[argyllcms] Re: Generated camera profile too bright

  • From: Chris Lilley <chris@xxxxxx>
  • To: Elle Stone <l.elle.stone@xxxxxxxxx>
  • Date: Tue, 3 Apr 2012 19:39:33 +0200

On Tuesday, April 3, 2012, 5:54:16 PM, Elle wrote:

> (I wrote):
>> Looking at the NEF (raw) file in Nikon Capture NX2 at default settings, I am
>> seeing the black patch at rgb 27, 27, 28 and the white at rgb 223, 225, 224.
>> (The working RGB space is Beta RGB). So the exposure seems reasonable to me;
>> it is the conversion to linear TIFF that is causing the big drop in
>> lightness.

ES> Linear output looks dark when there is not a proper linear gamma
ES> profile embedded, because imaging software assumes sRGB (with sRGB
ES> somewhat greater than gamma 2.2 overall gamma) when there is no
ES> embedded profile.

Yes, I know what gamma correction is ;) 

(and by the way, I wasn't using sRGB, as noted above. BetaRGB has a 2.2 gamma). 

224 ÷ 255 = 0.878 in gamma 2.2 space
with a gamma of 1, that is 0.752

This is substantially different to the 16% (0.16) value for white that Graeme 
found in the TIFF. Thus, the TIFF was not created correctly.

ES> I honestly don't know what Nikon Capture NX2 default settings are, but
ES> I rather suspect they cater to NEF files and "by default" do what
ES> needs to be done to make the processed raw file look good.

Uh, no, not really.

My point is that the exposure, as shot, was reasonable. The conversion to a 
linear space was not correct and has given substantially wrong values.

 Chris Lilley   Technical Director, Interaction Domain                 
 W3C Graphics Activity Lead, Fonts Activity Lead
 Co-Chair, W3C Hypertext CG
 Member, CSS, WebFonts, SVG Working Groups

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