[CREATE] Lens correction database

[Pablo d'Angelo]

Hi Andrew (and others),

I'm very happy that there is something happening here, since it is unlikely 
that I will have the time to do much work in this area.

Here are some comments (I haven't had a very close look at the interface yet).

The lf_corrector_apply does not allow different transformations for the 
color channels (which is required for TCA correction). TCA behaviour of most 
lenses is close to linear, but some specific ones (especially some fisheye 
lenses) exhibit a non-linear relationship. I provided Michel Thoby with some 
software to measure the TCA, and he has put a lot of work into testing 
different lenses and TCA correction tools. A comparison of tools is 
available at:
http://michel.thoby.free.fr/Compare_Fisheyes/Correction_TCA/Illusion_and_reality_TCA.html

To me it seems that photoshop is doing more than just a mere scaling of red 
and blue channels.

Also, TCA should not vary with different aperture settings. It will probably 
vary a little bit with the subject distance.

There is a nice paper (which I don't have handy right now :-( ) on how the 
k1 distortion coefficient can be interpolated across different focal 
lengths, by fitting a exponential function to the measured (distortion,focal 
length) values. This is much better than linear interpolation. I have been 
doing some work on the stability and estimation procedure for the radial 
distortion part, but I need some more work to reach final conclusions. 
However, the traditional k1, k2 model seems to work quite well for most 
rectilinear lenses.

I have written some example xml files and a tool to convert the PTLens 
database that might be helpful for inspiration, they are available at:
http://hugin.svn.sourceforge.net/viewvc/hugin/lensdb/trunk/

ciao
   Pablo

Andrew Zabolotny wrote:
> Here I am again :)
> 
> I thought I'd post a progress report on this task, such that interested
> persons would know it's not dead :-)
> 
> I have finished the first implementation of database management, it
> supports database loading (from multiple file/locations automatically,
> like I suggested before), saving to XML, supports several types
> of database queries (including fuzzy lens search).
> 
> The doxygen docs for the library interface can be found here:
> 
> http://cs.ozerki.net/zap/lensfun/doc/
> 
> Here's a simple example applications that loads the database, does a
> few queries and saves the results to a new xml file:
> 
> http://cs.ozerki.net/zap/lensfun/tfun/
> 
> You can see a sample of database format in the file saved by the
> example application.
> 
> Comments are still welcome, as the interface has not been frozen
> yet ;-) (hey, it's just a week since I started ;-).
> 
> The image correction logic is totally missing at this time, just a
> draft of the interface is present. I'll proceed with this today.
> 
> As soon as project stabilizes a bit, I'm going to upload it to
> sourceforge.
> 
> 
> 
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