Luminous Landscape Forum
Raw & Post Processing, Printing => Digital Image Processing => Topic started by: Mark D Segal on June 12, 2005, 06:25:08 pm
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When you tweak individual channels the histogram of each color channel is being re-distributed. Moves of the Upper Sliders to the left push color toward the color of the channel you are working in, moves to the right will push toward the opposite color.
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Jonathan, people would respect your intelligence and the contributions you make much more if you would just pay some attention to your bedside manners. Do you talk to your customers the same way you talk to some people on this Forum? Think back when you first got into digital photography - was all this stuff so immediately obvious to you, or did you have to read, learn, think and experiment? Professional Photoshop authors spend pages of their books explaining exactly the sort of stuff "hovis" is asking about, because they know it is necessary for the large numbers of newcomers to digital imaging. Today we are fortunate enough to have websites like this that can complement or substitute for these books and help short-cut the learning curve. If those of us who have something to contribute cannot do so in a manner that retains the self-respect of those who come here for help, it is not serving its purpose. Michael didn't initiate this Forum for people to be demeaned. He is a dedicated educator and he put this resource at our disposition for educational purposes. Let us respect that. Forum users range in expertise all the way from bare beginners to professionals, and all comers deserve to benefit from this Forum without fear or trepidation.
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Jonathan, the internet - at least on this continent - is an open sesame, and that includes the freedom to ask what you may think are avoidable questions about issues that nonetheless trouble other people who are at least trying to understand, perhaps without doing enough of their own homework - but why should you or I stand in judgment of that? Who are they hurting and how badly? We don't need I.Q. police on the L-L Forum.
And I wouldn't put questions about the results of moving color histograms in the same league as questions about the wisdom of sleeping with Michael Jackson. But be careful here as well, there are those........................we don't want to go there.
Your objectives of stimulating logical thought can be much more effectively achieved with a more positive approach that gets the person at the other end to say "hey, why didn't I think of that?", instead of "back-off you rude S.O.B."
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In case you didn't recognise yourself Eric, the first paragraph was an impersonation of you. You can take comfort in the fact that I've sunk to the right level.
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Adjusting each individual channel is basically using Auto Levels, which you can use more effectively in the Levels dialogue box with the Options button. Set it to Snap Neutral Midtones and Find Dark and Light Colors, set the clipping to around .02% for starters, and save that as the default. That will give a good starting point for neutralizing color casts and getting the full tonal range without clipping any channel. (And anyone with more experience with it, please jump in.)
The best advice I can give, though, is to get Blatner/Fraser's "Real World CS" or Eismann's "Restoration and Retouching," both of which have good, clear segments on using Levels and correcting color and why it all works as it does.
By the way, I gotta speak up for Jonathan. He answered my early questions when few others would, and his first reply to you wasn't meant to be offensive, just an aside from a pro.
Picture someone in a movie asking Clint Eastwood or Anthony Hopkins a question about levels, and the old photographer says, "That would be because altering the value of one color channel vs another is how you get different colors in a digital image. Your question is sort of like asking why fire is hot."
In the movie, the person would go, "Oh..." and go off and think about it 'cause he'd know he was just clued into something very basic that he had to learn.
But hovis, you shot back with "Learn some manners, if that's the best reply you can come up with don't bother."
We all react badly to misperceived slights, and it's one of the dangers of forums, but since you asked for help, and just joined the site this month, I'd say the manners should start with you.
Good luck with it all, and see you on the boards.
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I think the really important thing here, aside from answering a legitimate question, is that there is some discussion about the rudeness. Sometimes posters could do a bit more homework before asking an "obvious" question, and I think it is perfectly correct to politely suggest that they might do a search on the topic. But IMHO there is absolutely NO reason to EVER be rude or abusive.
Michael
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But IMHO there is absolutely NO reason to EVER be rude or abusive.
One could certainly make the case that calling someone "the poster child for bad behaviour" is rude. Even if true, it's no less derogatory or inflammatory than my fire comment.
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I would have thought so too, which is why I advised what I did earlier in the thread, but when I dug deeper and actually put the specific issue to the test, twice over the colour values (but obviously not the luminosity) of the mid-tones remained what they were. Now I admit this is only a sample of two trials, but they do seem to be saying something and have tweaked my curiosity enough to probe further. Stay tuned!
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While we're on the topic of curve adjustments and colour balance, why do you induce colour changes when you do curve moves on all three channels at once?
I assume the answer is that you must be changing some channels more than others, but why? How? Darn it, it says "RGB".
