# Photoshop Lab Color- P5

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## Photoshop Lab Color- P5

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Photoshop Lab Color- P5: LAB has a reputation for enormous power, yet virtually all reference materials that advocate its use illustrate its capabilities with a single class of image. This chapter introduces the basic LAB correction method and explains why it is so extraordinarily effective. if you happen to have a picture of a canyon.

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## Nội dung Text: Photoshop Lab Color- P5

1. Entering the Forest: Myths and Dangers 121 C D E F G H
2. 122 Chapter 6 things to worry about than bit depth and con- anywhere. 210R210G40B, which is a fairly sub- versions. As for Figure 6.8, I state categorically dued greenish yellow, isn’t likely to be found. that the two versions are identical for any con- And neither is anything with higher red, green, ceivable professional purpose. or both, coupled with an equal or lower blue. If you agree, 86,756 possible colors have just And Why Not Look, If You Please? fallen on their swords. Pastel blues, brilliant A boring bystander is unfortunate enough to be greens, and all cyans are also among the miss- confronted by an enraged Cyrano, who imag- ing. Plus, there may be some luck of the cards. ines the man is staring at his nose. 50R50G10B might easily be found in the woman’s jacket, but there’s no guarantee that even a CYRANO. What do you think? Is it not a single pixel will have exactly that value. phenomenon? Some programs can analyze exactly how THE BORE. But I knew better than to look! many discrete colors such a ﬁle contains, but I CYRANO. And why not look at it, if you don’t own one. My guess is that in this image it’s please? a lot more like 10 million than 17 million. THE BORE. I was… But now, let’s take it into LAB for the ﬁrst time. CYRANO. Does it disgust you? There should be around 256 values in the L, THE BORE. Monsieur… granted. But there won’t be anything like that in CYRANO. Perhaps you do not like its color? the A or B. With no really brilliant colors in the At the close of the scene, the bore is lucky to image, it would be surprising to see values more escape by being smacked on the side of the than 50 in the AB channels. So, there are head rather than being run through with an maybe 100 values in each one, tops. épée. A good cufﬁng might do wonders for his Having just said goodbye to 14,217,216 colors, counterparts in the Photoshop world, those who it only gets worse. As the L gets closer to its end- are so certain of their ground that they know points, the AB possibilities are sharply reduced. better than to look. By the time we’re at 5L or 95L we may be down And now that we have looked, and know the to only 20 real possibilities in each AB channel. correct answer, it must be conceded that some- To be generous, let’s shortcut a lot of arith- times the theory seems so obviously true as to metic and estimate that for each L value there render any alternative inconceivable. This is a are 60 possibilities in the A and B. If that wild compelling example. How can moving to LAB guess is exactly correct, there are 921,600 pos- not cause damage? We’re throwing away (so sible colors in the LAB version. Since it isn’t, let’s they say) a third of the colors! call it a million. And we estimate that the RGB Translation: the original RGB ﬁle consists of picture contains 10 million colors. We are throw- three channels, each of which has 256 possible ing 9 million of them away by converting, no? values, or levels of tonality. If we consider two This is much worse than the advertised loss of channels simultaneously, each of the 256 values Figure 6.9 These two images, one digital, one from in the ﬁrst channel has 256 more possibilities in ﬁlm, are joined in one ﬁle. Originally they were quite the second, for a total of 65,536 possibilities. If light, but instead of correcting in one pass, this drastic we add a third channel, each of these 65,536 change was done in seven separate steps. In one has 256 more, for a grand total of 16,777,216 version, all steps were done in 16-bit RGB; in another, possible combinations. 8-bit RGB; and a third and fourth were done in 16-bit and 8-bit respectively, but after each of the seven steps, I don’t know how many distinct colors are the ﬁle was converted into LAB and back into RGB. On in the original version of Figure 6.8, but it isn’t the opposite and next two pages, the four versions are 16,777,216. For example, there’s no bright yellow shown in random order. Can you tell which is which?
