50% Gray in RGB, Lab ang Gray-gamma 2.2: why are different?

Started Jan 15, 2013 | Discussions thread
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technoid Senior Member • Posts: 1,596
Re: Here's Why

One can certainly do a good B&W correction with a custom tone curve instead of an ICC profile. There really is no difference between profiling with an ICC profile or your own tone curve in B&W. The ICC profile is more standardized and can be used the same way in B&W and color and is widely supported in photo editing s/w.  The problem with B&W ICC profiles is finding s/w that generates good ones specifically for B&W.  What I do is generate a set of B&W patches from (0,0,0) to (255,255,255) (256 patches), print the patches then scan them with an X-rite IO. I load the 256 L values into Matlab along with a random printer profile then replace the white black points and 3DLUTs with a smoothed, interpolated table and write out the new profile. It's not too hard to do in Matlab but does require a good working familiarity with both Matlab and ICC profile structure. I wish there were specific programs people could do this with but I haven't found any.

Most of my printing is repro printing, where a print needs to match other art so I use absolute colorimetry a lot.  A L*a*b tri-value needs to match it's printed value as close as I can get it. The interesting thing about B&W is that getting high quality profiles is a lot easier because one only has to look at one dimension, the tone curve. 256 patches gives the best possible estimate. A full color set of all possible patches would be over 16 million patches which isn't doable.

I'm interested in seeing what your testing shows.

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