The Dimensions of Colour - a colour theory discussion thread - Page 8
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  1. #211
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    OK. It's not mutiplied by 0.45, it's to the power 0.45:

    http://www.huevaluechroma.com/092.php

    0.25 to the power of 0.45 is 0.5358867.

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    Smile Thank You

    Thank You man, I really think You are the best.
    I am sorry I am getting all easy now, but I would just like to express how fortunate I feel I am to know You; to have found out about Your knowledge and thinking, and even be able to speak to You.
    You are like a lexicon, an encyclopedia of the history, the present, and the future on artists' understanding and use of color in regards to understanding light.
    A pioneer for catapulting painting- and drawing artists' understanding of colors and light to a new level, a deeper level.

    The more You know, the more You don't just paint and draw from experience, but paint from actually knowing why You use the colors You use, how they make sense with the environment, or how to realistically and believably give a desired effect, while making everything else in the painting look consistent in the context of it. And You feel more and more safe doing what You are doing. Eliminating moments of frustration, where You don't know how to achieve a desired effect, or why, where and how to put a certain color at a certain area.
    And if You paint from life, You will be able to observe the laws You learned about at work.
    You won't just see different colors next to each other and their relationships, but You start to see a system in it all. A system from which You draw the most important necessary values, like basic colors of surfaces, all possible lightsources, all areas of reflected or bounced light, position of lightsources, level of specular reflection, layering of specular reflection on diffuse reflection, angles of surfaces, effects on color the light has (wavelengths present in the lightsource, and their peaks, wavelength absorbance and reflectance in the surfaces and their peaks), intensity of light with inclination angle, intensity of point light sources with distance, fresnel reflection ... . It's a beautiful thing.

    You have given me so much I always wanted to know, and I am sure many others too. And not just that, but You sparked my interest in a lot of other areas again too. Making me go out and research and read on a lot of things I never even thought of before. I really feel my life has been enriched and made more colorful.

    Thank YOU, briggsy. From the bottom of my heart.

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  4. #213
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    David Briggs is the best art teacher of all time. - I expect a 50% discount off next term for that comment.

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    Stephen Davidson
    "Imagination is more important than knowledge. For while knowledge defines all we currently know and understand, imagination points to all we might yet discover and create." Albert Einstein
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  5. #214
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    Question Light source through colored surface

    But Briggsy, there is still one thing I don't quite understand yet.
    I asked the question about light shining through a transparent colored cover, like plastic or glass, before; so I know what happens is substractive mixing of the colors of the light and those of the transparent surface.

    What I now don't understand, is how in a lot of colored lamps the inside, the center of the light source, or rather, the lightsource itself, can often still be seen in bright white, although behind the colored transparent cover.

    What's happening here? Is it so, that our eyes can only see each color to a certain maximum level, and beyond that, the color can't get any brighter? So that, let's say, in the case of a red cover, the spectral reflectance curve is highest on the red value, and becomes lower and lower to the blue side of the spectrum. Now a strong bright light source easily reaches that maximum red level. Now if compared to red it would only reflect let's say 8% of the blue wavelengths the light source emits (if the curve meant relative reflectance in relation to the other colors and there is a maximum possible level of recognition for a colors in our eyes), then at some point we'd have the brightest for our eyes recognizable red, and at some point we would eventually have the brightest possible version of every color, effectively making it appear white, although the actual relation of the colors in the reflectance curve hasn't changed, we just can't recognize them beyond a certain point of amount?

    I don't know, that's the only way I could explain it to myself.
    Which would mean, that any surface with exposure to a strong enough light would appear white. Unless it's black. The same way of thinking also explained to myself, that specular reflection of light sources is usually always 100% the 'lightness' of the light source. Although most surfaces (unless metals) have a generally low level of specular reflection at 0 degrees incident angle (I think about 8 percent, for glass for example). As in: 8 % of the brightness of the sun still looks completely white to us. 8% of other surfaces however, which are not illuminants, is much dimmer.

