[HN Gopher] Why does the chromaticity diagram look like that?
___________________________________________________________________
Why does the chromaticity diagram look like that?
Author : samwillis
Score : 254 points
Date : 2024-07-26 18:19 UTC (1 days ago)
(HTM) web link (jlongster.com)
(TXT) w3m dump (jlongster.com)
| SirMaster wrote:
| It's probably good to start with XYZ, but we have much better
| colorspaces now that do a better job at correlating with our
| vision.
|
| Mainly CIE 1976 L',u',v' and even more recently ICtCp from Dolby
| research.
| refulgentis wrote:
| CAM-16. When in doubt, ask the color scientists :)
| anon2345252 wrote:
| Oklab is even better.
|
| https://bottosson.github.io/posts/oklab/
| refulgentis wrote:
| No, it's not, by definition. It's one matrix multiplication
| to do an approximation of it. More here:
| https://news.ycombinator.com/item?id=41081832
|
| The only claim to superiority it makes is gradients, and
| that's a category error: they blend polar opposite hues in
| the Cartesian space (i.e. x / y / z), rather than polar
| (i.e. h/s/l). Opposite hues mean lerp'ing in cartesian
| brings it through the center of the circle, 0 saturation.
| Thus, blue and yellow _do_ combine to a off-white.
| Engineering around it indicates something fundamentally
| off, much less that it is _better_. I don 't ascribe ill
| intent but I do worry very much about how widely this is
| misunderstood.
| contravariant wrote:
| The xyY colour space is designed such that the colours of light
| you get by blending two points all lie on the line between the
| two corresponding points. This makes it extremely helpful when
| you want to figure out which colours you can make with a
| particular set of primaries. Similarly you can draw the colours
| corresponding to pure wavelenghts and figure out the entire
| space of physically possible colours by taking its convex
| closure.
|
| These features are not really replicable in any other colour
| space, at best you can use a linear transformation of it (which
| XYZ already is, and it has almost all properties you could want
| of a choice of basis).
| JKCalhoun wrote:
| That's true. And I will add that the viewable color gamut of
| a display can be depicted with a simple triangle on the xyY
| plot. All you need to know are the three chromaticity values
| for the reg, green and blue phosphors -- they make up the
| three corners of the triangle.
| paipa wrote:
| I don't think starting with XYZ and color matching functions is
| a good idea. LMS and cone response functions are a more
| fundamental and intuitive description of human color response,
| so if you're going to bother with XYZ at all, you should arrive
| there from first principles, via LMS.
| qingcharles wrote:
| And when you're done with XYZ, check out XYB used in JPEG XL
| and jpegli:
|
| https://giannirosato.com/blog/post/jpegli-xyb/
| hoherd wrote:
| Here's another really interesting exploration of color spaces
| https://ericportis.com/posts/2024/okay-color-spaces/
| kurthr wrote:
| I prefer this. The failure to discus Lab and OKLab in the main
| link is quite odd.
|
| Also, I'd mention to those who think that violet/magenta aren't
| "real" colors that the red X decays more slowly than the blue Z
| at short wavelengths so you can get saturated violet/magenta
| single wavelength colors (not well represented on the standard
| chroma charts) below 400nm at high power. Of course they aren't
| efficient for monitors (even blue isn't) and they're dangerous
| to look at for any length of time. But if you see a (single
| wavelength) violet/magenta laser, it's time to look away or
| shut your eyes.
| jlongster wrote:
| Yeah that article is better. I'm the author and I wrote this
| only for me as I studied it, it's not great as a way to
| describe it to others
|
| I wanted to start from the very beginning and as far as I
| know Lab and OKLab didn't come later. Studying the 1931
| studies and such was a start, and I wanted to later bring up
| all the other things we've learned since then, but haven't
| had time to write more about it
| PaulHoule wrote:
| That particular version of the chromaticity diagram makes it look
| like the colors missing from your display are various shades of
| laser pointer green as opposed to all the shades of red and blue
| that are missing because really saturated red and blue primaries
| are too dim (per unit of energy) to use.
|
| See https://nanosys.com/blog-archive/2012/08/14/color-space-
| conf...
|
| I learned a lot more about color management than I wanted to know
| in the progress of making red-cyan stereograms because I found
| when I asked for sRGB red I was getting something like
| (180,16,16) on my high gamut monitor which resulted in serious
| crosstalk between the channels.
|
| Right now I am working with a seamstress friend on custom printed
| fabrics and I have a flower print where yellow somehow turned to
| orange in the midst of processing the image and I want to get it
| debugged and thoroughly proofed before I send out the order... I
| am still learning more than I want to know about color
| management.
| ThrowawayTestr wrote:
| >Right now I am working with a seamstress friend on custom
| printed fabrics and I have a flower print where yellow somehow
| turned to orange in the midst of processing the image
|
| That's why Pantone makes so much money.
| lolinder wrote:
| And, importantly, that's why Pantone isn't just a freeloader
| making money off of nothing the way that some of the more
| clickbaity parts of the internet represent them. They're not
| solving an easy problem, if they were they wouldn't get paid.
| ttoinou wrote:
| Aren't they (present humans) simply profiting from work
| done decades ago (past humans, not them) through patents or
| others kinds of IP / protections granted by governments ?
| Surely it was original and useful before but now it's part
| of humans knowledge base
| itishappy wrote:
| Somewhat, but that work has utility to this day. You can
| find competing products out there if you don't want to
| pay Pantone's rates, but there's a reason many people
| still work with Pantone. It's a universal language of
| sorts.
