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anneke87
Joined: 03 Jun 2010
Posts: 1
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 Posted: Thu Jun 03, 2010 2:02 am Post subject: Lab-values/ colorprofile |
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I've a photo with a ref. card.
Now I want to match the lab values on the screen (photo) with the lab values of the original. how can I do this?
Can I do this in the colorprofile? |
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Myer
Joined: 05 Jun 2010
Posts: 10
Location: South Florida
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| Posted: Fri Jun 11, 2010 12:17 pm Post subject: |
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Somebody else who uses lab.
Can you scan the photo?
I imagine the reference card has something black, white or grey. Correct? Actually, black ,white and grey are all the same color. Neutral. Just darker and lighter.
If you can do a good scan of the photo you can use the eye dropper to determine values for l, a and b in specific places.
I assume you are trying to correct the color.
I would be tempted to take a different approach. Most photos have something in a relatively prominent place that is known to be white (I would say black as well but it's very difficult to differentiate between different very dark colors).
Using the eye dropper I would make a couple of selections of white objects. Then go into the a and b chanels, grab the center point and move it up or down until you get almost zero. |
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thehermit
Joined: 05 Mar 2003
Posts: 3987
Location: Cheltenham, UK
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| Posted: Fri Jun 11, 2010 4:42 pm Post subject: |
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Firstly, welcome to the forum anneke87 and indeed you Myer.
If you are trying to find the black and white points in a photograph, you could use a Threshold Adjustment Layer. Add the Adjustment Layer and move the slide to the left and mark the white point with the Colour Sampler Tool, also do the same to find the black point. Finding the grey point is another matter 
Really though I would recommend colour correcting by the numbers.
Incidentally when you say reference card do you mean a step wedge or does it have numbers?
_________________
If life serves you lemons, make lemonade! |
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Myer
Joined: 05 Jun 2010
Posts: 10
Location: South Florida
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| Posted: Fri Jun 11, 2010 7:54 pm Post subject: |
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I'm not suggesting she find the black (darkest) and white (lightest) portions of the image. You mentioned black and white points but really this will show the light and dark points. Not the same.
I'm assuming the image requires color correcting. Therefore I'm suggesting she locate some parts of the image that should be white (and or black).
She can select one or two spots that she determines "should" be white and mark them using the Color Sampler Tool.
Then, since she's in LAB, she can move the center point of the "a" and "b" channel curves up or down changing the value of each to 0.
The assumption is that since she changed what should be white to actual white, the other colors will fall into place.
The advantage of LAB is that she can modify the "a" (green/magenta) and "b" (blue/yellow) channels without changing the luminosity (light/dark). |
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Myer
Joined: 05 Jun 2010
Posts: 10
Location: South Florida
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| Posted: Sun Jun 13, 2010 10:39 am Post subject: |
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Let me guess. By reference card maybe the op means a grey card (or hopefully anything neutral).
As I mentioned earlier, to me white and black are the same color. Neutral. Well, that also applies to grey. They are all Neutral. Just different amounts of luminosity (light/dark).
LAB is great for this as you totally separate color (a and b channels) from luminosity (light/dark in the l channel).
Open the image. Go into LAB, put a marker on the grey reference using the color sampler tool.
Then, go into Curves and first the "a" channel and then the "b" channel. Move the center point in each of both channels up or down until the value is "0" (zero). That means the reference is now Neutral without changing the luminosity (light/dark).
That should color correct the image.
Maybe that's what the op meant. |
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