    The second thing I was thinking about, was where the glow around light sources comes from. You know, not the glow created by particles, or gases in the air, of the actual light. This glow around the light disappears, if You for example stand away from the light source, and in Your view to the light, cover the light with Your finger. Just hold a finger in front of You to cover the light source in the distance (not actually covering it, just in Your view).
    I thought this probably had to do with the adaptation of the eye between levels of illumination. The light source being in the middle of dimmer areas, and the dimmer area being around the brighter light source, creating this gradient of sensitivity originating in the lightsource and steadily becoming weaker outwards.

    Related to that, I was wondering where the sparkle appearance of light sources sometimes comes from. Is it really because of light somehow interacting with the eyelashes?

    Thank You

    EDIT: It's not easy to find out about something, when You don't know the correct name for it to begin with. lol But through some deeper research and reading between the lines of articles about other related things, I found what I was seeing is called a "glare" (http://en.wikipedia.org/wiki/Glare_(vision)).

    And I was partly right with what I was thinking.

    Last edited by Shindoh; January 1st, 2012 at 08:37 PM. Reason: Answer
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  6. #215
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    HDR; light through transparent barrier; questions

    bbshrmn, in reply to the question on post #174 about using HDR images as colour references: I wouldn't recommend it. First, there are a dozen different ways of creating HDRs; there isn't a standardised way that's "closer to reality". Second, people often over-do them, leading to artefacts like halos, lack of shadow depth/contrast, and over-saturation.


    Shindoh -

    Is your question "How come a white light will sometimes shine through as white when behind a coloured translucent barrier, instead of taking on the barrier's colour?"?

    If so, as I understand it, it can't be about our eye's exposure range maxing out (like it would be a camera, blowing out the highlights), because even if we look at the sun we can still see colour in it, and that's millions of times brighter than an indoor light. Heck, the sun is so bright that it damages our eyes if we look at it, and we can still see it being yellow.

    So, we're imagining a white light source behind a transparent but coloured barrier. The barrier will absorb some of the coloured light. Let's say it's a wall of red jello. Why is it the case that sometimes, the light source looks white (when it's strong), and sometimes it looks red (when it's weak)?

    My wild guess -- which might be wrong -- is that this is to do with the fact there's a non-linear relationship between perceived brightness and radiance. (See here.) If you remove 10% of the non-red light photons (radiance) from a dim white light, it will look slightly red, whereas if you remove 10% of non-red light from a bright white light, the result will look more white than the dim one.

    For a bright source, losing 10% makes less difference visually than losing 10% when it's dimmer.

    Another wild guess is that it's simply colour-correction. We see it as white because it's the whitest thing in our field of view.


    The "sparkle appearance of light" you mention in your second question does seem to be from the eyelashes (at least, if I squint and then move my eyelashes out of the way, the effect seems to go away -- hardly scientific but not sure what else it would be).


    Briggsy -

    I had to go through the whole site a couple of times and make notes to get it, but boy was it worth it. Thank you so much for opening my eyes to this fascinating subject.

    My main criticism is that it doesn't always define technical words before using them (or sometimes, at all), so I'd find myself having to guess what you mean. I'll try to write my own simplified take on it, to check understanding and hopefully make some parts more accessible. (And try the sphere exercise, of course.)

    Couple questions:

    1. What is the origin of the mapping from hues to different greyscale lightnesses?
    Some hues, like yellow, are lighter than other hues, like violet-blue. Even the camera exposure meter picks this up (one meters a middle-grey tone for correct exposure, but red and grass-green works too, and yellow would make the image under-exposed). What makes some hues lighter/brighter than others? Do cameras only pick this up because they're made to emulate how we perceive things? (I assume this can't be something objective to do with frequencies, because we don't see frequencies -- metamerism and all that. So.. it's about our eyes? What about them?)

