There are more issues like the interpolated slice is completely overriden, even if content of other labels are already present in the slice. In contrast, AcceptAllInterpolations() works in all conditions so we should probably also implement OnAcceptInterpolationClicked() in terms of AcceptAllInterpolations(), resp. refactor it into a common method that can be used by both single and all slices interpolation.

Definetly the interpolator has to rework and the different accept function refactored/streamlined.

To the problem of the task. I guess the results of the InterpolationController are always binary, so have the pixel value 1.

QmitkSlicesInterpolator::Interpolate line 848ff directly copies the binary result into m_FeedbackNode.

OnAcceptInterpolationClicked (line 932) directly uses this image to modify the segmentation. Therefore always 1s are added to the segmentation and only Label 1 is modified.

The whole boiler plate code should be removed and static SegTool2D::WriteBackSegmentationResult(...) should be used. Before that it has to be ensured that the FeedbackNode content has been changed to the right pixel value. E.G. with TransferLabelImageContent mapping from 1 to the active label value.

Damn... if I just would have more time...
May be it helps you

I have added a fix for this in D810. It only includes a fix to OnAcceptInterpolationClicked and does not refactor AcceptAllInterpolations, which is still quite clunky. But this should at least solve the buggy behavior.