[argyllcms] Re: Camera profiling with bracketing

  • From: Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 24 Feb 2011 11:45:22 +0300

Thu, 24 Feb 2011 18:29:09 +1100 Graeme Gil wrote:

> Nikolay Pokhilchenko wrote:
> > I'll be happy if Argyll will work with bracketed targets too. I had compute
> > combined target in OpenOffice scalc.
> > The bracketing is usefull for digital camera profiling, mostly in
> highlights.
> You'll have to explain a bit what you mean by bracketed targets. By
> this do you mean something similar to HDR capture, where you take
> several shots at different exposures, and then merge them back into
> a single photo/chart ?

Yes. I'm taking several shots of the same target under the same illumination. 
For example, aperture values from 2.8 to 
11: 2.8, 4.0, 5.6, 8.0, 11 with fixed shutter speed.

>  Were you assuming linear behavior with exposure time ?
> (ie. that the shutter time is accurate ?)

I have to compute exact exposure values per chart (I change the aperture, 
rather the shutter speed because the shutter 
speed is slightly vary the CCD or CMOS matrix behavior).
I'm choosing the reference chart with 1.0 exposure and computing the exposure 
coefficients for all the rest charts.
I'm computing the gamma (for raw files it's about 1..1,18) of the sensor (one 
for all charts), computing the flare and the 
exposure coefficients (per chart) simultaneously. Then I computing XYZ 
"stimuls" for each patch of every chart by 
subtracting flare and multiplying the XYZ by approptiate exposure coefficient. 
As a result, I have much more patches than 
the original target have. It helps to achieve predictable profile output for 
bright and saturated areas.
In many cases it's hard to optimize all parameters (exposure, gamma and flare) 
together, the results may be abnormal. 
In such cases I have to fix the parameter (for example the gamma=1 or flare=0) 
and compute the rest.
Another thing: there is the problem when trying to compute the input profile by 
such hihg DR data: Argyll colprof can't 
define the correct white point if the highest "Y" value have non-neutral patch. 
I have to delete some measurements or to 
compute virtual white patch whth highest "Y".

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