fix(planar): map dynamic-volume flattened indices to their group-local slice (#2801)

getVolumeImageIdIndexWorldPoint assumed a one-to-one imageId to k-slice
mapping, but StreamingDynamicImageVolume flattens its imageIds across
all dimension groups while dimensions[2] is the slice count of a single
group. A referenced image or explicitly carried initial slice from any
later dimension group therefore had a flattened index of at least
dimensions[2], and the clamp sent every such image to the final slice.
Convert the flattened index through the volume's own
flatImageIdIndexToImageIdIndex mapping (duck-typed; static volumes have
no such method and pass through unchanged) before resolving the IJK
world center.

Author: sedghi

packages/core/src/RenderingEngine/GenericViewport/Planar/planarSliceBasis.ts

@@ -344,7 +344,21 @@ export function getVolumeImageIdIndexWorldPoint(
   }
 
   const dimensions = imageData.getDimensions();
-  const k = Math.min(Math.max(0, imageIdIndex), dimensions[2] - 1);
+  // Dynamic (4D) volumes flatten their imageIds across dimension groups while
+  // dimensions[2] is the slice count of a SINGLE group, so an image from any
+  // later group has a flattened index >= dimensions[2] — clamping that raw
+  // index would pin every such image to the final slice. Map it to its
+  // group-local slice first (the k axis repeats per group).
+  const flatToGroupLocal = (
+    imageVolume as {
+      flatImageIdIndexToImageIdIndex?: (flatImageIdIndex: number) => number;
+    }
+  ).flatImageIdIndexToImageIdIndex;
+  const groupLocalIndex =
+    typeof flatToGroupLocal === 'function'
+      ? flatToGroupLocal.call(imageVolume, Math.max(0, imageIdIndex))
+      : imageIdIndex;
+  const k = Math.min(Math.max(0, groupLocalIndex), dimensions[2] - 1);
 
   return imageData.indexToWorld([
     (dimensions[0] - 1) / 2,

packages/core/test/planarSliceBasis.jest.js

@@ -851,6 +851,28 @@ describe('resolvePlanarVolumeImageIdIndex', () => {
 });
 
 describe('getVolumeImageIdIndexWorldPoint', () => {
+  it('maps a flattened dynamic-volume index to its group-local slice', () => {
+    // A 4D volume flattens its imageIds across dimension groups (here 3
+    // groups of 6 slices) while dimensions[2] stays the per-group slice
+    // count, and exposes the flat -> group-local mapping.
+    const volume = createOrthonormalVolume();
+
+    volume.flatImageIdIndexToImageIdIndex = (flatImageIdIndex) =>
+      flatImageIdIndex % 6;
+
+    // Flattened index 14 = group 3, local slice 2 — NOT the last slice (5)
+    // that a raw clamp against dimensions[2] - 1 would produce.
+    expectPoint3Close(
+      getVolumeImageIdIndexWorldPoint(volume, 14),
+      [102.25, 203.5, 304]
+    );
+    // Indices inside the first group pass through the mapping unchanged.
+    expectPoint3Close(
+      getVolumeImageIdIndexWorldPoint(volume, 2),
+      [102.25, 203.5, 304]
+    );
+  });
+
   it('returns the exact IJK slice center for an imageId-list index', () => {
     const volume = createOrthonormalVolume();