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color corection by the numbers

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  #21  
Old 07-21-2005, 03:56 PM
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It's with some trepidation that I weigh in here again. Here's an interesting thread you should check out http://www.retouchpro.com/forums/sho...ght=sharpening

One of my posts is a close up of a blue Heron. The second image was sharpening in LAB then converted back to RGB. The third image was sharpened in RGB, faded and changed to Luminosity. To my eyes there is a very noticeable increase in black content in the feathers in the third image. To be fair, I often try them both and get very similar results. But when there have been differences, it has "always" been in favour of the LAB adjustment. Just my 2 cents "again!". To me it's worth working in LAB for sharpening but it will always be a debatable point.

Dave

Last edited by Duv; 07-21-2005 at 10:18 PM.
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  #22  
Old 07-22-2005, 12:08 AM
pjanak pjanak is offline
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One of the first things you should do is purchase a color corection plugin. For example one called Color mechanic Pro. All you need to do is look at the original image. Decide which object in the image is most definately supposed to be white. then in two more steps the entire image is color corrected.
Take a look at th my example. The left image is a digital camera set for the wrong lighting type. The right image is after a white balance. Whie balancing is the process of telling your camera "this is supposed to be white" your camera then takes that knowledge and displays all other color data properly.
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  #23  
Old 07-22-2005, 12:16 AM
pjanak pjanak is offline
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Quote:
Originally Posted by Duv
Welcome aboard!! I would recommend as a starting point getting some good books on the subject. Two books I would suggest is Katrin Eismann's Photoshop Restoration & Retouching and Michael Kieran's Photoshop Color Correction. They're the real pros at explaining color correction which can be very simple or extremely difficult. Both books come with a CD of images that you'll find very helpful in working thru the color correction problems.

Cheers
Dave
I read a lot of great reviews on this Katrina's book. but one reviewer complained: "Further, there is too much restoration and not enough retouching here, and the very important Liquify Filter recieves only scant coverage."

The Lquify filter important? I've never ever used it. HAs anyone else? And what would put it at the level of "important"?
Pete Janak
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  #24  
Old 07-22-2005, 01:09 AM
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Gary Richardson Gary Richardson is offline
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I can think of a few instances where I'd use the liquify filter, but not too many involved with restoration, and in retouch, only really with "Beauty" jobs, with any regularity.

Depends on whether you do a lot of beauty work as to whether this is a "major" tool for you or not.

But if you're wondering whether to purchase Katrin's book or not, my advice is buy, it's worth every penny.
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  #25  
Old 07-24-2005, 09:30 AM
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Well Iíve ordered Katrin Eismann's Photoshop Restoration & Retouching Book. and Iíve worked my way through that tutorial/workflow. Itís certainly complete if not a little overboard and controversial (LAB). But Iíve found the bits that I will find useful. The one thing that workflow does not cover is colour correction so I re-read Victoriaís Link.
http://www.ledet.com/margulis/PP7_Ch02_ByTheNumbers.pdf

Iíve read this before but itís now beginning to make some sense. All the Proís seem to work in CMYK. And when talking about skintones they talk about Ratios. I can understand why they prefer to talk about ratios rather than actual numbers But I donít understand why CMYK is the chosen mode. Why does no-one seem to work in RGB. Surely those ratios would still work in RGB and I would get the same results?

I did find one reference to RGB skintones and it was talking about ratios of
Red 150
Blue 100
Green 90

Will I get the same results?
As you can probably tell. Iím trying to avoid this RGB>CMYK conversion. Iím not sure why.


Is Michael Kieran's Photoshop Color Correction that Duv Suggested the best book to buy?


Ken
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  #26  
Old 07-24-2005, 10:12 AM
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Quote:
Originally Posted by Cameraken
As you can probably tell. Iím trying to avoid this RGB>CMYK conversion. Iím not sure why.
You're quite right in avoiding an unnecessary conversion to CMYK, because every time you convert, you lose colour data.

Most pros correct in CMYK because that's exactly what they'll use to print - CMYK presses. Photographers tend to use RGB because inkjet printers prefer RGB data, and wet process digital labs use RGB. Unfortunately there's not much been written specifically for photographers though!