    2. How does one learn to bypass one's interpretation of colour constancy? Learn the Munsell system? Learn the situations where colour constancy problems come up? Other tricks like imagining the shadows are actually paint rather than different lighting? All of the above? (I found this to be one of the most amazing ideas mentioned in the site. That's friggin' awesome.)

    And, optionally:
    3. Does this bypassing of colour constancy affect the qualia ('what it is like' to see a colour), or is it an intellectual thing (i.e. you know the colour is one thing, but it still feels like what colour constancy has it as)?

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  7. #216
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    The second graph, the black and white one in the original post. I don't know if anyone'll answer this... Oh whatever, at the risk of sounding clinically retarded I want to know, what does the graph itself mean or what format is it in? I do not comprehend the notation.

    Everything else is gratuitiously helpful and appreciated it's just I can't understand that one graph.

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  8. #217
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    Quote Originally Posted by Jarlix View Post
    The second graph, the black and white one in the original post. I don't know if anyone'll answer this... Oh whatever, at the risk of sounding clinically retarded I want to know, what does the graph itself mean or what format is it in? I do not comprehend the notation.

    Everything else is gratuitiously helpful and appreciated it's just I can't understand that one graph.
    I'm sure you're not the only one, Jarlix. It's all explained here:
    http://www.huevaluechroma.com/012.php

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  9. #218
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    Quote Originally Posted by stevejjd View Post
    David Briggs is the best art teacher of all time. - I expect a 50% discount off next term for that comment.
    I've organized your discount, but they're bringing in a 100% surcharge for smartarses this year, so it'll work out the same. See you next week!

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  10. #219
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    Thanks very much for the praise, Shindoh. Very nice to be appreciated!

    Quote Originally Posted by Shindoh View Post
    What I now don't understand, is how in a lot of colored lamps the inside, the center of the light source, or rather, the lightsource itself, can often still be seen in bright white, although behind the colored transparent cover.
    It seems to me that with a strongly coloured filter the colour is still visible even using the strongest artificial light I have handy (I haven't tried it with the sun, and neither should you!). But I agree that there is some reduction in apparent saturation, or in other words, some approach to the appearance of white light. I think your explanation applies exactly to why the light could appear white in a photograph, and is kind of right for the eye as well, except that here it is not just the instantaneous response to the light that is involved. You are looking at the light with a desensitized patch of retina, caused by localized bleaching of the photopigment, which also causes the persistent afterimage that you see when you look away (dark against a light background, light against a dark background).

    Quote Originally Posted by Shindoh View Post
    The second thing I was thinking about, was where the glow around light sources comes from. You know, not the glow created by particles, or gases in the air, of the actual light.
    .....
    EDIT: It's not easy to find out about something, when You don't know the correct name for it to begin with. lol But through some deeper research and reading between the lines of articles about other related things, I found what I was seeing is called a "glare" (http://en.wikipedia.org/wiki/Glare_(vision)).

    And I was partly right with what I was thinking.
    As I understand it the main factors are scattering within the media of the eye, and at their interfaces, plus scattering within the retinal epithelium, and stray light bouncing around within the eye. Plus the effect of your eyelashes if your eye are half shut. I think twinkling is caused by turbulent movement of air masses in the atmosphere, though.

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  11. #220
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    Quote Originally Posted by Lulie View Post
    1. What is the origin of the mapping from hues to different greyscale lightnesses?
    Some hues, like yellow, are lighter than other hues, like violet-blue. Even the camera exposure meter picks this up (one meters a middle-grey tone for correct exposure, but red and grass-green works too, and yellow would make the image under-exposed). What makes some hues lighter/brighter than others? Do cameras only pick this up because they're made to emulate how we perceive things? (I assume this can't be something objective to do with frequencies, because we don't see frequencies -- metamerism and all that. So.. it's about our eyes? What about them?)
    The maximum chroma version of yellow is lighter than that of all other hues partly because to be bright yellow an object needs to strongly reflect a particularly large band of the spectrum (ROYG), partly because many yellow materials approach this ideal quite closely (more than cyan and magenta materials do), and partly because the ROYG band of the spectrum spans the part of the spectrum that looks brightest to us (i.e. that our cone cells respond most strongly to), which peaks in yellow-green. Cameras pick this up because they measure luminance, which is light energy weighted according to the effect of each spectral band on the human visual system (no sense in responding to ultraviolet or infrared radiation, right?)

