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Thread: The Dimensions of Colour - a colour theory discussion thread

  1. #326
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    So i was wondering, i was reading about the eye and specifically in regards to its ability to compress brightness in order to adjust the image.

    Would you know how much they adjust the brightness for different times of the day like sunrise and sunset etc?


    Heres what I found out so far.

    There is a Logarithmic relationship between luminance and subjective brightness perception in the human eye.

    Human eye uses a low pass filter and the band pass filter to compress the illuminance coming into the eye.

    The fall off for light compression is halved every time the frequency doubles.
    it can also divides 1/4 every time between the frequency doubles.

    But how does the frequency apply to the amount of light in the eye? Is the frequency the amount of light in the eye?

    From what i understood i thought there may be a significant decrease in the amount of compression that happens in low light circumstances.

    thanks again

    Edit: FOund the info i was looking for.
    Last edited by Siphonophores; January 12th, 2014 at 08:18 PM.
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  3. #327
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    New question,

    How does the principle of uniform saturation account for the brezold brucke shift? Does it account for the abney effect? and if it doesnt how could we incorporate this?
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  4. #328
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    Mister Janichan,

    The striped sphere in white light looks really good, but your method only seems to be letting you paint setups where the main light and the shadow illumination are the same colour. The way I showed lets you show multiple light sources of different colours.

    Siphonophores,

    Great, those last blocks look a lot more convincing than the spheres. One thing to at least be aware of though is that the gamut of digital colours is quite differently shaped to the range of commonly occurring natural colours, and extends well beyond the latter for many hues, especially in the greens and in the red-magenta-deep blue range (not so much in the yellows and greenish blues). So unless you tone down some colours below the maximum brightness and saturation possible, they'll tend to look a bit unnatural or even fluorescent (as in your green and magenta blocks).

    Quote Originally Posted by Siphonophores View Post
    New question,

    How does the principle of uniform saturation account for the brezold brucke shift? Does it account for the abney effect? and if it doesnt how could we incorporate this?
    My guess is that you'll get the same perceptual hue shift (if any) in the series of RGB colours of a given [I]digital[/I] "Hue" (H) and saturation as you would get in a series of colours of the same chromaticity in nature; in other words I think you'd automatically get the same hue shift happening.
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  5. #329
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    thanks! yea this is definetly something to consider, I was thinking about it but i wanted to keep the high saturations in to simulate artificial colors and surfaces.

    Now i read somewhere that a way to compensate for it could be to select your shading series in a kind of curve rather than a linear selection. The one question i had after reading your post was,

    If the shift happens naturally would it only happen if your computer monitor was in Direct sunlight or is it a property of the Intensity of the light itself?


    So say we select a color with 100 brightness (which i think equals 1000 Lux) Will this because of its intensity automatically create the brezold brucke effect, or is this only effect of the high (v high) intensity of Direct sunlight?
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  6. #330
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    Sorry, I haven't really gone into the BB effect in great detail myself because to be honest it seems a rather arcane issue in the context of practical painting. There's even some debate over whether the effect exists at all in object colours, as opposed to lights. I just meant that within the range of brightness of RGB colours, if there is any BB effect then I assume you would get that automatically as colours of a particular "Hue" and "Saturation" got brighter. If what you are concerned about is how to depict the BB shift of a light that gets brighter than that (?), then yes presumably you would have to adjust the hue appropriately. Any BB effect would of course be overprinted and possibly swamped by a difference in colour of the illumination between the shadow and light areas.
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  7. #331
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    Thanks for the swift reply.

    Thats interesting to think about. definetly laid those questions to rest.

    I have another one.

    I was researching atmospheric perspective. My hunch was that it increases exponentially as the distance travels back in space.

    It has been described in places that the atmospheric particles decrease in amount exponentially the higher you get in the atmosphere.

    However i couldnt find anything relating to distance along the z axis.

    I figured it would be exponential for two reasons;

    inverse square cube law / power law

    and because of the height to amount relationship described above.

    Then i stumbled across this;

    The inverse square law mentioned in the comments doesn't really come into play here because the fall-off in light energy is exactly balanced by the reduction in apparent size of the object. (An object twice as far sends only a quarter of the light energy to the eye, but also only occupies a quarter of the visual area, so the light energy per area of the visual field is the same). -David briggs


    It was from james gurneys blog.