|
| https://www.culturehustleusa.com/products/freetonebook
| marcosdumay wrote:
| What defines them as "leeches" or not is if they keep
| adding value to their work.
|
| Passively profiting from some work done decades ago makes
| them something you want to destroy; increasingly
| improving their catalogue with valuable new tones and
| knowledge makes them something you want to protect.
| Dylan16807 wrote:
| They solve a very real problem with most of their products.
| Universal physical references and supplies are great.
|
| But charging for the libraries that list a basic sRGB or
| CMYK code for each Pantone color is a pain in the ass
| leech.
| derefr wrote:
| > as opposed to all the shades of red and blue that are missing
| because really saturated red and blue primaries are too dim
| (per unit of energy) to use.
|
| Would another way to put that be, that the chromaticity diagram
| _could_ keep going southeastward (i.e. the XYZ color-space
| _could_ have the X and Z activation functions extended leftward
| and rightward), but due to the frequencies continuing on the
| spectral line, that area of the diagram would necessarily be
| made mostly of infrared and ultraviolet frequencies that we can
| 't see?
| ProllyInfamous wrote:
| From your linked article:
|
| >The colored region of the plot, labeled "visible region",
| contains points in the space representing colors humans can
| see, while the non-colored region contains points representing
| physically impossible cone stimulations.
|
| Visual perception goes _way way way beyond just what the eyes '
| cones can physically see_ -- you've got an entire brain back
| there trying to interpret optical nerve phenomena!
|
| For a nice brain tickler, look up _physically impossible to
| conically detect_ Chimerical Colors (e.g. stygian blue; self-
| luminous red; hyperbolic orange), which can be seen _without
| actually having been seen_ +
|
| +: https://www.perplexity.ai/search/please-provide-
| chimerical-c...
|
| W: https://en.wikipedia.org/wiki/Impossible_color
| ahazred8ta wrote:
| > Right now I am working on custom printed fabrics
|
| You almost certainly need to work in the vendor's CMYK/SWOPv2
| color space profile to get the colors to come out right. Ask
| them. Saturated RGB doesn't convert well. Plan B: desaturate to
| a slightly grayish yellow.
|
| One example: https://ctnbee.com/en/upload
| refulgentis wrote:
| TL;DR: Imagine color space has 3 dimensions in polar coordinates.
|
| - hue, the angle. your familiar red, orange, yellow, green,
| blue...
|
| - saturation/chroma, radial distance from center. intensity of
| the pigment
|
| - lightness, top to bottom, white to black
|
| The XY diagram shows 3d color space, from the top, in XYZ.
|
| XYZ is a particular color space that Nathan Myhrvold picked at
| Microsoft in the early 90s.
|
| There is no privileged "correct" color space, they're developed
| based on A/B tests and intuition by color scientists.
|
| However, there are _more_ correct color spaces, in that color
| science matters and is a real field. Commonly agreed state of the
| art is CAM16.
|
| It's a significant mistake that Oklab is the first space with
| significant mindshare since HSL, it was a quick hack by a ex-game
| developer to make something akin to CAM16 with just one matrix
| multiplication.
|
| CAM16 conversions involve significantly more than one matrix
| multiplication. But, its ~400 lines of code, and you can do
| millions a second on modern hardware.
|
| The lightness scale is _way_ off from scientific color spaces,
| and thus it can't be used to create simple rules like "40 delta
| L* ~= 3.0 contrast ratio, 50 delta L* ~= 4.5 contrast ratio".
| Instead you're still manually plugging colors into a contrast
| checker :(
|
| Then again, its still a step forward. It's even more maddening
| HSL was used for so long: it's absolutely absurd, ex. lightness =
| average of two highest RGB components. Great for demo hacks in
| 1976, not so great in 2016.
| klysm wrote:
| I think a good explanation of color spaces might be starting at a
| camera sensor with a bayer array and how that's processed.
| avidiax wrote:
| I'm not so sure.
|
| I think it's really important to understand spectral colors and
| metamerism.
|
| If you start at the Bayer array, you are going to have an odd
| discussion about how the bayer filters have spectral transfer
| functions that aren't directly related to the cones in our
| eyes, nor any color space's primaries, etc.
|
| It's going in the deep end.
| contravariant wrote:
| Starting with the function of the human eye seems like a good
| choice, I mean how else would you explain why a Bayer matrix
| only has 3 colours?
| JKCalhoun wrote:
| Or why there are two green pixels in the Bayer pattern for
| every red and blue...
| radicality wrote:
| Kinda related, but does someone maybe have a good set of links to
| help understand what HDR actually is? Whenever I tried in the
| past, I always got lost and none of it was intuitive.
|
| There's so many concepts there like: color spaces, transfer
| functions, HDR vs Apple's XDR HDR, HLG vs Dolby Vision, mastering
| displays, max brightness vs peak brightness, all the different
| hdr monitor certification levels, 8 bit vs 10bit, "full" vs
| "video" levels when recording video etc etc.
|
| Example use case - I want to play iPhone-recorded videos using
| mpv on my MacBook. There's hundreds of knobs to set, and while I
| can muck around with them and get it looking close-ish to what
| playing the file in QuickTime/Finder, I still have no idea what
| any of these settings are doing.
| wongarsu wrote:
| HDR is whatever marketing wants it to be.
|
| Originally it's just about being able to show both really dark
| and really bright colors. Something that's really easy if each
| pixel is an individual LED, but that's very hard in LCD
| monitors with one big backlight and pixels are just dimmable
| filters for that backlight. Or alternatively on the sensor side
| the ability to capture really bright and really dark spots in
| the same shot, something our sensors are much worse at than our
| eyes, but you can pull some tricks.