3. A B C D
4. E F Figure 6.10 Views of the four versions of Figure 6.9 at various sizes. Left to right, the magniﬁcations are 200%, 250%, 400%, J and 500% (showing the green channel only). K N P S T
5. G H L M Q R U V
6. 126 Chapter 6 a third of the colors. We’ve lost nine-tenths of can apply statistical measures to verify that the them! Surely, it is madness to suggest that impossible is indeed true. converting into LAB is safe! This is becoming surreal, and we haven’t even A great theory, seemingly irrefutable. And yet hit the clincher yet. Create a new RGB file. there’s Figure 6.8, big as life, laughing at us, Choose a couple of unlike colors for Foreground demonstrating that there’s no loss at all, not and Background Colors, activate the gradient even after 25 conversions to and from LAB. tool, and create a vignette. Make a copy of the ﬁle. Convert it to LAB, and then back to RGB. When the Impossible Happens Hideous! Banding in several areas. This book assumes that our RGB is the variant Seems fairly conclusive—but then again, known as sRGB, a choice of convenience, not an there’s Figure 6.9. Tortured almost beyond belief, endorsement. Many professional photographers converted again and again, when it’s a real believe that sRGB is unduly limiting. Its deﬁni- picture and not a computer-generated gradient, tions of the primary colors are relatively dull. all four versions are so close as to be indistin- Those who subscribe to this criticism generally guishable for any practical purpose. prefer the deﬁnition Adobe RGB, which permits Every logical way of looking at it suggests that more brilliant colors at the expense of some the LAB versions have to be much worse than subtlety. A few feel that Adobe RGB isn’t wide- the RGB originals. But they aren’t. Therefore, gamut enough and use an even more brilliant something about the reasoning is incorrect; it deﬁnition. only remains to ﬁgure out what part. An Adobe RGB user who wishes to work on a Faced with things I don’t understand, I ﬁnd it ﬁle that was prepared for sRGB has to convert it, useful to curse at the monitor. If that fails to using Image: Mode > Convert to Proﬁle, just as resolve the problem, Armagnac, or on extremely we LAB users need to convert out of whatever rare occasions a cigar, may make an appearance our own RGB is to do our thing. to help the thought process along. I forget how So, here’s the challenge. Suppose Figure 6.8 much of this was necessary ten years ago, when was prepared not by converting sRGB to LAB to I ﬁrst tried to ﬁgure out how there could possibly sRGB 25 times in a row, but rather by converting not be a visible loss when going to LAB. Anyhow, sRGB to Adobe RGB to sRGB 25 times. How there are two basic answers: much closer to the original version would it be 1. In mathematics, the symbols and do than the image with multiple LAB conversions? not mean the same thing. Adobe RGB is certainly a much closer relative 2. In a photograph, the blood does not curdle to sRGB than LAB is. It does waste a certain at the thought of altering a single comma. amount of real estate on colors that don’t exist in sRGB, but still, if there are 10 million distinct Of Salaries and Pixels colors in the sRGB version of Figure 6.8, I’d have Numbers make excellent servants, poor masters. to suspect that there would be 9 million in an An overweening and unwarranted belief in the Adobe RGB version. So it has to be less damaging power of their precision has been the hallmark to convert to Adobe RGB than to LAB—right? of those who cry data loss every time there’s a Wrong. minor move in the image. If you do this test—and I have—a most per- John Jones makes $50,000 per year. How plexing thing occurs. The multiple-LAB conver- much does he make per week? sion is closer to the original than the multiple- A computer programmer would answer, is it Adobe RGB version is. None of the three versions a leap year, or not? A statistician would answer, can be easily told apart, at least I can’t, but we about$1,000.