    Now i might be wrong, however i thought about this and thought,

    Even if the light ENERGY is 1/4, the light INTENSITY is the same due to the strength of the light? So the light that reaches our eyes would be greater than if we calculated using the energy as our benchmark.

    what do you think?



    Also: just wondering do you know if/how LRV's (light reflectance values) relate to brightness or values? Just been wondering about the connection. can a connection be made?

    Here is a study i did using a photo.
    Attached Images Attached Images  
    Last edited by Siphonophores; January 15th, 2014 at 03:33 PM.
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  8. #332
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    Quote Originally Posted by Siphonophores View Post
    The inverse square law mentioned in the comments doesn't really come into play here because the fall-off in light energy is exactly balanced by the reduction in apparent size of the object. (An object twice as far sends only a quarter of the light energy to the eye, but also only occupies a quarter of the visual area, so the light energy per area of the visual field is the same). -David briggs


    It was from james gurneys blog.

    Now i might be wrong, however i thought about this and thought,

    Even if the light ENERGY is 1/4, the light INTENSITY is the same .....
    That's exactly what I'm saying - the intensity (luminance) is the same because the smaller amount of energy is concentrated in a correspondingly smaller visual field.

    Quote Originally Posted by Siphonophores View Post
    Also: just wondering do you know if/how LRV's (light reflectance values) relate to brightness or values? Just been wondering about the connection. can a connection be made?
    Sure, it's the nonlinear luminance to lightness relationship that is given as a cube root formula for CIEL* (L = 116 (Y/Yn)1/3 - 16), so for example an L* of 75 corresponds to an LRV of about 48.
    http://www.huevaluechroma.com/081.php

    With your spheres, wouldn't you expect the shadow side to be a different hue to the light side, given that there is probably a difference in colour between the sunlight and the light from the opposite side of the sky?
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  9. #333
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    Name:  Color balls.jpg
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    haha! Yea i think i was just confused with that. Thanks.

    Also big thanks for this post. This just gave me a Huge leap in my thinking. I was getting confused between parameters.

    Just got a few (well not so few) questions.

    I found this definition and liked it i was wondering if you thought it was right?


    standard theories of visual surface perception, for instance, posit that brightness (“perceived luminance”) and lightness (“perceived reflectance”) constitute the perceptual counterparts to the physical dimensions of luminance (light intensity physically registered by the eye) and diffuse surface reflectance (ratio of physically incident and reflected light intensity)

    http://www.journalofvision.org/content/13/1/14.full




    Also what do you think about this?

    Anchoring theory assumes that lightness is a psychological continuum that extends upwards beyond the physically
    possible reflectance range (0-1), and that high values on this continuum represent surfaces that appear to glow

    http://www.journalofvision.org/content/13/1/14.full


    Do you know about gamut relativity? What do you think about this theory?

    A key aspect of gamut relativity is the redefinition of brightness and lightness as computationally defined modes, rather than dimensions, of vision. According to this view, the brightness mode corresponds to global anchoring (λ = 1) and the lightness mode to local anchoring (λ = 0).


    Is blackness bias a principle we can use in painting? Would it apply in direct sunlight?



    ALso The information on relative brightness was a real eye opener. Is there any info about the different value steps for the different colors? or any general rules?


    hmmmm yea those spheres were a tough one. I think maybe the shadow would be a little colder due to the lessening light of the sky, but also with a slight warm band for the alpenglow.


    Here are some more Studies i was doing, Though i did them before reading this post.


    Also how would you deal with the three modes of vision in one picture? say there was a dark area then like a spotlight?
    Last edited by Siphonophores; January 19th, 2014 at 02:27 PM.
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  10. #334
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    The Theory of Colour - Lecture series, Art Gallery of New South Wales
    For anyone who is in Sydney in November, you might be interested in this series of public lectures I'm giving for the Art Gallery of New South Wales. Please introduce yourself if you're a CAer!

