You seem to have answered most of your own question. Levels does use a composite RGB histogram, the "OR" of the individual rgb histograms.
Perhaps the confusion is with how levels operates There is no luminosity calculation. Levels operates directly on individual channel values. The values of the RGB sliders are applied directly to the individual channels.
This is why adjusting the RGB sliders normally causes shifts in saturation and hue, and should be avoided. In my role as der curvemeister, I should also add that curves are superior to levels.
Thanks for your reply. However: I may be as thick as a whale omelette, but I am still having problems understanding. Would you mind if I asked you a couple more questions? If you can answer them it would be really helpful. (So far no-one else has been able to).
1) In the histogram Palette ( and indeed in Levels) do the color channel histograms represent luminosity ( which I take to be brightness) or color saturation? (These are presumably separate entities).
2)Does the black RGB histogram represent a composite of the 3 color channels( as I think you’ve said ) or the grey tonal values? If the former, why do we use it to adjust tones ie. make the image darker or lighter,increase contrast etc?
3) What is the luminosity option for in the Histogram Palette and why is there not one in Levels?
4)In the channels palette, do the greyscale images for each channel represent colors, or are there really greyscale images behind each color channel?
5) Do Curves adjustments increase saturation or brightness ie. Luminosity.
6) Why is there a black channel in CMYK if each channel has its own greyscale image?
As you can see, I am having problems understanding what precisely color channels are, and also what exactly histograms are telling us.
I realise this might be a tall order for a forum such as this but I would appreciate any help you can give.
On Sat, 11 Oct 2008 12:38:05 -0700, wrote:
Good questions. It’s no wonder that the histogram tool is a bit confusing, since very little is said about it in the PS documentation. There are five histogram "modes", RGB, red, green, blue, luminosity, and colors.
1) In the histogram Palette ( and indeed in Levels) do the color channel histograms represent luminosity ( which I take to be brightness) or color saturation? (These are presumably separate entities).
Speaking only of RGB images, the red, green, and blue channel histograms are the first ones to sort out. They represent the number of values (0-255) of each color channel. The Color histogram is a superimposed display of red, green, and blue histograms. The absolute height of the peaks in the histogram is determined by the height of the maximum peak, and this is done on a per channel basks.
2)Does the black RGB histogram represent a composite of the 3 color channels( as I think you’ve said ) or the grey tonal values? If the former, why do we use it to adjust tones ie. make the image darker or lighter,increase contrast etc?
The black RGB histogram is a combined view of the histograms for each of the three channels, however all the channels are vertically scaled by the same amount, instead of being scaled individually per channel. This means that the location of the peaks for each individual channel stays the same, but that the height of each peak will be smaller for two of the channels.
3) What is the luminosity option for in the Histogram Palette and why is there not one in Levels?
Luminosity is calculated by taking the weighted sum of the red, green, and blue values for each pixel. The weight is smaller for blue, since it is the darkest color, and largest for green. This corresponds closely to the histogram seen in many camera displays, and is the reason that clipping can occur, but not show in the histogram.
4)In the channels palette, do the greyscale images for each channel represent colors, or are there really greyscale images behind each color channel?
Each channel is a grayscale image. For RGB, each image can be thought of as providing a certain amount of red, green, or blue to the image.
5) Do Curves adjustments increase saturation or brightness ie. Luminosity.
Again, speaking only in RGB, Curves can increase saturation and luminosity in a variety of ways, since saturation, hue, and luminosity information is mixed together in each of the red, green, and blue channels. Many people find that working in the Lab color mode is simpler, since the L channel contains all the luminosity information, and a and b contain independent color information. Try Lab – you might like it very much indeed.
6) Why is there a black channel in CMYK if each channel has its own greyscale image?
The black channel borrows data, more or less equally, from the C, M, and Y images. This produces a fourth channel, called K, that provides excellent control over shadows and textures. Since the CMY channels contain more color information, they provide a very sensitive adjustment for color, which is good for things like skin tone variation.
As you can see, I am having problems understanding what precisely color channels are, and also what exactly histograms are telling us.
