Rec. 709

ITU-R Recommendation BT.709, more commonly known by the abbreviations Rec. 709 or BT.709, standardizes the format of high-definition television, having 16:9 (widescreen) aspect ratio. The first edition of the standard was approved in 1990.

CIExy1931 Rec 709
BT.709 primaries shown on the CIE 1931 x, y chromaticity diagram. Colors within the BT.709 color gamut will fall within the triangle that connects the primaries. Also shown is BT.709's white point, Illuminant D65.

Technical details

Pixel count

Rec. 709 refers to HDTV systems having roughly two million luma samples per frame. Rec. 709 has two parts:

Part 2 codifies current and prospective 1080i and 1080p systems with square sampling. In an attempt to unify 1080-line HDTV standards, part 2 defines a common image format (CIF) with picture parameters independent of the picture rate.

Part 1 codifies what are now referred to as 1035i30 and 1152i25 HDTV systems. The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled ("square-pixel") systems. The 1152i25 system was used for experimental equipment in Europe and was never commercially deployed.

Frame rate

Rec. 709 specifies the following picture rates: 60 Hz, 50 Hz, 30 Hz, 25 Hz and 24 Hz. "Fractional" rates having the above values divided by 1.001 are also permitted.

Initial acquisition is possible in either progressive or interlaced form. Video captured as progressive can be transported with either progressive transport or progressive segmented frame (PsF) transport. Video captured as interlaced can be transported with interlace transport. In cases where a progressive captured image is transported as a segmented frame, segment/field frequency must be twice the frame rate.

In practice, the above requirements result in the following frame rates ("fractional" rates are specified in commonly used "decimal" form): 25i, 25PsF, 25p, 50p for 50 Hz systems; 23.976p, 23.976PsF, 24p, 24PsF, 29.97i, 29.97p, 29.97PsF, 30PsF, 30p, 59.94p, 60p for 60 Hz systems.

Digital representation

Rec. 709 defines an R’G’B’ encoding and a Y’CBCR encoding, each with either 8 bits or 10 bits per sample in each color channel. In the 8-bit encoding, the R’, B’, G’, and Y’ channels have a nominal range of [16..235], and the CB and CR channels have a nominal range of [16..240] with 128 as the neutral value. So in R’G’B’, reference black is (16, 16, 16) and reference white is (235, 235, 235), and in Y’CBCR, reference black is (16, 128, 128), and reference white is (235, 128, 128). Values outside the nominal ranges are allowed, but typically they would be clamped for broadcast or for display. Values 0 and 255 are reserved as timing references, and may not contain color data. Rec. 709's 10-bit encoding uses nominal values four times those of the 8-bit encoding. Rec. 709's nominal ranges are the same as those defined in ITU Rec. 601.[1]

Primary chromaticities

RGB color space parameters[2]
Color space White point Primaries
xW yW xR yR xG yG xB yB
ITU-R BT.709 0.3127 0.3290 0.64 0.33 0.30 0.60 0.15 0.06

Note that red and blue are the same as the EBU Tech 3213 primaries while green is halfway between EBU Tech 3213 and SMPTE C (two types of Rec.601). In coverage of the CIE 1931 color space the Rec. 709 color space (and the derivative sRGB color space) is almost identical to Rec. 601 and covers 35.9%.[3]

Standards Conversion

When converting between the various HD and SD formats, it would be correct to compensate for the differences in the primaries (e.g. between the Rec. 709, EBU Tech 3213, and SMPTE C primaries). In practice, this conversion is rarely performed because the difference is negligible in real-world scenes, except the ones with large patches of very saturated colors.[4]

Luma coefficients

HDTV according to Rec. 709 forms luma (Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722. This means that unlike Rec. 601, the coefficients match the primaries and white points, so luma corresponds more closely to luminance. Some experts feel that the advantages of correct matrix coefficients do not justify the change from Rec. 601 coefficients.[5]

Transfer characteristics

Rec. 709 specifies the OETF (opto electrical transfer function) of HDTV encoding in reference to the camera, known as camera gamma (sometimes indicated as "scene-referred"[6] gamma). The Rec. 709 transfer function from the linear signal (luminance) to the nonlinear (voltage) is linear in the bottom part and then transfers to a power function for the rest of the range:[7]

The conversion to linear is as follows.