    The Theory of Colour

    The idea that all colours are made from yellow, red and blue was widely accepted in science until the late nineteenth century, when it was overturned by a revolution in our understanding of colour. This new understanding not only led to modern colour printing, photography, and cinema, and eventually to digital photography, painting and rendering, it transformed our understanding of what colour is as radically as Darwin’s theory of evolution transformed our understanding of biology over the same period. These lectures will examine the nature, classification and teaching of colour from these two very different viewpoints.
    The nature of colour (Saturday 8 November 2014, 2-3 pm)
    The structure of colour (Saturday 15 November 2014, 2-3 pm)
    The teaching of colour (Saturday 29 November 2014, 2-3 pm)
    Advance booking strongly recommended. Further details:
    http://www.artgallery.nsw.gov.au/cal...colour-theory/

    By the way, I'm still happy to enter into discussions on this thread, but as I think I've said earlier here, anyone who comes back with ten questions when I answer one will have to be ignored!
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  11. #335
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    Hi Briggsy,

    First of all, I want to say that your site on color is amazing and I'm just starting to study it. I have a question though -- one I'm not sure has been answered here or anywhere else. Roughly speaking, I'm trying to build a mathematical model for the collection of energy in light and connect that to the color picker in Photoshop. I could play around with values and models forever (and I'm willing to try that), but I figured it would be a bit helpful if I could ask you this question first.

    Essentially, the color picker (when in hue mode) has 100 values for brightness that scale upwards and downwards vertically (please excuse the simplicity of the question and my explaining -- it's the only way I can figure out how to ask it!). The same is true for saturation on a horizontal scale -- 100 values. What I am trying to figure out is what number to use for the value of light energy when it emits from a light source (say, the sun)(perhaps in lumens? Not sure...), how to calculate how much energy is lost during the travel time to the object being lit (Earth, in this question -- or objects on Earth), how to calculate how much energy is absorbed by the first contact between light and an 'object' (the atoms that construct the object), and then, after following the light ray through the environment, where that final value (the amount of energy lost) will cause the picker to exist on the color square in Photoshop (essentially, how much brightness and saturation).

    It sounds simple, but I have no idea where to start studying for this kind of calculating. I've done searches on 'raytracing', but am immediately confronted by obscure terminology and a steep learning curve. I figured that I could create various models and arbitrarily give a value to light energy at the moment it emits from the sun, give an arbitrary value for the amount at which energy is lost per unit of measurement (93 million miles from the sun, I believe), then arbitrarily give an absorption rate (light reflectance value (LRV)? Not sure if this is the right term, but I saw it on the net), then arbitrarily say where the color picker exists at that moment in the 'life' of the ray -- say, at 80 on the brightness scale and at some arbitrary location on the saturation scale. Then, I could give an equally arbitrary representation value for what the 100 percentages on the color square (in Photoshop) covers in terms of the light values.

    Essentially, let's say I arbitrarily said the value out of the sun was 6000 (lumens, lux or whatever term is used to measure light energy), then said it was 5200 when it got to earth, then said it was 4000 after being absorbed and reflected by a 'red' material (shiny red apple, let's say -- diffuse specular, I believe), then said it was at 89 (fully saturated (100) and at 89 on the brightness scale). THEN, said what the 100 percentages stood for -- 100 lumens of change or 3000 lumens of change. That would tell me, roughly speaking (if I could follow all of the bounces) what brightness level to paint the object at. Obviously, I'd say that the 100 percentage points on the color square in Photoshop would represent a wide range of numerical values (of light changing in value). I doubt it would line up 100 for 100. It's likely to cover a large numerical value range of light energy values... but the problem is that I have no idea where to start pulling these values from.

    Equally as much, I realize that raytracers are probably calculating light like this and that we HAVE figured out the values (for absorption, light loss over unit of measurement and the value to give light at its 'emittance point' (if that's a term)). I just have no entry point in the current books and information on the net that I've found. I need some kind of nudge to get to the next step or I'm going to be playing around A LOT with arbitrary value changes in a 'roughly' mathematical model for how light works.

    I realize that my question is very rudimentary. For this, I apologize, but I have actually worked for a while to even get to this point (sadly, haha).

