What every coder should know about gamma (2016)

https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/

by sph • 1 month ago

127 46 1 month ago

blog.johnnovak.net

LoganDark5• 1 month ago

Pixels with RGB values of (128, 128, 128) emit about half as much light as pixels with RGB values of (255, 255, 255).

Technically, this is not always incorrect, if your working color space is linear and 0 is no light. The problem only comes if you hand that same data to routines or surfaces expecting sRGB or another nonlinear color space (or one where 0 is not no light).

nomel2• 1 month ago

or one where 0 is not no light

Oh, interesting. What's an example of this? Some sort of log space?

LoganDark5• 1 month ago

Oh, I was just listing the constraints. I'm not directly aware of a color space where value 0 is not no light. It would however mean that even if linear, doubling a value relative to 0 wouldn't necessarily double the amount of light.

pavlov• 1 month ago

Most video color spaces have black at a non-zero code value.

The most common 8-bit YUV format (e.g. in MPEG-2) uses a 16-235 range for valid luma values, so black is at 16 and white is at 235.

The reason for leaving this “headroom” and “footroom” had to do both with digitizing analog signals and avoiding clipping during processing.

Someone• 1 month ago

I would think the color spaces of most displays have that, don’t they?

The bevel of a black iPhone is darker than its screen, even when powered off. Similarly, switched off CRT displays aren’t truly black.

dheera• 1 month ago

On which image does the gradation appear more even? It’s the second one!

I felt the first one looked more even. On the first I could tell the difference between every two adjacent bars. On the second one I couldn't tell any difference between the first 4-5 bars.

tobr2• 1 month ago

Probably because it’s displayed on a white background. A gray or black background would have shown the difference more clearly.

account422• 1 month ago

No it's because #000 isn't actually black on a physical monitor, especially a backlit one so you loose contrast which affects the lower end of the range more than it does the brighter colors.

nitwit005• 1 month ago

I felt the same, and I saw multiple people did back in 2016 as well: https://news.ycombinator.com/item?id=12554887

Edit: Looking at the raw pixel data, the cells seem to be (0, 0, 0), (0, 0, 0), (1, 1, 1), (8, 8, 8), so there is indeed no practical difference between the first three.

zokier• 1 month ago

for lot of image processing linearizing srgb is only half-way measure, and you might want to reach for a better colorspace altogether. cielab is obvious example, or maybe oklab/jzazbz/xyb/something else.

raphlinus• 1 month ago

Exactly this, there's rarely a single "correct" colorspace, you make a choice based on expressive goals and constraints. For example, for gradients you almost never want linear, something like Oklab is indeed much better.

The gradient examples between high-chroma colors of similar luminance are highly misleading in my opinion. In that particular case, linear just happens to do well (and device RGB of course poorly), but in other cases linear is not great. For example, blue to white is especially bad, with hue shifts as well as lightness non-uniformity.

You can experiment with this in the interactive tester in my Oklab review[1].

[1] https://raphlinus.github.io/color/2021/01/18/oklab-critique....

leni5362• 1 month ago

Depends. Many times a linear colorspace is exactly what you want.

Espressosaurus• 1 month ago

Linear light, or linear perceptual?

nicebyte2• 1 month ago

there isn't much that is "arcane" about gamma correction itself (as in, the math itself is really simple).

at the fundamental level, if a surface is illuminated with one lightbulb and we add another light bulb, the difference is exteremely noticeable to the human eye. if we add one more lightbulb to a surface that is already illuminated by a hundred light bulbs, there will be no perceptible difference. the exact response can be modeled with a pretty simple power law (with a modification in the low range, as the article mentions).

that's all there really is to "gamma correction". it's a hack that exploits this quirk of the human visual system in order to more efficiently allocate bits for encoding different "lightness" values.

all of the confusion and bugs stem from one or more of the systems in the chain that forms the final image, making an incorrect assumption about what the others are doing. it's a bit like coordinate spaces in that regard.

nicebyte2• 1 month ago

general rule of thumb is that you never want gamma-corrected values if you intend to be doing any sort of math with them. the only exception is when you want to match the behavior of softwares that actually do all the math with pixel values in sRGB space (I think that's what photoshop does but don't quote me on that).

shaggie76• 1 month ago

ICC profiles present a similar problem and remarkably Instagram, ostensibly a photo-sharing web-site, does not respect them when scaling images (or at least it didn't a few years ago when I started posting photos to it from my PC); when I uploaded full-resolution SLR-resolution originals the skin-tones would get all screwed up. If I exported at the final resolution, even with the same ICC profile, they were fine.

blt• 1 month ago

I wonder why we use something polynomial-based instead of something exponential-based like the decibel scale?

mianos• 1 month ago

A power function seamlessly maps 0 to 0 and 1 to 1, easy for black to white.