It takes a while for all of this to sink in, but it does not require a PhD. Things like additive and subtractive color, and the idea that organizing color data into different color models, such as RGB, Lab, HSB, and CMYK can provide leverage.
I realise this might be a tall order for a forum such as this but I would appreciate any help you can give.
I think it’s important to work these things out from basic principles, as you are doing. Grab a book by Dan Margulis, and some light bulbs will start to turn on regarding the different color spaces.
BTW – histograms are a poor tool for color correction. If there are problems with clipping or other issues, it is much better to look at the individual channels of the image, rather than the histogram.
4)In the channels palette, do the greyscale images for each channel
represent colors, or are there really greyscale images behind each >
color channel?
Each channel is a grayscale image. For RGB, each image can be thought of as providing a certain amount of red, green, or blue to the image.
In strict technical terms, these "images" are merely a representation of the spatial correlation of a pixel’s given component value, meaning the amount of how much a color component contributes at a given pixel location. They don’t exist as separate data in the image, but are intrinsically derived. Since these values are considered to be between 0 and 1, you should best imagine them as multipliers to the maximum possible channel value.
Mylenium
Another way to think of the RGB channels – if they were projected on the wall, with red, green, and blue filters, you would see the final color image.
This was done, a century and more ago, to capture very high quality color images for the Czar of Russia, and you can use Photoshop to put them back together:
http://www.loc.gov/exhibits/empire/
Karl,
"…In the histogram palette menu there are red, green and blue histograms to give the saturation of those colors…"
You can cut to the quick on most of your questions by getting a better handle on SATURATION. That is really an inappropriate word for the parameter to which it has long ago been assigned, leading you to ascribe inapplicable properties implied by the word itself. You are equating it to BRIGHTNESS and/or LUMINOSITY when, in fact, it is very different.
Like a cloth which has sponged up all the water it can hold, it is intuitive to think of a pixel as saturated when it is as bright as it can be. Yet, an essentially black pixel (RGB 2/1/0) is 100% saturated while a blindingly bright pixel (RGB 255/254/253) is less than 1% saturated.
A better designator of this thing we call saturation would have been purity or dilution. It is a measure of the extent to which a pixels color has been diluted by gray. No gray whatsoever is fully saturated. All gray is totally unsaturated. The gray portion is that component containing equal levels of red, green, and blue. What remains after discarding the gray might appropriately be called the chromatic component. The latter is what determines the pixels hue. The gray and chromatic components together make up the composite color. Gray includes the black and white extremes and all intervening shades.
You will never be led astray if you use the formula for saturation:
S(%) = 100 x (1 – L/H)
where L and H are, respectively, the lowest and highest of the RGB color values. The middle color value affects hue but is irrelevant in establishing saturation. As you can see from the formula, if any one of the three RGB channels has a zero value, the pixel is 100% saturated no matter what the other channel values are provided, of course, they are both not also zero. Likewise, if all three components are non-zero, 100% saturation cannot be reached.
You can also see from its formula that saturation is a pixel property, not a color channel property. You cannot know what the saturation is without knowing the color values in each color channel and ferreting out which of the three values is high and which is low.
Yes, "red saturation", "blue saturation", and "green saturation" sliders are so named in camera raw dialog boxes, but here again we encounter very misleading nomenclature. This raw terminology is not literal—it means something like this (taking red for example): "For those pixels having red as the highest value, and ONLY THOSE PIXELS, use this slider to adjust the saturation. The slider leaves the saturation as is for all pixels where red is not dominant."
There is no such thing as "red or green or blue color saturation". You could say that’s oxymoronic. But you can limit saturation adjustments to those areas in which the red or green or blue hue predominates (or C/M/Y for that matter). Since it would be awkward, if not preposterous, to label a slider with a long explanation of what it does, Photoshop designers obviously opted for shorthand albeit confusing labels. As a consequence, we now contend with legions of literal-minded users misinterpreting the slider notations they encounter in camera raw and believing that a color channel all by itself can be "saturated" without regard to the status of other channels.
Why do you all make it so difficult? The histogram shows the proportion of pixels with a particular value, from 0 – 255. That’s all there is to it.