    I did the same thing with linear perspective when I was studying it. I took everything all the way to matrix math and gradually realized that it's impossible to use matrix math as an artist for linear perspective. I found it liberating to know how perspective ultimately worked in computer graphics so that I could bend the rules and calculate things more roughly than exactly. I'm trying to do the same thing for light here, but lack that ability to make 'connections' -- connections between the color picker setup in Photoshop and the basic mathematical setup for light energy loss/absorption/reflection and, ultimately, color vision.

    If you're able to give me any help, I'd really appreciate it.

    Thanks so much!
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  12. #336
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    Well, your quest to track things back to the sun sounds like complete madness to me, but the site should help you to understand exactly what brightness and saturation mean in the colour picker.
    http://www.huevaluechroma.com/012.php#brightness
    http://www.huevaluechroma.com/012.php#saturation

    Note that "brightness" here is actually relative brightness compared to the maximum possible for a given hue and saturation, and is very different from perceived lightness (= L). Note also that both "brightness" and "lightness" are scaled according to human perception, not physical energy.

    Our real task is to take the light at each point in the visual field and create a set of image colours that at least suggest the effect of those lights. This almost inevitably involves substantial "fudging", in that to preserve the physical difference in energy coming from lights or highlights we would often have to paint everything else much darker than most people would want to.
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    Thank you so much for the reply, Briggsy.

    I'll examine the site locations you linked to. I realize now that it is too difficult to light in this way. I just needed some advice to help me on my trek with art. I'm going to work out a more simplistic model for lighting as a concept artist.
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    Hello Everyone,
    Kavan here. Thanks for the good thread. I have some general questions about color I would like to ask:


    1) Highlight (specular reflection of source)
    When a colored object (say a red apple) is illuminated, we can see some spots on the object that look white, i.e. have the same color as the source illuminating the object. These spots, called highlights, are explained as regions of specular reflection. In general, reflected light is diffusely reflected, i.e. scattered, in all various directions if the surface is not perfectly smooth. Why does specular reflection instead occur at these regions called highlights? Are these spots smoother than other spots? Or does more light fall on these spots? Specular reflection (angle of incidence= angle of reflection) redirects light is specific direction


    2) Saturation
    A more saturated color appears "brighter" than a less saturated color, correct? Why?


    3) Metamerism

    • Lights with different spectral content can be perceived by the human eye (which has 3 light sensitive receptors, the 3 types of cones) as identical. Are we talking about being really identical perceptions of just very similar? When a pure monochromatic light enters the eye, 3 values are always generated (one for each cone).



    • Metamerism, based on my understanding, consists in the fact that certain objects show a different perceived color under different illuminations (which have different emission spectra). Every object has a certain reflectance curve which together (multiplied?) with the spectrum of the source (and the eye) determine the "color" of the object.



    • How do certain objects manage to maintain the same color under different illuminations?



    4) White, black and gray

    • White black and gray are essentially the same from a spectral point of view: they all have the same uniform spectrum but the luminosity is different.



    • At what point does a white start becoming a gray? What is the intensity threshold to perceive gray? What about gray and black?



    • Why are there some many different types of whites each having different spectra?
    • Brown is not a really new color. I think it is what we perceive orange at low luminosity


    5) RGB


    • Do you know of a free internet applet that lets me control the proportion of RGB to see the resulting color?


    6) Lightness vs Brightness
    Regardless of the intensity of the illuminating source, certain colors appear to stay the same. Lightness is that term to describe the appearance of a surface.
    I have read about lightness constancy: the color of an object is determined by the relative intensity coming from each object. There is also lightness contrast: the lightness of a object is influenced by the lightness of the surrounding areas. Based on my understanding, brightness is the amount of light and the sensation it produces (dark to bright). Lightness is different: it is the % of incident light reflected by a surface (grayness, blackness, whiteness). To change brightness we simply adjust the intensity knob of the illuminating source. But to change lightness we must make it less reflecting? How do we make a surface less reflecting? If we texturized the surface we would make it rougher but diffuse reflection would still be there. How do we make a surface less reflecting? Lightness of a surface seems to be independent of the illumination intensity (a white piece of paper appears white even in lower light conditions and does not appear gray. I think that is called light constancy)... I would like to gain some clarity on this topic if possible.

    Thank you!
    Kavan
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