[Bug] createSlabThicknessSynchronizer initial slice index issue

[salkz]

Describe the Bug

We noticed a peculiar issue with createSlabThicknessSynchronizer - if you add this synchronizer to a viewport then the initial loaded slice index will be some very weird number instead of the usual middle slice.

Steps to Reproduce

Adding synchronizer to a viewport immediately on a VIEWPORT_DATA_CHANGED event it works unexpectedly:

const synchronizerId = 'SLAB_THICKNESS_SYNCHRONIZER_ID';
const synchronizer = synchronizers.createSlabThicknessSynchronizer(synchronizerId);
const renderingEngineId = 'OHIFCornerstoneRenderingEngine';

...

useEffect(() => {
  cornerstoneViewportService.subscribe(
    cornerstoneViewportService.EVENTS.VIEWPORT_DATA_CHANGED,
    event => {
      synchronizer.add({
        renderingEngineId,
        viewportId: event.viewportId,
      });
    }
  );
}, []);

However, when added with a setTimeout(..., 2000), it works as expected:

useEffect(() => {
  cornerstoneViewportService.subscribe(
    cornerstoneViewportService.EVENTS.VIEWPORT_DATA_CHANGED,
    event => {
      setTimeout(() => {
        synchronizer.add({
          renderingEngineId,
          viewportId: event.viewportId,
        });
      }, 2000);
    }
  );
}, []);

The current behavior

The initial slice indexes are some weird number as seen on the image:

The expected behavior

The initial slice indexes are for the middle slices in all viewports.

OS

macOS 14.5 (23F79)

Node version

v18.20.4

Browser

Version 130.0.6723.70 (Official Build) (arm64)

[vlasn]

After experiencing very similar issues in one of our workflows that programmatically removes & restores viewports,
I was able to reproduce the issue (or something extremely similar) with the currently live OHIF version (
3.9.1, commit hash b417063) by

1. Navigating HERE
2. Executing the below code block in the browser console
3. Resizing the window ever so slightly

Repro code:

((cvs) => {
    const firstViewportID = cvs.getViewportIds()[0];
    const firstViewport = cvs.getCornerstoneViewport(firstViewportID);
    firstViewport.setSlabThickness(5);
    firstViewport.render()
})(services.cornerstoneViewportService)

Expected behaviour: Viewports get rescaled according to new window width, mostly retain camera configs & the currently scrolled positions
Current behaviour: The (axial) viewport that I updated slice thicknesses in jumped to a different location in the volume

Notes:

• no synchronizers are involved in the repro
• if I destroy all implicit synchronizers (mpr, sameFORId) via cornerstoneTools.SynchronizerManager.destroy(id), the issue still persists
• We later also found a no-code repro by
  • activating crosshairs
  • using one of the inner handles to change a viewport's slice thickness
  • resizing the window

[vlasn]

After some binary search with your published docker images, it looks like the issue was introduced in v3.9.0-beta.78

[sedghi]

I pushed a fix, but the real solution is to recompute the image index when the slab thickness changes. We shouldn't keep the old slice numbers when the slab changes; instead, we should recalculate the number of slices after the slab thickness changes.