(OT) Stupidist question I ever heard was in a retail store: "Can I put this on layaway until it goes on sale?" :p
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Bottom line: in principle you are correct: any curve shift alters the color balance, BUT in practice the impact depends on the specific situation. Quote: "If you move the endpoints of the individual channel curves in such a way that the midpoints don't move much, it won't affect midtone color balance." This is probably what was happening in the tests I performed, because on the other hand - quote: "Anytime you make differential edits to individual channels, you'll get color shifts somewhere. That's usually the reason for doing so, in fact."
You have attained wisdom. The degree and type of color shift depends on the exact adjustment you make. If you adjust all color channels equally, only luminosity will be affected. If you adjust the channels unequally, color balance will be affected. The degree of color shift corresponds to the degree of inequality of the adjustment of one color channel to relative to the others. This may range from blatantly obvious to extremely subtle, or anywhere in-between, depending on the exact adjustment performed.
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Just checked, and while it seems you can't get it in Levels (the boxes aren't big enough to show those numbers), you can go to the Info Palette - click on the little arrow for Options - and select "Show 16 values."
No, that's not correct, at least not for the CS tryout I had temporarily installed, nor for CS2, which is what I'm using right now.
Here are the options available for the info palette:
(http://folk.uio.no/jani/tmp/pics/CS2InfoPaletteOptions.gif)
However, if you click on the eyedropper icon, then you can change between 8, 16 and 32 bit values in CS2:
(http://folk.uio.no/jani/tmp/pics/CS2InfoPaletteEyedropperOptions.gif)
Note that 8-bit and 16-bit values are completely ga-ga in a 32-bit image. The 32-bit eyedropper lets you see values in floating point between 0 and 1 with up to three significant digits:
(http://folk.uio.no/jani/tmp/pics/CS2InfoPalette32bpc.gif)
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Hi
I have two questions, both are related to the levels tool:
Firstly, I was wondering why; when working in 16 bit mode in both C1 pro and Photoshop are there only 255 tonal increments shown in the levels chart instead of the 65 000 or so that exist and why if when converting a 12 bit image, the 4000 levels of this image aren't shown as having gaps in the tonal scale?
Secondly, what are the advantages/disadvantages of adjusting the levels of each colour (RGB) individually?
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Firstly, I was wondering why; when working in 16 bit mode in both C1 pro and Photoshop are there only 255 tonal increments shown in the levels chart instead of the 65 000 or so that exist.
As a side comment, the only place in PS where it is possible to actually see the RGB values in 15 bits images is the information window.
You will need to modify some options for the value to be displayed this way though.
Regards,
Bernard
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OK, the fire comment was unnecessary. My apologies.
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I'm sure nobody would find it particularly helpful or useful or relevant if I posted a thread about how the sun rises in the east or how rocks are a bad choice of lens cleaning implements; where do we draw the line? I don't hate anyone, the primary goal of what I write is to get people to think logically and use more common sense. A little bit of that would go a long way toward answering a lot of the questions posted here and elsewhere.
Jonathan,
That reminds me of a scientist colleague many years ago who's spouse couldn't understand how the International Date Line works. My colleague tried many times to explain it and always gave up in exasperation.
As a college professor I sympathize with your occasional lack of patience with students who don't think things through. And I always appreciate the wealth of sensible information and reasoning that you bring to so many questions on the LL forum.
Eric
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How I love to watch the plebeians go about their daily chores, it amuses me to see their tiny brains tested by the most meagre task. They pass their lives away with such crass and vulgar interactions as drinking, fornicating, fighting, it is with a most troubled heart that we sometimes find it necessary - for the good of the many - to crush the weakest and teach a more noble path.
I am not completely without sympathy for any frustrations Jonathan may have with the questions on this forum but this is not a university course, we are not already into the second year of a physics degree and still asking what the difference between conduction and convection is. This is a public forum meant for the betterment of our knowledge of digital photography, it's also supposed to be fun. It was for this reason I wrote my reply to Jonathan and he graciously apologized (hopefully he wasn't being disingenuous). The fact is that no one is beyond mistake, I can see that my question was foolish but when you talk to people on the internet you are doing so blind, you have no knowledge of their background, of the way they have conducted their life. Yet it is so easy to treat them which condescension.
So Eric, if you're going to try and ingratiate yourself in that manner could you do it in private.
