[argyllcms] Re: Camera Profiling using ArgyllCMS

  • From: Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 28 Nov 2008 00:02:54 +0300

> There is also another difference between how LProf and Argyll do this 
> compared 
> to ProfileMaker.  Specifically ProfileMaker appears to create the same CLUT 
> curve for all three channels which means that no correction is done for 
> response shifts in the individual channels at different luminosity levels.  
> Where as LProf and Argyll create distinct curves for each channel.  I know 
> that the author of UFRAW believes that using a single curve for all three 
> channels is the correct approach and it appears that authors of ProfileMaker 
> agree him.  At some point I will add a switch to LProf to allow users to 

Graeme Gill wrote:
> Without understanding the rationale, I wouldn't be following them blindly.

After some considering I found one reason for introducing that switch: in many 
cameras the ADC after the color sensor is only one for all channels. The color 
subpixels charges are digitized by one the same ADC sequentially. So there is 
no difference in non-linearity between all channels in RAW file data. There may 
be sensitivity mismatch between color subpixels (white unbalance) but there 
will be full match by sensitivity curve shape. So for the cases with known 
camera ADC design, the single curve (may be corrected by sensitivity factor) 
for all channels will be useful. The optimization assuming channel curves 
equivalence may be more reliable and pricise. There is no other nonlinearity 
sources in camera except common charge amplifier and ADC. The written above is 
right only for unprocessed or unified processed RAW channel data.

ADC=Analog to Digital Converter.

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