|
| Once you have that ability you notice that 8 bits of brightness
| information isn't that much. So you go with 10 bit or 16 bits.
| Your gamma settings also play a role (the thing that turns your
| linear color values into exponential values).
|
| And of course the people who care about HDR have a big overlap
| with people who care about colors, so that's where your color
| spaces, certifying and calibrating monitors to match those
| color spaces etc comes in. It's really adjacent but often just
| rolled in for convenience.
| radicality wrote:
| More bits to store more color/brightness etc makes sense.
|
| I think my main confusion has usually been that it all feels
| like some kind of a... hack? Suppose I set my macbook screen
| to max brightness, and then open up a normal "white" png.
| Looks fine, and you would think "well, the display is at max
| brightness, and the png is filled with white", so a fair
| conclusion would be thats the whitest/brightest that screen
| goes. But then you open another png but of a more special
| "whiter white", and suddenly you see your screen actually can
| go brighter! So you get thoughts like "why is this white
| brighter", "how do I trigger it", "what are the actual limits
| of my screen", "is this all some separate hacky code path",
| "how come I only see it in images/videos, and not UI
| elements", "is it possible to make a native Mac ui with that
| brightness".
|
| In any case, thanks for the answer. I might be overthinking
| it and there's probably lots of historical/legacy reasons for
| the way things are with hdr.
| wongarsu wrote:
| > there's probably lots of historical/legacy reasons for
| the way things are with hdr
|
| That's pretty much it. If you use a HDR TV it will usually
| work like you describe. It would display the same white for
| a normal white PNG and an "even whiter" "HDR" PNG.
|
| Apple's decision makes sense if you imagine SDR (so not-
| HDR) images as HDR images clipped to to some SDR range in
| the middle of the HDR range (leading to lots of over- and
| underexposure in the SDR image). If you then show them
| side-by-side of course the whitest white in the HDR range
| is whiter than the whitest white in the SDR image. Of
| course that's a crude simplification of how images work,
| but it makes for a great demo: HDR images really pop and
| look visually better. If you stretched everything to the
| same brightness range the HDR images wouldn't be nearly as
| impressive, just more detail and less color banding. The
| marketing people wouldn't like that
| crazygringo wrote:
| It is all extremely hacky.
|
| Because HDR allows us to encode brightnesses that virtually
| no consumer displays can display.
|
| And so deciding how to display those on any given display,
| on a given OS, in a given app, is making whatever "hacky"
| and totally non-standardized tradeoffs the display+OS+app
| decide to make. And they're all different.
|
| It's a complete mess. I'm strongly of the opinion that HDR
| made a fundamental mistake in trying to design for "ideal"
| hardware that nobody has, and then leaving "degraded"
| operation to be implementation-specific.
|
| It's a complete design failure that playing HDR content on
| different apps/devices results in output that is often too
| dark and often has a telltale green tint. It's ironic that
| in practice, something meant to enable higher brightness
| and greater color accuracy has resulted in darker images
| and color that varies from slightly wrong to totally wrong.
| duskwuff wrote:
| > So you get thoughts like [...] "what are the actual
| limits of my screen" [...]
|
| Some of the limitations, at least in Apple's displays, are
| thermal! The backlight cannot run at full brightness
| continuously across the full display; it can only hit its
| peak brightness (1600 nits) in a small area, or for a short
| time.
| suzumer wrote:
| While one commenter had it somewhat right that HDR has to
| do with how bright/dark an image can be, the main thing HDR
| images specify is how far ABOVE reference white you can
| display. With srgb, 100 percent of all channels is 100
| percent white (brightness of a perfect lambertian
| reflector). Rec 2100 together with rec 2408 specify modern
| hdr encoding, where 203 nits is 100 percebt white, and
| above that would be anything brighter (light sources,
| specular reflection, etc). So if a white image encoded in
| sdr looks dimmer than hdr for non specular detail, that is
| probably encoding or decoding error.
| dahart wrote:
| One motivation for HDR is having absolute physical units,
| such as luminance in candelas per square meter. You can
| imagine that might be a floating point value and that 8
| bits per channel might not be enough.
|
| The problem you're describing is that color brightness is
| relative, but if you want physical units and you have a
| calibrated display then adjusting your brightness is not
| allowed, because it would break the calibration.
|
| Another reason for HDR is to allow you to change the
| "exposure" of an image. Imagine you take a photo of the sun
| with a camera. It clips to white. Most of the time even the
| whole sky clips to white, and clouds too. With a film
| camera, once the film is exposed, that's it. You can't see
| the sun or clouds because they got clamped to white. But
| what if you had a special camera that could see any color
| value, bright or dark, and you could decide later which
| parts are white and which are black. That's what HDR gives
| you - lots of range, and it's not necessarily all meant to
| be visible.
|
| In computer graphics, this is useful for the same reason -
| if you render something with path tracing, you don't want
| to expose it and throw away information that happens to get
| clamped to white. You want to save out the physical units
| and then simulate the exposure part later, so you don't
| have to re-render.
|
| So that's all to say- the concept of HDR isn't hacky at
| all, it's closer to physics, but that can make it a bit
| harder to use and understand. Others have pointed out that
| productized HDR can be a confusing array of marketing mumbo
| jumbo, and that's true, but not because HDR is messed up,
| that's just a thing companies tend to do to consumers when
| dealing with science and technology.
|
| I was introduced to HDR image formats in college while
| studying physically based rendering, and the first HDR
| image format I remember was Greg Ward's .hdr format that is
| clever- 8 bits mantissa per channel and an 8 bit shared
| exponent, because if, say, green is way brighter than the
| other channels, you can't see the dark detail in red &
| blue.