An exponential/log function requires arbitrary clamping or offsets because you cannot represent pure black, 0, on a pure log scale without hitting negative infinity.

Basically, it fits better, aside from a good map of human perception

hatthew• 1 month ago

This feels like less of a gamma issue and more of a color space and monitor calibration issue. Possibly affected by the fact that oklab didn't exist when this article was written and the general awareness of color spaces was lower.

20k3• 1 month ago

This article contains some pretty major errors, which is kind of surprising to see!

The transformation used to represent the physically linear intensity data either generated synthetically via an algorithm or captured by a linear device (such as a CMOS of a digital camera or a scanner) with the discrete values of the perceptually linear scale is called gamma encoding.

This isn't super correct, and it underscores the biggest issue in this article:

sRGB (and its gamma encoding function) has absolutely nothing to do with perceptual linearity. sRGB is not perceptually linear! The original gamma encoding as far as I'm aware was made to compensate for the nonlinear transfer function of CRTs back in ye olde days. Its true that human vision is nonlinear, but sRGB is not a particularly good match to the perceptual linearity of human vision. Its a really common error to make, and leads to people wondering why we can't use sRGB to blend in if the reason why it was invented is because its perceptually linear

The article goes to compound on this mistake, which is why this is such a problematic misconception:

Interestingly, Photoshop antialiases text using γ=1.42 by default, and this indeed seems to yield the best looking results (middle image). The reason for this is that most fonts have been designed for gamma-incorrect font rasterizers, hence if you use linear space (correctly), then the fonts will look thinner than they should.

This is where the mistakes start to add up

Consider what you're trying to achieve during antialiasing: when rasterising a line, lets say we discover that a pixel is only 40% covered and want to darken it. This means that we want our pixel's brightness to decrease by 40% to a human being. We don't want to emit 40% less light, because that's not what antialiasing is trying to achieve!

Both sRGB and linear colour are the wrong colour spaces to use. You want to blend in a perceptually linear colourspace, and photoshop's 1.42 gamma exponent probably maps better to human vision than 2.2 or 1.0 while being cheaper than a LUV conversion

The standard gamma (γ) value to use in computer display systems is 2.2. The main reason for this is because a gamma of 2.2 approximately matches the power law sensitivity of human vision

The gamma transfer functions are also wrong. Its worth getting hung up on because it actually causes nontrivial errors, especially in the age of hardware accelerated sRGB conversions where doing it correctly is free

vektorsigma• 1 month ago

When antialiasing, I think you do want to model light: a fully black object occluding 40% of a pixel should cause it to emit 40% less light. Linear intensity representations of colour should therefore be used.

When doing a smooth fade-to-black in video, you may want to gradually decrease the amount of emitted light from the whole frame in a way that is smooth to a human. Here I think you should consider how a perceptual space can help.

20k3• 1 month ago

When antialiasing, I think you do want to model light: a fully black object occluding 40% of a pixel should cause it to emit 40% less light. Linear intensity representations of colour should therefore be used.

The antialiasing value you get represents how much the pixel is covered by the glyph in question, and directly represents a desired change in perceptual brightness. There's no physical underlying lighting process, so it doesn't make sense to use physical light units

Blending in linear RGB models different strength lights being mixed together. This is why you do want to blend images together in linear RGB, but not fonts - because its not an underlying light based transport process

To take a direct example: Imagine two cases

1. We blend a white font on a black background

2. We blend a black font on a white background

Using perceptual blending, the antialiasing will be exactly the same efficacy in both cases. Using blending in linear space, these two test cases will look very different and render incorrectly!

adrian_b2• 1 month ago

As you, say the original purpose of gamma was strictly to compensate the non-linearity of CRTs.