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This has been blown up way out of proportion. I thought Jonathan's rebuke was very mild compared to some of his really inventive invective, which I might add I often find quite entertaining .... provided I'm not on the receiving end :D .
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...to encourage people to learn how to fish instead of simply giving a fish to everyone who requests one....
I hear ya. My favourites are:
"How big can I print with a Canxxx"
"I've got these funny spots on my image, what should I do?"
"Is my 24-70 2.8 IS L soft?"
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Shifting midpoint via the center gamma box or changing the black and/or white point doesn't matter. If you do any of these things to one color channel, you are altering the values of that channel relative to the other channels and will get a color balance shift.
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All that guarantees is that the brightest pixels will be neutral white and the darkest pixels will be neutral black; there's zero guarantee that you'll get neutral midtones. In fact, unless you do careful tweaking of the gamma value for each color channel, you're pretty much guaranteed to introduce a color shift to the image.
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Jonathan, Jack et. al.
OK, I have gone to the Temple Mount, communed with an authoritative Guru (who will remain un-named because I didn't ask permission to attribute) and thought further about this myself.
Bottom line: in principle you are correct: any curve shift alters the color balance, BUT in practice the impact depends on the specific situation. Quote: "If you move the endpoints of the individual channel curves in such a way that the midpoints don't move much, it won't affect midtone color balance." This is probably what was happening in the tests I performed, because on the other hand - quote: "Anytime you make differential edits to individual channels, you'll get color shifts somewhere. That's usually the reason for doing so, in fact."
It was an interesting exercise and somewhat insightful.
Cheers,
Mark
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Thanks for the acknowledgement Jonathan - no harm attaining a bit of wisdom now and then - and I think the points you added also answer Peter McLennan's query with the added clarification that unless the shapes of the histograms of the channels underlying the composite are identical, when you tweak the composite channel that tweak is a uniform move operating on each of the underlying channels that by definition have different starting structures, hence the actual resulting change to each channel will differ thereby altering the color balance.
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Hi,
Anyone care to explain the difference in terminology between tonal range and dynamic range? I assume that dynamic range relates to bit depth from black to white, where tonal range refers to just the range from black to white?? These terms seem to be used interchangably at times.
Thanks, Byron
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It isn't feasible to show 65000 levels in the chart, but if the image identification pane at the top of the Photoshop window says RGB16 you are nonetheless working in "16 bit", but in fact Photoshop in its 16-bit mode works with 32000+ levels, not 65000+. I don't know whether or not the conversion process from 12 bit images to 16 bit mode leaves or smooths gaps, but you wouldn't see them anyhow, the gradations are so many. In levels you adjust the composite RGB channel for changing overall luminosity (contrast and brightness). You would adjust the individual R,G,B channels for two reaons: (1) also to change luminosity, but making sure you don't clip any highlights or shadows in any of the three channels, or (2) to intentionally rebalance colors of an off-color image. "Curves" gives you much more control and flexibility for all of these purposes, but using Levels is often sufficient and convenient.
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I thought that adjusting the RGB's indivually would be good for changing the luminosity, however, when I tried moving the levels of each colour individually to their upper and lower limits (without clipping any of them) it alters the colour balance. Do you have any idea why this would happen?
That would be because altering the value of one color channel vs another is how you get different colors in a digital image. Your question is sort of like asking why fire is hot.
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Think back when you first got into digital photography - was all this stuff so immediately obvious to you, or did you have to read, learn, think and experiment?
In this instance, yes it was. Given that just about any color perceivable by human vision can be reproduced by altering the mixture of red, green, and blue color channels, it is totally illogical to think that altering one color channel by itself would not affect the color balance of an image. It's fairly simple logic:
Premise 1: RGB images encode a wide range of colors by mixing various amounts of red, green and blue. You can change one color to another by changing the red, green, or blue color values.
Premise 2: Using the levels control on one color channel in an RGB image will change the color values for that channel compared to the other channels.
Conclusion: The levels control, when applied to a single color channel, will change the color balance of the image, because the values for one color channel are being increased or decreased in relation to the other two color channels.
Think back to the recent thread about polarized sunglasses. The original poster claimed to be familiar with the operation and effects of a polarizer, and said he knew that his sunglasses were polarized, yet was sufficiently amazed by the fact that the polarized lenses actually did what they were designed to do that he felt it worthwhile to post his "discovery" to share it with the rest of us. But again, simple logic should make that instinctively obvious:
Premise 1: Polarizers can reduce or eliminate reflections on water.
Premise 2: My sunglasses' lenses are polarizers.