| bscphil wrote:
| > Originally it's just about being able to show both really
| dark and really bright colors.
|
| Sort of. The primary significance of HDR is the ability to
| specify absolute luminance.
|
| Good quality SDR displays were already very bright and
| already had high native dynamic range (a large ratio between
| the brightest white and darkest black). The issue was that
| the media specifications did not control the brightness in
| any way. So a 1.0 luminance pixel was just whatever the
| brightest value the display could show (usually tuned by a
| brightness setting). And a 0.0 luminance pixel was just the
| minimum brightness the display could show (unfortunately,
| usually also affected by the brightness setting thanks to
| backlighting).
|
| What HDR fundamentally changes is not the brightness of
| displays or their dynamic range (some HDR displays are worse
| than some older SDR displays when it comes to dynamic range),
| but the fact that HDR media has absolute luminance. This
| means that creators can now make highlights (stars,
| explosions) close to the peak brightness of a bright display,
| while diffuse whites are now dimmer.
|
| Prior to HDR, a good bright display was just a calibrated
| display with its brightness incorrectly turned up too high,
| making _everything_ bright. With HDR a good bright display is
| a display calibrated with _correct_ brightness and the
| ability to show highlights and saturated colors with much
| more power than a normal white.
|
| You're right about higher bit depths, though. Because HDR
| media describes a much wider dynamic range on a properly
| calibrated display (though not necessarily on a typical over-
| bright SDR display), it has a different gamma curve to
| allocate bits more appropriately, and is typically without
| any banding in only 10 or 12 bits.
| https://en.wikipedia.org/wiki/Perceptual_quantizer
| ttoinou wrote:
| You can start by understanding the physics behind high dynamic
| range. Any real world analog value can have a tremendous
| dynamic range, it's not just light : distances, sound, weight,
| time, frequencies etc. We always need to reduce / compress /
| limit / saturate dynamic range when converting to digital
| values. And we always need to expand it back when reconverting
| to an analog signal
| jasomill wrote:
| A summary, with pictures:
|
| https://cdn.theasc.com/curtis-clark-white-paper-on-hdr-asc.p...
|
| To better understand the "knobs", consider opening up your
| iPhone videos in DaVinci Resolve[1] and playing around with the
| scopes and tools in the color panel.
|
| [1]
| https://www.blackmagicdesign.com/products/davinciresolve/tra...
| meindnoch wrote:
| Mostly correct, but I don't understand what the author is trying
| to do in the last section, where they try to fill the locus by
| generating spectra with two peaks and projecting it into the
| chromaticity diagram. Why do it like that?
|
| This is how you should do it:
|
| - You pick a Y value. This is going to be the luminance of your
| diagram.
|
| - For each pixel inside the area bounded by the spectral locus
| (and the line of purples - the line connecting the two endpoints
| of the locus) you take its x, y coordinates.
|
| - Together these 3 values specify your color in the CIE xyY color
| space. Converting from xyY to XYZ is trivial: X = Y / y * x, Y =
| Y, Z = Y / y * (1 - x - y)
|
| - You map these XYZ values into your output image's color space
| (e.g. sRGB). If a given XYZ value maps outside the [0,1] interval
| in sRGB, then it's outside the sRGB gamut, and you may clip the
| values to the closest valid value inside the gamut.
| jlongster wrote:
| Author of the article here; I wasn't able to understand how to
| get that to work, and I talked about that in the post. This
| demo is doing that: https://jlongster.com/why-chromaticity-
| shape#block-31f373
|
| The problem is "for each pixel inside the area". I could have
| done that, and then clipped the output by that shape. The
| problem is this doesn't answer why the shape is this way at all
| because you are using the shape itself to clip the output. It
| felt fake.
|
| I do think this is what is most common though. I was trying to
| understand a more rigorous approach, and the one where you
| generate spectra and try to fill it is described here:
| https://clarkvision.com/articles/color-cie-chromaticity-and-...
|
| That feels like a more rigorous approach, but clipping is
| probably "good enough" too
| meindnoch wrote:
| Ok, so you're asking why every visible color _has to_ lie
| within the bounds of the spectral locus in the chromaticity
| diagram?
|
| The reasoning is simple:
|
| 1. Spectral colors are basis vectors of the color spectrum.
| I.e. every possible spectrum can be thought of as a weighted
| sum of infinitely many Dirac deltas. With nonnegative
| weights, in particular, so it's a so-called _conical_
| combination (i.e. linear combination with nonnegative
| weights).
|
| 2. Taking the inner product with color matching functions is
| a linear transformation from this infinite dimensional space
| spanned by spectral colors to a 3-dimensional space.
| Linearity means that weighted sums are preserved, that is:
| every possible color spectrum's XYZ values are going to be
| the weighted sum of the spectral colors' XYZ values. And
| because the XYZ color matching functions are nonnegative
| everywhere, conical combinations are also preserved.
|
| 3. And finally, the conversion from XYZ to xyz is such, that
| it turns conical combinations into _convex_ combinations
| (i.e. conical combinations where the weights sum to 1). It 's
| easy to verify this with pen and paper.
|
| It follows, that every color on the xy chart is going to be a
| convex combination of xy points corresponding to spectral
| colors, which geometrically means that they're going to lie
| inside the spectral colors' convex hull.
| _wire_ wrote:
| This article illustrates the theory and math that lead to the
| horseshoe diagram in a very approachable style that is as simple
| as possible without being too simple.
|
| A Beginner's Guide to (CIE) Colorimetry -- Chandler Abraham
|
| https://medium.com/hipster-color-science/a-beginners-guide-t...