The power function of the majority of the gamma curve is 0.45, but because it is offset by the linear section the resulting equivalent gamma is more approximate to 0.50-0.53 (the inverse of which is approximately gamma 1.9-2.0 to convert back to linear).

While Rec. 709 does not specify the display referred gamma, display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as a gamma of 2.4. This is a higher gamma than the 2.0 the math shown above would indicate, because the television system has been deliberately designed with an end-to-end system gamma of about 1.2, to provide compensation for the ‘dim surround’ effect. Therefore, the monitor gamma is not the inverse of the camera gamma. [8]

It is worth noting that Rec. 709 and sRGB share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred with a gamma of 2.2. [9]

In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired aesthetic look, as viewed on a reference monitor with a gamma of 2.4 (per ITU-R BT.1886) in a dim reference viewing environment (per ITU-R BT.2035).[10][11][12]

See also

References

  • ITU-R BT.709-6: Parameter values for the HDTV standards for production and international programme exchange. June, 2015. Note that the -6 is the current version; previous versions were -1 through to -5.
  • [3]: Poynton, Charles, Perceptual uniformity, picture rendering, image state, and Rec. 709. May, 2008.
  • sRGB: IEC 61966-2-1:1999
  1. ^ ITU-R Rec. BT.601-5, 1995
  2. ^ ITU-R Rec. BT.709-5 page 18, items 1.3 and 1.4
  3. ^ ""Super Hi-Vision" as Next-Generation Television and Its Video Parameters". Information Display. Archived from the original on 2013-01-12. Retrieved 2013-01-01.
  4. ^ [1]: Chan, Glenn, "HD versus SD Color Space".
  5. ^ [2]: Poynton, Charles, "Luminance, luma, and the migration to DTV" (Feb. 6, 1998)
  6. ^ https://www.provideocoalition.com/scene_vs_display_referred_profiles/
  7. ^ ITU-R Rec. BT.709-6 page 3, item 1.2 http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.709-6-201506-I!!PDF-E.pdf
  8. ^ EBU Tech 3320 page 11, https://tech.ebu.ch/docs/tech/tech3320.pdf
  9. ^ Poynton, Charles (2012). Digital Video and HD Algorithms and Interfaces. Burlington, Mass.: Elsevire/Morgan Kaufmann. p. 321. ISBN 978-0-12-391926-7.
  10. ^ ITU-R Rec. BT.709-6 page 3 footnote 1
  11. ^ ITU-R BT.1886 http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1886-0-201103-I!!PDF-E.pdf
  12. ^ IITU-R BT.2035

External links

Colour banding

Colour banding, or color banding (American English) is a problem of inaccurate colour presentation in computer graphics. In 24-bit colour modes, 8 bits per channel is usually considered sufficient to render images in Rec. 709 or sRGB. However, in some cases there is a risk of producing abrupt changes between shades of the same colour. For instance, displaying natural gradients (like sunsets, dawns or clear blue skies) can show minor banding.

Colour banding is more noticeable with fewer bits per pixel (BPP) at 16–256 colours (4–8 BPP), where not every shade can be shown without dithering.

Possible solutions include the introduction of dithering and increasing the number of bits per colour channel. Blurring does not fix this, unless one actually increases the number of levels available so that the blur can render color in intermediate levels.

Fraps

Fraps (derived from frames per second) is a benchmarking, screen capture and screen recording utility for Windows developed by Beepa. It can capture from software that uses DirectX and OpenGL, such as PC games.

HCL color space

HCL (Hue-Chroma-Luminance) is a color space model designed to accord with human perception of color. HCL has been adopted by information visualization practitioners to present data without the bias implicit in using varying saturation.

ITU-R

The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU) and is responsible for radio communication.

Its role is to manage the international radio-frequency spectrum and satellite orbit resources and to develop standards for radiocommunication systems with the objective of ensuring the effective use of the spectrum.ITU is required, according to its Constitution, to allocate spectrum and register frequency allocation, orbital positions and other parameters of satellites, “in order to avoid harmful interference between radio stations of different countries”. The international spectrum management system is therefore based on regulatory procedures for frequency coordination, notification and registration.

ITU-R has a permanent secretariat, the Radiocommunication Bureau, based at the ITU HQ in Geneva, Switzerland. The elected Director of the Bureau is Mr. François Rancy of France; he was first elected by the ITU Membership to the Directorship in 2010.

