[argyllcms] Re: Misc questions about color management
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 16 Mar 2006 23:24:15 +1100
Frédéric wrote:
So, the camera has its own sensor profile, to convert from the sensor
colorspace to AdobeRGB98/sRGB. Do external programs act in the same way ?
> They have a profile of the camera to convert from raw to specifed
> colorspace ?
Yes they must. The accuracy of the "profile" they use may be had to
figure out though.
I read that calibrating a camera is only usefull when shooting in a studio,
where you always use same params (time exposure, aperture, iso...). Or do
you think I can create a profile useable in all cases ?
Conventional profiles tend to be only usable in fixed situation, if
you expect them to do complete output referenced rendering for
you. If the raw sensor is well behaved though, then something
like a matrix/shaper profile may be more useful in converting
sensor output to a working space, in which you can then apply
output rendering. Creating a good profile, can be difficult though.
The sort of thing a good camera manufacturer does to create a
conversion from the sensor to a conventional RGB space can be
quite complex. Typically the better ones would want to measure the
spectral sensitivities of the camera sensors, and then apply
various statistics on typical real world color spectra in order to
optimize the matrix values. They may even go to the trouble of
arriving at several different matrices, one for each type of "typical"
lighting (ie. daylight, incandescent, fluorescent, flash etc.),
and then using a heuristic to decide which proportion of
each matrix to use for a given situation. (Adobe raw
does something a little like this). Really good manufacturers
will measure the actual sensors of each camera, rather than using
batch average estimates.
This is all designed to work around the fact that typical
camera sensors do not have quite the same spectral sensitivities as
the human eye.
For most people, measuring the spectral sensitivity of a camera
sensor is beyond their means, and some sort of test chart is
all that is available. The lighting will then be whatever
you use to take the photo, and the spectral statistics of
the chart may or may not resemble the real world colors
you are actually going to shoot with the camera.
Graeme Gill.
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