Comment by fireattack
5 years ago
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).
But they are ! Like other lossy forms of compression, chroma subsampling takes advantage of the human visual system's lower acuity for color differences than for luminance.
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...
> It absolutely should
The GP just pointed out that the CRT link is not needed to motivate the talk about linear vs non-linear.
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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.)
sRGB gamma is often approximated to 2.2 [1], but the actual function has a linear section near 0, and a non-linear section with gamma of 2.4, possibly to avoid numerical difficulties near 0.
The document you cite claims that CRT gamma is typically between 2.35 and 2.55.
Human eye gamma can probably be approximated with cieLAB, that is designed to be a perceptually uniform colorspace, which seemingly has a gamma of 3 [2], although it also has a linear section, so maybe slightly lower overall gamma. ciaLAB is not state of the art though in perceptually uniform colorspaces.
[1] https://en.wikipedia.org/wiki/SRGB
[2] https://en.wikipedia.org/wiki/CIELAB_color_space
What I don't like about this whole CRT/gamma topic:
1. It brings in perceptually uniform colorspaces to the discussion, while it's completely unnecessary. Perceptually uniform colorspaces are mostly unsuitable for arithmetic on colors like any other non-linear colorspace.
2. While the sRGB colorspace and a colorspace defined by a CRT monitor's transfer function are closer to perceptually uniform than a linear colorspace, they are still pretty damn far from it. sRGB still does a decent job to prevent banding in dark areas.
3. The sRGB colorspace is not identical to a colorspace defined by a CRT monitor's transfer function.
4. "gamma" is a crude approximation for transfer functions, assuming they follow a power function on all of their domain.
5. This whole thing about CRTs and gamma doesn't matter if you just want to understand that if you want to do arithmetic on color components, then you most probably want it represented in a linear colorspace (didn't even talk about it yet), and most color values you encounter is actually encoded in sRGB, so you want to convert that to linear first, then convert the result back, depending on what your output requires. This is the most widespread bug in computer color and you don't need the history of CRTs to do that, and in fact this has nothing to do with perceptually uniform colorspaces.
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> 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
If you don't explicitly specify the color space you're working in, then you're using some implicitly defined color space in which case you basically need to know what that is (at least roughly).
So traditionally in Windows, way back, when you created a bitmap, wrote some data to it and then drew it, there was no explicit mention of a color space. Instead it was implicit, and it was de-facto non-linear.
These days you can specify[1][2] the color space you're working in, and Windows will then transform the colors into the specified device color space. So then you can specify if you want to work in linear RGB space or say non-linear sRGB.
Unity has something similar[3], which affects how you write shaders, how your textures should be saved etc.
[1]: https://docs.microsoft.com/en-us/previous-versions/windows/d...
[2]: https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf...
[3]: https://docs.unity3d.com/Manual/LinearRendering-LinearOrGamm...
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