Bob thanks for checking in with that great story. The internet is just amazing that way-- it has connected us in ways that we couldn't have dreamed of just a few years ago. Please send your two young friends my warm regards.
~cj
Bob thanks for checking in with that great story. The internet is just amazing that way-- it has connected us in ways that we couldn't have dreamed of just a few years ago. Please send your two young friends my warm regards.
~cj
I'm a big fan of LAB for color corrections, and I've read Dan Margulis' great book on the subject.
I do have a big concern with this technique though.
For those of you unfamiliar with the details of LAB, 0 is neutral in the color channels (A and B). Negative values in A are green, positive values magenta, and in B negative values are blue and positive values yellow.
The technique described above takes every color value in LAB between -10 and +10 and neutralizes it. This is a BIG deal, because you're taking literally 10,000 color values (10 x 10 x 10 x 10 for the pos and neg values in each color channel) and rendering them neutral. In fact it's more than 10,000, because you also have the 256 luminosity values (in 8-bit), so you're actually taking 2.56 million colors and neutralizing them.
The following image shows colors that all have A and B values that are between -10 and 10. The luminosity values are different, but they're irrelevant. Every single one of these colors, whether from a color cast or not, would become gray with this tecnique.
Furthermore, there are many colors that have a value of under 10 (or -10) in one channel but greater than 10 in the other channel. Like a yellow-green or a purple. If you neutralize the value that's under 10 but not the one that's greater than 10, you will get a color shift.
Now, I do use LAB for color casts, but I use a different technique -- I measure neutral objects for any color bias and kill that specific cast. Again, neutral objects (white or gray) should have values of 0 (or very near 0) in both the A and B channels if they're neutral.
After measuring them, simply take a single point on the A and B curves and drag it up or down until you've removed any color cast. So if your white object has an A value of -6 (making it green), drag -6 up to 0 (you can plug in the numbers too). This will kill the green cast.
Last edited by DrPablo; 6-Aug-2006 at 10:52.
Hi Chris:Originally Posted by chris jordan
You outline a nifty technique and this is in no way intended as a flame, but in addition to the concerns brought out by Dr. Pablo above, I wanted to address the comment you made above because it is not accurate.
In RGB mode if you add a curves adjustment layer above your image and set the blend mode of that curves layer to "saturation," you in fact have exactly a "saturation curve" tool.
If you flatten that curve on all channels to a full horizontal line, all outputs at 0 (or all outputs the same), you will get a completely grayscale image. Alternatively, you can of course also set points for any single hue and pull them down to any desired degree of saturation.
Cheers,
Last edited by Jack Flesher; 6-Aug-2006 at 11:02.
Dr. Pablo, I think you might have misread my original posting. My technique is to lock down the 10,10 values, so that they are unaltered. Only the values lower than that get neutralized-- the 2,2's, 4,4's, etc. Usually I don't neutrallize the 8's all the way-- they get taken from 8 down to 3 or 4 or so. And then, after that, I fade the curve back to taste, which is usually 50% or so. I talked about all of that in my post; hopefully it was more clear to others...
And one other thing: My technique is not for removing color casts-- that is easy to do in any of several different ways in RGB mode. The point of the technique I am describing is to reduce the saturation of the neutrals, which cannot be done in RGB mode.
Ah yes, I did misread the lockdown at 10. This would be better, but there are still some subtle colors that would be neutralized (all these have A and B values from -4 to 4).
But you certainly could make the colors more subtle, like reprogramming -2 to -1 and -4 to -2, etc, making the color curve shallower without truly neutral. This would preserve some color information. Of course it's to some degree subject specific.
Chris: It can be done in RGB and quite easily. Please read my post directly above -- you probably missed it while responding to Dr. Pablo.
Cheers,
Last edited by Jack Flesher; 6-Aug-2006 at 11:08.
Hi Jack, yeh that technique kind of works, but not the same. You can change the saturation of any particular hue, but you can't divide out the saturated colors from the unsaturated colors, and adjust them separately along a curve, which is what you can do in LAB. There is no way in RGB to desaturate the neutrals while leaving the saturated colors untouched.
The other thing is, all I'm offering here is another tool for the tool box, to be used however it helps. It doesn't work with every image, and it may not be useful for some people at all. It works for me, though, most frequently with images made on transparencies in low light, where the neutrals are too saturated but otherwise the image looks properly color balanced.
Actually you can by building the curve much the same way you build your Lab curve, locking down points you don't want to change -- plus it is easier to load a mask at the same time in this process allowing even more control over the areas you are affecting.
Again, I am only pointing this out for clarity and am in no way challenging the veracity of your Lab method -- both are indeed good tools to know and which tool one chooses to use will be dependant upon their normal image processing workflow.
Cheers,
Jack I can't get it to work in RGB-- where do you place your points? In LAB, the neutrals are all right around zero regardless of their brightness; but in RGB the neutrals are all up and down the curve depending on their brightness. So how do you lock down the saturated colors and tweak just the neutrals?
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