Saturation has nothing to do with it. Although one could discuss the true meaning of the concept "saturation", the point is moot because the histogram doesn’t show saturation, except indirectly in very special cases.
Freeagent,
Because the OP’s questions were multi-faceted and a number of apparent misconceptions could be dispelled by an understanding of saturation, which was interwoven in the thread and mutilated from the getgo. IMHO, the implied saturation questions were hardly moot.
George
a number of apparent misconceptions (…) interwoven in the thread and mutilated from the getgo
OK, I can see what you were saying, and it’s all correct of course.
It’s just that I’ve been watching this thread and waiting for someone to say, "wait a minute, back up a little here"… I guess it was that word saturation – which is not, in any way, reflected in the histogram. That’s the basic, and very simple, misunderstanding.
George,
I thought your answers were spot on not only for the OP, but for others reading this forum to gain a better understanding of Photoshop’s operations. The precision of your language is also appreciated.
George, I thought your answers were spot on not only for the OP, but for others reading this forum to gain a better understanding of Photoshop’s operations. The precision of your language is also appreciated.
Add me to the appreciative of both George’s and Curvemeister’s responses in this thread. Some of it I already knew, much of it was put into valuable new context. Bravo gentlemen. Thank you.
karl, your great questions made it happen. My thanks to you also.
I obviously missed something here, LOL… I’ll reread the whole thread carefully and keep my mouth shut.
I was just thrown by this, from the OP:
there are red, green and blue histograms to give the saturation of those colors
I couldn’t make any sense of that.
Hi, I am grateful for all the interest, but I am somewhat embarrassed to relate that I am still a little unclear.I know I must sound a bit slow, but my background is in Classics and Art History, not Maths and Science ( thats my excuse!).I appreciate clarity, and explanations appropriate to my schoolboy level in this subject.I must say that none of the books I have read so far have explained this matter; they tend to reinforce my mistaken view that saturation simply means strength of colour, which is the way it appears when I move the saturation sliders in PS CS3
Is it the case then that:
Hue= what we normally call colour.
Luminosity = Brightness i.e. how near white a pixel is, and Saturation = a mixture of hue and luminosity ie. if the hue remains at the same intensity and we make the pixel brighter the saturation will increase?
Have I got the hang of it or not? And if so, I have to ask the next question:
If no grey whatsoever is fully saturated, how then can it be that an essentially black pixel is 100% saturated??
I’ve done quite a lot in Photoshop, and I’m now trying to understand how it works.I’ve obviously got a long way to go…..
Paulo and John,
Thank you for your feedback. Verification that someone has been helped—the lifeblood of any forum—is always gratifying. That’s especially so when an uninvolved "lurker" takes the trouble to post an acknowledgment, the non-obligatory nature of which suggests it is truly heartfelt.
Karl,
"…If ‘no grey whatsoever is fully saturated’, how then can it be that ‘an essentially black pixel is 100% saturated’?? "
Well I guess I’m also guilty of poor nomenclature in tabbing RGB 2/1/0 "black". The modifier "essentially" is my defense.
RGB 2/1/0 is really a brownish orange of hue 30 degrees. Because its brightness is so low, however, it looks black for all practical purposes. A pixel of any hue will look black if its brightness is sufficiently low.
Yet, it’s true this black-looking 2/1/0 pixel has no gray component because you cannot subtract equal amounts of R, G, and B if B is zero. Using the saturation formula yields S = 100 x (1 – 0/2) = 100%. And that is how an essentially black (zero brightness) pixel can be 100% saturated.
BTW, changing the blue channel to 1, giving RGB 2/1/1, produces a gray component of 1/1/1, reduces the saturation to 50%, and changes the hue to red (0 degrees).
George
Karl,
"…Have I got the hang of it or not?…"
Not meaning to offend you and with apologies for contradicting Mike Russell, I have to say you are not even close to a meaningful, non-trivial definition of any of those parameters.
