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Author Topic: Colour Correction Using Gretag CC and Curves  (Read 4636 times)

Dinarius

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Colour Correction Using Gretag CC and Curves
« on: August 08, 2006, 07:49:37 am »

I document a lot of fine art for repro.

I have been using a Gretag Color Checker and a Curves layer to correct/remove colour casts.

Using the Colour Sampler tool, I take (5x5 pixel) samples of four of the grey scale patches - White, the two Middle-Grey patches, and Black. If I could take more samples, I would, but the Colour Sampler tool only allows four.

I then correct these four patches, in seperate RGB channels, until they match the values listed here>

http://www.babelcolor.com/download/RGB%20C...olorChecker.pdf

(Scroll down to page 5. Bablecolor Average table)

i.e. White: 245, 245, 240. Grey(1): 159, 160, 160. Grey(2): 119, 121, 121. Black: 53, 53, 54.

Everything is done in 8 bit Adobe 1998.

The results I have been getting are excellent. I am merely wondering whether I can improve this method in any way?

e.g. Once the four patches are balanced, the values for the Red, Green and Blue patches are off, albeit only slightly. I have tried sampling these patches and correcting, but the results are never as 'clean'.

I am using a DELL PC and Eye-One corrected LCD monitor.

Thanks.

D.
« Last Edit: August 08, 2006, 07:55:31 am by Dinarius »
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Jonathan Wienke

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Colour Correction Using Gretag CC and Curves
« Reply #1 on: August 08, 2006, 12:42:07 pm »

Quote
Everything is done in 8 bit Adobe 1998.

The results I have been getting are excellent. I am merely wondering whether I can improve this method in any way?

Do a Google search to find Tom Fors' free Camera RAW calibration script, which uses a RAW image of a Color Checker to adjust Camera Raw's calibration sliders to dial in the color so that it comes out of the RAW converter already correct. All you have to do is set the proper white balance, which is trivial if you shoot a Color Checker or WhiBal during the session. Using curve layers to fix profile problems is a kludge at best. I've found it quite valuable when trying to match colors between images shot with multiple cameras on a shoot.

Use 16-bit mode. You'll be able to do much more aggressive level and curve adjustments before you start having banding or posterization.
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Dinarius

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Colour Correction Using Gretag CC and Curves
« Reply #2 on: August 09, 2006, 05:10:23 am »

Jonathan,

Thanks for the suggestion.

I did try the Fors script a while back and I didn't like the results. To be honest, I'm wary of a 'one size fits all approach'. I know I'm not alone in this. For those who haven't tried it, you can read about it here.......

http://www.fors.net/chromoholics/download/

I've also tried Bruce Fraser's more hands-on approach to calibration, described here....

http://www.creativepro.com/story/howto/21351.html

....and I didn't like that either.

I guess I'm biased towards correction that is specific to the job in hand. i.e. the Gretag CC is present in the exact lighting setup at the time of the shoot.

I also hate ACR, much preferring to convert straight using Capture One and then adjusting in CS2.

You are correct when you say that using Curves is a bit of a trudge, but the results speak for themselves in my experience.

I am merely wondering if I can improve the technique I am using. I would certainly perfer if the Colour Sampler Tool could take at least six samples, rather than the current four.

Thanks for the reply.

D.

ps....One last thing........an interesting exercise is to correct using the four patches I mentioned above. Then do the same thing using the red, green, blue and mid-grey patches and compare the results.
« Last Edit: August 09, 2006, 08:01:37 am by Dinarius »
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Jonathan Wienke

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Colour Correction Using Gretag CC and Curves
« Reply #3 on: August 09, 2006, 12:53:19 pm »

I've had good results using the Fors script (it's been updated several times) and ACR; the trick is to get a good reference RAW so the script has a good base image to work with. I've done things like yearbook shoots and weddings where the only color correction necessary was to manually set the white balance (which can be done by groups of shots taken in a specific lighting condition). If you use Capture One, you can create a custom camera profile for Capture One specific to a particular lighting setup. Somebody was offering that as a service a while back, but I forget the guy's name. You might want to look into it. A properly-profiled RAW converter will save you an incredible amount of time spent doing unnecessary color corrections. It may sound too good to be true, but once you've experienced it, you'll never want to go back.
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Dinarius

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Colour Correction Using Gretag CC and Curves
« Reply #4 on: August 15, 2006, 05:08:42 am »

http://www.babelcolor.com/download/RGB%20C...olorChecker.pdf

Can someone please explain to me why the RGB values in 16bit are expressed in five digit numbers (and not the usual three digit numbers) on page 6 of the above link.

How, if at all, can I use these numbers when editing in Photoshop? e.g. in Levels or Curves or any colour channel correcting.

Many thanks.

D.
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Dinarius

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Colour Correction Using Gretag CC and Curves
« Reply #5 on: August 17, 2006, 09:14:21 am »

Two points..........

1. Regarding the 5 digit numbers used for expressing 16bit RGB values in the link I gave above.....

To convert these numbers to a usable three digit number, simply divide by 256.

2. Someone recommended that instead of correcting using Curves I should correct using a Selective Colour Adjustment Layer.

I did so and the difference is astonishing.

I can correct as many patches on the Gretag CC as I wish (I have been correcting four on the grey scale and red, green and blue) and they all fall into place, without affecting each other at all. All I am doing in C1 or ACR is getting the grey scale patches as near as possible before conversion.

Fantastic!

D.
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32BT

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Colour Correction Using Gretag CC and Curves
« Reply #6 on: August 17, 2006, 10:21:36 am »

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2. Someone recommended that instead of correcting using Curves I should correct using a Selective Colour Adjustment Layer.


Forgive me for yelling: DO NOT, I REPEAT, DO NOT USE SELECTIVE COLOR ADJUSTMENTS for this. This is not a selective color correction problem.
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Dinarius

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Colour Correction Using Gretag CC and Curves
« Reply #7 on: August 20, 2006, 01:19:41 pm »

Quote
Forgive me for yelling: DO NOT, I REPEAT, DO NOT USE SELECTIVE COLOR ADJUSTMENTS for this. This is not a selective color correction problem.
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Please explain why not?

Or let me put it another way:

If the RGB readings for all the selected patches (up to eight in my case) on the info palette correspond exactly to what they should be (that's ALL three values for each patch), and (less scientifically) if the image on my calibrated monitor looks exactly like the painting on my wall in my studio, then why do you say it's not the way to do this?

I'm trying to reduce this exercise to one of strictly ones and zeros. If the red patch is exactly 152, 52, 59, the blue 53, 64, 144 and the green is 102, 148, 78, (all 16bit, Adobe 1998, with the other patches being equally accurate) where's the problem?

Thanks.

Denis
« Last Edit: August 20, 2006, 01:23:21 pm by Dinarius »
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