Comment by labawi
5 years ago
> However, sRGB is not linear, meaning that (0.5,0.5,0.5) in sRGB is not the color a human sees when you mix 50% of (0,0,0) and (1,1,1). Instead, it’s the color you get when you pump half the power of full white through your Cathod-Ray Tube (CRT).
While true, to avoid confusion, it might be better rephrased without bringing human color perception or even colors into the mix.
sRGB uses non-linear (gamma) values, which are good for 8-bit representation. However, operating on them as normal (linear) numbers using average (expecting blur) or addition, multiplication (expecting addition, multiplication of corresponding light) - gives nonsensical, mathematically and physically inaccurate results, perceived by any sensor, human or animal as not what was expected.
RGB colorspace in it's linear form is actually very good for calculations, it's the gamma that messes things up.
In simplified monochrome sRGB/gamma space, a value v means k·v^g units of light, for some k and gamma g = 2.2. Attempting to calculate an average like below is simply incorrect - you need to degamma¹ (remove the ^g), calculate and then re-gamma (reapply ^g).
(k*v1^g + k*v2^g)/2 = k*(v1^g + v2^g)/2 != k*((v1+v2)/2)^g
¹ gamma function in sRGB is a bit more complex than f(x) = x^2.2
I always feel we have way too many historical burdens (which were good compromises at the time) in (digital) image/video field.
In no particular order (and some are overlapping), I can immediately think of gamma, RGB/YCbCr/whatever color models, different (and often limited) color spaces, (low-)color depth and dithering, chroma subsampling, PAL/NTSC, 1001/1000 in fps (think 29.97), interlaced, TV/PC range, different color primaries, different transfer functions, SDR/HDR, ..
the list can go on and on, and almost all of them constantly cause problems in all the places you consume visual media (I do agree gamma is one of the worst ones). Most of them are not going anywhere in near future, either.
I often fantasize a world with only linear, 32-bit (or better), perception-based-color-model-of-choice, 4:4:4 digital images (similar for videos). It can save us so much trouble.
That's a bit like saying that we shouldn't use lossy image/video compression.
Do you realize the amount of 'waste' that a 32bpc†, 4:4:4, imaginary color triangle gamut picture would have ?
†it looks like the human eye has a sensitivity of 9 orders of magnitude, with roughly 1% discrimination (so add 2 orders of magnitude). So, looks like you would need at least 37 bits per color with a linear coding, the overwhelming majority of which would be horribly wasted !
I have no issue with lossy compression; but things like 4:2:0 aren't really typical lossy compression. It's like calling resizing an image to half its resolution "compression".
Also lossless compression can reduce plenty of "wasted" bits already (see: png vs bmp).
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I don't think of colorspaces as "historical burdens". I don't like that CRT monitors are brought up every time sRGB is mentioned though. I know it has historical relevance, but it's not relevant anymore, and it's not needed to understand the difference between linear and non-linear colorspaces.
I misunderstood what you mean. Please ignore. On a side note, by colorspace I mainly meant that we can just stick with one with ultra-wide gamut. There are indeed other reasons to have different color spaces.
(Below is my original comment for transparency.)
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Gamma isn't really about CRT; or I should say, they're two different things. The fact CRT has a somewhat physical "gamma" (light intensity varies nonlinearly with the voltage) is likely just a coincidence with the gamma we're talking here.
The reason gamma is used in sRGB is because human eyes are more sensitive to changes in darker area, i.e. if the light intensity changes linearly, it feels more "jumpy" in darker end (which causes perceptible bandings). This is especially an issue with lower color depth. To solve this, we invented gamma space to give darker end more bits/intensity intervals to smooth the perceptive brightness.
>it's not needed to understand the difference between linear and non-linear colorspaces
It absolutely should, since any gamma space would have problem with "averaging", as explained by the GP. Actually, it's so bad that almost all the image editing/representing tasks we have today are doing it wrong (resizing, blurring, mixing..).
This topic has been discussed extensively on Internet, so I'm not going to go into detail too much. A good start point is [1][2].
[1] https://www.youtube.com/watch?v=LKnqECcg6Gw [2] http://blog.johnnovak.net/2016/09/21/what-every-coder-should...
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I don't see why you would avoid talking about it.
As far as I've understood, CRT monitor gamma has basically evolved to become the inverse of human eye gamma :
http://poynton.ca/PDFs/Rehabilitation_of_gamma.pdf
(With some changes for a less accurate, but more visually pleasing/exciting replication of brightness levels ?)
Now, with many modern, digital screens (LCD, LED, e-ink?), as far as I've understood the electro-optical hardware response is actually linear, so the hardware actually has to do a non-linear conversion ?
I'm still somewhat confused about this, as I expected to have to do gamma correction when making a gradient recently, but in the end it looked like I didn't have to (or maybe it's because I didn't do it properly : didn't do it two-way?).
Note that the blog author might be confused there too, as just after he says :
> With these conversions in place, dithering produces (more) accurate results:
– you can clearly see that the new dithered gradient doesn't correspond to the undithered one ! (Both undithered gradients seem to be the same.)
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Are you sure about sRGB being already good enough for averaging ? (As long as we don't want to go to a wider color space of course.)
We have been recently taught how to do it 'properly', and we had to go through CIELAB 76 (which, AFAIK, is still an approximation, as human perception is actually non-euclidean).
If you want physically accurate averaging (resize, blur etc), then RGB is fine, as long as you use linear values (or do long-winded transformed math). AFAIU it is by definition 100% physically accurate. As was said, sRGB uses gamma values, where typical math creates ill effects, as in many if not most typical programs.
If you want to do perceptually uniform averaging of colors, color mixing / generating / artistic effects, that's something else entirely.
I'm not sure what you mean by "physically accurate" ?
No color reproduction is going to be physically accurate, except by accident, since color is an (average) human qualia, not a physical observable.
And especially because whatever the way that the colors are going to be reproduced, there's no guarantee that they will correspond to the same light spectrum than the original, since there's an infinity of spectra corresponding to the same impression for a specific color.
And if you want to do perceptually accurate averaging, you're going to have to work in a perceptually uniform color space ! Which (even linear) sRGB isn't.
All that said, it's perfectly possible that linear sRGB is just good enough for most use cases of averaging, even despite not being perceptually uniform. Especially in OP's example with just 2 shades : black and white.
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My bad, I was wrong – can't have both linearity and perceptual uniformity, and I guess that in use cases like dithering, linearity is more important ?
https://news.ycombinator.com/item?id=25648490