Prerequisites
Volumes to be fused need to start as different image sets in the same SIS file
The volumes must be of the same bit depth
While volume fusion can work with images of different dimensions, the images must be of the same bit depth. Bearing in mind that higher bit depths lead to larger file sizes, it may be preferable to scale down higher bit depth images to match a lower bit depth volume than the other way around. Bit depth scaling is done through the Transformation Gallery (Data> Transformation Gallery).
If both images have the same bit depth no scaling is necessary.
Calibrations need to be correct for the sample
Registering using landmarks considers the image spatial calibrations to work out the scaling required (registering using surface transformations ignores calibrations in favor of the user-defined scaling). This is usually not an issue when fusing images captured using the same microscope and the same magnification. However, if the two volumes are imaged using different imaging devices the calibrations could be different and this could lead to the fused volumes having a scaling offset. Also, when doing multimodal imaging where the sample may receive additional preparation steps for the second acquisition, the sample can shrink or grow resulting in a calibration that is technically correct, but effectively wrong. In such cases, we can either correct the calibrations to account for changes in the sample or use the surface transformation to disregard the calibration and adjust the scaling as required.