    Quote Originally Posted by Lulie View Post
    2. How does one learn to bypass one's interpretation of colour constancy? Learn the Munsell system? Learn the situations where colour constancy problems come up? Other tricks like imagining the shadows are actually paint rather than different lighting? All of the above?
    Yes, excellent summary! I'd especially stress the second one for people who worry that they will never learn to see through these illusions. Even if you can't avoid making a mistake initially, if you know about these effects you can get it right on the second or third attempt, instead of going around in circles.

    Quote Originally Posted by Lulie View Post
    And, optionally:
    3. Does this bypassing of colour constancy affect the qualia ('what it is like' to see a colour), or is it an intellectual thing (i.e. you know the colour is one thing, but it still feels like what colour constancy has it as)?
    Very, very good question. Without doubt some of the tricks for bypassing colour constancy affect the qualia, but unsurprisingly, since they involve a substantial change in the visual stimulus (e.g squinting, or using a reduction screen with two apertures). Certainly it can be a purely intellectual recognition, involving a conscious switch between seeing the colour of an object in an image, and judging the image colour. Most interestingly, a dramatic change in the qualia can sometimes be effected simply by changing between global and attentive viewing, which can bring about the latter switch without us necessarily knowing consciously that this is what we are doing.

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  13. #221
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    What causes hue lightness

    Quote Originally Posted by briggsy@ashtons View Post
    The maximum chroma version of yellow is lighter than that of all other hues partly because to be bright yellow an object needs to strongly reflect a particularly large band of the spectrum (ROYG)
    Not sure how this could contribute, because: What about a yellow laser? It would only emit one frequency, but we still see it as yellow and still see it as lighter than other hues.

    Surely whether a hue looks light or not can't be different for the pure spectral hue and non-pure spectral frequency version of the hue, because metamerism says they look the same?

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    Quote Originally Posted by Lulie View Post
    Not sure how this could contribute, because: What about a yellow laser? It would only emit one frequency, but we still see it as yellow and still see it as lighter than other hues.
    The third part of my explanation still applies to a yellow laser, which indeed is seen as brighter at a given energy level than all other monochromatic hues except yellow green (see attachment). The lightest high chroma object colour is a different question, to which the first two parts of my explanation become relevant.

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  15. #223
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    Quote Originally Posted by briggsy@ashtons View Post
    The third part of my explanation still applies to a yellow laser, which indeed is seen as brighter at a given energy level than all other monochromatic hues except yellow green (see attachment). The lightest high chroma object colour is a different question, to which the first two parts of my explanation become relevant.
    So does the brightest hue of light (at given energy level) happen at a different hue from the lightest hue of a surface?

    Also, why doesn't a yellow-green laser look even brighter at a given energy than a yellow one?

    To go back to a thing you mentioned earlier:

    Quote Originally Posted by briggsy@ashtons
    to be bright yellow an object needs to strongly reflect a particularly large band of the spectrum (ROYG)
    Why must this be the case? Couldn't an object reflect a single frequency of yellow, in principle? (I realise real objects may not, but I'm curious about the 'ideal'.)


    On the previous topic of bypassing colour constancy: the whole idea of seeing past illusions blows my mind. If anyone has any links or book recommendations for how to learn this skill, please do post them.

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  16. #224
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    What I said was that yellow-green is the brightest hue for monochromatic light. For broadband light, yellow is the lightest hue, as for object colours. For both of the latter, the range of wavelengths that contribute to the hue sensation is critical.

