Luma (video)


Luma (video)

As applied to video signals, luma represents the brightness in an image (the "black and white" or achromatic portion of the image). Luma is typically paired with chroma. Luma represents the achromatic image without any color, while the chroma components represent the color information. Converting R'G'B' sources (i.e. the output of a 3CCD camera) into luma and chroma allows for chroma subsampling, enabling video systems to optimize their performance for the human visual system. Since human vision is more sensitive to luminance ("black and white") detail than color detail, video systems can optimize bandwidth for luminance over color.

Luma versus Luminance

Luma is the weighted sum of gamma-compressed R'G'B' components of a color video. The word was proposed to prevent confusion between luma as implemented in video engineering and luminance as used in color science (i.e. as defined by CIE). Luminance is formed as a weighted sum of "linear" RGB components, not gamma-corrected ones [Charles Poynton, "YUV and luminance considered harmful: a plea for precise terminology in video," [http://poynton.com/papers/YUV_and_luminance_harmful.html online] ] . SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote luminance. [Engineering Guideline EG 28, "Annotated Glossary of Essential Terms for Electronic Production," SMPTE, 1993.]

Use of luminance

While luma is more often encountered, (photometric) luminance is sometimes used in video engineering when referring to the brightness of a monitor. The formula used to calculate luminance used coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (i.e. the original NTSC primaries, SMPTE C, Rec. 709). For the Rec. 709 primaries the linear combination, based on pure colorimetric considerations and the definition of luminance (relative) is:

:Y = 0.2126 R + 0.7152 G + 0.0722 B

The formula used to calculate luma in the Rec. 709 spec arbitrarily also uses these same coefficients, but with gamma-compressed components:

:Y' = 0.2126 R' + 0.7152 G' + 0.0722 B', where the prime symbol ' denotes gamma correction.

Rec. 601 luma versus Rec. 709 luma coefficients

For digital formats following CCIR 601 (i.e. most digital standard definition formats), luma is calculated with the formula Y' = 0.299 R' + 0.587 G' + 0.114 B'. Formats following ITU-R Recommendation BT. 709 use the formula Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. Modern HDTV systems use the 709 coefficients, while transitional 1035i HDTV formats may use the SMPTE 240M coefficients (Y' = 0.212 R' + 0.701 G' + 0.087 B'). These coefficients correspond to the SMPTE RP 145 primaries (also known as "SMPTE C") in use at the time the standard was created [Charles A. Poynton, "Digital Video and HDTV: Algorithms and Interfaces", Morgan–Kaufmann, 2003. [http://books.google.com/books?vid=ISBN1558607927&id=ra1lcAwgvq4C&pg=PA289&lpg=PA289&dq=luma+luminance+709+601&sig=HXmxZa35yf3Zqj_Eb4PIlrU10Fo online] ] .

The change in the luma coefficients is to provide the "theoretically correct" coefficients that reflects the corresponding standard chromaticities ('colors') of the primaries red, green, and blue. However, there is some controversy regarding this decision. [ [http://poynton.com/papers/SMPTE_98_YYZ_Luma/index.html Luminance, luma, and the migration to DTV] ] The difference in luma coefficients requires that component signals must be converted between Rec. 601 and Rec. 709 to provide accurate colors. In consumer equipment, the matrix required to perform this conversion may be omitted (due to cost reasons), resulting in inaccurate color.

As well, the Rec. 709 luma coefficients may not necessarily provide better performance. Because of the difference between luma and luminance, luma does not exactly represent the luminance in an image. As a result, errors in chroma can affect luminance. Luma alone does not perfectly represent luminance; accurate luminance requires both accurate luma and chroma. Hence, errors in chroma "bleed" into the luminance of an image.

Due to the widespread usage of chroma subsampling, 'errors' in chroma typically occur when it is lowered in resolution/bandwidth. This lowered bandwidth, coupled with high frequency chroma components, can cause visible errors in luminance. An example of a high frequency chroma component would be the line between the green and magenta bars of the SMPTE color bars test pattern. Error in luminance can be seen as a dark band that occurs in this area. [ [http://poynton.com/notes/video/Constant_luminance.html Constant Luminance] ]

References

ee also

*chroma subsampling
*chrominance
*gamma correction


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Luma — may refer to : * Luma (plant), a genus of plants in the myrtle family * Luma (video), a signal used in video technology * A star species from Super Mario Galaxy. * One hundredth Armenian dram * Life Unity Music Amplified, , a jazz/funk band from… …   Wikipedia

  • Video home system — L’expression anglaise Video Home System (système vidéo domestique), plus connue sous son sigle VHS, désigne une norme d’enregistrement de signaux vidéos sur bande magnétique de 1/2 pouce mis au point par JVC à la fin des années 1970. Sa diffusion …   Wikipédia en Français

  • Video codec — A video codec is a device or software that enables video compression and/or decompression for digital video. The compression usually employs lossy data compression. Historically, video was stored as an analog signal on magnetic tape. Around the… …   Wikipedia

  • Video a composantes separees — S Video Une prise vidéo à composantes séparées de type standard. S Video (ou Super Video ou Y/C) est un acronyme désignant un signal vidéo analogique dont la luminance et la chrominance sont véhiculées séparément. Le mode S Video exploite ainsi… …   Wikipédia en Français

  • Vidéo en composantes — S Video Une prise vidéo à composantes séparées de type standard. S Video (ou Super Video ou Y/C) est un acronyme désignant un signal vidéo analogique dont la luminance et la chrominance sont véhiculées séparément. Le mode S Video exploite ainsi… …   Wikipédia en Français

  • Vidéo à composantes séparées — S Video Une prise vidéo à composantes séparées de type standard. S Video (ou Super Video ou Y/C) est un acronyme désignant un signal vidéo analogique dont la luminance et la chrominance sont véhiculées séparément. Le mode S Video exploite ainsi… …   Wikipédia en Français

  • Video Home System — L’expression anglaise Video Home System (système vidéo domestique), plus connue sous son sigle VHS, désigne une norme d’enregistrement de signaux vidéos sur bande magnétique de 1/2 pouce mis au point par JVC à la fin des années 1970. Sa diffusion …   Wikipédia en Français

  • Video synthesizer — A Video Synthesizer is a device that electronically creates a video signal. A video synthesizer is able to generate a variety of visual material without camera input through the use of internal video pattern generators, as seen in the stillframes …   Wikipedia

  • S-Video — Infobox connector name=S Video type=Analog video connector logo= caption=A standard 4 pin S Video cable connector, with each signal pin paired with its own ground pin. designer= design date= manufacturer= production date= superseded= superseded… …   Wikipedia

  • Composite video — Not to be confused with Component video. Composite video On consumer products a yellow RCA connector is typically used for composite video. Type Analog video connector …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.