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Rene:  "... i want to have a knowledge-sharing debate. ..."


Superb!  Me, too!


Yes, of course you are correct that there is more information in a raw file than in a JPG or TIF.  But the real question is does this extra information help significantly in the problem at hand, separation of hair from a background.  What I have been saying is that the amount of improvement provided by making the luminosity mask using raw data vs making one using the image itself is minimal. 


So, let's consider two situations. For the moment, let's only consider B&W images to avoid the complications / alternatives offered by color. In the first situation, say you have a nice JPG or TIF in which the hair is somewhere in the middle of the tonal range (eg, well lit blonds, redheads, light brown hair) and the brightness of its immediate background is also of reasonable brightness.   So, to make a luminosity mask that will separate the two, one can use curves (or other tools) to exaggerate the difference between the hair and its immediate background.   Sure, one may get a bit better mask if you have a 16 bpc (bit per channel) or raw file compared to an 8 bpc JPG, but the difference in quality of the mask is minimal unless the tonality difference between hair and background is very small, say, only a couple of levels out of 256.  This is what I am defining as "the second situation".


In the case of small local tonality differences between the hair and the background, sure, the increased bit depth provided by having raw data available can in principle help in separating the two, but the point is that the end viewer would likely never be able to see such small differences.  In addition, if the difference in tonality between hair and background is that small, often there will be much larger variations in brightness from one area of the image to another that will be much larger than the hypothetical couple of levels (out of 256) that we are trying to separate, so one really can't make use of the increased bit depth one gets with a raw file and simple processing technique such as you suggested in your tutorial.  Instead one has to use one of the adaptive or other techniques (eg, a mask based on differences in hue, etc.) that I mentioned in my earlier email.


What do you think?


Tom 




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