7. Entering the Forest: Myths and Dangers 127 Someone who thinks that converting to LAB is it’s off by as much as .02, it could conceivably be damaging would answer, $958.9041095890. reporting something as 437 that actually should We need clariﬁcation. Does what we have have been rounded to 436 or 438. And if you been told really, literally mean that he makes say yes, the device is really that good, I’ll ask$50,000.00, not a penny more or less, in the whether it’s good enough to know the difference course of one non-leap year? Or is $50,000 between 436.4999999, which rounds to 436, merely shorthand for somewhere between and 436.5000001, which should be reported as$45,000 and $55,000? Or between$49,000 437; and I’ll keep adding decimal places until and $51,000? you give up and admit that it’s theoretically pos- Knowing as little as we do, the statistician’s sible that 437 is not technically the correct value. answer is correct. It really sounds like$50,000 is Back in the real world, the results are re- some kind of rough estimate. Any answer more ported on a scale of 0 to 255, or 256 values in precise than $1,000 a week makes an unwar- all. We use this scale because 256 happens to be ranted assumption about the reliability of the the number of possibilities that can be described data.$958.9041095890 sounds ever so much with eight bits of computer data. That is, a single more authoritative, and so impresses some bit is either on or off, yes or no, 0 or 1. Two bits Photoshop authorities that they call the \$1,000 give us four possibilities: 00, 01, 10, and 11. answer “quantization error.” In fact, from the Three bits permit eight, since any of the above statistical point of view, it’s far more accurate four two-digit numbers could be followed by than making unwarranted assumptions about either a 0 or a 1. Each time a new bit is added, how many signiﬁcant digits we start with. Any- the possibilities redouble. Four bits allows 16, ﬁve thing other than the ﬁrst digit after the dollar sign 32, six 64, seven 128, and eight 256. is a random number, for all we know now. All modern capture devices nominally use The same analysis applies to digital images. more bits. They may think they’re getting 1,024 Cameras and scanners do not return perfect values, or even 4,096. The question is whether data. We should have more conﬁdence in the the numbers are particularly accurate. Some reliability of midtone captures than those of people are so buffaloed by arithmetic and so in extreme lights and darks; in less saturated rather awe of any kind of measurement by machine than brilliant colors; in the green channel rather that they forget to ask it. than either of the other two. But in any case, the No computer program can verify whether a very act of capturing the image has introduced given pixel is correct. We have only our gut feel- unwanted variation. ings as to how accurate the capture is. My own Even if the data is very good (and how would is, I don’t think any devices can make accurate you prove that it is?), it can never be fully reliable. real-world captures in more than thousandth- Suppose you own the ﬁnest camera or scanner part increments, and that’s only under the very in the world. You claim that it’s capable of re- best conditions at certain levels of lightness. If it’s solving 1,000 different levels of gray, and that a a digital capture taken in relatively dark condi- certain pixel measures 437, and that’s the correct tions, I don’t think the camera gets even close to value, period, amen. 256 accurate values. Under better conditions, I The response is, how can you be so sure? The think most cameras record accurately to within a device is actually trying to juggle a lot more than level of the ideal, particularly in the critical green 1,000 values, and it’s doing some rounding. channel. That is, if the camera records 128G, I What 437 really tells us is that the pixel mea- doubt, but don’t rule out the possibility, that sures somewhere between 436.51 and 437.49. 126G or 130G would have been more accurate. But is the device actually that good? Because if A difference of one level, that’s another story.
9. Entering the Forest: Myths and Dangers 129 begs for banding or other evil consequences. things. If you had to choose which of these cities Gradients should be created in the same color- to live in based solely on their climates, you space as the output device—in this case, CMYK . would certainly choose them in the order of lowest standard deviation—even if you don’t The Most Useful Statistic know precisely what standard deviation means An architect planning to build something in a or how it is computed. strange city needs to know what temperatures In fact, almost everything having to do with are likely to be encountered, so that appropriate process control in the graphic arts amounts to heating and air conditioning systems can be a struggle to reduce the standard deviation, ordered. The information that the average noon- because variation is bad and variation is what time temperature in my New Jersey home town the standard deviation measures. For example, is around 53 degrees Fahrenheit would not be the printer of this book, whose presses are run enough for that purpose. That average tempera- by mortals, sometimes prints jobs lighter or ture is similar to that of Kansas City, Missouri, darker than his average. I am hoping very hard which, not being close to any ocean, has more that his standard deviation is low and that this extreme heat and cold. Yet summer days where book will fall close to the mean when printed. I live are frequently hotter than in San Juan, Once enough data exists for a standard devi- Puerto Rico, which has a much higher average ation to be computed, it can be used to predict temperature overall. As a matter of fact, Fair- the likelihood of various events. For example, banks, Alaska, is sometimes as hot as