hmm, wouldn't that tool make more sense as part of lensfun rather than DT ?
we have seen with profiled-denoise that DT is in a very good position to
collect hardware data from our users... probably in a better position than
projects like ufraw or lensfun...
A tool that helped/automated lens profiling would be awesome, though it
would make more sense as an add-on like our noise-profiler rather than an
IOP
iop would only really make sense if each individual lens was different.
Since distorsion is per-model it makes more sense to collect the info into
aproject (lensfun in this case)
On Mon, Apr 1, 2013 at 1:21 AM, Torsten Bronger <
[email protected]> wrote:
> Hallöchen!
>
> Daniel J Blueman writes:
>
> > In my thesis a while back [1], I developed a strong algorithm and
> > tool [2] to automatically generate LCA correction data from an
> > image. These correction values hold for the given set of lens
> > parameters usually encoded in the image's EXIF fields.
>
> I appreciate that people are working on TCA correction (I avoid the
> acronym "LCA" because it may be misinterpreted as "longitudinal
> CA"). It is still sub-optimal in Darktable, so any improvement is
> welcome.
>
> Let me first review the current situation.
>
> Darktable has two means to reduce TCA. The automatic approach works
> often poorly according to many user reports. The profile-based
> approach using LensFun is reliable, but LensFun has an extremely
> small TCA profiles database.
>
> LensFun profiles are generated by running tca_correct over a sample
> photograph. This works very well. tca_correct's method is
> described at <http://hugin.sourceforge.net/tutorials/tca/en.shtml>.
> Basically, it finds control points and creates distortion
> polynomials for the red and blue channels respectively.
>
> You can use the coefficients in LensFun. Alternatively, you can use
> fulla to TCA-correct your photograph. If you use fulla, the
> polynomial is allowed to be more complex, and decentering of the
> lens is taken into account. You can reimport the picture in
> Darktable. However, this is awkward.
>
>
> Adobe Lightroom used to use profile-based TCA. Its database is much
> larger that LensFun's. But for whatever reason, Adobe decided to
> give up this method. Instead, Lightroom prevents most of TCA
> already while demosaicing, without any profile. I have no idea how
> it achieves this. After that, you can enable CA correction, which
> is simply a desaturation of edges. It works well, and allows for
> removal of axial CA as well.
>
> > I wanted feedback onsome ideas to bring this tool to a wider audience:
> > - reduce runtime/memory usage
> > - generating lib lensfun-compatible output
> > - ability to work on a directory of images, averaging across related
> settings
> > - moving from python to C/C++
>
> If I understood your text correctly, you calculate difference images
> between the channels and total these differences up. Then, you
> calculate the distortion coefficients that minimize this integrated
> difference. This sounds costly. In particular, I doubt that it can
> be used for realtime correction of images in Darktable.
>
> I suggest the following: Write a C program that reads a TIFF and
> writes a LensFun <tca> tag line. This would be very helpful. While
> tca_correct works well, your method really seems to be very robust
> and accurate. Its slowness doesn't matter when it comes to
> generating of profiles. I would use your tool in my own calibration
> tutorial/program, and the LensFun people probably would link to it,
> too.
>
> > Finally, I find that LCA correction is depenendent on aperture and
> > potentially focus settings (as lens elements are moved); this isn't
> > encoded in the existing lensfun correction model [3].
>
> TCA is weakly dependent on focal length (of a zoom lens). See
> <http://en.wikipedia.org/wiki/Breathing_(lens)> for the cause. It
> is very weakly dependent on aperture. In fact, if I browse through
> the reviews at <http://www.photozone.de/Reviews/overview>, I doubt
> that there is a measurable dependence beyond systematic errors.
> (I'd really like to know why there is no dependence, by the way.)
>
> Be that as it may, I don't think that it is senseful to extend
> LensFun this way.
>
> Tschö,
> Torsten.
>
> --
> Torsten Bronger Jabber ID: [email protected]
> or http://bronger-jmp.appspot.com
>
>
>
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