[Qgis-developer] Affine transformation code in Georeferencer -- incomplete?

[Benjamin Ducke]

FYI,

The pure C (+Java bindings) version of the transformation
methods in the QGIS georeferencer is now available here:

https://svn.code.sf.net/p/gvsigce/code/trunk/libraries/libCTL

I realize that this won't be of immediate interest for QGIS
folks, but I would nevertheless like to thank you all for
taking the time to answer my questions on this list.

If anyone here does find my C/Java conversion useful, then
I'd be thankful for any tests/comments.

Cheers,

Ben

On 05/05/15 11:41, Benjamin Ducke wrote:
> On 05/05/15 11:20, Olivier Dalang wrote:
>> Hi,
>>
>> If I'm correct, the QgsLeastSquares::affine() method is used when
>> creating a worldfile, and it seems it does not account for rotation (at
>> least the generated worldfiles don't).
> 
> Ok, that explains a lot.
> 
>>
>> Not really sure of what you're trying to do,
> 
> I am trying to extract the transformation methods
> to plain C. I find that they are nicely designed and
> compact, with GNU Scientific Library (also a plain C
> lib) being the only dependency. I am not aware of any
> other simple C lib for such transformations and I would
> like to make the excellent QGIS implementations of
> Helmert and projective transformations available to
> C and Java (via JNI bindings) programmers. Granted,
> affine transformations are usually easier to find,
> but for the sake of completeness...
> 
>> but just in case, here's my
>> implementation to compute worldfile parameters with rotation too (I had
>> no time to port to core yet) : 
>>
>> https://gist.github.com/olivierdalang/ba97fc986ade4545068d
>>
> 
> Thanks! I will take a look. Maybe this already contains
> all I am searching for.
> 
> Cheers,
> 
> Ben
> 
>> Best,
>>
>> Olivier
>>
>>
>>
>>
>> 2015-05-04 10:35 GMT+02:00 Benjamin Ducke <benducke at fastmail.fm
>> <mailto:benducke at fastmail.fm>>:
>>
>>     Dear Devs --
>>
>>     I am currently converting the transformation methods
>>     in the QGIS Georeferencer to plain C.
>>     So far, I have been able to extract and convert the
>>     projective and Helmert transformations.
>>
>>     However, the code for the affine transformation seems
>>     incomplete.
>>
>>     Looking here:
>>
>>     https://github.com/qgis/QGIS/trunk/src/plugins/georeferencer/qgsleastsquares.cpp
>>
>>     I find this method:
>>
>>     void QgsLeastSquares::affine()
>>
>>     It computes a 6x6 transformation matrix but stops there.
>>     In analogy to "QgsLeastSquares::helmert()", I _think_
>>     the next step should be to extract the rotation, scaling,
>>     translation and shearing vectors using "gsl_vector_get()".
>>
>>     It seems to me that "QgsLeastSquares::affine()" has been
>>     abandoned code for a while and that the affine transformation
>>     is now delegated to gdal_warp(?).
>>
>>     Does anyone here still remember how to extract the affine
>>     components from the 6x6 matrix? I have done some extensive
>>     searching on the web but came up empty. Any pointers would
>>     be greatly appreciated.
>>
>>     Cheers,
>>
>>     Ben
>>     _______________________________________________
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>>     Qgis-developer at lists.osgeo.org <mailto:Qgis-developer at lists.osgeo.org>
>>     http://lists.osgeo.org/mailman/listinfo/qgis-developer
>>
>>
> 
> 
> 



-- 
Dr. Benjamin Ducke
{*} Geospatial Consultant
{*} GIS Developer

Spatial technology for the masses, not the classes:
experience free and open source GIS at http://gvsigce.org