[Insight-developers] Image::TransformPhysicalPointToIndex

[Damion Shelton]

Hi...

>     if somebody call RebuildTransform() that will
>    crash because the RebuildTransform() method
>    assumes an "AffineTransform"

Perhaps we should remove "RebuildTransform()" altogether. With the 
assumption that origin and spacing are only defined for an affine image, 
the affine transformation could be rebuilt whenever the origin/spacing are 
set, and at no other time.

> 2) If we change the current state, and Nobody (not even
>     RebuildTransform() initialize the transform... then
>     the image will not have a Transform at all and any
>     call to TransformIndexToPhysicalPoint() will be
>     pointless.

We could initialize the origin to 0xN and the spacing to 1xN in the 
constructor, prior to creating the default affine transform (which would 
use these values). So, if you do nothing at all and start transforming 
points, you at least get something that works.

> I will follow Jim's suggestion that the image should
> not be responsible for managing conversions from
> pixels to world coordinates.   Let the image be a
> grid like container and let some other class take over
> the task of interpreting how this grid maps to physical
> space.

<snip>

> This has the drawback (as Damion pointed out before) that
> conversions that are done now with one line of code will
> require three or four....

Afraid I still stick to this position. I think it's safe to assume the 
following:

1) A default affine transformation with origin 0 and spacing 1 is not "bad" 
- if the user intends something else, they should be smart enough to 
specify it by setting the transform explicitly.

2) If the user does not specify a transform explicity, but instead modifies 
the values of origin and spacing, they intend to modify the default affine 
transformation.

3) If the user specifies some other kind of transform, then the image 
should leave well enough alone and not mess with it.

As long as these assumptions are true, there's no reason to move the 
computations offboard.

Agree/disagree?

-Damion-