gefillmore
30th December 2006, 03:41 AM
Color casts
This is a partial summary of Chapter 4 in Dan Margulis' book Professional Photoshop.
(for much more advanced text, examples and discussion, please see Dan Margulis’ book, Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace).
As stated previously, the center point is neutral (white, gray, or black). However, if you have a particular cast to your photograph you can correct it by moving off the center point. In the image below, the ‘white’ is not. By a minor adjustment to the LAB curves, the red/yellow cast to the moisturizer bottle and sheet can be removed without greatly affecting the other colors.
In the lightness channel there is a basic adjustment for contrast. In the a and b curves, there is a standard LAB adjustment with a very small movement of the center point from ‘zero’ away from magenta in the a curve and away from yellow in the b curve.
In this particular image, I would have dealt with the raw file in ACR by adjusting the white balance, but am doing it in this fashion to demonstrate a LAB curve adjustment.
http://gefillmore.smugmug.com/photos/119712199-M.jpghttp://gefillmore.smugmug.com/photos/119712574-M.jpg
Curves applied to change color cast to white
http://gefillmore.smugmug.com/photos/119712786-M.jpg
Curves adjustment
Contrast adjustment
Again stated previously, you generally do not use LAB adjustments on bright colors. However, reading Margulis, LAB lightness curve adjustments work well on bright colors. Let’s use the bottle image again.
Here are images of the RGB channels of the image:
http://gefillmore.smugmug.com/photos/119713273-M.jpg
Red channel---------------------blue-----------------------green
This is the lightness channel in LAB:
http://gefillmore.smugmug.com/photos/119711837-M.jpg
There is a wide disparity from light to dark in the RGB channels with an image with bright colors, making it difficult to adjust contrast by steepening a curve in RGB mode. In the lightness channel, it is a short throw. This shows that contrast adjustment is easier in the lightness channel in the LAB mode than dealing with contrast in RGB mode. But, it also means that the adjustments need to be small.
Please see the images in the color casts section above for contrast improvement through the lightness channel curve adjustment.
Targeting specific colors
Because the a and b curve in LAB deal specifically with color and not contrast, specific colors can be adjusted without affecting the image as a whole and without affecting the other colors.
We'll use the bottle image again:
http://gefillmore.smugmug.com/photos/119712574-M.jpg
In the first example, the red/magenta/orange will be transformed to green:
http://gefillmore.smugmug.com/photos/119711504-M.jpg
The a curve is the magenta/green curve. The lower half of the curve is moved from magenta to green, transforming the red/magenta/orange to green.
In the second example, the blue/teal will be changed to red/magenta.
First, we have to change the blue/teal to a green:
http://gefillmore.smugmug.com/photos/119713108-M.jpg
The blue half of the b curve is moved into the yellow portion changing the blue to green.
Then, the green is transformed to red/magenta:
http://gefillmore.smugmug.com/photos/119713148-M.jpg
The green portion of the curve is moved into the magenta portion, completing the transformation of the teal/blue to red/magenta.
This is a partial summary of Chapter 4 in Dan Margulis' book Professional Photoshop.
(for much more advanced text, examples and discussion, please see Dan Margulis’ book, Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace).
As stated previously, the center point is neutral (white, gray, or black). However, if you have a particular cast to your photograph you can correct it by moving off the center point. In the image below, the ‘white’ is not. By a minor adjustment to the LAB curves, the red/yellow cast to the moisturizer bottle and sheet can be removed without greatly affecting the other colors.
In the lightness channel there is a basic adjustment for contrast. In the a and b curves, there is a standard LAB adjustment with a very small movement of the center point from ‘zero’ away from magenta in the a curve and away from yellow in the b curve.
In this particular image, I would have dealt with the raw file in ACR by adjusting the white balance, but am doing it in this fashion to demonstrate a LAB curve adjustment.
http://gefillmore.smugmug.com/photos/119712199-M.jpghttp://gefillmore.smugmug.com/photos/119712574-M.jpg
Curves applied to change color cast to white
http://gefillmore.smugmug.com/photos/119712786-M.jpg
Curves adjustment
Contrast adjustment
Again stated previously, you generally do not use LAB adjustments on bright colors. However, reading Margulis, LAB lightness curve adjustments work well on bright colors. Let’s use the bottle image again.
Here are images of the RGB channels of the image:
http://gefillmore.smugmug.com/photos/119713273-M.jpg
Red channel---------------------blue-----------------------green
This is the lightness channel in LAB:
http://gefillmore.smugmug.com/photos/119711837-M.jpg
There is a wide disparity from light to dark in the RGB channels with an image with bright colors, making it difficult to adjust contrast by steepening a curve in RGB mode. In the lightness channel, it is a short throw. This shows that contrast adjustment is easier in the lightness channel in the LAB mode than dealing with contrast in RGB mode. But, it also means that the adjustments need to be small.
Please see the images in the color casts section above for contrast improvement through the lightness channel curve adjustment.
Targeting specific colors
Because the a and b curve in LAB deal specifically with color and not contrast, specific colors can be adjusted without affecting the image as a whole and without affecting the other colors.
We'll use the bottle image again:
http://gefillmore.smugmug.com/photos/119712574-M.jpg
In the first example, the red/magenta/orange will be transformed to green:
http://gefillmore.smugmug.com/photos/119711504-M.jpg
The a curve is the magenta/green curve. The lower half of the curve is moved from magenta to green, transforming the red/magenta/orange to green.
In the second example, the blue/teal will be changed to red/magenta.
First, we have to change the blue/teal to a green:
http://gefillmore.smugmug.com/photos/119713108-M.jpg
The blue half of the b curve is moved into the yellow portion changing the blue to green.
Then, the green is transformed to red/magenta:
http://gefillmore.smugmug.com/photos/119713148-M.jpg
The green portion of the curve is moved into the magenta portion, completing the transformation of the teal/blue to red/magenta.