The theory is the same - find a neutral white point, black point, and midtone. Look at the RGB numbers, and any time the numbers are approximately the same, (ie. r128, g128, b128) you've got a neutral (it's actually easier to remember than CMYK!). And better still, if you get those points neutral, your skin tones should be spot on too! You can always set the eyedropper to show RGB and CMYK numbers in the info pallette if you want to double check against CMYK numbers.

It mostly comes with trial and error, and a bit of experience, but if you follow those guidelines, you should be in the right ball park. Work on adjustment layers (levels or curves are your best bet), and then if you make a mistake you can easily change it.

I tend to use 1 levels layer to adjust the picture to neutral (white, mid, and black), and then put another layer on top to add a little red and yellow back into the midtones/highlights as I prefer a warmer look.

Victoria
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  #27  
Old 07-24-2005, 10:31 AM
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Ken, I would suggest the following. General color correction is perhaps easiest in RGB. Matching numbers for shadows, highlights and midtones (blacks, whites and greys) is pretty easy to comprehend and get acceptable results.

Most prepress work for fashion, glamour etc is done in CMYK and I guess it is why more info is written for skin tone correction in CMYK. This also gives you an additional ink to work with.

Flipping back and forth between RGB and CMYK can cause some degradation. How much and can you notice it should be considered.

There is a way to have the best of both worlds without degradation. It's called "Correcting in CMYK without converting the original file to CMYK". Here's how:

Select Image > Duplicate, then Image > Mode > CMYK.
Make your skin tone adjustments
Select All > Edit > Copy
Activate the original RGB file and select Edit > Paste.
Change the pasted layer's mode to Color.

The end result is a color corrected file in RGB that was done in CMYK without the degradation. Just one of many tidbits in Eismann's book.

Michael Kieran's book does not deal with skin tone correction per se but is a definitive guide to Color quality of digital images in RGB, CMYK, LAB and HSB.
It may be better suited to the intermediate to skilled but if you have any understanding at all about color correction, you should still find lots of nuggets of good information.

Dave
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  #28  
Old 07-24-2005, 01:27 PM
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Cameraken Cameraken is offline
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Thanks for your replies.

I am quite happy setting Black and White points But I disagree that this always puts the skin right. I cannot judge colours by eye and must check numbers. I can make Colour casts disappear just by staring at them.

I have just tried Duvís suggestions.

I followed the steps and my skin tone looked great. I left the magenta as it originally was (40) and corrected the cyan from 0 to 13 and yellow from 20 to 50.
Superb I thought, so I pasted the corrected layer back into my RGB picture and set the mode to colour.
I guess setting the mode to colour is to keep the quality of the original RGB image.

I have checked the numbers and my highlights and shadows have now slipped a bit.
Should I now add a layer mask to keep the original highlights and shadows (IE just use the skin tones from CMYK) OR should I do another levels adjustment.
Or maybe I should have done the colour correction Before I adjusted the levels but I donít think that is the correct order.


Thanks Duv. It looks like I ordered the right book.

Ken
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  #29  
Old 07-24-2005, 01:51 PM
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Ken, you're now into tweaking which is a good thing in that you are trying to "fine tune" your image. Reworking your shadow numbers a bit shouldn't have too much effect on the skin tone. By the way, did you get some color shifts or did the numbers stay equal but went up or down. ie. did it go 25 25 25 to 25 30 19 or did it go to 31 31 31? Also be careful if you haven't sharpened your image yet because that can also cause color shifts. Your suggestion for a mask sounds good, just be very careful with what you have acheived. I've screwed up a ton of photos trying to tweak them to the nth degree.

Dave
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  #30  
Old 07-24-2005, 02:47 PM
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Duv. Thank you

My numbers for the Highlights and shadows did not change that much but now I donít know which are right.

Shadows have gone from 8,10,10 to 4,13,0
Highlights have gone from 252,249,253 to 247,250,255
Skin has gone from 239,174,172 to 227,184,150

But this does make the eyes and teeth a touch Blue (in theory). Also the hair has changed a touch
Now I donít know which is correct. Would you recommend to I make a skin mask or should I flatten the image and do another levels correction for the highlights and shadows?

I was just thinking could a similar method be used for RGB>LAB for sharpening.

Ken
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