Why does setting Shadow, Highlight & Neutral points help Remove Color Casts

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dkrassen
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Joined: Sat Dec 04, 2010 12:03 am

Postby dkrassen » Sat Feb 05, 2011 8:58 pm

Hello.

I am curious as to the exact reason why setting a shadow, highlight and neutral helps to correct color casts.  That is, how do the curves change in LAB and RGB when setting a shadow, highlight and neutral?  If I understand correctly setting a shadow and highlight usually brings the ends of the curve inward, but the neutral seems to perform more complex changes to the curves; what are these changes and how does it know what changes to apply to the curves?  Also, why can you set multiple neutrals in RGB and only one in LAB?

Thank you
David


ggroess
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Postby ggroess » Mon Feb 07, 2011 4:29 pm

David,
Color cast in an image can be different based on brightness.  As you saw in the class there are times when you can have multiple things going on within a single image.  Setting the Shadow and Highlight sets the boundaries for the overall brightness range of the image and changes the visual perception of the colors we see. 

The curves change in RGB and L channel of LAB when you set the Shadow and Highlight by moving the end points of the curve in closer to the center of the the curve grid;   if there is room in the image for that to happen.  If the image is too dark or too light there may be no room on the ends of the curve line to allow for this When you have that you have to make other adjustments to the image before you can use a curve to set the boundaries.  Those adjustments can include a screen layer, adjusting the Gamma of the image or using a multiply layer.  There are many more....The usual end result of a Shadow highlight move is that the image gains contrast and brightness overall; and the pixels in the image are more evenly distributed across the entire brightness range.

Setting a neutral in LAB simply sets the center of the A and B channels to a new value and adjusts the entire channel to the new zero settings.  There can only be 1 neutral in LAB.  That is the way the color space is designed.

For example:  If you look at the A or B channel as a (X, Y) grid and the center axis is (0,0) but your image has a color cast; you need to correct it so you set a new neutral based on your visual perception of the image. Telling CM to move the 0,0 point to say (+2, -4) now re-sets as the center of the X,Y grid and the point you have chosen becomes 0,0.  Every Pixel in the image is adjusted by that amount and the color cast is removed.  In LAB there is no change in the brightness since the color is separate. 

In RGB the same thing happens; but on the RGB channels you are telling CM to set the RGB values to be the same so the pixels under the sample are neutral.  Since RGB can have multiple areas where the pixels are neutral based on brightness; you can have multiple neutrals.  It is best if they are separated in brightness by say 30 to 50 points in value so that they do not compete with each other.  This is how we can achieve a "By the Numbers" type correction.  This is taking advantage of the design of RGB to remove color cast based on brightness.

How does CM Know what to do??    Kidding...There are complex math processes associated with these color spaces that CM takes advantage of to re-process the image with your adjustments applied.  It's all in the math really...Each color space has a conversion "from or to" the original color space and this is done on the fly in CM and Photoshop.

Great Questions...
Greg


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