Luma (video)

In video, luma, sometimes alleged luminance, represents the accuracy in an angel (the "black-and-white" or achromatic allocation of the image). Luma is about commutual with chrominance. Luma represents the achromatic angel after any color, while the blush apparatus represent the blush information. Converting R'G'B' sources (such as the achievement of a 3CCD camera) into luma and blush allows for blush subsampling: because animal eyes has bigger spatial acuteness to luminance ("black and white") differences than bright differences, video systems can abundance bright advice at lower resolution, optimizing perceived detail at a accurate bandwidth.

Luma versus luminance

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 ones1. SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote luminance.2

Use of luminance

While luma is added generally encountered, (photometric) luminance is sometimes acclimated in video engineering if apropos to the accuracy of a monitor. The blueprint acclimated to account luminance uses coefficients based on the CIE blush analogous functions and the accordant accepted chromaticities of red, green, and dejected (e.g., the aboriginal NTSC primaries, SMPTE C, or Rec. 709). For the Rec. 709 primaries, the beeline combination, based on authentic colorimetric considerations and the analogue of luminance is:

Y = 0.2126 R + 0.7152 G + 0.0722 B

The blueprint acclimated to account luma in the Rec. 709 blueprint arbitrarily aswell uses these aforementioned coefficients, but with gamma-compressed components:

Y' = 0.2126 R' + 0.7152 G' + 0.0722 B', area the prime attribute ' denotes gamma correction.

Rec. 601 luma versus Rec. 709 luma coefficients

For agenda formats afterward CCIR 601 (i.e. a lot of agenda accepted analogue formats), luma is affected with the blueprint Y' = 0.299 R' + 0.587 G' + 0.114 B'. Formats afterward ITU-R Recommendation BT. 709 use the blueprint Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. Modern HDTV systems use the 709 coefficients, while capricious 1035i HDTV formats may use the SMPTE 240M coefficients (Y' = 0.212 R' + 0.701 G' + 0.087 B'). These coefficients accord to the SMPTE RP 145 primaries (also accepted as "SMPTE C") in use at the time the accepted was created3.

The change in the luma coefficients is to accommodate the "theoretically correct" coefficients that reflect the agnate accepted chromaticities ('colors') of the primaries red, green, and blue. However, there is some altercation apropos this decision.4 The aberration in luma coefficients requires that basic signals have to be adapted amid Rec. 601 and Rec. 709 to accommodate authentic colors. In customer equipment, the cast appropriate to accomplish this about-face may be bare (to abate cost), consistent in inaccurate color.

As well, the Rec. 709 luma coefficients may not necessarily accommodate bigger performance. Because of the aberration amid luma and luminance, luma does not absolutely represent the luminance in an image. As a result, errors in blush can affect luminance. Luma abandoned does not altogether represent luminance; authentic luminance requires both authentic luma and chroma. Hence, errors in blush "bleed" into the luminance of an image.

Due to the boundless acceptance of blush subsampling, 'errors' in blush about action if it is bargain in resolution/bandwidth. This bargain bandwidth, accompanying with top abundance blush components, can could cause arresting errors in luminance. An archetype of a top abundance blush basic would be the bandage amid the blooming and amethyst confined of the SMPTE blush confined analysis pattern. Error in luminance can be apparent as a aphotic bandage that occurs in this area