Author Topic: Scanners and Color Management  (Read 9693 times)

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Xepher

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Scanners and Color Management
« on: October 23, 2007, 12:12:33 am »
So, I browse a lot of art sites on a regular basis. Not being an artist myself, I rarely have much in the way of useful commentary. However, I've recently noticed that a lot of people's art posts are accompanied by complaints like "My scanner ate the colors!" or "The greens really don't show up after the scan."

Computer imaging IS something I have a fair amount of experience with. Especially lately, I've been playing with my new digital camera and working a lot with color profiling and the like. As such, I think I could probably pull together a good primer on such a thing. Before I go and take the time to make a fancy article on it, I figured I'd ask around and see if anyone would actually find that useful, or if there's already some good/obvious ones out there that I should just point people at instead. Either way, my point is that there's no excuse for letting a scanner eat the colors of your artwork. :-)

The example below is as simple as enabling the color profile for my camera when processing things.

Default sRGB output:


Nikon (Camera) ICC profile output:



EDIT: Changed "at the colors" to "ate the colors" so it makes more sense. :-)
« Last Edit: November 03, 2007, 07:04:36 am by Xepher »

Gwyn

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Re: Scanners and Color Management
« Reply #1 on: November 03, 2007, 03:44:50 am »
I'd be interested, I might even bring it to school to had out. I'm doing a class on scanning.
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Desh

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Re: Scanners and Color Management
« Reply #2 on: November 04, 2007, 01:39:26 am »
I'm also interested. I tried to read about color management once but I can't say it made much sense.

Xepher

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Re: Scanners and Color Management
« Reply #3 on: November 04, 2007, 04:05:49 am »
Alright, I'll work on a short tutorial. I need to set my scanner up again anyway. :-)

Xepher

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Re: Scanners and Color Management
« Reply #4 on: November 04, 2007, 07:09:21 am »
Alright, let's start this with a tutorial on getting your MONITOR somewhere near to standard output. I'm going to assume you're on windows or linux or some other "standard" computer. The web/windows/linux gamma standard is 2.2. If you're using a mac, the value is probably 1.8, but no one else in the world uses that, so you should still try and match 2.2 if you want your images to look the same to the rest of the world as they do to you.

So we start with a test...


The idea is to back up (or unfocus your eyes) enough that you don't see lines, but just a solid, completely uniform grey square. If you see banding or color variation, then your monitor needs adjustment. If it looks like a uniform grey, then you're good to go and can skip the rest of this post. Note that if you're using an LCD monitor, they can look a LOT different if you're looking at them from even a slightly different angle. Thus, for this whole process, make sure you're viewing from the same angle, and that angle is the one you normally view your screen at. If you back up instead of blur your eyes, try and back up in a straight line from your normal eye position... That is, don't go stand up in the back of the room, then expect it to look the same when you're sitting at a lower height. If you've got an old CRT type monitor (the glass tube things) then it should look the same from pretty much any angle.

Now, assuming your monitor isn't correct, let's continue. Do you have photoshop installed? If so, it often installs a helper program called "Adobe Gamma Loader" that loads monitor adjustments on start up. If you have this, you need to go read the photoshop docs and do this whole process their way, or disable the loader. Usually it goes in the start menu, in the "startup" group. You can remove the icon from there and it won't run. If you do that, reboot your computer to make sure it's not already made it's adjustments though. (Newer photoshop might not have this, I don't know. But if photoshop is trying to do it's own correction, then we go and correct again, we'll just make things worse. Odds are, if you haven't ever done a calibration in photoshop, the loader is just setting everything to "normal" anyway and we can ignore it. I may try and write up the photoshop way of doing this later, if I get the time. For now, we go with the more manual way of doing this.)

First, try to minimize any external lighting on your screen. At night in a dark room is best, but at the very least, don't have any lights shining directly on your screen. Set the contrast to something high-but-not-maxed... E.G. 90% or so. Also, if you can adjust the color temperature on your monitor, set it to 6500K. Now turn the Brightness (black level) on your monitor all the way down and look at the image below.


There's a big black bar on the right, and in that are two not-quite-black bars (a and b.) We're looking at bar A. Slowly turn the Brightness up until Bar A has just become visible (it appears to raise as you adjust) at the 2.2 gamma level as marked on the left of the image. The rest of bar A above the 2.2 point should be totally black and blend in with the surrounding black. You've now set the proper black level for your monitor.

The next part is a bit trickier, and is going to vary a lot based upon your video card. You basically need to go into your display settings, and find the advanced options for gamma. If your video card drivers let you directly adjust the separate RGB values, that's best. If it only gives you a single control, we can still work with it though.

