[Insight-users] ITK Image Coordinates / Problem with PhysicalPoint to Index Conversion

[Steve M. Robbins]

Hello Andreas,

I think you're right: the docs are correct but the code is buggy.


On Tue, Jan 08, 2008 at 10:37:32AM +0100, Andreas Keil wrote:

> As mentioned in my earlier post (see below), the documentation and
> implementation differ in this point. The two proposed solutions are either
> fixing the documentation (ITK Software Guide, p. 40) and defining the
> origin to lie in the "lower-left(-back)" corner of the pixel with index
> 0/0(/0) or fixing the implementation of the respective methods
> (IndexToPhysicalPoint, PhysicalPointToIndex, etc.) and defining the origin
> to lie in the center of this pixel.

Right.  I find the centre-based coordinate convention to be much more
natural; e.g. when writing out formulas for interpolation.  If you're
taking votes, put me down for the centre.

I think the centre-based coordinate convention is more common with
image processing folk, whereas the corner-based convention is more
common in graphics circles.  For what it's worth, VTK's image data is
said to use the centre-based coordinate system [1].  On the other
hand, VTK uses corner convention for screen pixel coordinates, and I
suspect this confusion underlies the problems in VTK's coordinate
conversion routines [2].


[1] http://public.kitware.com/pipermail/vtk-developers/2005-November/003825.html
[2] http://www.vtk.org/Bug/view.php?id=5111

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