Volume transformation snapping to orientation closest to voxel orientation

[Antmaker (Anthony)]

I’ve noticed this following behavior when I transform a volume then hide-and-unhide the volume, the volume will snap back to the orientation closest to the voxel orientation. This happens even when I harden the transform.

From what I’ve read on the forum, this thread indicates that hardening a transform updates the direction of the axes, and this thread explains that the sliders show a relative transformation (and only one slider can be non-zero at one time).

I don’t quite understand why the transform doesn’t persist when I hide-and-unhide the volume. The hide/unhide feature is a part of my workflow as I assume it works like how layers are hidden/shown in Photoshop or GIMP.

Could you point me in the right direction to understand why hiding/unhiding a volume affects the edits (i.e., transform) made on the volume?

Example 1 — rotate less than 45º

This is a demo data, and I assume this default orientation is the voxel orientation.

If I were to rotate the cross-section plane by less than 45º as shown…

Hiding-and-unhiding the volume would result in the volume snapping back to the original orientation.

Example 2 — rotate greater than 45º

Here I rotated the same demo data by 74.4º along the cross-sectional plane.

Hiding-and-unhiding the volume results in the volume snapping back to an 90º rotation aligning the voxel orientation.

[Antmaker (Anthony)]

I think I found the answer and am going to describe it here for posterity. Please do correct me if my understanding or vocabulary is incorrect.

Conclusion

The transformation (whether hardened or not) changes the direction of axes of the global coordinate. This can be demonstrated by observing how an ROI created pre-transform rotates in the views.
Regarding “snapping to the closest voxel orientation,” it is because I was viewing each of the slice view in the “Reformat” view (as seen at the top of each of the slice viewports).

Some screenshots below to help explain what is going on.

Example 1 — rotating less than 45º

An ROI is created pre-transform with the ROI borders aligning with the current global coordinate axes.

The volume is transformed via a rotation of less than 45º without hardening.

Hide-and-unhide the volume in the “data module” and the ROI that was drawn in the orientations of the global coordinate system is not moved. Even though the volume has “snapped to the voxel orientation,” the rotation actually did happen. Observe that the viewport is in “Reformat” view.

Example 2 — rotate by greater than 45º

An ROI box is created pre-transform.

Transform the volume via rotation of greater than 45º without hardening.

Hide-and-unhide the volume in the “data module” and the ROI has rotated accordingly. However, the volume has “snapped to the closest voxel orientation” but the transform did happen because the ROI box has moved. Observe that the viewport is in “Reformat” view.