Joint Photographic Experts Group

The Joint Photographic Experts Group is the joint committee between ISO/IEC JTC 1 and ITU-T (formerly CCITT) that created and maintains the JPEG, JPEG 2000, and JPEG XR standards. It is one of two sub-groups of ISO/IEC Joint Technical Committee 1, Subcommittee 29, Working Group 1 (ISO/IEC JTC 1/SC 29/WG 1) – titled as Coding of still pictures. In the ITU-T, its work falls in the domain of the ITU-T Visual Coding Experts Group (VCEG). ISO/IEC JTC1 SC29 Working Group 1 (working together with ITU-T Study Group 16 – SG16 and previously also with Study Group 8 – SG8) is responsible for the JPEG and JBIG standards. The scope of the organization includes the work of both the Joint Photographic Experts Group and Joint Bi-level Image Experts Group.In April 1983, ISO started to work to add photo quality graphics to text terminals. In the mid-1980s, both CCITT (now ITU-T) and ISO had standardization groups for image coding: CCITT Study Group VIII (SG8) – Telematic Services and ISO TC97 SC2 WG8 – Coding of Audio and Picture Information. They were historically targeted on image communication. In 1986, it was decided to create the Joint (CCITT/ISO) Photographic Expert Group. The JPEG committee was created in 1986. In 1988, it was decided to create the Joint (CCITT/ISO) Bi-level Image Group (JBIG). The group typically meets three times annually in North America, Asia and Europe. The group often meets jointly with the JBIG committee.

Libvpx

libvpx is a free software video codec library from Google and the Alliance for Open Media (AOMedia).

It serves as the reference software implementation for the video coding formats VP8 and VP9, and for AV1 a special fork named libaom that was stripped of backwards compatibility.

As free software it is published also in source code under the terms of the revised BSD license. It ships with the commandline tools vpxenc/aomenc and vpxdec/aomdec that build on its functionality.

List of RAL colors

Below is a list of RAL Classic colors from the RAL colour standard. The CIE L*a*b* and RGB Web colors shown are approximate and informative only.

List of color spaces and their uses

This is a list of color spaces and their uses.

Log profile

A log profile, or logarithmic profile, is a shooting profile, or gamma curve, found on some digital video cameras that gives a wide dynamic and tonal range, allowing more latitude to apply colour and style choices. The resulting image appears washed out, requiring color grading in post-production, but retains shadow and highlight detail that would otherwise be lost if a regular linear profile had been used that clipped shadow and highlight detail. The feature is mostly used in filmmaking and videography.

Luma (video)

In video, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance. Luma represents the achromatic image, while the chroma components represent the color information. Converting R′G′B′ sources (such as the output of a three-CCD camera) into luma and chroma allows for chroma subsampling: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store and transmit chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.

Ostwald color system

In colorimetry, the Ostwald color system is a color space that was invented by Wilhelm Ostwald. Associated with the Color Harmony Manual, it comprises a set of paint chips representing the Ostwald color space. There are 4 different editions of the Color Harmony Manual. Each manual is made up of charts, with each chart being a different color space.

RG color space

The RG or red-green color space is a color space that uses only two colors, red and green. It is an additive format, similar to the RGB color model but without a blue channel. Thus, blue is said to be out of gamut. This format is not in use today, and was only used on two-color Technicolor and other early color processes for films; by comparison to a full spectrum, its poor color reproduction made it undesirable. The system cannot create white naturally, and many colors are distorted.

Rec. 2020

ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including picture resolutions, frame rates with progressive scan, bit depths, color primaries, RGB and luma-chroma color representations, chroma subsamplings, and an opto-electronic transfer function. The first version of Rec. 2020 was posted on the International Telecommunication Union (ITU) website on August 23, 2012, and two further editions have been published since then. It is expanded in several ways by Rec. 2100.

Rec. 601

ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 (or its former name, CCIR 601) is a standard originally issued in 1982 by the CCIR (an organization which has since been renamed as the International Telecommunication Union – Radiocommunication sector) for encoding interlaced analog video signals in digital video form. It includes methods of encoding 525-line 60 Hz and 625-line 50 Hz signals, both with an active region covering 720 luminance samples and 360 chrominance samples per line. The color encoding system is known as YCbCr 4:2:2.