| GrantMoyer wrote:
| In my opinion, plotting chromaticity on a Cartesian grid -- by
| far the most common way -- is pretty misleading, since
| chromaticity diagrams use barycentric coordinates (and to be
| clear, I blame the institution, not the author). The effect is
| that the shape of the gamut looks skewed, but only because of how
| it's plotted; the weird skewedness of a typical XYZ chromaticity
| diagram doesn't represent anything real about the data.
|
| Instead, a chromaticity diagram is better thought of as a 2D
| planar slice of a 3D color space, specifically the slice through
| all three standard unit vectors. From this conception, it's much
| more natural to plot a chromaticity diagram in an equilateral
| triangle, such as the diagram at [1]. A plot in a triangle makes
| it clear, for instance, that the full color gamut in XYZ space
| isn't some arbitrary, weird, squished shape, but instead was
| intentionally chosen in a way that fills the positive octant
| pretty well given the constraints of human vision.
|
| [1]: https://physics.stackexchange.com/questions/777501/why-is-
| th...
| drmpeg wrote:
| Here's a video that shows the concept. Each frame shows the
| allowable colors for a particular brightness in Rec. 709 YCbCr
| space.
|
| https://www.w6rz.net/chromacity.mp4
| jacobolus wrote:
| The CIE 1976 u'v' chromaticity diagram is skewed to be closer
| to perceptually uniform, and is probably overall a better
| picture than the xy chromaticity diagram or than the
| equilateral triangle picture.
|
| X, Y, and Z dimensions are somewhat arbitrary, so plotting xy
| inside an equilateral triangle isn't inherently more correct
| than plotting it inside an isosceles right triangle.
|
| If you want you can use something closer to the cone cell
| responses as your coordinates, and the result may be
| conceptually clearer, but any 2d picture like this is going to
| be misleading to viewers who don't have a sophisticated
| understanding of how vision works and what the diagram means.
| meindnoch wrote:
| Indeed.
|
| But the chromaticity diagram is not a slice through the color
| space; rather a central projection from the origin onto the
| X+Y+Z=1 plane.
| VanillaCafe wrote:
| I thought this might be a useful article because I've often had a
| similar question. But there's a diagram that has text:
|
| > _More simply put: imagine that you have red, green, and blue
| light sources. What is the intensity of each one so that the
| resulting light matches a specific color on the spectrum?_
|
| > _..._
|
| > _The CIE 1931 color space defines these RGB color matching
| functions. The red, green, and blue lines represent the intensity
| of each RGB light source:_
|
| This seems very oddly phrased to me. I would presume that what
| that chart is actually showing is the response for each color of
| cone in the human eye?
|
| In which case it's not a question of "intensity of the light
| source" but more like "the visual response across different
| wavelengths of a otherwise uniform intensity light source"?
|
| ... fwiw, I'm not trying to be pedantic, just trying to see if
| I'm missing the point or not.
| jlongster wrote:
| I'm the author of the article and the intensity is referring to
| the level of the light source used in the study to generate the
| data. See the study explained here: https://medium.com/hipster-
| color-science/a-beginners-guide-t...
|
| but you're right, the intensity needed of each R, G, and B
| light sources to produce the correct color is directly related
| to how our eyes perceive each of those sources, so yes you are
| correct
| GrantMoyer wrote:
| The wording on the article is correct, despite being confusing.
| The CIE 1931 RGB primaries each stimulate multiple types of
| cone in human eyes, so the RGB Color Matching Functions (CMFs)
| don't represent individual cone stimulations.
|
| However, the CMFs for LMS space[1] _do_ directly represent
| individual cone stimulations over. Like the CIE RGB CMFs, the
| LMS CMFs can also be thought of as the required intensities of
| three primariy colors required to reproduce the color of a
| given spectrum. The reason these two definitions coorespond for
| LMS space is that each primary would stimulate only one type of
| cone. However, unlike CIE RGB, no colors of light which
| stimulate only one type of cone physically exist.
|
| Finally, CIE RGB and LMS space are linear transformations of
| each other, so the CIE RGB CMFs are linear combinations of the
| LMS CMFs, so each CIE RGB CMF can be though of as representing
| a specific linear combination of cone stimulations (the
| combination excited by the primary color).
|
| I often find it easiest to reason about these color spaces in
| terms of LMS space, since it's the most physically
| straightforward.
|
| [1]: https://en.m.wikipedia.org/wiki/LMS_color_space
| carlosjobim wrote:
| I think the explanation is simple: Color is light and it is
| linear going from ultraviolet to blue to green to yellow to red
| to infrared. It's just a line.
|
| In physical reality, there exists no purple light. Our minds make
| up all the shades of purple and magenta between blue and red when
| our eyes receive both red and blue light.
|
| So in order to include the magentas, you need to draw another
| line between blue and red. Meaning you have to bend the real
| color line. And that's what we see in the chromaticity diagram.
| tobinfricke wrote:
| Wavelength (or frequency) is linear but light, in general, is
| made up of many wavelengths -- an entire spectrum.
| carlosjobim wrote:
| Each wavelength of visible light corresponds to a color on
| the gradient from blue-green-yellow-red. Purple or magenta
| colors do not exist as light and only exists in our minds.
| That's why rainbows do not contain any of these colors.
| ianburrell wrote:
| Purple totally exists, but isn't a single wavelength of
| light. It is multiple wavelengths of light. Physical colors
| are all blends of wavelengths.
|
| Displays are tricking the eye by showing three single
| colors that look like real color.
