lab色彩空间一书各章节总结 3
转贴Chapter 3 is about preparing for expanding on the benefits of LAB that were introduced in the symmetrical adjustment recipe.p [|R7AfW
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[size=4][b]REVIEW:[/b][/size]
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The chapter starts with a review of that recipe. LAB separates colorand contrast. After steepening the A and B curves to improve the rangeof colors in the image, adjusting the L curve involves finding therange that most needs the adjustment. While in L curves, mouse over theimage (click and drag) and notice the area of the L curve that isactivated. This is the area that you need to steepen. You can do thiswith a simple curve that steepens that area. Sharpening is then appliedto the L channel.;RH Ld[4s-yKQ
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Images that have L curve problems (too dark, too light, etc.) also tendto be colorless. You can't just lighten a dark image, because chancesare it's too monotone, as well. You need to increase the range ofcolors as well.
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[size=4][b]NEW CONCEPT:[/b][/size]
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The next element that Margulis adds is this: steepening the A curvewill intensify the green and magenta, the B curve the blue and yellow.You can use this to control the color balance. As long as you keep thecurve straight at this point, and passing through the center point, theimage will remain otherwise neutral.
One example he gives is for images that are predominantly greenfoliage. If you increase both A and B equally, you'll end up withgreens that are a bit too yellow. Simply reduce the steepening on the Bcurve, and you'll reduce the yellows. Keep in mind that this will alsodecrease the blues. b O vy L+w~A/d m
In the Chapter 1 thread, I got ahead of myself and was trying tocorrect this image of my daughter. The skin tones are too magenta tostart with. I could tell this by the numbers by checking the CMYKvalues, knowing that unless you want the skin to look sunburned yellowshould always be greater than magenta.[attach]50298[/attach]
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When I increased both A and B equally, the magenta in the skin tonesgot even stronger. Here is the result of steepening both equally:[attach]50299[/attach]
If I apply different curves to A than to B (and in this case I actuallymade the A curve LESS steep), I reduce the magenta, increase the yellowand produce more pleasing skin tones:[attach]50300[/attach]K-QNk x3G
Here are the curves I applied:[attach]50301[/attach][attach]50302[/attach])Z eH RR/\.N
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You can see that reducing the steepness of A and increasing it in Bmade the image overall more yellow than magenta, but it kept the blueand yellow balanced.
For the purpose of going over this new tool that we have, I'm ignoringthe L curve (which I modified as well) since in LAB the colors areindependent of Luminosity.4zu Ik k[,am0I Q
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This is a powerful, new overall color correction that we have.*H0V DR5TW)J`
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Margulis spends a bit of time talking about our perception of thegreens of nature and skin tone. These are two situations where our mindsees most differently from the camera. We're sure that the greens arericher and more detailed than they really are. And we are also surethat skin is more tanned, bronzed than it really is. His argument forthese two situations is that we should satisfy our perceptions of thesetypes of images, not the reality. If we are too literal with greens orskin, our perception of the image will that it is flat and washed out.
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[size=4][b]IMPORTANT TERMS:[/b][/size]
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[u][b]Full Range.[/b][/u] This means that the lightest and darkestparts of the image are handled right. Appropriately is the word heuses. This is also called setting highlight and shadow, which we're allfamiliar with. In RGB and CMYK achieving a neutral color is a matter ofbalancing the numbers. Neutral colors in RGB have the same value foreach of the channels. In CMYK, the channels that need to be the sameare magenta and yellow, with a bit more cyan.zEs1~ Q F
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In LAB you can set your highlight with the L channel and then just makesure that you have a neutral color by making sure that A and B both are0. This means that you can make huge changes very quickly in LAB. Thisis because L is, as Margulis calls it, a bull. It's a very powerfultool. It can correct serious problems with contrast. This also meansthat RGB and CMYK will handle some situations better, since they arenot a bull in a china shop. Those instances will be covered later inthe book.
The point of "Full Range" is to "maximize range while retaininghighlight and shadow detail." The point here is very clear: becautious, especially if you will be doing final work in RGB or CMYK,where you can further adjust the highlights and shadows. Once you'velost highlight or shadow information it's gone, never to be regained.So treat it with care.
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[u][b]Impossible colors[/b][/u] are colors that just aren't right. Youdon't always have the luxury of a neutral color to balance from, butunderstanding LAB numbers (as we'll all get better at as we workthrough this), you can think through an image and make intelligentchoices. The large the A or B number (positive or negative) the morecolor. The closer those values are to 0, the more neutral they are./?] YYc!kq5K/D
What I talked about above in setting the L channel curve; that's called [u][b]Allocation of Contrast.[/b][/u]This concept basically says that you should give contrast (tonalityrange) to the part of the curve where the image lives. Or, morespecifically, the part of the image that you want to focus theattention of the viewer on.s#sq7_] L1ww7I?
