Has anyone answered Andreas's questions below in this thread or offline?
On Sat, Sep 17, 2011 at 12:06 PM, egoman <[email protected]>wrote: > ** > Great to have someone from Arri here! Unfortunately that post left me more > confused than I was before [image: Smile] > > To start with, when I said "Arri's own Log C LUT (as well as the original > Cineon LUT), are really designed for video output", I wasn't referring to > the LogC to REC 709 LUTs. I meant that the concept of a black point is > related more to output-referred rather than scene-referred image states. > When using the LogC or Cineon formulas, anything below 95/1023 will be > mapped to negative values. No scene-referred encoding of a physical scene > should ever contain negative values. No matter how much one increases the > exposure of such an image, the negative data can never be regained. > > I'm not saying that the LogC Log to Lin is wrong in any way, I'm just > saying it's more geared towards making a pretty picture (when viewed through > the appropriate monitor LUT) than it is to giving color values suitable for > VFX work. I therefore expressed an interest in using the Josh Pines math for > reading these images instead. The question was what parameter values would > best decode the LogC data. As is stated in that pdf you referred to, 18% > gray is mapped to 400/1023. The other things that one needs to know is the > negative gamma and negative density per log code value. These are of course > film properties (as the formula is designed for cineon film scans), but > unlike pseudo-log formats (such as FilmStream, ARRI Log F, Panalog, S-Log) > Log C should have corresponding values. Assuming a density of 0.02, some > empirical testing gives a negative gamma of 0.45 to match the look of an > image converted by the ARRI equation. This is what we are using on our > current production. > > We can leave that question for now though, as reading the documents you > recommended has left me with new questions [image: Smile] > > You say that the math in the ALEXA Log C Curve pdf is correct, and the one > used in Nuke. You also say that the math in the Alexa Color Pipeline for > Nuke pdf is not up to date, and should not be used. But as far as I can > tell, it's this latter formula that is actually used in Nuke (6.3v2)! > > Another interesting discrepancy is the ALEXA Wide Gamut RGB primaries. When > using the Nuke Colorspace node to convert from the Alexa primaries to CIE > (or anything else), one gets a somewhat different matrix than is given in > the ALEXA Log C Curve pdf. If The Foundry has gotten this wrong, perhaps > someone should inform them? > > Andreas Bravin Karlsson > Compositing Supervisor > > _______________________________________________ > Nuke-users mailing list > [email protected], http://forums.thefoundry.co.uk/ > http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users >
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