    Many, many people assume that we see colour by detecting wavelengths, and consequently that if a yellow object reflects a lot of red and green light, then these red and green components are "impurities" in the yellow. We don't, and they aren't: the red and green reflectances combine additively to make most of the yellow stimulus. An object that just reflected a "single frequency of yellow" would only reflect a tiny proportion of the light falling on it, and so would be black or very dark olive! You can see from the fact that a bright yellow object is close to white in value that it must be reflecting most of the light falling on it.

    Last edited by briggsy@ashtons; January 23rd, 2012 at 06:23 PM.
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    Your website is just amazing and full of information!

    I'm a complete idiot on color theory and painting, and all this time, I've thought Saturation/Chroma of color shifts a lot in shadows. Those tutorials from Deviantart.....

    I've read your website thoroughly three times but I'm still confused haha. I think I need to read it a hundred times more.

    So are my understandings 'basically' correct?

    -- Chroma of colors don't shift much throughout the full light-shadows except the rapid transition from highlight to full light?

    -- Skins generally have higher chroma in the shadows, little more reddish?

    Some questions I have though:

    -- On the section that talks about Relative Brightness, you have said that in a painting with B=50 gray representing white, any color with Brightness greater than 50 will be bright and lower will be dark, so is it important to first decide the white value of a painting, then choose highlight/shadow colors based on that Grayscale value?

    -- If above is the case, then would it be important to set the white balance value pretty high, say B=70ish, so that your painting will have wide range of colors available, without making the colors look like light sources?

    -- On the Figure 10.17 on http://www.huevaluechroma.com/109.php, the top colors with max Brightness look very bright, real world colors are not like them right?

    -- On your website, you have said, "The series of colours we use to represent such a surface, here called a shading series, should therefore lie along a line of uniform saturation; such lines radiate from the black point of the colour solid (Figure10.1). Along such a line, chroma decreases steadily as lightness decreases, at the precise rate necessary to keep the saturation of light from the surface constant."

    What do you mean by ''chroma decreases steadily as lightness decreases"? I thought the saturation doesn't change along the shading series?

    Last edited by sinefinehabitarevolo; March 14th, 2012 at 09:09 PM.
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    sinefinehabitarevolo -

    This was actually one of the main things I struggled with at first too. The thing that made it click for me was the realisation that there's a difference between describing surfaces (e.g. paint on a canvas) and describing light (e.g. a computer monitor or a light source). You can see this in the diagram on this page of briggsy's site.

    There is often a chroma shift in shadows (if you're already at the max chroma, for example, it's not possible to stay at that chroma at lower values because they are by definition not as pure). But there is not a saturation shift (assuming the ambient light is the same hue as the main light).


    Saturation is about purity of light. A colour is desaturated when it's mixed with other light and makes it look greyer to the eye. If an object is just in shadow, there isn't other light being mixed in, it's just dimmer. It's the same photons, there are just less of them.

    Chroma is about intensity of colour, but refers to surfaces. Most hues have a peak chroma at a particular value -- so getting darker would lower its chroma.



    So to answer your first two questions:

    1. If I understand correctly, chroma of colours do shift slightly in full-light. But saturation does not. (Yes, except when it's a specular highlight -- this is because specular highlights are a reflection of the light source, so they take on its colour.)

    2. My initial guess based on the above would be no, skin doesn't generally have higher chroma in shadows, but then skin is translucent and has sub-surface scattering which might do that, I'm not sure. Another thing is skin next to skin might influence the ambient light hitting it and make it more saturated/chromatic.
    If I were you I'd just colour-pick a few photos, or pay attention to your reference or just look out for it in real life. Keep in mind that the colour of ambient light in real life may well be different from the main light (e.g. the blue sky might reflect blue ambient light, or a sitter on an orange sofa will have light reflected from it which saturate skin tones).


