ScRGB

2025-03-29T12:44:51Z

88.113.179.115: Clarify the main point, devs need to understand that DXGI scRGB HDR isn't clamped between -0.5 and 7.5, which wasn't otherwise that clear

{{Short description|Wide color gamut RGB color space}}
{{lowercase|title=scRGB}}
{{Infobox technology standard
| title = scRGB
| long_name = IEC 61966-2-2:2003
| native_name = {{Plainlist|
* scRGB
* IEC 61966-2-2:2003
}}
| native_name_lang = English
| image = ScRGB.svg
| caption = Intersection of the scRGB [[gamut]] with the X+Y+Z=.6443 plane (the gamut varies depending on the intensity as it is a cube with a non-zero origin).
| status = Published
| year_started = 
| first_published = {{Start date and age|2003|01|23}}<ref name="iec-standard">{{Cite web|url=https://webstore.iec.ch/publication/6171|title=Multimedia systems and equipment - Colour measurement and management - Part 2-2: Colour management - Extended RGB colour space - scRGB|date=2003-01-23|access-date=2021-04-18|author1=HP|author-link1=Hewlett-Packard|author2=Microsoft|author-link2=Microsoft|author3=IEC|author-link3=International Electrotechnical Commission|editor=IEC|website=IEC}}</ref>
| version =
| version_date =
| preview =
| preview_date =
| organization = {{Plainlist|
* [[Hewlett-Packard|HP]]
* [[Microsoft]]
* {{abbr|[[International Electrotechnical Commission|IEC]]|International Electrotechnical Commission}}<ref name="iec-standard" />
}}
| committee = '''{{abbr|TC|Technical Committee}}'''/'''{{abbr|SC|Sub-committee}}''': TC 100/TA 2
| editors =
| authors = {{Plainlist|
* [[Hewlett-Packard|HP]]
* [[Microsoft]]
}}
| base_standards = [[sRGB]]
| related_standards = [[Adobe RGB color space]]
| abbreviation =
| domain = [[Color space]], [[color model]]
| license =
| website = 
}}

'''scRGB''' is a wide color [[gamut]] [[RGB color space]] created by [[Microsoft]] and [[Hewlett-Packard|HP]] that uses the same color primaries and white/black points as the [[sRGB]] color space but allows coordinates below zero and greater than one. The full range is −0.5 through just less than +7.5.

Negative numbers enables scRGB to encompass most of the [[CIE 1931 color space]] while maintaining simplicity and [[backward compatibility]] with sRGB by not changing the [[primary colors]]. However this means approximately 80% of the scRGB color space consists of [[imaginary color]]s. Numbers greater than 1.0 allow [[high dynamic range]] images to be represented, though the dynamic range is less than other formats.<ref>[http://www.anyhere.com/gward/hdrenc/hdr_encodings.html "HDR Encoding Comparison". Greg Ward. Retrieved 2010-10-25.]</ref>

== Encoding ==
Two encodings are defined for the individual primaries: a linear 16 bit per channel encoding and a nonlinear 12 bit per channel encoding.

The 16 bit '''scRGB(16)''' encoding is the ''linear'' RGB channels converted by {{math|8192''x'' + 4096}}. Compared to 8-bit sRGB this ranges from almost {{frac|2|1|2}} times the color resolution near 0.0 to more than 14 times the color resolution near 1.0. Storage as 16 bits clamps the linear range to {{math|−0.5..7.4999}}.<br>

The 12-bit '''scRGB-nl''' encoding is the linear RGB channels passed through the same opto-electric conversion function as [[sRGB]] (for negative numbers use {{math|−f(−''x'')}}) and then converted by {{math|1280''x'' + 1024}}. This is exactly 5 times the color resolution of 8-bit sRGB, and 8-bit sRGB can be converted directly with {{math|5''x'' + 1024}}. The linear range is clamped to the slightly larger {{math|−0.6038..7.5913}}.

A 12-bit encoding called '''scYCC-nl''' is the conversion of the non-linear [[sRGB]] levels to [[yCbCr|JFIF-Y'CbCr]] and then converted by {{math|1280''{{prime|Y}}'' + 1024}}, {{math|1280''Cb'' + 2048}}, {{math|1280''Cr'' + 2048}}. This form can allow greater compression and direct conversion to/from [[JPEG]] files and video hardware.

