The 3D Tools (e.g. Threshold, FastMarching 3D, Region Growing 3D) compute a preview for the current settings. The preview can be confirmed and written into the slice (slices for 4D segmentations).
This will overwrite any existing segmentation mask - also if this mask was e.g. created by the simple Add-tool or even the same tool (e.g. using Threshold twice).
So in contrast to the 2D tools it is not possible to add up / combine different segmented patches one after another, like it can be easily done with the 2D tools.
This is especially problematic for static 4D segmentations: here you can not use the 3D tools (e.g. Threshold) to segment a slice and add another segmentation on the next timestep - it will overwrite the segmentation from the previous timestep.
To reproduce:
- load a 3D image, open the segmentation plugin, create a new segmentation (labelsetimage)
- use the Threshold-tool to create a segmentation mask
- confirm the preview and see how the segmentation is written into the slice
- use e.g. the UL Threshold to create another segmentation mask
- confirm the preview and see how the new segmentation is written into the slice and the previous mask is completely removed
- load a 4D image, open the multilabel-segmentation plugin, create a new STATIC segmentation (labelsetimage)
- create a new label on the first layer
- create another label on the first layer (distinct color)
- use the 2D Add-tool to create a segmentation mask for the second label
- use the 3D Threshold-tool to create a segmentation mask for the first label
- see how the new segmentation is written into the slice and the previous mask is completely removed
- change the timestep
- see how the threshold segmentation mask does not fit the underlying image anymore that well
- use the 3D Threshold-tool to create a segmentation mask for the first label on the new timestep
- see how the new segmentation is written into the slice and the previous mask is completely removed
- change back to the previous timestep
- see how the threshold segmentation mask does not fit the underlying image anymore that well
There is a checkbox "Create as new segmentation" but this will only allow to create a new segmentation node and prevent overwriting - but it does not change the behavior when it comes to "combining masks one after another".