    As for the brightness thing, everything you said sounds good to me but I'm curious what the more knowledgable people on this forum have to say.


    (And in answer to your last question: chroma decreases, saturation stays constant -- see above explanation.)

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    Thank you for the detailed explanation!

    Ooh chroma is different from saturation? My reading comprehension obviously fails. I thought chroma and saturation were basically the same thing. I will read that part again. So chroma is changing when you change the value of a color with saturation staying the same. Now it makes perfect sense.

    So how would ambient light affect the main saturation of an object?

    If it's in the same hue, saturation stays the same

    If the hue of an object is red and the ambient light is blue, then it would turn purplish because of subtractive mixing, (am i right?) but would it lose saturation or stay about the same?

    If the hue of an object is red and the ambient light is yellow, would it.... um.... be orange-ish.... um.... I've been thinking about it but I don't know what I'm thinking about.

    I was testing out with Multiply layer as the website instructed, but is that really the case? seems like Soft Light layer mode or Screen mode is what it's supposed to be.... How do you simulate colored light on dark surface (Real world surface and not full saturation RGB colors) in photoshop?

    I've attached a sample photoshop file. http://www.mediafire.com/i/?o232oxpb8og99b8

    Last edited by sinefinehabitarevolo; March 15th, 2012 at 07:36 PM.
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    I'm gong to go against the grain a little bit and say that chroma does not decrease by default in areas of shadow or blocked light. The color of shadow is entirely dependent on the properties of the light source illuminating the shadow side. That could be a secondary light source, bounced light, or ambient background light. For example, a red ball with one light source placed in a gray box will have a chroma shift in the core shadow since gray bounced light will be illuminating the core shadow. However if you place that same red ball in highly saturated yellow room the core shadow will be very saturated in color with bright yellow illuminating the core shadow. Our eyes depict gray when there is an equal level of Red, Blue, and Green light reaching our eyes at the same time. When the levels of RGB light shift away from equal then colors become more saturated. Play with the RGB sliders in Photoshop to see what RGB colors make color.

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    sinefinehabitarevolo

    Lulie seems to have cleared up your confusion of saturation and chroma, but I'd just add that Figure 9.8 on this page also seems to help people struggling with this.
    http://www.huevaluechroma.com/093.php

    If you read through the site at least one more time knowing this, thigs should make a lot more sense!

    As for brightness, in any natural scene, a white object will be represented by different greyscale values in different parts of the scene, though the highest such value needs to a step or two under 100 if you want to be able to show specular reflections.

    Real coloured paints never reflect 100% of any part of the spectrum, or (quite) 0% of other parts, so you're completely right that those digital colours look too bright (and saturated) for actual paints.

    Last edited by briggsy@ashtons; March 15th, 2012 at 08:41 PM.
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    Quote Originally Posted by sinefinehabitarevolo View Post
    So how would ambient light affect the main saturation of an object?

    If it's in the same hue, saturation stays the same

    If the hue of an object is red and the ambient light is blue, then it would turn purplish because of subtractive mixing, (am i right?) but would it lose saturation or stay about the same?

    If the hue of an object is red and the ambient light is yellow, would it.... um.... be orange-ish.... um.... I've been thinking about it but I don't know what I'm thinking about.

    I was testing out with Multiply layer as the website instructed, but is that really the case? seems like Soft Light layer mode is what it's supposed to be.... How do you simulate colored light on dark surface (Real world surface and not full saturation RGB colors) in photoshop?

    I've attached a sample photoshop file. http://www.mediafire.com/i/?o232oxpb8og99b8
    It's impossible to give a precise answer to these questions because this is a subtractive mixing process, and as a subtractive processe it depends on the actual, wavelength-by wavelength spectral distribution of the light source and the reflectance of the object. A yelow laser, a yellow filtered fluorescent lamp, and yellow filtered daylight for example could give all give quite different results.

    Using multiply mode gives a mathematically accurate calculation of what you would get using a light source and object with very simple spectral power distributions, but it should only be taken as giving an indication of the general kind of result that might be expected.