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Margulis goes through two examples at this point. A dark, dense forestand head shot of a model. His basic point here is that the way we seethese scenes is different than the camera. We see more in the shadows,more shades of green, more intense green, and a tanner face. Our job aspost-processors is to not capture the scene accurately from ascientific standpoint, but from the way that we think we see. We havelocalized contrast and a much wider tonal range available to us as wewalk through this world. We need to overcome the limitations of thephotographic image by making the image optimized to reproduce ourmemory of the scene.2l'^-i!o)zL
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Understanding the L curve is easy. Dark is dark and light is light. TheA and B curves are more difficult to keep straight. Once you get inyour head that green is opposite of magenta and that blue is theopposite of yellow you'll know which colors are paired. After that justremember B = Blue. Then you'll know that the B curve is blue andyellow. You could also add: A = mAgentA. There's no "a" in blue oryellow.*s.N g S2h*I7^
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Another thing to get clear on is that in LAB positive values are warm(magenta and yellow) and negative values are cool (green and blue). Iknow that yellow and green can't be opposites; they are too closelyrelated. That helps me remember that yellow is paired with blue.
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Anyway, it helps me. Y:{7l?'x6e
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It's time to make sure that we're all working on the same page. Some ofthe settings that Margulis recommends are important, others are justwhat he uses. His suggestion is to do as he does, since it will lead toless confusion when reading the book.
First, the eyedropper is set by default to point sample. [attach]50303[/attach]
This is not very useful since it could pick up a bit of noise or dust.When you've got the eyedropper tool selected you can change this in theupper left. 3 by 3 or 5 by 5 is fine.[attach]50304[/attach]
The info palette is also very important. There are little eyedroppersnext to the colorspaces on each side. This is a drop down menu that youcan change. The left side is set by default to Actual Color. Leave itas is. The right side you should set to the colorspace that you aremost familiar with and understand the numbers best. Margulis says thatfor him that is now LAB, but for most of us (especially those of us whohave to read this book!) we'll find RGB or CMYK most natural.*b/X @~P,d&n a4Q~
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Having said that, there are different RGB and CMYK colorspaces, all ofwhich will yield different results. For the purpose of working withthis book he recommends that you set your RGB to sRGB and your CMYK toU.S. Web Coated (SWOP)v.2. You can change these in your Color Settings,found in the Edit menu.
In order to change between RGB and LAB, Margulis uses the Convert toProfile command. His reasons are pretty esoteric and I'm pretty surethat we can all just use the Image>Mode menu.
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If you want to use the Curves dialog eyedroppers to set valuesautomatically (black and white points), he suggests the followingsettings:qG%]m-aN FZ
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Black: 6L 0A 0Bt9r gL,c(iy#s
White: 97L 0A 0B
You set these numbers by double clicking on the eyedroppers in the Curves dialog box and changing the values for LAB there.[attach]50305[/attach]B3q![5yE(~(]V
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We all see color differently. Especially color-blind people, who cannotdistinguish the green/magenta very well. This section of the chapter isreally only useful if you read it and study the images. The interplayof A and B is so crucially important to what we do, that as muchexperience you can get in playing with the numbers in your head as youlook at images is important. Take a look at the images that he's put inthis part of the chapter, read along and you'll gain more insight intohow A and B work together.l8}Q c U'_F
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Check this out and let me know what you think:*P8BYq"?j`h&`$V
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Here's the original RAW conversion:[attach]50306[/attach]:D|.o/LUii V
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If I apply equal changes to the A and B curves[attach]50307[/attach][attach]50308[/attach]"y'g%Y8j x ~1| gw)y
and this L curve: [attach]50309[/attach]
I get this:[attach]50310[/attach]rhwZ5}G5v N:]
So, following Margulis' logic I lowered the steepness of the B curve to compensate for the yellow in the green:[attach]50311[/attach][attach]50312[/attach]
to get this:[attach]50313[/attach],k3B6yTsf!P
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My question is this: to my eye the even changes in A and B, the moreyellow version, looks better, as this is evening sweet light. Inaddition, there's not much skin tone, but my son's hand there gets toomagenta in the second version.p yK4r t:{;d(R
Are there times that it would be desirable to leave the accentuated yellow in the greens of nature? Which do you all prefer?&x7mzYJvn
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[[i] 本帖最后由 honghu 于 2007-3-11 16:40 编辑 [/i]] 有图，还勉强看，纯E文的就不行了:L 要是中文该多好:victory: 很有用,虽然看着比较累但总算是明白了.谢谢鸿鹄. My question is this: to my eye the even changes in A and B, themoreyellow version, looks better, as this is evening sweet light.Inaddition, there's not much skin tone, but my son's hand there getstoomagenta in the second version. \"g n6pH~
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Are there times that it would be desirable to leave the accentuated yellow in the greens of nature? Which do you all prefer?6eL%E)B X7A)O? } Z
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俺做了两个，然后选择天空部分用那个toomagenta的天空替代不就行了吗？
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难道可以只用一层就调出想要的效果 more yellow version 's greens of nature+ toomagenta sky?6O1?(j O \p"Kf7I
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