My definitions (below) are consistent with Photoshop usage
Hue is an angle (0-360 degrees)on the color wheel reflecting the relative mix of the highest and middle RGB values after subtracting the gray component, the latter being all of the lowest color and matching amounts from the other two colors. (I am witholding the hue fomula because it is cumbersome and more likely to turn you off than turn you on.
Brightness, as used in the PS color picker, is simply the highest color-channel value, normalized to range from 0 to 100 (rather than 0 to 255).
Luminosity (not Luminance or Lightness) is the weighted average of all R/G/B values, the weighting factors being 0.3/0.59/0.11. Note that these weighting factors add up to 1, so that the luminosity of white 255/255/255 is 255. Keep in mind that these factors, although used by PS, are not universally used and other weighting schemes exist in the field.
Saturation as used by PS has been defined earlier. To say that saturation is a mixture of hue and luminosity (where you have equated luminosity to brightness) is not only incorrect but useless. In the color picker’s HSB breakout the three parameters are independent and thus no one of them can be said to be a mixture of the others. If that were valid, only two parameters rather than three would be required to define a color.
George
….
Hue= what we normally call colour.
Luminosity = Brightness i.e. how near white a pixel is, and Saturation = a mixture of hue and luminosity ie. if the hue remains at the same intensity and we make the pixel brighter the saturation will increase?
These are characteristics of the HSB color model, which is one of several color models (RGB, Lab, CMYK, …)
Uh-oh I feel an historical reference coming on. HSB was invented by a great guy named Alvy Ray Smith. Two cool things about HSB are that it is intuitively very similar to the behavior of mixing paint pigments, and it is easy to compute on the slow things we used to call computers of years ago. Alvy also invented the alpha channel, is one of the founders of Pixar, and incidentally had a big hand in the software on which Photoshop
1.0 was based. Alvy also argued, to no effect, in favor of storing all
images in gamma 1.0 format. OK – enough about history already… no, wait, check out Alvy’s home page:
http://alvyray.com/ Back to the points you were making. You are correct on Hue and Brightness. You’re very close on Saturation. Think of Saturation as the amount of white that is mixed with the color. No white = 100 percent saturation.
If no grey whatsoever is fully saturated, how then can it be that an essentially black pixel is 100% saturated??
From an intuitive standpoint, mixing black with a pure color decreases the Brightness, bit it does not change the saturation.
Mixing white and black, so as to keep the Brightness the same, will decrease the Saturation. This is where George’s comment that adding gray changes the saturation comes from.
Just to sum up: adding pure color controls the hue, adding white decreases the saturation, and adding black reduces the Brightness.
I’ve done quite a lot in Photoshop, and I’m now trying to understand how it works.I’ve obviously got a long way to go…..
I like to think of it another way. It’s as if you are Aladdin, in the magic cave, and you’ve gathered up all the treasure you can hold for now. Be happy, and you can always make another trip to the cave. There is one unassuming little magic lamp in the cave, however, that you should be sure to find room for on one of your trips: Lab.
—
Mike Russell – www.curvemeister.com
I thought this thread was interesting enough (well, my part anyway, LOL) to propagate to the curvemeister forum. There has been a follow up by the illustrious Greg Groess, dealing more broadly with the question of color spaces, that may be of general interest:
http://www.curvemeister.com/forum/index.php?topic=2183.0 —
Mike Russell – www.curvemeister.com
Thank you, George. I am not in the least offended; I am well aware that I know comparatively little about this subject, which is why I joined this forum–to ask others more knowledgeable than myself. Having said that, I in my turn must say that I hope you are not offended when I point out that, whilst you obviously have a deep knowledge of these matters, your talent for explanations at a level which people who don’t share your knowledge can understand is not so evident….which brings me to yet more questions:
1) Mike Russell says, "You are correct on Hue and Brightness." You say, "You are not even close to a meaningful, non-trivial definition of any of those parameters. " Should this blatant contradiction fill me with confidence in your answer or his?
2)You say that luminosity, brightness and luminance are not all the same thing, yet award-winning PS gurus like Colin Smith ( "PSCS3 for digital photographers" page 73 with reference to index )say they are. Why is that? ….Please don’t imagine I am saying you are wrong; I am just trying to understand what appears to be increasingly complex.