Take a look back at the big chart. You're gonna do the same blurry-eyes thing as the initial test image, and adjust the main gamma value (or all three RGB values by the same amount) until you get as close as you can to 2.2. That is, with fuzzy-vision, the color should look like a uniform grey at the 2.2 mark, across the entire left half of the image. Obviously, this has some room for error in it, as your eyes are constantly adjusting themselves too. It doesn't have to be perfect, just get it close. Then go back to the original test image and see if it looks solid grey now. If you see a sort of "banding" across the width of it (that is, there's a couple of vertical bands) then you've got to tweak further. If the bands look perfectly grey and just lighter or darker, then adjust the main gamma. If the look to be slightly colored (such as having a greenish tint) then adjust that individual color. This takes a LOT of trial and error. Note that increasing one value can have almost the same effect as decreasing the other two. It's about balance. Usually it's best to bump one up a little, and if that gets you closer, but not quite there, bump the other two down. That keeps the total gamma as close as possible to what you calibrated the main value to at first.

If you get it so the bands look perfectly grey (not tinted any color), but still show up, tweak the main gamma value (or all three at the same time) a bit up or down and see if you can get the banding to disappear completely. As I said, this whole process takes a while, and is really just trial and error. You'll frustrate yourself if you try to get TOO perfect with each individual step. I'd advise you to get pretty close, stop for a few minutes (go browse some other pages) and then check the top test image again. It'll probably look a LOT closer than you thought it was. If you concentrate too hard, you'll always find some variation or imperfection, so a short break helps. Odds are, you can get about 95% accurate in about 10 minutes of fiddling. Don't worry about the final 5%, human eyes just aren't that accurate.

A lot of this info is at http://www.normankoren.com/makingfineprints1A.html#gammachart which has way more detail on the subject, as well as the charts I'm using here.

Also, a note to all linux users. The program to adjust gamma is "xgamma" and you can set CORRECTION values (that is, 1.0 is normal gamma) with it via "xgamma -gamma 1.2" or "xgamma -rgamma 1.2 -ggamma 1.2 -bgamma 1.2" for the tricolor way.  You can add the final value to your xorg.conf monitor section with the line "Gamma 1.2 1.2 1.2" for RGB values respectively.
« Last Edit: November 04, 2007, 07:17:35 am by Xepher »

Desh

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Re: Scanners and Color Management
« Reply #5 on: November 04, 2007, 09:44:05 pm »
Do you have photoshop installed? If so, it often installs a helper program called "Adobe Gamma Loader" that loads monitor adjustments on start up. If you have this, you need to go read the photoshop docs and do this whole process their way, or disable the loader.

I have the Gamma Loader from Photoshop CS I think.  I found Adobe Gamma in the Control Panel on Windows XP.  It offered either a wizard or control panel for the adjustments.  I used the panel to adjust to your images pretty easily.

The panel:


Xepher

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Re: Scanners and Color Management
« Reply #6 on: November 04, 2007, 11:05:53 pm »
Good to know. Looks like you just need to pick the right "phosphors" (are there LCD options? They don't have phosphors) and make sure the white point (color temperature) is set the same as your monitor itself. 6500K is optimal, but not all monitors have that as an option, so if yours doesn't, adobe has an extra correction for that. I'm betting the "wizard" will step people through pretty much what I'm showing manually here, so any of you with photoshop might be better off trying that route. No matter what method you use, if the gamma is properly balanced at 2.2, the test image above should still look perfectly gray at the end.

Desh

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Re: Scanners and Color Management
« Reply #7 on: November 05, 2007, 01:37:38 am »
I've got another screen of the options in the panel plus an animation of the wizard.  It's seven simple steps with a little text about each option.

I don't know about the phosphors, it might be automatically detected.  I don't know anything about displays, but I'm pretty sure my monitor isn't an LCD.  My monitor has the 6500K option so I'm happy there.

Options:


Wizard:


Xepher

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Re: Scanners and Color Management
« Reply #8 on: November 05, 2007, 02:15:45 am »
Hmm... doesn't look to have any options specifically for LCD, so the standard HDTV one (or whatever is automatically detected) is probably fine for anyone that has an LCD monitor. The rest of the wizard seems to step you through exactly what I've described above, so if you've got that gamma control panel on your computer, just use that, and ignore my tutorial. :-)

Bill Nye moment follows:
FYI, there's two types of monitors... the glass tube type are "CRT" (cathode ray tube) where an electron gun (or guns) fires at the back of the screen, and that excites phosphors, which glow either red, green, or blue. The new flat-panel monitors are LCD (liquid crystal display) They have a light behind a screen, and color filters that let red, green, or blue light through at particular sub-pixels. In front of that is the actual liquid crystal, which is a bunch of little cells of the same stuff that makes calculator or digital watch screens. The crystals float in liquid and are thin, kinda paper-like if you magnified them. Electricity makes them turn one way or another. If they're all turned so you're seeing them edge-on, then they're almost invisible and nearly all the light gets through. You see a bright pixel. As they turn toward the flat side, they block more and more of the light. When they're turned so you're seeing them fully face on, they block almost all the light, and so that pixel goes black. There is also a display technology called "plasma" but computer monitors (almost) never use that, it's mainly used for big HDTV sets. It works with each pixel basically being a tiny little neon sign... a little electricity makes it glow.