[Insight-users] Position given in Physical, index or pixels?

[motes motes]

As I understand a position in an image can be expressed in 3 ways:

1) Pixels coordinates.

2) Index coordinates.

3) Physical/Space coordinates (in millimeters).
where a physical point is obtained as:

 physical = Index * spacing + origin

So when spacing is 1.0 in all dimensions the physical coordinates
equals the index coordinates + origo.




Based on section 6.9 Geometric Transformations in the itkSoftwareGuide
I understand that the following types are used:


Spacing:                              Physical/Space coordinates (mm).
Origin:                                 Physical/Space coordinates (mm).
Image Size :                        Pixels coordinates.
Transform parameters :         Physical/Space coordinates (mm).

As I understand the units for all axis in the Figures in section 6.9
are pixels assuming a unit spacing of 1.0,1.0.

When looking at the output image in Figure 6.50 the size of the image
is 40*45 pixels. This does not match well with the axis. But I assume
that its because spacing=(1.0,1.0) are used for the axis while
spacing=(4.0,4.5) are used for the image.

I don't understand the difference between pixel coordinates and index
coordinates, are they the same?