Negative index ?

Beginner Questions
1

Hello,

I’d like to set the position (0,0) at the center of an image. That would imply using negative indexes.

Is it possible (that could be: the [] operator may be overloaded) ?

Thanks.

2

ITK does support negative indices - indices are signed, sizes are unsigned. But that is not necessary - you can set origin to negative values to have your image centered on the origin of the physical coordinate system.

3

Origin corresponds to the position of voxel with index (0,0,0).

4

It isn’t related to physical coordinate system.
I want to run an algorithm from the center, and it would be simple to understand in the code if the origin (in pixel) is at the center.

5

You can accomplish that with something like this:

auto image0 = ImageType::New();
auto origRegion = image->GetLargestPossibleRegion();
image0.SetRegions(origRegion);
image0.CopyInformation(image);
image0.SetPixelContainer(image->GetPixelContainer)  // shallow copy pixel buffer

// now adjust it so center index is 0
ImageType::IndexType centerIndex = ...; // half the size
auto centerPoint = image->TransformIndexToPhysicalPoint(centerIndex);
auto region = origRegion;
centerIndex *= -1;
region.SetIndex(centerIndex);
image0->SetRegions(region);
image0->SetOrigin(centerPoint);

For some operations (e.g. halving the size, inverting center index coordinates) you will probably have to do it in a for loop over dimension, the above will probably not compile.

2 Likes
6

Well…
As I am beginning with ITK, maybe I’ll avoid this hack for now. Thanks.

1 Like
7

This sounds amazing can you pls share more details about this?