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    Hello

    I think I really understand the concept now, but is the part about "using Multiply layer mode to depict the effects of colored light on real world surfaces" true?

    I found that Screen mode seems to work the best when I tested with Photoshop.

    Here's Multiply mode


    Here's Screen mode


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    Hello

    I'm really understanding all these color theory concepts. This is great!

    But is it true that using multiply layer is what is happening to the objects with colored lights on them?

    Shouldn't screen mode with lowered opacity be used to depict this? I really don't know, though.




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    Quote Originally Posted by Jason Ross View Post
    I'm gong to go against the grain a little bit and say that chroma does not decrease by default in areas of shadow or blocked light. The color of shadow is entirely dependent on the properties of the light source illuminating the shadow side. That could be a secondary light source, bounced light, or ambient background light. For example, a red ball with one light source placed in a gray box will have a chroma shift in the core shadow since gray bounced light will be illuminating the core shadow. However if you place that same red ball in highly saturated yellow room the core shadow will be very saturated in color with bright yellow illuminating the core shadow. Our eyes depict gray when there is an equal level of Red, Blue, and Green light reaching our eyes at the same time. When the levels of RGB light shift away from equal then colors become more saturated. Play with the RGB sliders in Photoshop to see what RGB colors make color.
    Jason, all of this may agree perfectly with what I've said (if you fix up the bit about "gray" light!), depending on what you mean by "by default". Chroma does not automatically or invariably decrease in the shadow areas; as you say, if there is a change in spectral composition between the main light and secondary lights dominating in the shadow, it's likely to cause a departure from the path of uniform saturation/chromaticity that would otherwise apply.

    Last edited by briggsy@ashtons; March 15th, 2012 at 08:01 PM.
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    Quote Originally Posted by sinefinehabitarevolo View Post
    Hello
    But is it true that using multiply layer is what is happening to the objects with colored lights on them?
    It does, but you get some strange-looking results if you use digital colours at full brightness and saturation, i.e. colours that have R, G and B values of 0 and 100%, because these are much more saturated than natural lights and the reflectance of natural objects. Try again using light and object colours no greater than 90 in brightness and saturation and you should get more natural looking results. (In retrospect I should have done this on my own diagrams on the site!).

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    The below responses are an attempt at help with understanding the concepts of what's going on, but keep in mind that in real life the results of subtractive mixing depends on the spectral distribution (which we can't see directly) like briggsy said.

    Quote Originally Posted by sinefinehabitarevolo View Post
    So chroma is changing when you change the value of a color with saturation staying the same.
    Almost but not quite: Saturation does decrease if you make the value lighter. It just stays the same if you make it darker. Think about what's happening to the light: If you make it lighter, you need to add more photons, and if you're already at max saturation (say only your computer mointor's red phosphors are glowing), you'll need to add photons of other frequencies (blue and green parts of the pixel activate -- in other words you're adding more white light).

    If it's in the same hue, saturation stays the same
    Assuming it's not already at max saturation, it wouldn't stay the same but increase in saturation. It would stay the same saturation if the ambient light was white.

    If the hue of an object is red and the ambient light is blue, then it would turn purplish because of subtractive mixing, (am i right?) but would it lose saturation or stay about the same?
    If I understand correctly, that's right, and it would stay the same saturation unless the ambient light is the opposite colour (because as in all colour mixing, combining opposites go to neutral).

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    Quote Originally Posted by Lulie View Post
    Almost but not quite: Saturation does decrease if you make the value lighter. It just stays the same if you make it darker. Think about what's happening to the light: If you make it lighter, you need to add more photons, and if you're already at max saturation (say only your computer mointor's red phosphors are glowing), you'll need to add photons of other frequencies (blue and green parts of the pixel activate -- in other words you're adding more white light).
    Actually sinefinehabitarevolo was right. Perceived saturation will stay the same while the value gets higher as long as the eye is adapted to the general level of lighting. With a camera, however, saturation will begin to decrease once the brightness of a colour reaches the limit of the RGB gamut. We can't paint with colours brighter than the adaptation level of the eye, but one way to "fake" their effect is to desaturate the lightest lights in the manner of an overexposed photograph.