3)Colin Smith says, "Luminance is the brightness of the color"(reference to this in the index is to luminosity; thus he equates all three as I have done).
He goes on to say, "The effect is often confused with saturation. When darkening a color, it may appear to become more saturated;lightening the color may appear to make it less saturated. The truth is you are really just changing the brightness." No doubt I am misinterpreting his words through lack of knowledge, but this sounds more like my "common sense", but obviously mistaken, understanding.
4)When I use the Saturation slider in "Hue and Saturation" it seems just to remove the color, leaving the greyscale image behind. Yet you imply that this is not so; that saturation is, at least partly, about darkening of pixels….so that an almost black pixel with no apparent color is 100% saturated. How can this be?
In none of this am I questioning your knowledge, which is plainly extensive, but your explanations do not explain..not to me anyway.You are like a primary school teacher who explains mathematics to a small child by speaking in Latin.
I apologise if I appear critical; I am grateful that informed people on this forum bother to take the time out to help people like myself. While there is anyone left who is prepared to continue to offer help I shall keep on asking questions…which brings me to some more questions 🙂
1)Can anyone tell me in terms I can understand what is the difference between Brightness, Luminosity and Luminance?
2)ditto for explaining what Hue and Saturation actually are.
3)When I adjust the sliders in Camera Raw’s HUE, LUMINANCE and SATURATION tabs what exactly is happening?
4)ditto for Camera Raw’s Saturation and Vibrance sliders
4)ditto for the Hue and Saturation adjustment layer.
Many thanks,
Hey there Karl,
In the words of the irascible John McEnroe, YOU CAN"T BE SERIOUS!!
You have sought confirmation (and, surprisingly, got it from Mike) on such totally trite definitions as "Hue = what we normally call color". Yes, Karl. And opium puts people to sleep because of its dormitive effect.
After being advised that Brightness in PS is nothing more nor less than the highest of the RGB values (normalized to the 0-100 range) you respond by re-asking what it is.
Likewise, after being told that Luminosity is a weighted average of R, G, and B values and being given the weighting factors, you ignore that answer and cry out for help all over again. No-one can put it any simpler or more precisely. If you can’t understand that definition, honestly—maybe you should siick to the arts.
The same for saturation. Its grade-school formula is as simple as math gets and is comprehensible even to my sub-teen grandchildren. That formula tells you everything you need to know about saturation. Sorry, I just can’t do any better in explaining it.
Given that we can’t reach a meeting of the minds on these elementary things, I must in all honesty lay off on undertaking to answer your many other questions. With empathy for you, my suggestion would be for you to mull over the answers I have already given until the light bulb comes on.
George
I suppose falling back upon pomposity and arrogance, not to mention insult and abuse, is one way of avoiding answering embarrassing questions. We can all be arrogant about our own subjects and qualifications, no matter how sad and pathetic it is to do so. I don’t need your "empathy".
I have struggled alone to learn Photoshop from books and videos for some time, with considerable success.I joined this forum as I hoped to get some help from people who know more than I.I have shown politeness throughout.I never asked you personally to help, and if I had known an arrogant self-obsessed buffoon like you was going to respond I wouldn’t have bothered joining.
Much of what I have written I gleaned from PS gurus who are probably more qualified than you.I am aware they may all be wrong; I don’t know enough to say…..that is the nature of being a raw amateur rather than a professional.
As far as the level of maths your children are at, I home educated my son from the age of five in all subjects; God help your kids if you are teaching them.
As for my questions, the second list was not addressed to you. Therefore if anyone wishes to help me by answering them I shall be very grateful.If, on the other hand, I am turfed out because of the tone of this response, so be it.
….and this could all have been avoided if the discussion had been kept at the appropriate level right from the start.
Mike and George, no one questions your understanding of the subject. But it was clear from the beginning that the OP’s question was of a much more basic nature. He was asking about a light switch, and you threw Einstein at him… 😉
So there. I couldn’t keep my mouth shut after all. But I did re-read the whole thing.
