RGB CMYK LAB and Dan Margulis !

[KR1156]

Originally Posted by pixelzombie

i'm trying to figure this out but can't seem to get a handle on it, how would i use the smart object to add K only to a RGB file?

what i do is double click on the base layer of the CMYK smart object and turn all chanels off except the K, then save. when i get back in RGB, i do the necessary blending of that K plate.

[cricket1961]

Originally Posted by pixelzombie

ok, i got that far but i can get a similar result by just using selective color on the RGB file itself...

True, but you are not pushing in just black. You are pushing in an interpretation of black. At least you can dial in exactly the amount of black you want.

[pixelzombie]

i'm not sure what you mean by interpretation as i'm getting the same numbers when i read the cmyk values...

[edgework]

Originally Posted by pixelzombie

i'm not sure what you mean by interpretation as i'm getting the same numbers when i read the cmyk values...

Those are the numbers you'd get if you converted your RGB file to CMYK. But as long as it's an RGB file, your shadow detail is blended throughout the three color channels. The difference is that when you manipulate a K curve in CMYK you are dealing with only shadow information in a way that RGB and LAB cannot match. I've used the K plate as a layer mask to refine my shadow moves in RGB, but it's not quite the same. That's why Dan always says that CMYK is the best space to work with shadow detail. And nothing beats the effect of sharpening the K plate only, not possible in RGB.

[Markzebra]

i'm not sure what you mean by interpretation as i'm getting the same numbers when i read the cmyk values... - This is a useful technique - what Chris means by interpretation is a re-separation of the K channel back into RGB. Its the same as if you had duplicated your RGB document, converted the duplicate to CMYK, copied the K channel, and pasted this copy from the clipboard back into your original RGB document. If you now look at your three RGB channels you will see the K channel dropped into the R, G and B in equal amounts - because photoshop has made your copy into a new desaturated layer. Each of these channels is identical to the K channel in CMYK.

Glad everyone finds this stuff interesting and useful, Smart objects after the release of CS2 took me a year or so too for me to get my head around, because they were so new. But they are a pretty important usability breakthrough I think

[Markzebra]

Oops sorry edgework,I duplicated your answer there. Yes Its about shadow detail density. In CS3 You can of course use a Smart filter on your new K layer to sharpen it - you could for example apply a blend mode to the K layer, maybe multiply? sort of like the opposite of a shadow highlight, bottom end density control with a scrubby slider.

[pixelzombie]

definitely something to master but i can't justify doubling a file size everytime i need to add black to an RGB image, at least not at my current gig...

[transoptic]

I had two questions:

first, I like your tip edgework on sharpening the black plate. How would you do this? I tried to copy it on a new layer, run high pass, and change to overlay blending. It looks interesting that way. Is there a superior method you use?


Second question is, I'm fully aware of how each mode has its own seperate gamut, but I've never come across color shifts unless printing or uploading to web, etc. Except when switching modes with lots of layers and masks. So can someone give me reasons, or better yet examples where switching modes under normal circumstances are bad?

[edgework]

Originally Posted by transoptic

I had two questions:

first, I like your tip edgework on sharpening the black plate. How would you do this? I tried to copy it on a new layer, run high pass, and change to overlay blending. It looks interesting that way. Is there a superior method you use?

You're thinking too hard. I meant sharpen the K channel. Literally. Go to your channel's palette, activate only the k channel and sharpen it. Works particularly well for faces since there is virtually none of the textural detail in the black plate to get grunged up. Eyes and hair can take a heavy hit and always look better for it. Do this on a duped layer to blend as desired.

Second question is, I'm fully aware of how each mode has its own seperate gamut, but I've never come across color shifts unless printing or uploading to web, etc. Except when switching modes with lots of layers and masks. So can someone give me reasons, or better yet examples where switching modes under normal circumstances are bad?

Not bad, necessarily, but the reality is that CMYK is incapable of producing many of the colors you take for granted in RGB. Find a picture of a vivid blue dress with lots of variations in blues and greens for detail. You'll find that with the default conversion in Photoshop, which is Relative Colormetric, all those subtle shifts in tone will pretty much turn to mud. Blues and greens suffer particularly, as well as vivid purples, turquoise and oranges and virtually all pastels; they are simply out of the normal printing gamut for CMYK. (Reds fare better and Yellow is actually purer in CMYK.) Photoshop converts colors that are in gamut precisely, and the stuff that's outside the range gets crunched at the edges. Hence the blue mud. The other useful conversion mode is Perceptual which recreates the relationship between all the colors but which will necessarily alter all of them in order to do so. It's useful when you're not going to get the original RGB color anyway and at least want to maintain detail and a reasonable facsimile of the original intent. Other options would be to pull out the lightness channel from LAB, or one of the RGB channels that captures the needed detail, copy it before converting, then layering it in on top of the image in luminosity mode. You'll need to play around with moves like this, and you'll send your colors way south, but, again, the colors weren't there to begin with. If it's a toss-up betwen bad color and bad detail, or color that's off but which has clear detail, go with the second.

Of course, not everything goes to a printing press these days, but if you need 1000 copies of something, you'll need to navigate the mysteries of conversion. (Just remember that every beautiful image you've ever seen in a book, magazine or poster has managed to get around these limits, so all is not lost.).

[Markzebra]

Yes -everything that edgework has said is 100%. Having some sort of production experience helps a lot in knowing what a good CMYK file is, and that just converting an Adobe RGB file to PhotoshopSWOP default is a no-no. The key is the black plate. Each press varies immensely in how it handles the black ink: whats the ink limit? what kind of GCR do you need? ie how HEAVY is the black plate as it stretches into the lighter tones of the image. Images vary too, more monochromatic images can benefit from a stronger K plate which helps their detail and highly saturated images a lighter K.

By the way Margulis is a good teacher in these areas because thats his background I reckon. Edgework has given some good advice there about moves on the L channel (which can now be a smart L) and a lum blend on this, thats great technique which I use. Luminosity blend uses LAB calculations fairly non destructively without any conversions actually you can just use adjustments with a Luminosity blend with tremendous power.

[amica999]

Hi guys and girls,
I am reading my way through the LAB book and found some difficulty following the calculation instructions....it states that in the calculation screen choose green channel and it should be applied to the whole picture (all other channels if none is selected). In CS3 I do only have the option the choose red, blue, gray etc. Would that in conclusion mean that I have to dup the layer twice and apply the green channel in the first dup to the red channel, and in the second copy to the blue channel? Wouldn't a channel mixer be more appropriate? Please let me have your thoughts, as I am lost here. Thanks