[Tig] Creating a 10 bit Log Lut

John Quartel john
Wed Feb 2 15:56:50 GMT 2005


Kevin Wheatley wrote:

>Cineon/DPX files hold somethign called printing density which has a
>different spectral sensitivity curve used to measure it, than Status-M
>(Neg) or Status-A (Print), the files typically hold 10 bits of
>printing density, they are conceptually base 10 log encodings of the
>transmission of light through a film filtered by a set of 3 spectral
>weightings
>
>SMPTE RP180 describes printing density which is used in 268M (DPX file
>format)
>
>  
>
Unless you have some sophisticated equipment with which to measure the 
spectral product of your Oxberry, then the RP180 document will be of no 
use to you at all. Moreover, there is no commercially available 
densitometer which will measure RP180 printing density, so you're not 
likely to ever know how close your scanner comes to this standard. A 
status M densitometer, which is commercially available, also doesn't 
help you much because the relationship between status M and printing 
density is stock dependent.

My advice is, unless you have the ability to measure anything else, 
always make a simple LUT using the native density of your scanner. If 
the scanner was designed sensibly, then its colour separation will be 
sufficient to produce a reasonable image. That is, just convert you 
linear output (x say) to log (X say) with use a LUT like

   X = -(1/0.002) * log10(x)

and offset the result according to clear base or a grey patch (using the 
values stated by Peter Swinson earlier on this thread).

John Quartel
FilmLight Ltd





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