| carlosjobim wrote:
| As a hue, magenta and purple shades do not physically
| exist in the electromagnetic spectrum. All hues on the
| gradient blue-green-yellow-red exist and can be generated
| by a single wavelength of radiation.
|
| You can test this in physical reality with a prism, which
| will never show purple shades, because it is an
| extraspectral color that is made up in our minds.
|
| Color can thus exist as pure in physical reality.
| However, our eyes can maybe not perceive colors purely,
| since our receptors overlap each other.
| ianburrell wrote:
| Colors are not single wavelengths. If you are redefining
| what color means, you should use a different word to
| reduce confusion. Maybe spectral color.
|
| Secondary colors are colors. Notice that the hue on color
| wheel includes magenta and purple because it includes
| mixtures of the primary. Magenta and purple exist on
| electromagnetic spectrum but not as single wavelengths.
|
| There are imaginary colors that are represented in color
| space but not by physical light spectrum. But purple and
| magenta are not imaginary. As can tell from the Roman
| emperors' clothing.
| carlosjobim wrote:
| The discussion on this specific graph is fairly
| scientific and not about cultural color perception. I
| changed the wording to "hue" to be more clear. Magenta
| and purple absolutely do not exist on the electromagnetic
| spectrum as their own wavelengths. Every other hue of
| other colors we can perceive does.
|
| "Primary" and "secondary" colors are not scientific
| terms, but cultural terms.
|
| The evidence is right there in physical reality, a
| rainbow or a prism will not include magenta/purple,
| because the colors between red and blue are not part of
| the spectrum. It is an amazing thing that we can make up
| these shades in ur minds.
|
| But I was wrong in my original comment, because the red-
| blue connection can also be done by making a color wheel,
| ie making a complete curve.
| mncharity wrote:
| > Color is light
|
| For an ELI5 on a "maybe teach color better by emphasizing
| spectra?" side project, I went for hard disjointness on "color"
| vs "light". Distinguishing world-physics-light from wetware-
| perception-color. Writing not "red light", but "\"red\" light".
| So physical spectra were grayscale, on nm and energy. Paired
| with perceptual spectra in color, on hue angle and luminosity.
| And both could be wrapped around a 3D perceptual color space
| (tweening the physical spectra from nm to hue). Or along a 2D
| non-primate mammalian dichromat space, to emphasize the wetware
| dependence. Misconceptions around color are so very pervasive,
| K-graduate, that extreme care for clarity seems helpful.
| Leftium wrote:
| The https://oklch.com color picker shows another way to represent
| colors:
|
| - The 3D version looks like a mountainscape of colors
|
| - L(ightness), C(hroma), and H(ue) are orthogonal 2d slices of
| this mountainscape
|
| ---
|
| And this software renders 3D chromaticity (gamut?) diagrams:
| https://youtu.be/FdFpJFSTMVw?t=679
| tylerneylon wrote:
| This page is also a beautiful explanation of color spaces, with
| chromaticity explained toward the end:
| https://ciechanow.ski/color-spaces/
|
| Note that many of the diagrams are interactive 3d graphics (I
| didn't realize that at first, and it makes the page more
| interesting.)
| tylerneylon wrote:
| I have a question for fellow color science nerds. I've been
| reading through Guild's original data:
| https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1932...
|
| However, I'm having trouble understanding the meaning of the
| numbers in table 4. Does anyone understand all the columns there?
|
| What I'm particularly interested in is finding the unnormalized
| coefficients from the color matching experiments, or some way to
| un-normalize those coefficients. (By "those coefficients," I mean
| the trichromatic coefficients u{a,b,c}_\lambda listed in table
| 3.) I don't know if that data is in table 4 so maybe those are
| two separate questions.
| mncharity wrote:
| Does anyone know of a nice "pedagogical" color space? That is,
| one optimized for teaching and learning, for correctness rather
| than for simple math? Where the space's highly-noticeable
| characteristics are actual features of human perception, rather
| than the usual mess of "nope, that too is a model artifact"
| (mostly from optimizing for computation). And full-gamut, well
| behaved out to spectral locus. And with at least somewhat linear
| hues and color combination. Sort of the Munsell niche, but full
| gamut, and this century.
|
| I wasn't able to find anything even close, for a "maybe teach
| color better by emphasizing spectra?" side project, so I kludged.
| CAM16UCS as state-of-the-art for perceptual color, untwisted with
| Jzazbz for linear hues (it also sanity checked absolute
| luminosity), with a rather-unprincipled mashing down of CAM's
| IIUC-non-perceptual near-locus silly blue tail. Implemented as
| lookup tables. If there is any related work out there, I'd love
| to hear of it. Tnx.
| suzumer wrote:
| Cam16 (as opposed to cam16 ucs) is perception based. It
| calculates chroma, lightness, and hue, and is based on the
| munsell color system. Hellwig and Fairchild recently simplifed
| the model mathematically, improving it's chroma accuracy.(
| http://markfairchild.org/PDFs/PAP45.pdf) Another, simpler,
| model is CIELAB, which outputs paramters L, a, and b, where L
| is lightness, hypot(a,b) is chroma, and arctan2(b,a) is the
| hue.
| mncharity wrote:
| Thanks! IIRC(fuzzily - it's been a while), I chose -UCS for a
| more euclidean color difference metric - I should review
| that. My even fuzzier recollection, is CIELAB's visible gamut
| shape is very artifacty[1], perhaps misleadingly representing
| the volume outside sRGB/P3 for instance.
|
| The pedagogical objectives of playing well with full visible
| 3D gamut, and spectral locus, and of avoiding shape artifacts
| (concavities, excursions), are... non-traditional.
