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Author Topic: levels  (Read 8292 times)
Mark D Segal
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« on: June 12, 2005, 05:25:08 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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Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
Mark D Segal
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« Reply #1 on: June 14, 2005, 01:16:28 PM »
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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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Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
Mark D Segal
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« Reply #2 on: June 14, 2005, 09:31:12 PM »
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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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Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
hovis
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« Reply #3 on: June 14, 2005, 11:16:35 PM »
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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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sc21
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« Reply #4 on: June 15, 2005, 12:36:11 AM »
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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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mcanyes
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« Reply #5 on: June 15, 2005, 07:40:37 AM »
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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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Michael Canyes
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Jonathan Wienke
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« Reply #6 on: June 15, 2005, 01:11:55 PM »
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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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Mark D Segal
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« Reply #7 on: June 15, 2005, 03:45:49 PM »
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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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Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
Peter McLennan
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« Reply #8 on: June 15, 2005, 10:32:06 PM »
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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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Jonathan Wienke
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« Reply #9 on: June 16, 2005, 12:24:46 AM »
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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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jani
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« Reply #10 on: June 17, 2005, 03:11:38 PM »
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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:


However, if you click on the eyedropper icon, then you can change between 8, 16 and 32 bit values in CS2:



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:

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Jan
hovis
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« Reply #11 on: June 12, 2005, 01:43:06 PM »
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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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BernardLanguillier
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« Reply #12 on: June 14, 2005, 12:59:57 AM »
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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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A few images online here!
Jonathan Wienke
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« Reply #13 on: June 14, 2005, 02:56:48 PM »
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OK, the fire comment was unnecessary. My apologies.
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Eric Myrvaagnes
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« Reply #14 on: June 14, 2005, 08:10:57 PM »
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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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-Eric Myrvaagnes

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hovis
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« Reply #15 on: June 14, 2005, 11:00:22 PM »
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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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Ray
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« Reply #16 on: June 15, 2005, 02:39:00 AM »
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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  Cheesy .
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Tim Gray
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« Reply #17 on: June 15, 2005, 11:01:30 AM »
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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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Jonathan Wienke
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« Reply #18 on: June 15, 2005, 01:00:51 PM »
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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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Jonathan Wienke
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« Reply #19 on: June 15, 2005, 02:48:04 PM »
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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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