Conclusion: My sunglasses' lenses will reduce or eliminate my ability to see reflections on water when I wear them.
I don't mind answering questions that indicate the questioner put some thought and effort into posing the question, but when questions are asked that indicate either a total lack of understanding of the most basic, foundational concepts, or (more often) a near-complete non-use of critical thinking skills, it makes me (and others) feel like the driving instructor whose student asked how wheels work. At some point, it becomes difficult to distinguish legitimate questions from silliness posted with the sole purpose of being annoying. I know people say there is no such thing as a stupid question, but that isn't true; there really are stupid questions:
"What's the number for 911?"
"Do you think I should let my kids sleep with Michael Jackson?"
"My stripper girlfriend is cheating on me. If I married her, do you think she'd stop?"
I'm sure nobody would find it particularly helpful or useful or relevant if I posted a thread about how the sun rises in the east or how rocks are a bad choice of lens cleaning implements; where do we draw the line? I don't hate anyone, the primary goal of what I write is to get people to think logically and use more common sense. A little bit of that would go a long way toward answering a lot of the questions posted here and elsewhere.
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Yes, OK, he could have done some more homework, and Yes, many people would find it obvious that recomposing color channels changes pixel values, while some don't; BUT THAT IS NOT THE POINT. No need to move on guys - just CHILL-OUT and be cool.
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hovis,
It takes some understanding of what the Levels histogram is telling you in order to anticipate the likely result of a manipulation. Let’s look at what happens to a grey pixel when we fiddle with an individual color channel in the Levels dialog. Assume our example pixel is 8 bit RGB mid-grey, that’s R=128, G=128, B=128. Now select the red channel and move its mid-point to the left about half way to black. Photoshop will redistribute all pixels with a non-zero red value to a new brighter/higher red value. So our grey pixel now looks more red since the old value R=128 has been reassigned a some new higher value (~R=180 or something close to this). Also note that the Green and Blue values have not changed.
Try it. Pick any file you like and locate the Histogram pallet so you can see it while you manipulate the Levels dialog. Use the Histogram flyout menu and turn on [All Channels View] and [Show Channels in Color]. Now open the Levels dialog, select the Red channel and move the mid point up and down the scale. Watch what happens to the red channel histogram. Note this changes the white balance of the whole image since you are changing the relationship of RGB for each pixel.
I call it learn by fiddling. After four or five-hundred fiddles even I get it.
paul
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In case you didn't recognise yourself Eric, the first paragraph was an impersonation of you. You can take comfort in the fact that I've sunk to the right level.
Dear Hovis,
I am sorry if my post offended you. Perhaps I was too subtle. I was not trying to insult you, I was simply acknowledging that Jonathan sometimes seems to have a short fuse, which is really too bad, because most of his posts are truly helpful.
The fact that you felt so insulted by my post really astonished me, so I went back to reread the entire thread to see what had happened. As I see it, you asked two very reasonable questions in your first post, and MarkDS gave what I thought were good answers to both. You then thanked him, and added a new question that did suggest to some of us that you really needed some basic help with color theory. MarkDS again gave a thoughtful response, as did Bernard.
Then Jonathan made his comment, that seems to me to be a bit over the edge. But your reply to him sounds to me as if you are escalating the conflict. Your comment and MarkDS's next comment both sound to me less courteous than Jonathan's original comment; nevertheless, he graciously apologized.
At this point mcanyes jumped in with an even more insulting and, to my way of thinking, unnecessary comment. The whole thread (at least the name-calling) should have stopped there, but it didn't. Jonathan tried to explain where he was coming from, and in so doing he poured a bit more gasoline on the fire. I added my comment, which was milder than any of the previous six comments (except for Jonathan's apology -- note that he is the only one who has apologized so far, except for me, and I really didn't mean to belittle your requests for information, so again I apologize.)
Jack then put his two cents in in support of Jonathan, who was being rudely pilloried by this time. And MarkDS rightly suggested it was time for all to "just CHILL-OUT and be cool."
You then took two more posts to insult me, when I was in no way trying to offend you.
One of the nice things about this forum is that there is, indeed, a place for questions at all levels. Stef-T is a beginner who has asked many good, beginner-level questions in a way that have prompted helpful responses. And there are certainly times when I have come close to asking questions that will bring down Jonathan's scorn upon my head. But I strongly urge you not to take any of this personally. For some people (such as Jonathan), a lot of these technical things do come really easy, so they have a hard time understanding why it's not so easy for other people. My brother is very much like that, and he can be a real pain in the sensor. In Jonathan's case, his posts that have good, accurate, useful information greatly outnumber the ones in which he loses his cool. I strongly recommend simply ignoring the latter and paying attention when he does say something you may find helpful.