| Characteristics which could be happily traded away in
| traditional uses of color spaces, for characteristics like
| model math and simplicity which here have near-zero value
| (lookup tables satisficing). And were - most spaces have "oh
| my, that's a hard downselect" bizarre visual hulls, and
| topologies outside of P3 or even sRGB can get quite strange.
| Thus the need to untwist CAM16's curving hue lines - they're
| not bad within sRGB, but by the time they hit visible hull,
| yipes, I recall some as near parallel to hull.
|
| Having a color space to play with as a realistic 3D whole,
| seems not the kind of thing we collectively incentivize. A
| lot of science education content difficulty seems like that.
|
| [1] https://commons.wikimedia.org/wiki/File:Visible_gamut_wit
| hin...
| suzumer wrote:
| CAM16's hue lines are curved by design. Hue is not linear
| with regards to xy chromaticity, as evidenced by the Abney
| effect[1].
|
| [1] https://en.wikipedia.org/wiki/Abney_effect
| mncharity wrote:
| But maybe not _this_ [1] non-linear? Fun if real. But
| perhaps fitting was done within a gamut folks care more
| about, and model math then induced artifacts at the
| margins of the full visible gamut? I'd really love to
| know if that blue tail represents real perception.
|
| [1] https://www.researchgate.net/profile/Volodymyr-
| Pyliavskyi/pu... [png] from https://ojs.suitt.edu.ua/inde
| x.php/digitech/article/download... [PDF dl] (Curiously,
| bing image search has this figure, but google doesn't.)
| Daub wrote:
| > Does anyone know of a nice "pedagogical" color space? Where
| the space's highly-noticeable characteristics are actual
| features of human perception
|
| When talking with students about color, I find the HSL space
| the easiest to employ. From a color maths point of view I have
| been told that it is very messy, which is one reason why Adobe
| stopped using it in version 3 of Photoshop. But from the
| perceptual point if view, it is the artists favorite.
|
| Perhaps a better option is the Muncell color space. Muncell
| chopped up the entirety of the color domain into thousands of
| small chunks. The distance between each chunk was a single
| units of 'barely perceptual difference' which he established
| through meticulous user testing. Hence the green domian was
| much larger than the yellow. The story behind his development
| of this space makes for facinatting reading. He was an artist
| (a painter) yet his work paved the way for more modern spaces.
| mncharity wrote:
| > When talking with students about color, I find the HSL
| space the easiest to employ. [...]
|
| Nod. I was exploring how/whether an emphasis on spectra might
| be used to more successfully explain and teach color.
| Motivated by observations of wide-spread profound failure,
| like first-tier graduate students saying "the Sun doesn't
| have a color; it's rainbow color", and color instruction
| content, err, exercising diverse artistic license. So I'd
| hoped for Munsell-like web interactives, but with principled
| coupling to spectra. Hopefully as easily understood as HSL,
| if less convenient as a more-abstracted artist UI. I like
| implicit curriculum ("things noticed in passing provide
| insight and invite exploration"), and am leery of science
| education graphics' traditional "some aspects done with great
| care for correctness, mixed with others of utter bogosity,
| with students unable to tell which is which". And so wanted
| to avoid the high-profile "no, that too is merely a model
| artifact, not reality" of say CIE 1931 chromaticity diagrams
| ("such tiny sRGB coverage!", "so much green!", "a prism
| shaped solid!"). And also its frequently sloppy graphics
| (incorrect colors, misleading handling of out-of-gamut
| colors, misplaced white point not intersected by blackbody
| curve). I had hope for linear-hue absolute-physical-
| brightness Jzazbz, but meh[1]. Perception-optimized wide-
| gamut recent CAM16 seems unsurprisingly closest to this
| vision, but for some "hmm, is that bit real or model?", and
| "how does this behave with changes in absolute
| illumination?". A tweaked CAM16-UCS can end up a seemingly
| unobjectionable swelling blob. The scattered instances of
| intensive use of Munsell in art instruction suggested at
| least some hope of clarity and utility. Thanks for the
| thoughts.
|
| [1] https://user-
| images.githubusercontent.com/181628/61452533-43... [png] from
| https://github.com/coloria-dev/coloria/issues/41
| akira2501 wrote:
| > I say "cursed" because I have no idea what that means. What the
| heck is that shape??
|
| Reminds me of frinklang.
|
| "The most-commonly used, CIE 1931, is long known to be off by a
| factor of 7 from average human perception at short wavelengths,
| (compare it to the 1978 definition at 400 nm) and is arbitrarily
| truncated before the limits of human perception. In addition, no
| one perceptually-weighted curve is possible because the human eye
| is differently sensitive for photopic (bright-light, cone cells)
| and scotopic (dark-adapted, rod cells), or if the illumination
| occurs over narrower or wider fields. Many incremental
| improvements on these systems have been proposed, but none are
| part of the authoritative, oversimplified definition of the
| candela, making it useless for unambiguous definitions that can
| be agreed upon or binding to any party. Pronouncements of the CIE
| are in no way binding on the BIPM, nor vice-versa, and the CIE
| has a proliferation of "standard curves," which all disagree with
| each other. Agreements to use one curve or another thus have to
| be agreed _outside_ the definitions of the SI, and, of course,
| parties can disagree on which curve to use. You can use CIE 1931,
| or CIE 1978, or the "CIE 1988 Modified 2deg Spectral Luminous
| Efficiency Function for Photopic Vision" or the 2005 improvements
| by Sharpe, Stockman, Jagla & Jagle, or ISO 23539:2005(E), or
| something else..."
