[argyllcms] Re: scanin and perspective distortion
- From: "Gerhard Fürnkranz" <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 06 Feb 2008 15:55:00 +0100
-------- Original-Nachricht --------
> Datum: Wed, 06 Feb 2008 11:01:57 +0100
> Von: Pablo dAngelo <Pablo.dAngelo@xxxxxx>
> An: argyllcms@xxxxxxxxxxxxx
> Betreff: [argyllcms] Re: scanin and perspective distortion
> I agree that an automatic estimation of the parameters would be
> required for this process. However, since the "relative brightness"
> of the target patches is already known,
You are assuming, that a particular luminance ratio between two patches also
implies that you obatin the same linear raw RGB ratios for these patches from
the sensors. This is however not granted in general, but is only granted for a
pair of patches with the same shape (but different magnitude) of the spectral
reflectance, due to observer metamerism between a camera and the human vision.
Thus I'm rather inclined to use only patches with the same color (e.g. white
ones, or identical gray tones (border of an IT8 target?)) for this purpose,
since they can be expected to have all approx. the same spectral reflectance
too, so that Y ratios and RGB ratios will be the same.
For a first rough guess, comparing luminance ratios (from the reference file)
and say linear raw green channel ratios may possibly suffice (in conjunction
with fitting the data to a (smooth) vignetting model, and sorting out outliers).
Whether a subsequent iterative refinement would converge, needs IMO further
investigations (The idea is to convert RBG to XYZ using the profile from the
previous iteration, then fitting and compensating the Y ratios to the
vignetting model, creating the next (hopefully better) profile, and so on).
Such an iterative approach might also work for non-linear RGB data which are
not power functions.
Quite some time ago I had already extended lprof's color picker to add the
spatial location of the patches to the RGB measurement file, in order to use
them subsequently for compensation of spatial variations, and I had also
started some initial investigations, but at some time this stalled, due to lack
of time... (it is still on my list of things I'd like to continue if I find
enough free time). Initially I was rather focussed on lighting variations (so I
had started investigating a lighting model with two point sources), but in the
mean time I think that vignetting possibly plays an even more important role
for camera profiling than lighting variations, since the latter can be
addressed to a good deal by setting up a suitable geometrical arrangement of
target, light sources and camera.
Regards,
Gerhard
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