The reason why gamma has been preserved in digital television even after CRTs have become obsolete is that it happens to perform a dynamic range compression that allows the use of 8 bits for luminance or for color components without making too visible the steps between adjacent color values.

If you want to encode the color components linearly, you need to use more than 8 bits, preferably the FP16 format, which was originally introduced in GPUs especially for this purpose.

So today the only purpose of gamma is as a method of data size compression that is specific to images, by allowing the reduction of the number of bits per pixel, while keeping acceptable the degradation of the image quality.

It is probable that the standard gamma curves are not optimal for data compression, but the slight improvements in image quality that could have been attained with other curves are not worth the complications that would have been created by abandoning the compatibility with legacy recordings.

arghwhat2• 1 month ago

Just a nit: Post-CRTs, there is no longer a "standard gamma curve", but many different transfer functions and many errors stem from misunderstanding this.

Even within "SDR"/"sRGB", many mistakes crop up from people erroneously mixing content encoded with the piecewise sRGB transfer function with content encoded according to a plain gamma 2.2 transfer function. And this is before we are getting into e.g., incorrect blending spaces or mismatched primaries.

But yes, it is purely a matter of compression, with many options for exactly what dynamic range you need and how you want your content defined (e.g., sRGB, gamma2.2, scRGB, HLG, PQ, ...), with linear light primarily reserved as an intermediate space for color conversions and blending - something your display server and any software working with arbitrary color spaces will be using.

dmitshur• 1 month ago

[...] the only reason to use gamma encoding for digital images is because it allows us to store images more efficiently on a limited bit-length [...]

It's interesting how this part of the trade-off changes when using float16 for color components (as is common when HDR is involved) rather uint8.

Good timing that Safari 27 adds support for srgb-linear and display-p3-linear color spaces.

jbritton• 1 month ago

I thought you corrected gamma in the monitor settings, not in your application code. Am I wrong.

evilturnip• 1 month ago

I work on the open-sourced Jedi Academy game engine, which is based on id tech 3. It has special logic to specifically adjust the gamma of the physical monitor during launch, then restore it upon exit.

This is why when these older games crashed on your PC, the monitor would look all washed out due to the manual gamma adjustments the game made that didn't get restored.

joedrago2• 1 month ago

Nice! I remember being an early Nvidia adopter (RIVA 128, TNT) when all of my fellow LAN party buddies were Voodoo users, and I had to download a custom opengl32.dll which actually adjusted the gamma into tolerable levels so you could see what was going on in Quake.

Dwedit• 1 month ago

This kind of thing is why you need a separate watchdog process that gets notified when the other process terminates.

HappMacDonald• 1 month ago

Erlang/Elixir coder identified :)

joedrago2• 1 month ago

For those interested in this topic, I found minutephysics' treatment of this subject to be really pleasant and easy to grok. I had a talk I gave at work once or twice discussing color conversion and I would play a 90 second snippet from this (attributed appropriately) as it would very quickly crash course people on the reasoning behind an EOTF in a really cute and layman's terms kind of way:

https://www.youtube.com/watch?v=LKnqECcg6Gw

tomhow• 1 month ago

Previously:

What every coder should know about gamma (2016) - https://news.ycombinator.com/item?id=27721094 - July 2021 (50 comments)

What every coder should know about gamma - https://news.ycombinator.com/item?id=12552094 - Sept 2016 (183 comments)

Const-me• 1 month ago

On which image does the gradation appear more even? It’s the second one!

Can’t reproduce. Tested on two monitors on my desk, designer-targeted Benq and cheap laptop. On the Benq, darkest 3 segments are indistinguishable, the 4-th one barely distinguishable. On the laptop, darkest 4 segments are indistinguishable, the 5-th barely distinguishable. However, on the “emitted light intensity” all bars are clearly visible.

Image resizing

“Unsurprisingly, C gives the correct result” On my computers B very similar to A, just a tiny bit darker. While the “correct” C result is a lot lighter than A.

Also from the same section:

B the result of resizing the pattern by 50% directly in sRGB-space (using bicubic interpolation)

Bicubic interpolation is only applicable when enraging images; downsampling is very different problem from interpolation.

What every coder should know about gamma (2016)

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