Colour Question.......?

[Dinarius]

A question for you colour whizz-kids! ;-)

If I photograph a Gretag Macbeth CC and balance the bottom two rows of patches (Grey Scale and RGBYCM), then the Red value, for example, will be 152, 52, 59 in Adobe 1998 16bit. The green will be 102, 148, 78, etc....

If the Gretag is included in a photograph of a studio setup and I decide that the image needs, for example, to be a bit darker, then the RGB values will reduce correspondingly. Of course, having already balanced these RGB values, I can bring them back up by simply increasing the Lightness channel in Hue/Sat.

But, what I want to know is this:

Is the Red patch, for example, only 152, 52, 59 when the Grey Scale patches are correct? i.e. As in a Fraser profiling exercise?

Or is the Red patch *always* 152, 52, 59, regardless of the overall gamma of the image?

In short, should the Gretag CC be readjusted to take changes in the image into account?

I hope this makes sense!

Many thanks.

D.

ps...If you dispute my values for Red and Green, I don't mind. That's not the point of the question! ;-)

[Tim Lookingbill]

Those original numbers for red you posted were mapped that way under a set of viewing and color space conditions that form the luminance, sat/hue for the given preview. Your eyes don't care squat about these conditions and the numbers. Your edits by eye will now introduce a variable that changes these conditions but will be mapped with an algorithm that changes the numbers specified by the color space of choice.

You'ld have to be a programmer to predict what these numbers mean and will do under any given editing space and condition.

If you're trying to come up with a consistant color workflow calibrating to a Gretag CC you must stay with one condition=(lighting, capture device and target/color space). If those conditions don't give you satisfactory default results visually, adjust and save your edits and apply to other images captured under the same conditions and forget about the numbers. Or change the conditions at input by adjusting exposure which were the original conditions that created the numbers in the first place.

[Jonathan Wienke]

First of all, RGB color numbers are completely meaningless without specifying a color space to go with them. RGB 146,22,67 in ProPhoto is not the same color as RGB 146,22,67 in sRGB; the luminance, hue and saturation are all different. If you google around, you can find the LAB values for a Color Checker, and adjusting tonality will affect L values, but not the A and B values, which control hue and saturation. I'd suggest going into your info palette settings and changing the second color readout to LAB instead of proof color so you can see the LAB values associated with a given pixel without having to convert the image to LAB. That way you have a consistent color reference no matter whether you're working in ProPhoto, Adobe RGB, or sRGB. As long as the LAB values match, the color will be the same, no matter what the RGB values happen to be.

Tom Fors has an ACR calibration script that uses a Color Checker to aet the values in ACR's calibrate tab for the most accurate color possible from a given digital camera. A properly-exposed shot of a Color Checker in direct sun will work well in a variety of lighting conditions, but if you have trouble with a particular lighting condition you can shoot a calibration shot in that lighting and get a set of calibration adjustments for that lighting condition. Once you have a good set of calibration adjustments, all you need to do is take a shot with a Color Checker or some other white reference in it, click-white-balance on that reference in ACR, and no further color adjustments are typically necessary. If you're shooting with a camera supported by ACR, I highly recommend it. You'll save yourself an incredible amount of time not doing unnecessary color adjustments.

[Dinarius]

Thanks for the replies.

Yes, I am aware that all this is colour space dependent. That's why I quoted values for A1998 in the second line of my question.

But, my point is simply this........

Calibrating a Gretag CC, by first setting highlight, shadow and mid-tones (the bottom row), and only then adjusting RGBYCM, will give a result that is as clean as a whistle, but a bit flat and, usually, a bit bright too. As Fraser wrote (Real World Camera Raw....p. 97);

"You'll usually need to bring the Brightness and Contrast down to the point where the image looks extremely flat. .....this is normal, and the goal is simply to get the overall contrast t match the relatively low contrast of the [Gretag] colour checker. Once you've tweaked the colour response in the Calibrate tabs, you're free to tweak the tonality without affecting hue and saturation."

My understanding of this is that if you adjust tonality *after* correcting RGBYCM, even though H/S rise or fall, they do so in tandem and therefore colour is *not* altered. Which is why a simple adjustment of the Lightness channel will bring them back up (or down) to their corrected levels.

All I want to know is, does one have to use the Lightness channel to readjust/recorrect the H/S levels (which has no affect on the new tonality settings) or is it OK (as Fraser seems to be implying) to leave them be.

In short, is the colour still correct, albeit now a bit brighter/darker. I'm assuming it is, I only want confirmation.

If you think you've already answered this question, then I apologize. ;-)

Thanks.

D.

[Jonathan Wienke]

My experience is that if the A and B values are correct, you can play with the L value quite a bit and still keep natural-looking color. My general methodology is to shoot RAW and convert in ACR, which is calibrated with Fors' script. The only color adjustment I usually do is white balance tweaking, and I use the Curve tab in ACR to do global tonal and contrast adjustments. That way, I'm adjusting tonality/luminance without altering hue/saturation. Once I'm happy with things in ACR, then I bring the image into Photoshop, process noise, sharpen, remove dust spots, etc.

[Tim Lookingbill]

"Is the color still correct?"...

In reference to the numbers?...I couldn't tell you. How it looks on your display?...You'ld have to decide.

I'm not familiar the Bruce tutorial but from what you've indicated I think what he was trying to go for is a kind of gammaless linearization of the scene. The flatness and brightness descriptions suggests something similar to what I have to do on my scanner to evenly spread out and equalize the high bit data in reference to the histogram so when I apply a scene correct tonal correction curve-(specifically shaped curve) I don't get the familiar hue/sat shift inherent editing in a gamma encoded RGB environment.

Whenever I'm forced to apply an oddball shaped tonal curve rather than a simple bow/umbrella shaped curve on scenes that have uneven hot spots in luminance/hue/saturation, I get these color anomolies. Flattening and brightening to even these hotspots out allows applying only a simple symetrical bow shaped or S-curve depending on low/high key scenes eliminating this color anomolie effect with the least amount of edits.

Your camera is a far better and more accurate device for capturing linearly than my cheap scanner and adjusting only the luminance channel if available in your situation is the thing to do. It's very much like working in LAB space. Now trying to predict what the numbers should be and if they are correct after doing this is beyond my ability or knowledge. If it looks good who cares about the numbers.

For instance big moves can be made to the numbers before a difference can be seen when adjusting a flat color patch in PS's Color Picker in AdobeRGB. Try it. So I find it quite difficult to understand how exactly matching the numbers in a Gretag CC can be a way to achieve accuracy. It's probably meant as a rough guide in regards to the numbers and the eyes can do the rest.

I'm not a professional and I couldn't shoot a pro image if my life depended on it but I've been observing and researching for several years tinkering with several different RGB devices and digital image processes to a point I've developed a kind of intuition. So take what I've said here with a grain of salt.