And I hope you can lift yourself above the level you felt it necessary to sink to in responding to my post. And good luck getting a handle on the color management stuff. I for one don't find it easy.
Eric
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(...) and just joined the site this month, I'd say the manners should start with you.
Just a minor comment:
That someone registered at a particular date doesn't mean that one hasn't been reading for a longer period.
I think it should be allowed to be a "lurker" and then register when you desire to post something, and not just because that's what I did. It's a long-standing tradition on most bulletin boards, going back a couple of decades.
Also, how long one's been posting is no excuse for poor manners.
Poor manners are poor manners, no matter who's displaying them.
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I would like to suggest to my virtual colleagues that we put the subject of civility to rest - as it has been exhausted - and at the risk of getting Jonathan and Eric's dander up we return to the technical aspects of Hovis' basic question, because it isn't either that dumb or that obvious. To quote Hovis from near the start of all this:
<<I thought that adjusting the RGB's indivually would be good for changing the luminosity, however, when I tried moving the levels of each colour individually to their upper and lower limits (without clipping any of them) it alters the colour balance. Do you have any idea why this would happen? I'm sure I'm missing something but it seems to me that unless one actually encroaches into a colour (thus removing active pixels) it wouldn't be possible to alter the colour balance of the whole image.>>
You see, he wasn't asking about shifting mid-points, he was asking about the impact on color balance of filling the luminosity range by stretching each channel to the end-points without clipping, and wondering why when he did this the colour balance gets altered. None of us have done Hovis a service in the technical answers we've provided, me included. The fact is that his basic premise is correct - doing what he did should NOT alter the colour balance of the image, only the luminosity. I think what happened to Hovis is one of two things: (1) when he saw his image fall into a proper range of contrast and brightness he mistook this for a change in colours and/or (2) his adjustments to the end-points of the histogram may have looked accurate on the monitor image of the histogram but may have been slightly off in the actual numbers.
To satisfy ourselves that what I am saying here is correct, the empirical test is best implemented in Curves, where we have the detailed data we need for this and we can replicate the identical moves we make in Levels by shifting the white point to the left and the black point to the right (assuming black is lower left) for each channel up to the point that there is no clipping of anything anywhere. I selected a RAW image that failed to fill the luminosity range (especially all values much too far below the white point) and has enough grey in it to easily anchor a colour-neutral point. So I dropped an anchor on a natural grey point that had a value of level 171 in each of the RGB channels - perfect. The hypothesis is that if exactly filling the luminosity range of each channel without clipping alters colour balance along the range, then my resulting neutral grey point should change to unequal values of R,G,B. To test it, I very carefully adjusted the white and black end points of each channel's curve as far as I could without clipping; then I went back to measure the value of my anchor point: 235, 235, 235 bang-on neutral grey but brighter.
Then I selected another more colourful image and replicated the same procedure selecting a coloured anchor point and observing the impact on percentage composition of each of R,G and B to total RGB for that colour. My pre and post-adjustment percentages compared very closely, with slight variances I suspect due to limitations of visual accuracy in adjusting the end-points to the clipping point.
The discussions in Blatner/Fraser and Eismann referenced elsewhere in this thread don't really address this issue directly. Blatner/Fraser (page 306 onward) focus on what happens when you make adjustments to the COMPOSITE curve where clipping does occur in the underlying colour channels, while Eismann mainly discusses using Levels for color correction.
So Hovis, your initial premise seems correct; therefore, you may wish to consider the other reasons I suggested about why perhaps you didn't see what you thought you saw!
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Shifting midpoint via the center gamma box or changing the black and/or white point doesn't matter. If you do any of these things to one color channel, you are altering the values of that channel relative to the other channels and will get a color balance shift.
Yes of course you are correct in this useful clarification, but just to remind - that was neither the question nor the full content of the test. It is about what happens when you adjust all three channels so the end-points of the three histograms are just at the clipping frontier.
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Hey hovis, if you're still reading this, you wrote:
I was wondering why; when working in 16 bit mode in both C1 pro and Photoshop are there only 255 tonal increments shown in the levels chart instead of the 65,000 or so that exist.