San Juan the variation of noontime temperatures over in the summertime. the period of a year is likely to be slightly less The average temperature is not as important than six times the standard deviation, meaning as how much it ﬂuctuates. And the architect in my case that the hottest day is around 80º would need something better than all the tem- hotter than the coolest. Fairbanks, I am given to perature records of the last few years. For exam- understand, has the highest standard deviation ple, I don’t recall noontime temperatures of of any major city—around 140º difference be- higher than 95º in the last ﬁve years. However, tween the coldest and hottest days. I can also around 15 years ago, it hit a ghastly 106º and learn from the standard deviation that my town stayed there for several days. does occasionally have days in the 90s; that The supremely important statistic known as something on the order of the 106º heat wave is standard deviation would have informed the apt to occur every 20 years or so, and that a architect that such a heat wave was possible, reading of 115º would indicate that either the even if the only records available were for the thermometer is broken or the weather recording last two years. The concept applies whenever station is on ﬁre. there are many data points clustered more or less uniformly around a mean value, as the The Odds Are Against It weather is. If the mean is 53º, we’re equally likely As you may have conjectured, standard devia- to ﬁnd 63º as 43º; less likely but still equally tion can also be part of image analysis. Like the likely to ﬁnd 73º as 33º, and so on. histogram, I consider it worthless as an aid to I haven’t gathered the data or done the arith- image manipulation. Neither can tell us about metic, but I’m going to estimate that the stan- the visual quality of an image as accurately as dard deviation in my home town is around 14º, our own eyes do. and the cities mentioned above as follows: Both are, however, sometimes helpful in try- Kansas City, 17º; San Juan, 5º; Fairbanks, 24º. ing to ﬁgure out why something is happening High standard deviations are generally bad that we don’t understand, like, for example, why
10. 130 Chapter 6 converting to LAB is safe when logic seems to and harmless. (If you’re going to be converting dictate otherwise. ﬁles 50 times, though, you should turn it off, as To learn how close the two halves of Figure I did for these tests.) 6.8 are, I applied one to the other in Difference Further, if these numbers are correct, around mode. This blend, which can be done in several 80 percent of the pixels in the two green chan- ways, creates a black ﬁle, except in pixels where nels are identical, and essentially all others are the two images aren’t identical. one level apart. Variation of two or more levels For an RGB image, Photoshop offers six dif- would occur, if at all, less than one time in every ferent sets of statistics to accompany the his- 5,000 pixels. togram, in locations that vary with the version of Also, remember that we never see individual Photoshop. The most important stats are those pixels except on the monitor. When the image is for the green channel and for luminosity, which printed, there’s always an averaging process to is a weighted average of red, green, and blue. convert the original pixels into the form that the Photoshop reports that in the green channel the output device requires. This is true regardless of mean variation between the original of Figure how the image gets printed. In the case of this 6.8 and the version that went in and out of LAB book, the press requires halftone dots, tiny blobs 25 times is .15 levels and the standard deviation of cyan, magenta, yellow, and black ink. Each .36; in luminosity the numbers are .10 and .30. dot is calculated by averaging, usually, the val- Let me offer, er, a translation. The numbers ues of three or four pixels. Take a loupe to either indicate that the variation is approximately half of Figure 6.8, and if you have a few weeks to equivalent in impact to the soft noise or dither spare you’ll be able to count some 2.7 million that Photoshop by default inserts every time halftone dots, averaged down from around 7.5 an image is converted from one colorspace to million pixels in the CMYK Photoshop ﬁle. another. If you didn’t know that Photoshop does What would it take for us to notice roughness, so, you’re not alone—it’s undetectable, useful, any degradation in quality? I’d say, a dot, not a pixel, that varied from its proper value by at least two percentage The Torture Test: 25 Times Back and Forth points. Although printing dots (All variations from original are expressed in Cyrano Units; lower is better) are usually referred to in terms of sRGB to Red Green Blue Lum percentages, they in fact are con- structed on a 256-level scale, just LAB 1.62 1.16 1.71 0.95 as pixels are. Two percentage ColorMatch RGB 1.68 0.65 0.49 0.09 points equals ﬁve levels. But let’s be ultra-conservative Adobe RGB 3.96 0.88 1.99 3.18 and say that a dot might be de- Wide Gamut RGB 8.45 12.80 3.66 9.12 tected if it were only two levels larger or smaller than it should be. LAB (w/dither, 1 conversion) 2.62 2.13 2.82 1.38 Being that it’s camouflaged by three other correct dots of different LAB (w/dither, 25 conversions) 9.50 7.67 10.04 3.37 colors that are intersecting with it to some extent, it would be almost Figure 6.11 The original of Figure 6.8, in addition to the 25 conversions to LAB impossible to see, but let’s theorize and back, went through ﬁve other sets of conversions, in each case but one that we are going to edit the ﬁle so being converted in and out of the destination space 25 times. Variation from the original is expressed in terms of “Cyrano Units” as deﬁned in the text. All drastically that the difference might ﬁles except the ﬁnal two lines were converted with dither disabled. show up later.