[argyllcms] Re: Camera profiling with bracketing
- From: Iliah Borg <ib@xxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 24 Feb 2011 19:50:51 -0500
Dear Graeme,
Such shots are done using tripod and remote, so if user (or scanin) samples one
chart all other locations are the same. I prefer manual sampling (like it is
done in RPP) because that allows to avoid any imperfections right from the
start. It seems that profiles with manual sampling go easy to 6dE max, while
with scanin they are usually about 12 to 20 dE.
I would start with a merge utility that re-calculates spectral reference for
each individual .ti3 file based on the exposure of grey patches (around neutral
grey)
Flare (veiling glare) is from the chart, the lens, the chamber in the camera,
and the sensor itself. Flare is responsible for a lot of non-linearities in the
sensor response and in real world shooting it is strongly scene-dependent. For
cheaper lenses and poor filters veiling glare can reach 2/3 EV easily when
shooting a chart. Adding a simple black trap (hole) to a chart allows for a
good estimation of veiling glare.
On Feb 24, 2011, at 7:21 PM, Graeme Gill wrote:
> Nikolay Pokhilchenko wrote:
>> 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.
>
>> 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
>
> Hmm. I'm not sure that it would be possible to support this without adding a
> quite elaborate set of new code to do the fitting. Is the flare you are
> subtracting assumed to be the inter reflection of the test chart squares,
> rather than flare due to air particles or the camera optics ?
>
> It would be easy enough to add an option to scanin that lets you specify
> a scale factor (I was thinking as a numerator and denominator, so that
> it's easy to pick one of the shots and scale all the rest to it) that
> is then applied to the XYZ reference values, but this wouldn't take into
> account flare.
>
>> 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".
>
> Is that a problem when you use "colprof -u" ?
>
> Graeme Gill.
>
--
Iliah Borg
ib@xxxxxxxxxxx
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