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[re conversion of RGB to CMYK]
There are a host of technical issues with out of gamut colors. The ICC standard defines a "tag" that defines out of gamut colors.
But the gamut tag is not always included in a profile, and when it is, it is not used consistently by different manufacturers. So programs like Photoshop generally ignore the gamut tag, perferring instead to rely on color values calculated using the profile. One rather simple method of doing this is to see if converting the color back and forth gives the same result. For example, converting the out of gamut RGB (255,0,0) into CMYK and back again gives a different RGB value from the original (RGB(199,3,3), say). When this happens, that particular color is considered out of gamut.
There are other, better, ways to calculate out of gamut colors, and I suspect the Photoshop engine uses one of those. BTW, out of gamut colors are calculated by the engine, not by Photoshop itself. You can experiment by telling Photoshop to use the Microsoft (or Apple) ICM engine in your color prefs.
But this only says that the profile generates unique values for a particular color, and not whether the printer itself yields a different color when that color and colors close to it in value are printed. Given the limits of the current technology, particularly the poor support of the gamut tag, the only way to tell for sure is to print the colors, and scan them with a photometer or spectrophotometer. You can’t even look at the darn things to tell if they are different, because the differences are very subtle. No wonder there is so much confusion, so much being spent on calibration equipment, and relatively little result to show for it.
But most of us realize that our inkjet printers do a pretty good job. If printers were watches, at this time in history they would accurate to a couple of minutes a day. As was the case with watches, there are those who require (or think that they require) greater accuracy, and are willing to pay for it. As has happened already with watches, at some point in the future printers will probably be extremely accurate and inexpensive. Until that happens, most of us can easily live comfortably with less than perfection.
Here are some more thoughts that I hope will give pause to some of you who have bought into the conventional wisdom that larger gamuts are always better than smaller ones.
1) What was the last out of gamut object you photographed? Blue sky, for example, is not really that saturated, particularly near the horizon where most of our cameras are pointed. Red objects – even bright bird plumage, is a far cry from RGB(255,0,0). For most of us, the answer to the question is "none".
2) RGB is not the last word in color spaces. Consider pure yellow objects, which are not all that uncommon, are not at all well represented in the RGB color space. For most monitors RGB(255,255,0) is brighter, but less saturated, than CMYK(0,0,100,0) will be in print. Magenta and cyan objects have similar problems.
Recent monitor developments have made an end run around this problem, resulting in the apparent miracle of a monitor capable of displaying the Adobe RGB color space directly. This is achieved by filtering the RGB phosphors to create purer colors, sharpening and stretching the three corners of the RGB gamut (to see these shapes, check out Curvemeister’s Labmeter, a free gamut plotter image). But even purity can have its limits, and the extermely sharp spectral characteristics of these monitors are bringing a new problem, viewer metamerism, to the forefront. With this latest advance, color consistency is literally in the eye of the beholder. The color on these monitors simply look funny to some people.
3) Consider that a larger gamut such as Adobe RGB sacrifices color gradation. An Adobe RGB image uses a smaller number of color values to represent the same range of colors than an sRGB image. This issue may be addressed by working in 16 bits, but there is a more serious problem to working in a large gamut space: the high probability that someone else may look at your image on an sRGB monitor and conclude that your work is too drab. Both of these issues – particularly the second one – are reasons for working in sRGB, particularly since real world objects such as pure blue sky do not come close to exceeding the gamut of even sRGB.
4) Print has a much smaller gamut than a CRT. The big money in photography is still in printed images, whether they be published images, or wall-sized art.
My suggestion, as always, is to trust what you can see and verify, and not spend too much time or money on getting your "watch" to run within one second of correct. Todays color and printer technology is excellent, and most of us – the vase majority – can live with its imperfections, provided we understand them, and base our understanding on common sense. —
Mike Russell
www.curvemeister.com
[re conversion of RGB to CMYK]
By "Certain colors can not be reproduced in CMYK", I assume you meant out of gamut colors between color spaces. But some colors on a monitor that are not flagged as oog in PS still won’t show up in Epson inkjet prints, after all the fuzz of monitor calibration and custom printer profiles. Is the PS oog detection faulty? Aside from oog detection, are there other ways to find out what colors won’t print?
