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

  • From: Iliah Borg <ib@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 23 Nov 2012 13:36:20 -0500

Dear Ben,

On Nov 23, 2012, at 11:50 AM, Ben Goren wrote:

> On 2012-11-22, at 8:25 PM, Iliah Borg wrote:
> 
>> Additionally, I see clipping of the green channel on the portions of the 
>> whitest patch of the target itself
> 
> I just looked a bit closer, and I can only see a half-dozen pixels clipped in 
> the second green channel in patch N18, and they're all outside of the 
> BOX_SHRINK margins.

Flare still hits around those, and optical cross-talk too. I would definitely 
try to expose not that hot. Look at white point diagnostics from the colprof, 
is the white rendered neutral?

>> and general clipping on the darkest patch.
> 
> Actually, that's a good sign.

Maybe not. The black clipping in the camera is sort of unpredictable, there is 
a problem with uncertainty and accuracy at low levels, and forcing that all to 
be black may be not the best possible approach. Extrapolation often works 
better. Here is a sample from the black patch:

PNG image

>> Shooting against bright background for profiling may add unwanted flare to 
>> the shot. I recognize that those are your actual shooting conditions, still 
>> I would try to profile from a shot with a dark background and see if it 
>> helps. It certainly helped in a test I just made.
> 
> Hmmm...I can try turning off the background lights for the profiling 
> shot...still, they're contributing a fair bit of illumination to the overall 
> scene.

I understand that. But it does not hurt to try.
> 
>> Last, but not least - raw converter you used seems not fully adequate for 
>> the task as it treats highlights in a slightly crude manner.
> 
> Said converter is RAW Developer, which is supposed to use DCRAW for its 
> processing engine.

dcraw is not a converter, it is a raw file decoder with some limited conversion 
functions in it.

--
Iliah Borg
ib@xxxxxxxxxxx

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