Hello,
I'm not an image processing expert, but was trying to write a small program
to convert images to grayscale, just for the fun of it [1].
First I managed to get something working with image/draw, then I
discovered about github.com/disintegration/gift through an old post here.
I realized the output images had a different luminance, what led me to dig
a bit deeper to see how the implementations differed.
image/color [2]:
y := (299*r + 587*g + 114*b + 500) / 1000
gift [3]:
y := 0.299*px.R + 0.587*px.G + 0.114*px.B
The initially funny numbers, weights, match those described in Wikipedia
[4].
I went ahead and compared to what Matlab [5] and Octave [6] do and found a
similar formula.
I did not understand why image/color adds an extra 0.5 (500/1000) to y.
Could anybody give me a clue?
[1] https://gist.github.com/rhcarvalho/5e97f310701528f5a0610415e317b992
[2] image/color:
https://github.com/golang/go/blob/master/src/image/color/color.go#L249
[3] GIFT: https://github.com/disintegration/gift/blob/master/colors.go#L252
[4] https://en.wikipedia.org/wiki/Grayscale#Luma_coding_in_video_systems
[5] Matlab:
https://www.mathworks.com/help/matlab/ref/rgb2gray.html#expand_body_buiz8mj-9
[6] Octave:
https://sourceforge.net/p/octave/image/ci/default/tree/inst/rgb2gray.m#l43
Thanks,
Rodolfo Carvalho
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