[argyllcms] Re: Profile input white not mapping to output white

  • From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 26 Nov 2012 08:48:45 -0700

On 2012-11-25, at 7:02 PM, Graeme Gill wrote:

> Consider a situation where I have three bright light sources
> facing the camera.


That's a very good thought experiment, and it helped clarify things a good deal 
for me.

However, as Gerhard has pointed out, whether optimal or not, RAW engines will 
already do everything that can be done in today's workflow to deal with such 
situations at the stage when the engine applies the user's choice of white 
balance. Either the user will go for a 5000K white balance and the D50 light 
will get mapped to the neutral axis and the D65 light will be bluish; or the 
user will go with a 6500K white balance and the D65 will get mapped to the 
neutral axis and the D50 light will be reddish; or, as is very common in 
shooting in the "golden hour," the user will "split the difference" between the 
4500K direct sunlight and the 10000K skylight, and the highlights will look 
reddish and the shadows will look bluish and nothing actually photographed in 
the scene (except perhaps a tiny spot on a white sphere) will be on the neutral 
axis. And, of course, the same will apply to the spectrally flat patches on the 

But, again, as Gerhard has pointed out, all this is well out of the hands of 
the color management engine.

(On a side note, I'm sure I'm not the only one who'd love to see you work to 
incorporate color management directly into RAW processing -- but that's 
somewhat of a tangent from your current project. Still, should you and Iliah 
just happen to decide to work together on this....)

Upon further reflection, it seems to me that the proper course for the color 
management engine is to assume that R=G=B=255 should be treated the same as the 
user's choice of white balance -- or, perhaps, ideally, a measurement made with 
a spectrophotometer in ambient mode. The reported white balance might be 
problematic...RAW engines are notorious for reporting different color 
temperatures for the same effective RGB channel multipliers -- and, of course, 
the absolute channel multipliers will be different for different cameras in the 
same light.

Still, I'll try some experiments today with the i1 (you did fix the bug some 
time back where it didn't work in flash mode, right?) and see if that makes any 

> I've put a snapshot of what I've been working on here
> <http://www.argyllcms.com/colprof_osx_exe.tgz>
> <http://www.argyllcms.com/colprof_win32_exe.zip>

I'll give that a whack sometime today (tomorrow at the latest) and report back 
off-list with my findings.



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