Luma (video)
Encyclopedia
In video
, luma, sometimes called luminance, 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 without any color, while the chroma components represent the color
information. Converting R'G'B' sources (such as the output of a 3CCD 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 chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.
Luma is the weighted sum of gamma-compressed
R'G'B' components of a color video – the prime symbols (') denote gamma-compression. 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-compressed ones. SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote 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 uses coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (e.g., the original NTSC
primaries, SMPTE C, or Rec. 709
). For the Rec. 709 primaries, the linear combination, based on pure colorimetric considerations and the definition of luminance is:
The formula used to calculate luma in the Rec. 709 spec arbitrarily also uses these same coefficients, but with gamma-compressed components:
(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.
The change in the luma coefficients is to provide the "theoretically correct" coefficients that reflect the corresponding standard chromaticities ('colors') of the primaries red, green, and blue. However, there is some controversy regarding this decision. 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 (to reduce cost), resulting in inaccurate color.
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.
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
, luma, sometimes called luminance, represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
. Luma represents the achromatic image without any color, while the chroma components represent the color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
information. Converting R'G'B' sources (such as the output of a 3CCD camera) into luma and chroma allows for chroma subsampling
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....
: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.
Luma is the weighted sum of gamma-compressed
Gamma correction
Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems...
R'G'B' components of a color video – the prime symbols (') denote gamma-compression. 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
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
). Luminance is formed as a weighted sum of linear RGB components, not gamma-compressed ones. SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote 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 uses coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (e.g., the original NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
primaries, SMPTE C, or Rec. 709
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 aspect ratio. The first edition of the standard was approved in 1990....
). For the Rec. 709 primaries, the linear combination, based on pure colorimetric considerations and the definition of luminance is:
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 601CCIR 601
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector for encoding interlaced analog video signals in digital video form...
(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
ITU-R
The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....
Recommendation BT. 709 use the formula Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. Modern HDTV
High-definition television
High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
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.
The change in the luma coefficients is to provide the "theoretically correct" coefficients that reflect the corresponding standard chromaticities ('colors') of the primaries red, green, and blue. However, there is some controversy regarding this decision. 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 (to reduce cost), resulting in inaccurate color.
Due to the widespread usage of chroma subsampling
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....
, '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
SMPTE color bars
The SMPTE color bars are a type of television test pattern, and is most commonly used in countries where the NTSC video standard is dominant, such as those in North America. The Society of Motion Picture and Television Engineers refers to this test pattern as Engineering Guideline EG 1-1990...
test pattern. Error in luminance can be seen as a dark band that occurs in this area.