There are a host of technical issues with out of gamut colors. The ICC standard defines a "tag" that defines out of gamut colors.
But the gamut tag is not always included in a profile, and when it is, it is not used consistently by different manufacturers. So programs like Photoshop generally ignore the gamut tag, perferring instead to rely on color values calculated using the profile. One rather simple method of doing this is to see if converting the color back and forth gives the same result. For example, converting the out of gamut RGB (255,0,0) into CMYK and back again gives a different RGB value from the original (RGB(199,3,3), say). When this happens, that particular color is considered out of gamut.
There are other, better, ways to calculate out of gamut colors, and I suspect the Photoshop engine uses one of those. BTW, out of gamut colors are calculated by the engine, not by Photoshop itself. You can experiment by telling Photoshop to use the Microsoft (or Apple) ICM engine in your color prefs.
But this only says that the profile generates unique values for a particular color, and not whether the printer itself yields a different color when that color and colors close to it in value are printed. Given the limits of the current technology, particularly the poor support of the gamut tag, the only way to tell for sure is to print the colors, and scan them with a photometer or spectrophotometer. You can’t even look at the darn things to tell if they are different, because the differences are very subtle. No wonder there is so much confusion, so much being spent on calibration equipment, and relatively little result to show for it.
But most of us realize that our inkjet printers do a pretty good job. If printers were watches, at this time in history they would accurate to a couple of minutes a day. As was the case with watches, there are those who require (or think that they require) greater accuracy, and are willing to pay for it. As has happened already with watches, at some point in the future printers will probably be extremely accurate and inexpensive. Until that happens, most of us can easily live comfortably with less than perfection.
Here are some more thoughts that I hope will give pause to some of you who have bought into the conventional wisdom that larger gamuts are always better than smaller ones.
1) What was the last out of gamut object you photographed? Blue sky, for example, is not really that saturated, particularly near the horizon where most of our cameras are pointed. Red objects – even bright bird plumage, is a far cry from RGB(255,0,0). For most of us, the answer to the question is "none".
2) RGB is not the last word in color spaces. Consider pure yellow objects, which are not all that uncommon, are not at all well represented in the RGB color space. For most monitors RGB(255,255,0) is brighter, but less saturated, than CMYK(0,0,100,0) will be in print. Magenta and cyan objects have similar problems.
Recent monitor developments have made an end run around this problem, resulting in the apparent miracle of a monitor capable of displaying the Adobe RGB color space directly. This is achieved by filtering the RGB phosphors to create purer colors, sharpening and stretching the three corners of the RGB gamut (to see these shapes, check out Curvemeister’s Labmeter, a free gamut plotter image). But even purity can have its limits, and the extermely sharp spectral characteristics of these monitors are bringing a new problem, viewer metamerism, to the forefront. With this latest advance, color consistency is literally in the eye of the beholder. The color on these monitors simply look funny to some people.
3) Consider that a larger gamut such as Adobe RGB sacrifices color gradation. An Adobe RGB image uses a smaller number of color values to represent the same range of colors than an sRGB image. This issue may be addressed by working in 16 bits, but there is a more serious problem to working in a large gamut space: the high probability that someone else may look at your image on an sRGB monitor and conclude that your work is too drab. Both of these issues – particularly the second one – are reasons for working in sRGB, particularly since real world objects such as pure blue sky do not come close to exceeding the gamut of even sRGB.
4) Print has a much smaller gamut than a CRT. The big money in photography is still in printed images, whether they be published images, or wall-sized art.
My suggestion, as always, is to trust what you can see and verify, and not spend too much time or money on getting your "watch" to run within one second of correct. Todays color and printer technology is excellent, and most of us – the vase majority – can live with its imperfections, provided we understand them, and base our understanding on common sense. —
Mike Russell
www.curvemeister.com
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