I just read a bit on this the other day in "Real World CS":
"For outrageous precision, CS even displays the 16-bit values - ranging from 0 to 32,768 - when you work on high-bit files." It also mentions how they still work with the 8-bit values "at least until we get used to thinking of midtone grey as 16,384."
So there's gotta be a way to turn that on. Just checked, and while it seems you can't get it in Levels (the boxes aren't big enough to show those numbers), you can go to the Info Palette - click on the little arrow for Options - and select "Show 16 values."
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Thanks for all your replies, especially Mark for taking the time to really look at this.
It's been very interesting for me and I've learnt a thing or two, Jack's suggestion with the curves tool is very useful.
Thanks again all.
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Thanks for your reply.
I thought that adjusting the RGB's indivually would be good for changing the luminosity, however, when I tried moving the levels of each colour individually to their upper and lower limits (without clipping any of them) it alters the colour balance. Do you have any idea why this would happen? I'm sure I'm missing something but it seems to me that unless one actually encroaches into a colour (thus removing active pixels) it wouldn't be possible to alter the colour balance of the whole image.
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Hi Jonathan
learn some manners, if that's the best reply you can come up with don't bother. If you'd venture into someone elses world maybe you'd realise that you're not as bright as you think, or have you already tried that and this is the reason for your lack of courtesy. Things aren't always so straightforward for the uninitiated and remember that it's a person not a computer you are talking to.
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Jonathan,
You are the poster child for bad behaviour. There are a lot of comments on this forum, and other forums that are unnecessary.
Michael
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Hey guys...
I have to chime in on this one and defend Jonathan on this particular question. Simply stated, the question shows that the poster obvisouly did not even make an attempt to uncover a basic answer before posting his question.
Had he simply clicked the "help" button at the top right of the tool bar in CS, and then navigated to "index" then to "curves command" he would have seen several subheadings that give a great deal of insight to what the tool can do. Then had he clicked on one of the subheadings like "adjusting color balance and tonal range with" -- and gee, do I need to point out that this was more or less his original question -- he would have seen this as an initial explanation:
"The Curves dialog box, like the Levels dialog box, lets you adjust the entire tonal range of an image. But unlike Levels, which has only three adjustments (white point, black point, gamma), Curves lets you adjust up to 14 different points throughout an image’s tonal range (from shadows to highlights). You can also use Curves to make precise adjustments to individual color channels in an image. You can save settings made in the Curves dialog box for use in another image. See To save and reapply settings in a dialog box."
Maybe Jonathan and I need to move on...
Cheers,
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My ultimate goal here isn't so much to be the resident curmudgeon as to encourage people to learn how to fish instead of simply giving a fish to everyone who requests one. If people wish to debate my choice of techniques, that's fine, but I think we can all agree that most internet forums would improve significantly if more people spent a few moments checking the applicable help file or using the forum search tool or simply thinking through their question to see if it makes logical sense before posting.
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Paul,
That's a great experiment. I like your "learn by fiddling".
Eric
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Oh, Tim, there's more to match that. When I was a kid contributing to my income with part-time work at a photo retailer in downtown Montreal an angry customer wanted her money back because the flash unit we sold her failed to light up the city when she took a night shot from the top of Mount Royal; or the story from a late cousin who was a manager at Eastman Kodak's Information Centre in Grand Central Station New York - after carefully explaining to a suspicious customer why the image appears upside-down on the ground glass of a view camera the guy told him he had to be the most stupid person he ever met.
You see why I think there's lots of time for people who ask what may superficially seem to be obvious questions about histogram manipulation in Photoshop. :D
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EricM, when I was in school, I had a few (not many) rude and arrogant professors. I was not in school to be reminded that I needed an education. If you set yourself in life to be a teacher (or, in Jonathan's case, appoint yourself Teacher) you must expect to find some folks less educated than yourself. And among those less educated, you will find different levels of ability and desire to learn. You should accept that or change professions.
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For all that Jonathan apologised, just be glad it's not Didger on heat..... :p
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As for HOW to accomplish keeping the color balance in check when adjusting the curve channels independantly, here's one method:
You can tweak and adjust the individual channel curve points in any fashion you desire. This in and of itself will most certainly alter the color balance. HOWEVER, if you first make a layer, then make all the curve adjustments on the layer, you can change the blend mode of that layer to "luminosity" and the overall color-balance will remain the same as the original and only changes to the luminosity of the image will be made. (Alternatively, you can run the curves dialog on the main layer, but right after you hit "okay" go to "edit> fade curves" and here you can again set the blend mode to luminosity. )
Cheers,