    Last edited by briggsy@ashtons; March 17th, 2012 at 06:30 AM.
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    Quote Originally Posted by Lulie View Post
    Assuming it's not already at max saturation, it wouldn't stay the same but increase in saturation.
    Correct in terms of physical "saturation", i.e. spectral purity (though not necessarily in terms of perceptual saturation, because of the phenomenon of colour constancy: in a room lit only by a red light, red objects look surprisingly pale).

    Last edited by briggsy@ashtons; March 17th, 2012 at 06:27 AM.
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    Quote Originally Posted by Lulie View Post
    If I understand correctly, that's right, and it would stay the same saturation unless the ambient light is the opposite colour (because as in all colour mixing, combining opposites go to neutral).
    No, the question about saturation can't be answered because it depends entirely on the spectral distributions of the two colours. If the red and the blue were both saturated enough to have no wavelengths in common, the red object would appear black.

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    Quote Originally Posted by briggsy@ashtons View Post
    Actually sinefinehabitarevolo was right. Perceived saturation will stay the same while the value gets higher as long as the eye is adapted to the general level of lighting. With a camera, however, saturation will begin to decrease once the brightness of a colour reaches the limit of the RGB gamut. We can't paint with colours brighter than the adaptation level of the eye, but one way to "fake" their effect is to desaturate the lightest lights in the manner of an overexposed photograph.
    Ah, yes, I was thinking about this in terms of digital screens rather than physical objects.

    I've heard it's impossible to paint a red rose in bright sunlight accurately (as in both realistic values and realistic 'colours' - i.e. chroma/hues - at the same time). So is this the reason for it? As in, to achieve the effect of both high saturation and high brightness, you would have to dull everything else.

    Also, I recall you mentioning on your site that value had a specific meaning and you mostly used lightness or brightness (depending on whether you were talking about light or surfaces), but I couldn't find it. What is the technical definition of value? Is it just a shorthand for lightness/brightness and can mean either?

    No, the question about saturation can't be answered because it depends entirely on the spectral distributions of the two colours. If the red and the blue were both saturated enough to have no wavelengths in common, the red object would appear black.
    So, in practice when painting, we either have to just look at what's happening, or guess/work out the spectral distribution (based on things like material or how things have looked to us in similar situations)?

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    Quote Originally Posted by Lulie View Post
    I've heard it's impossible to paint a red rose in bright sunlight accurately (as in both realistic values and realistic 'colours' - i.e. chroma/hues - at the same time). So is this the reason for it? As in, to achieve the effect of both high saturation and high brightness, you would have to dull everything else.
    Yes, though I'd add that to preserve the colour relationships needed to create a realistic effect of light in most subjects, you need to paint most things somewhat darker than people first assume.

    Quote Originally Posted by Lulie View Post
    Also, I recall you mentioning on your site that value had a specific meaning and you mostly used lightness or brightness (depending on whether you were talking about light or surfaces), but I couldn't find it. What is the technical definition of value? Is it just a shorthand for lightness/brightness and can mean either?
    Value is a synonym of lightness. I didn't correct your use of "value' in this context because you can talk about the components of a subject either in terms of the brightness and saturation of the light coming from them, or the value/lightness and chroma of the paint you would use to represent them.

    Quote Originally Posted by Lulie View Post
    So, in practice when painting, we either have to just look at what's happening, or guess/work out the spectral distribution (based on things like material or how things have looked to us in similar situations)?
    I'd take it to mean that there is a (certain) range of colours that could result, and so we're free to use any of those (including the ones we get using multiply mode) and they should look right.

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