Converting RGB to grayscale intensity

Question

When converting from RGB to grayscale  it is said that specific weights to channels R  G  and B ought to be applied  These weights are  0 2989  0 5870  0 1140   It is said that the reason for this is different human perception sensibility towards these three colors  Sometimes it is also said these are the values used to compute NTSC signal   However  I didn t find a good reference for this on the web  What is the source of these values   See also these previous questions  here and here

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These values vary from person to person  especially for people who are colorblind

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Heres some code in c to convert rgb to grayscale  The real weighting used for rgb to grayscale conversion is 0 3R 0 6G 0 11B  these weights arent absolutely critical so you can play with them  I have made them 0 25R  0 5G 0 25B  It produces a slightly darker image   NOTE  The following code assumes xRGB 32bit pixel format  unsigned int  pntrBWImage  unsigned int    data pointer       assumes 4 width height bytes with 32 bits i e  4 bytes per pixel unsigned int fourBytes          unsigned char r g b          for  int index 0 index lt width height index                          fourBytes pntrBWImage index    caches 4 bytes at a time             r  fourBytes gt  gt 16               g  fourBytes gt  gt 8               b fourBytes               I Out index     r  gt  gt 2    g gt  gt 1     b gt  gt 2     This runs in 0 00065s on my pc and produces slightly darker results               I Out index    unsigned int  r g b   3        This runs in 0 0011s on my pc and produces a pure average

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is all this really necessary   human perception and CRT vs LCD will vary  but the R G B intensity does not   Why not L    R   G   B  3 and set the new RGB to L  L  L

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I found that this publication referenced in an answer to a previous similar question  It is very helpful   http   cadik posvete cz color to gray evaluation   It shows  tons  of different methods to generate grayscale images with different outcomes

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The specific numbers in the question are from CCIR 601  see the Wikipedia link below    If you convert RGB -  grayscale with slightly different numbers   different methods  you won t see much difference at all on a normal computer screen under normal lighting conditions -- try it   Here are some more links on color in general     Wikipedia Luma  Bruce Lindbloom  s outstanding web site  chapter 4 on Color in the book by Colin Ware   Information Visualization   isbn 1-55860-819-2  this long link to Ware in books google com may or may not work  cambridgeincolor   excellent  well-written  tutorials on how to acquire  interpret and process digital photographs using a visually-oriented approach that emphasizes concept over procedure   Should you run into  linear  vs  nonlinear  RGB  here s part of an old note to myself on this  Repeat  in practice you won t see much difference     RGB -   gamma -  Y -  L   In color science  the common RGB values  as in html rgb  10   20   30     are called  nonlinear  or Gamma corrected   Linear  values are defined as  Rlin   R gamma   Glin   G gamma   Blin   B gamma   where gamma is 2 2 for many PCs  The usual R G B are sometimes written as R  G  B   R    Rlin    1 gamma    purists tongue-click  but here I ll drop the     Brightness on a CRT display is proportional to RGBlin   RGB   gamma  so 50  gray on a CRT is quite dark   5   2 2   22  of maximum brightness   LCD displays are more complex  furthermore  some graphics cards compensate for gamma    To get the measure of lightness called L  from RGB  first divide R G B by 255  and compute  Y    2126   R gamma    7152   G gamma    0722   B gamma   This is Y in XYZ color space  it is a measure of color  luminance    The real formulas are not exactly x gamma  but close  stick with x gamma for a first pass    Finally    L    116   Y   1 3 - 16            aspires to perceptual uniformity  and  closely matches human perception of lightness   --   Wikipedia Lab color space

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Check out the Color FAQ for information on this  These values come from the standardization of RGB values that we use in our displays  Actually  according to the Color FAQ  the values you are using are outdated  as they are the values used for the original NTSC standard and not modern monitors

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Here s a paper on how these numbers  or similar ones  were derived   https   web archive org web 20160303201512 http   www cis rit edu mcsl research broadbent CIE1931 RGB pdf

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What is the source of these values    The  source  of the coefficients posted are the NTSC specifications which can be seen in Rec601 and Characteristics of Television   The  ultimate source  are the CIE circa 1931 experiments on human color perception  The spectral response of human vision is not uniform  Experiments led to weighting of tristimulus values based on perception  Our L  M  and S cones1 are sensitive to the light wavelengths we identify as  Red    Green   and  Blue   respectively   which is where the tristimulus primary colors are derived 2  The linear light3 spectral weightings for sRGB  and Rec709  are   Rlin   0 2126   Glin   0 7152   Blin   0 0722   Y  These are specific to the sRGB and Rec709 colorspaces  which are intended to represent computer monitors  sRGB  or HDTV monitors  Rec709   and are detailed in the ITU documents for Rec709 and also BT 2380-2  10 2018   FOOTNOTES   1  Cones are the color detecting cells of the eye s retina   2  However  the chosen tristimulus wavelengths are NOT at the  peak  of each cone type - instead tristimulus values are chosen such that they stimulate on particular cone type substantially more than another  i e  separation of stimulus   3  You need to linearize your sRGB values before applying the coefficients  I discuss this in another answer here

[image-processing] Converting RGB to grayscale/intensity

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