With the addition of an alpha channel with the same number of bits the 16-bit encoding may be seen referred to as 64 bit and the 12-bit encoding referred to as 48-bit. Alpha is not encoded as above, however. Alpha is instead a linear 0-1 range multiplied by {{math|2<sup>''n''</sup> − 1}} where {{math|''n''}} is 12 or 16.

The much newer [[DirectX Graphics Infrastructure|DXGI]] '''scRGB HDR swapchains''' store the linear sRGB channels as 16-bit [[half float]] and has a much larger range of over ±60,000, without any enforced clamps.{{cn|date=March 2025}} Storing linear sRGB values as floating point is very common in modern computer graphics software.

== Usage ==
The first implementation of scRGB was the [[GDI+]] API in [[Windows Vista]]. At [[Windows Hardware Engineering Conference|WinHEC]] 2008 Microsoft announced that [[Windows 7]] would support 48-bit scRGB (which for [[HDMI]] can be converted and output as [[xvYCC]]). The components in Windows 7 that support 48-bit scRGB are [[Direct3D]], the [[Windows Imaging Component]], and the [[Windows Color System]] and they support it in both full screen exclusive mode and in video overlays.<ref>{{cite web |url=http://download.microsoft.com/download/5/E/6/5E66B27B-988B-4F50-AF3A-C2FF1E62180F/GRA-T583_WH08.pptx |title=WinHEC 2008 GRA-583: Display Technologies |publisher=Microsoft |date=2008-11-06 |accessdate=2008-12-06 |url-status=dead |archiveurl=https://web.archive.org/web/20081227074524/http://download.microsoft.com/download/5/E/6/5E66B27B-988B-4F50-AF3A-C2FF1E62180F/GRA-T583_WH08.pptx |archivedate=2008-12-27 }}</ref><ref>{{cite web|url=http://news.softpedia.com/news/Windows-7-High-Color-Support-98741.shtml |title=Windows 7 High Color Support |publisher=Softpedia |date=2008-11-26 |accessdate=2008-12-06}}</ref>

== Origin of sc in scRGB ==
The origin of the sc in scRGB is shrouded in mystery. Officially it stands for nothing. According to Michael Stokes (the national and international leader of the International Electrotechnical Commission, or IEC, group working on scRGB), the name appeared when the Japanese national committee requested a name change from the earlier XsRGB (excess RGB). The two leading candidates for meaning are "specular RGB" because scRGB supports whites greater than the diffuse 1.0 values, and "standard compositing RGB" because the linearity, floating-point support, HDR (high dynamic range) support, and wide gamut support are ideally suited for compositing. This meaning also implicitly emphasizes that scRGB is not intended to be directly supported in devices or formats, since by definition scRGB encompasses values that are beyond both the human visual system and (even theoretically) realizable physical devices.<ref>{{Cite book |last=Anderson |first=Chris |url=https://dl.acm.org/doi/abs/10.5555/1237896 |title=Essential Windows Presentation Foundation (WPF) |date=April 11, 2007 |publisher=Addison-Wesley Professional |isbn=978-0-321-37447-9 |pages=227 |doi=}}</ref>

== References ==
{{reflist}}

== External links ==
* [https://webstore.iec.ch/publication/6171 The standard IEC 61966-2-2]
* [http://www.iec.ch/cgi-bin/getcorr.pl?file=iec61966-2-2-cor1{ed1.0}en.pdf Annex B: Non-linear encoding for scRGB: scRGB-nl]
* A [https://web.archive.org/web/20110725185444/http://www.colour.org/tc8-05/Docs/colorspace/61966-2-2NPa.pdf working draft] of IEC 61966-2-2 is available online.
* [https://web.archive.org/web/20130626023754/https://www.pcmag.com/article2/0,2817,13769,00.asp PCMag.com: Defining scRGB]

{{Color space}}

{{DEFAULTSORT:Scrgb}}
[[Category:Color space]]
[[Category:High dynamic range]]

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ScRGB