[argyllcms] Re: help with camera profile

  • From: Iliah Borg <ib@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 25 Jul 2012 12:37:59 -0400

On Jul 25, 2012, at 12:23 PM, Ben Goren wrote:

> On 2012-07-25, at 8:29 AM, Iliah Borg wrote:
>> With digital there is no simple way to get density or L* values, and 
>> linearization in RGGB domain does not work too well.
> If you're using Adobe Camera Raw or something else that works with DNG 
> profiles, you can (awkwardly) create a DNG profile with a custom tone curve. 
> Shoot a target with known Lab values, and then adjust the tone curve until 
> the output matches the measured values. On a Mac at least, you'll need 
> Adobe's DNG Editor and the Digital Color Meter application. If, while you're 
> at it, you also use DNG Editor's feature to create color tables from a 
> ColorChecker, the resulting output is already decidedly in the ``not bad'' 
> category. Profile those results with Argyll, and you get great results.
> RAW Developer would seem to make that process a bit easier as it lets you 
> edit the tone curve right there, and it gives you much more helpful pixel 
> readouts. What I suspect I'll be doing, at least for the time being, is 
> creating a tone curve that maps the neutral step patches to the corresponding 
> RGB values for the gamma of whatever working space I feel moved to use this 
> week. (I've been using ProPhoto RGB because that's been the least awful one 
> that Adobe lets you convert RAW to, but I'll probably move back to Beta RGB 
> with RAW Developer.)

You are editing in the output space, CIE. No, that is not how photography works 
;) We use density calibration to control output. Colour is a given, it is 
defined by the set of dies, pigments, colour filters in front of the sensor. It 
is easy to determine the spectral/colour properties of a CFA - all you need is 
a rather cheap monochromator, a light sphere, and a reference photodiode ;) Or, 
some inside information from the manufacturer. Or, some red, green, and blue 
patches to shoot. Look at characteristic curves, they state density for the 
exposure, and do not care for colour. Colour is the second step. The error most 
are making when trying to imitate film in raw processing is that they do not 
realize the curves are not taking colour into account.

> What I'm envisioning for Argyll is a modification to scanin such that it 
> still takes an RGB TIFF but then calculates the calibration curves at the 
> same time as it reads the patch values. You could then feed it a linear TIFF 
> output if your RAW developer supports such, or whatever else you can manage, 
> and let scanin be the one to worry about the tone curve.

This would be my wish list: First, equalization of light based on the target 
border (includes white balance and light un-evenness). Second, linearization 
using the grey step wedge. Third, direct coding of desired gamma or tone curve 
(say, tone curve based on an amp file using L* from it). Nothing is 
particularly hard if you divide it into small jobs (c) Henry Ford

Iliah Borg

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