[CREATE] Filling in LensFun database - solution (well, maybe)

[Bruno Postle]

On Wed 18-Jun-2008 at 13:01 -0400, Liam R E Quin wrote:
>On Sat, 2008-06-14 at 17:22 +0400, Alexandre Prokoudine wrote:
>
>> We need a tool that would help us filling LensFun database with
>> geometry distortions data, right? (Not speaking about CA right now,
>> sorry)
>
>Doesn't hugin/panotools already have a lot of such data, and an
>algorithm for starting with?  I see no reason to duplicate that.
>If it's deficient in some way, maybe the panotools mantainers
>would be interested in enhancing it??

The hugin lens correction tutorial hasn't been written.  I'll attach 
an email I wrote recently describing what this tutorial would look 
like.

A couple of points that I ought to add to this lens database 
discussion:

0. Lensfun already has an import of the old 'ptlens' database, so it 
is primed with a lot of older camera calibrations.

1. hugin does this lens calibration during the normal panorama 
stiching process - There is a potential for hugin to submit 
calibrations automatically (similar to freedb), see previous 
discussions on this list.

2. Printing out a target and photographing it isn't very practical.  
How do you get it flat?  How do you illuminate it evenly?  How do 
you deal with focus distance issues?

3. hugin SVN has a tool called tca_correct which can be used to 
determine chromatic aberration automatically, though not fast enough 
for interactive use, this needs some work but is very promising.

Here is my description of a (to be written) lens correction 
tutorial:

>> Calibrating a lens and saving it for use in later projects, this 
>> seems to be something that scares people.  Actually this isn't so 
>> hard, you can do it in three ways:
>> 
>> * By stitching a panorama, i.e. basically the two photos tutorial, 
>> perhaps with more photos, 50% overlap and a scene in the far 
>> distance.  This is also the only way to calculate photometric 
>> parameters.

http://hugin.sourceforge.net/tutorials/two-photos/

>> * Using the perspective correction technique with a modern gridded 
>> building.

http://hugin.sourceforge.net/tutorials/perspective/

>> There is a very old tutorial here:
>> 
>> http://www.path.unimelb.edu.au/~bernardk/tutorials/360/calibrate/
>> 
>> * There is a third technique which uses 'straight line control 
>> points' (which are not described anywhere in the tutorials), but 
>> otherwise it is similar to perspective correction - The advantage of 
>> this one is that it only corrects lens distortion, making it more 
>> accurate than a technique that has to correct perspective at the 
>> same time.
>> 
> 'Straight line' points are described here:
> 
> http://wiki.panotools.org/Straight_line_control_points
http://wiki.panotools.org/Hugin_Control_Points_tab#Straight_line_control_points

>> Actually the technique is described here, but there is no need to 
>> have a 'flat-on' image, I'm not sure why everyone recommends it:
>> 
>> http://www.erik-krause.de/index.htm?./verzeichnung/distort_en.htm
>> 
>> The 'straight line' technique doesn't involve correcting 
>> perspective, just a scene with straight-lines - A heap of barbeque 
>> sticks ought to do.
>> 
>> -- 
>> Bruno