Karl,
My apologies. I am used to frank give-and-take between colleagues in direct group discussions where body language and inflection convey intent and rule out meanness when not intended. I lost sight of the limitations of a forum in which the written word is the sole means of communicating. In that medium, I was rude and your reaction understandable.
George
Freeagent,
"…He was asking about a light switch…" The bulb never lit!
"…and you threw Einstein at him…" This is NOT rocket science!
George,
Just a figure of speech. Let’s move on. BTW, I thought your posts were pretty good, too. And Curvemeister’s, as always.
1)Can anyone tell me in terms I can understand what is the difference
between Brightness, Luminosity and Luminance?
Brightness is most often associated with HSB space, and is the max of the three RGB values, multiplied by 100/255. Luminosity is the result of the luminosity equation – a weighted average of R, G, and B. Luminance usually refers to a measured brightness, using an instrument to measure the actual light from a monitor.
2)ditto for explaining what Hue and Saturation actually are.
In HSB, hue is an approximate angle, calculated using the two largest RGB values, using a simplified approximation to trigonometry. The exact equations are available in a variety of places, including here:
http://www.tech-faq.com/hsv.shtml. HSB, by the way, is not the end all. There are other, better, ways to represent the same values.
3)When I adjust the sliders in Camera Raw’s HUE, LUMINANCE and SATURATION tabs what exactly is happening?
The RGB values in the image are converted to something approximating HSB, the HSB values are tweaked, and the result converted back to RGB again. It’s very likely that ACR works internally in Lab color space, where there are other ways to calculate Hue.
4)ditto for Camera Raw’s Saturation and Vibrance sliders
Vibrance is an interesting one. I believe it modifies saturation, using a brightness based mask calculated.
4)ditto for the Hue and Saturation adjustment layer.
This is the same as the HSB adjustment, only implemented as a layer instead of a dialog that directly modifies the colors.
BTW – this is a rather clannish group, requiring the hide of a rhino as well as asbestos underwear. I have "kidnapped" this rather interesting thread over to the curvemeister forum. There you can discuss it without the big guys using you as a punching bag – LOL.
http://www.curvemeister.com/forum/index.php?topic=2183.0
"My apologies. I am used to frank give-and-take between colleagues in direct group discussions where body language and inflection convey intent and rule out meanness when not intended. I lost sight of the limitations of a forum in which the written word is the sole means of communicating. In that medium, I was rude and your reaction understandable. "
Your apology is accepted in the same spirit in which it was offered.
" There you can discuss it without
the big guys using you as a punching bag."
I am at a loss as to what you mean by the phrase "big guys". Where I come from (UK) this phrase has definite connotations of enormous genitalia. The thought of you guys "hanging out"( another phrase pregnant with meaning)here comparing sizes evokes an image most unwelcome to my mind; nonetheless I would suggest such an image is entirely appropriate to the testosterone-laden atmosphere on this most
peculiar forum, with it’s overtones of pomposity and braggadocio.
In that appalling saying so beloved of you yanks: "I’m out of here!"
Quite a lot of posters (even in this thread) are not from the USA.
And stop implying that the Brits are obsessed with the size of their wedding tackle!
1)Can anyone tell me in terms I can understand what is the difference
between Brightness, Luminosity and Luminance?
Photometric units can be rather confusing and the references listed below might be helpful in clearing up some of the confusion. Brightness is a perception that can’t be measured directly, whereas luminance can be measured physically and expressed as candelas per meter squared. You are looking at the same thing from different perspectives.
Color FAQ <
http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html# RTFToC3>
Radiometry vs. photometry FAQ <
http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm>
george, as always, thank you for the concise explanations.
Good to hear from you, Dave, and to know you’re paying attention. Thanks for the support. 🙂
and to know you’re paying attention
I may not always understand it all, but i am paying attention. and those were pretty good explanations. maybe there’s a language barrier/issue?
Damn. This thread has everything you could want in a thread.
Yes, but it has strayed off-color. Before it peters out completely, the obsession cited should not be ascribed to Brits. It is a universal peccadillo not to be hung on any single genre.
