[geotk] Re: Mapping coordinate system axis order to i, j indices on a map images

Martin Desruisseaux martin.desruisseaux at geomatys.fr
Mon Apr 19 13:57:46 EDT 2010

Le 19/04/10 18:43, Jonathan Blower a écrit :
> My main question is - what happens if the transformation from real-world to grid coordinates is not affine?

Actually the most robust approach is to ensure that we never need to known if 
axes are swapped/flipped or not, especially since non-affine transforms provide 
no garantee that axes swapping/flipping do not vary with the position.

Once the axes swapping/flipped information is buried into the AffineTransform, 
there is often no need to extract them out of the transform. From that point, we 
try to design all the processing chain in terms of concatenations of 
MathTransform, without knowing if the transforms we are concatenating are affine.

The main points where we need to care about axes order/flipping are typically 
right at the moment where an image is read from a file, or when it is written in 
a particular format. For everything else (including rendering), we try to avoid 
explicit handling of axes swapping/flipping.

For example our renderer engine defines the CRS of the output device (we call 
that the "display CRS") and just ask the referencing module to figure out the 
transform from the coverage CRS to the display CRS.



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