|
| https://frinklang.org/frinkdata/units.txt
| ttoinou wrote:
| I've been having problems studying this topic for years now, is
| there actually an official scientific field with official books
| and an official consensus on this ? Seems hard to know who to
| trust on this wide-but-niche topic
| jasomill wrote:
| Yes. See, e.g.,
|
| https://en.wikipedia.org/wiki/Color_science
|
| https://en.wikipedia.org/wiki/Colorimetry
|
| https://en.wikipedia.org/wiki/Photometry_(optics)
| gorgoiler wrote:
| This is fantastic. It gave me an idea about colors, perception,
| and gamut.
|
| Put simply, imagine that there is a combination of wavelengths of
| light that causes you to perceive the smell of ripe cheese, and
| another that causes you to think that there is a bear behind you.
| Now your diagrams must be filled in not only with colored pixels
| but also include a small picture of a cheese and a bear at the
| points where those specific perceptions occur.
|
| I think, in real life, this is what magenta is: a non spectral
| color that's more of a _feeling_ or sensation that, in order for
| our brains to not get too overwhelmed, we simply perceive as
| another color. This is also, I believe, close to describing a
| real phenomenon for those living with varying degress of
| synesthesia or, if you will forgive a play on words, those on the
| synesthesia _spectrum_.
| meindnoch wrote:
| >this is what magenta is: a non spectral color that's more of a
| feeling or sensation that, in order for our brains to not get
| too overwhelmed, we simply perceive as another color.
|
| Ummm. What?
|
| Why would the brain get "overwhelmed" by non-spectral colors?
| You realize that spectral colors are pretty much non-existent
| in nature, right?
| vinnyvichy wrote:
| Bengtsson & Zyczkowski in the introduction (see p14-16) to their
| wonderful book make use of chromaticity diagrams to motivate
| their study of quantum states.
|
| https://www.researchgate.net/profile/Karol-Zyczkowski/public...
|
| >In a way tradition suggests that colour theory should be studied
| before quantum mechanics, because this is what Schroedinger was
| doing before inventing his wave equation.
| hilbert42 wrote:
| That link is so slow where I live that I had difficulty getting
| the site to work, but as far as I could judge it gives a rather
| nice and understandable explanation of what is a rather complex
| matter. It's in considerable contrast to those sections of my
| textbooks on color theory, they're so dry as to make one yawn,
| they're full of algebra, the complex operator and matrices with
| precious little other explanation of what it all means.
|
| Some of the comments have already covered most of what I'd have
| mentioned so I won't dwell on them now, although I'd add that I
| reckon _GrantMoyer_ is on the mark with his point about the
| inappropriateness of displaying chromaticity on Cartesian
| coordinates.
|
| It's worth noting that understanding the intricacies of
| chromaticity and color theory is difficult to the extent that its
| 'opaqueness' has been used to protect trade secrets (and likely
| still is for reasons I'll mention in a moment).
|
| Commercial lab printers that print masked color negative (neg
| film with the orange mask) to positives--color photos and color
| film print stock--go to great lengths to protect their matrices
| (precision resistor banks) against copying. Similarly, companies
| like Kodak do not publish the 'film terms' for their various
| emulsions ('film terms' being the unique matrix information for
| each film emulsion).
|
| The reason for this that to reverse-engineer the matrix with
| enough accuracy for a single film is a complex job let alone do
| so for a multitude of different films. Moreover, it's imperative
| the matrix be accurate if good color balance is to be achieved.
| Keeping this info secret provided a competitive edge, selling or
| licensing the info is worth money.
|
| I'd add that the destructive orange mask used in color negative
| film is a brilliant concept for reasons I cannot cover here,
| however what's relevant here is that the mask makes reverse-
| engineering the negative's film terms that much more complicated.
|
| I'm a bit out of touch these days but no doubt the same applies
| with inkjet printers and the like (matching coordinates to
| specific inks etc). So there's a modicum of truth to statements
| from Canon, Epson and HP when they say not to use third-party
| inks because the colors won't match properly (mind you, that's
| never stopped me at the exorbitant and outrageous prices they
| charge for inks).
|
| My point is that if it were possible to unravel and make this
| chromaticity stuff simpler to understand then many of these
| expensive commercial decisions would disappear.
|
| Ahh but alas, we're suck with it.
|
| --
|
| _BTW, for those who 've used scanner software like SilverFast
| the manufacturer provides a list of film emulsions to select from
| before the film is scanned. Selecting the correct emulsion type
| ensures the proper 'film terms' are used for the scan, this in
| turn ensures the color balance is optimal.
|
| I'm a bit cynical about SilverFast's approach to the problem,
| they've a limited range of film emulsions to select from (many of
| the old and important color negative types are missing).
| SilverFast's literature suggests that if one's color negative
| type is not listed then to select one that best suits. I am at a
| loss how one does that except to just make a guesstimate, so much
| for calibration. Also, one has to wonder why SilverFast has such
| a limited range given they've been in the business since many of
| said emulsions were still in production.
|
| There are similar issues with Hamrick's VueScan software but I've
| not time to address them here.
|
| Again, all these issues further illustrate the practical
| complexities surroundibg the chromaticity diagram._
| klodolph wrote:
| > What do you think a negative red light source means?
|
| It means that the subject turned a dial to add red light to the
| color being matched.
|
| Basically, you have an unknown color C, and then an R+G+B color.
| Sometimes, you can't match it, so you try matching C+R = G+B.
| This results in "negative" R, because you're adding R to the
| other side of the equation.
|
| The same happens with green and blue, but to a lesser extent.
___________________________________________________________________
(page generated 2024-07-27 23:11 UTC)