The Rec. 601 video raster format has been re-used in a number of later standards, including the ISO/IEC MPEG and ITU-T H.26x compressed formats – although compressed formats for consumer applications usually use chroma subsampling reduced from the 4:2:2 sampling specified in Rec. 601 to 4:2:0.

The standard has been revised several times in its history. Its edition 7, referred to as BT.601-7, was approved in March 2011 and was formally published in October 2011.

Standard-dynamic-range video

Standard-dynamic-range video describes images/rendering/video using a conventional gamma curve, and therefore presenting a dynamic range that is considered standard, as opposed to high-dynamic-range video. The conventional gamma curve was based on the limits of the cathode ray tube (CRT) which allows for a maximum luminance of 100 cd/m2. The first CRT television sets were manufactured in 1934 and the first color CRT television sets were manufactured in 1954.

Ultra-high-definition television

Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) today includes 4K UHD and 8K UHD, which are two digital video formats with an aspect ratio of 16:9. These were first proposed by NHK Science & Technology Research Laboratories and later defined and approved by the International Telecommunication Union (ITU).The Consumer Electronics Association announced on October 17, 2012, that "Ultra High Definition", or "Ultra HD", would be used for displays that have an aspect ratio of 16:9 or wider and at least one digital input capable of carrying and presenting native video at a minimum resolution of 3840×2160 pixels. In 2015, the Ultra HD Forum was created to bring together the end-to-end video production ecosystem to ensure interoperability and produce industry guidelines so that adoption of ultra-high-definition television could accelerate. From just 30 in Q3 2015, the forum published a list up to 55 commercial services available around the world offering 4K resolution.The "UHD Alliance", an industry consortium of content creators, distributors, and hardware manufacturers, announced during a Consumer Electronics Show (CES) 2016 press conference its "Ultra HD Premium" specification, which defines resolution, bit depth, color gamut, high-dynamic-range imaging (HDRI) and rendering (HDRR) required for Ultra HD (UHDTV) content and displays to carry their Ultra HD Premium logo.

White point

A white point (often referred to as reference white or target white in technical documents) is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit by incandescent lights, which are relatively orange compared to daylight. Defining "white" as daylight will give unacceptable results when attempting to color-correct a photograph taken with incandescent lighting.

YUV

YUV is a color encoding system typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently masked by the human perception than using a "direct" RGB-representation. Other color encodings have similar properties, and the main reason to implement or investigate properties of Y′UV would be for interfacing with analog or digital television or photographic equipment that conforms to certain Y′UV standards.

The scope of the terms Y′UV, YUV, YCbCr, YPbPr, etc., is sometimes ambiguous and overlapping. Historically, the terms YUV and Y′UV were used for a specific analog encoding of color information in television systems, while YCbCr was used for digital encoding of color information suited for video and still-image compression and transmission such as MPEG and JPEG. Today, the term YUV is commonly used in the computer industry to describe file-formats that are encoded using YCbCr.

The Y′UV model defines a color space in terms of one luma component (Y′) and two chrominance (UV) components. The Y′UV color model is used in the PAL composite color video (excluding PAL-N) standard. Previous black-and-white systems used only luma (Y′) information. Color information (U and V) was added separately via a sub-carrier so that a black-and-white receiver would still be able to receive and display a color picture transmission in the receiver's native black-and-white format.

Y′ stands for the luma component (the brightness) and U and V are the chrominance (color) components; luminance is denoted by Y and luma by Y′ – the prime symbols (') denote gamma compression, with "luminance" meaning physical linear-space brightness, while "luma" is (nonlinear) perceptual brightness.

The YPbPr color model used in analog component video and its digital version YCbCr used in digital video are more or less derived from it, and are sometimes called Y′UV. (CB/PB and CR/PR are deviations from grey on blue–yellow and red–cyan axes, whereas U and V are blue–luminance and red–luminance differences respectively.) The Y′IQ color space used in the analog NTSC television broadcasting system is related to it, although in a more complex way. The YDbDr color space used in the analog SECAM and PAL-N television broadcasting systems, are also related.

As for etymology, Y, Y′, U, and V are not abbreviations. The use of the letter Y for luminance can be traced back to the choice of XYZ primaries. This lends itself naturally to the usage of the same letter in luma (Y′), which approximates a perceptually uniform correlate of luminance. Likewise, U and V were chosen to differentiate the U and V axes from those in other spaces, such as the x and y chromaticity space. See the equations below or compare the historical development of the math.

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