and the white
605reference are:
606
607
608
609.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
610
611.. flat-table:: NTSC 1953 Chromaticities
612 :header-rows: 1
613 :stub-columns: 0
614 :widths: 1 1 2
615
616 * - Color
617 - x
618 - y
619 * - Red
620 - 0.67
621 - 0.33
622 * - Green
623 - 0.21
624 - 0.71
625 * - Blue
626 - 0.14
627 - 0.08
628 * - White Reference (C)
629 - 0.310
630 - 0.316
631
632
633.. note::
634
635 This colorspace uses Illuminant C instead of D65 as the white
636 reference. To correctly convert an image in this colorspace to another
637 that uses D65 you need to apply a chromatic adaptation algorithm such as
638 the Bradford method.
639
640The transfer function was never properly defined for NTSC 1953. The Rec.
641709 transfer function is recommended in the literature:
642
643.. math::
644
645 L' = 4.5L\text{, for } 0 \le L < 0.018
646
647 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
648
649Inverse Transfer function:
650
651.. math::
652
653 L = \frac{L'}{4.5} \text{, for } L' < 0.081
654
655 L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
656
657The luminance (Y') and color difference (Cb and Cr) are obtained with
658the following ``V4L2_YCBCR_ENC_601`` encoding:
659
660.. math::
661
662 Y' = 0.2990R' + 0.5870G' + 0.1140B'
663
664 Cb = -0.1687R' - 0.3313G' + 0.5B'
665
666 Cr = 0.5R' - 0.4187G' - 0.0813B'
667
668Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
669[-0.5…0.5]. The Y'CbCr quantization is limited range. This transform is
670identical to one defined in SMPTE 170M/BT.601.
671
672
673.. _col-sysbg:
674
675Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG)
676=========================================================
677
678The :ref:`tech3213` standard defines the colorspace used by PAL/SECAM
679in 1975. In practice this colorspace is obsolete and SMPTE 170M should
680be used instead. The default transfer function is
681``V4L2_XFER_FUNC_709``. The default Y'CbCr encoding is
682``V4L2_YCBCR_ENC_601``. The default Y'CbCr quantization is limited
683range. The chromaticities of the primary colors and the white reference
684are:
685
686
687
688.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
689
690.. flat-table:: EBU Tech. 3213 Chromaticities
691 :header-rows: 1
692 :stub-columns: 0
693 :widths: 1 1 2
694
695 * - Color
696 - x
697 - y
698 * - Red
699 - 0.64
700 - 0.33
701 * - Green
702 - 0.29
703 - 0.60
704 * - Blue
705 - 0.15
706 - 0.06
707 * - White Reference (D65)
708 - 0.3127
709 - 0.3290
710
711
712
713The transfer function was never properly defined for this colorspace.
714The Rec. 709 transfer function is recommended in the literature:
715
716.. math::
717
718 L' = 4.5L\text{, for } 0 \le L < 0.018
719
720 L' = 1.099L ^{0.45} - 0.099\text{, for } 0.018 \le L \le 1
721
722Inverse Transfer function:
723
724.. math::
725
726 L = \frac{L'}{4.5} \text{, for } L' < 0.081
727
728 L = \left(\frac{L' + 0.099}{1.099} \right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
729
730The luminance (Y') and color difference (Cb and Cr) are obtained with
731the following ``V4L2_YCBCR_ENC_601`` encoding:
732
733.. math::
734
735 Y' = 0.2990R' + 0.5870G' + 0.1140B'
736
737 Cb = -0.1687R' - 0.3313G' + 0.5B'
738
739 Cr = 0.5R' - 0.4187G' - 0.0813B'
740
741Y' is clamped to the range [0…1] and Cb and Cr are clamped to the range
742[-0.5…0.5]. The Y'CbCr quantization is limited range. This transform is
743identical to one defined in SMPTE 170M/BT.601.
744
745
746.. _col-jpeg:
747
748Colorspace JPEG (V4L2_COLORSPACE_JPEG)
749======================================
750
751This colorspace defines the colorspace used by most (Motion-)JPEG
752formats. The chromaticities of the primary colors and the white
753reference are identical to sRGB. The transfer function use is
754``V4L2_XFER_FUNC_SRGB``. The Y'CbCr encoding is ``V4L2_YCBCR_ENC_601``
755with full range quantization where Y' is scaled to [0…255] and Cb/Cr are
756scaled to [-128…128] and then clipped to [-128…127].
757
758.. note::
759
760 The JPEG standard does not actually store colorspace
761 information. So if something other than sRGB is used, then the driver
762 will have to set that information explicitly. Effectively
763 ``V4L2_COLORSPACE_JPEG`` can be considered to be an abbreviation for
764 ``V4L2_COLORSPACE_SRGB``, ``V4L2_YCBCR_ENC_601`` and
765 ``V4L2_QUANTIZATION_FULL_RANGE``.
766
767***************************************
768Detailed Transfer Function Descriptions
769***************************************
770
771.. _xf-smpte-2084:
772
773Transfer Function SMPTE 2084 (V4L2_XFER_FUNC_SMPTE2084)
774=======================================================
775
776The :ref:`smpte2084` standard defines the transfer function used by
777High Dynamic Range content.
778
779Constants:
780 m1 = (2610 / 4096) / 4
781
782 m2 = (2523 / 4096) * 128
783
784 c1 = 3424 / 4096
785
786 c2 = (2413 / 4096) * 32
787
788 c3 = (2392 / 4096) * 32
789
790Transfer function:
791 L' = ((c1 + c2 * L\ :sup:`m1`) / (1 + c3 * L\ :sup:`m1`))\ :sup:`m2`
792
793Inverse Transfer function:
794 L = (max(L':sup:`1/m2` - c1, 0) / (c2 - c3 *
795 L'\ :sup:`1/m2`))\ :sup:`1/m1`
796
797Take care when converting between this transfer function and non-HDR transfer
798functions: the linear RGB values [0…1] of HDR content map to a luminance range
799of 0 to 10000 cd/m\ :sup:`2` whereas the linear RGB values of non-HDR (aka
800Standard Dynamic Range or SDR) map to a luminance range of 0 to 100 cd/m\ :sup:`2`.
801
802To go from SDR to HDR you will have to divide L by 100 first. To go in the other
803direction you will have to multiply L by 100. Of course, this clamps all
804luminance values over 100 cd/m\ :sup:`2` to 100 cd/m\ :sup:`2`.
805
806There are better methods, see e.g. :ref:`colimg` for more in-depth information
807about this.