Mask subtraction is very different in the COM and .NET versions of DicomObjects
In the COM version of DicomObjects, two DicomImage properties associated with the Mask Subtraction operations are sometimes found a bit confusing to developers: Mask & MaskFrame.
This functions as a switch to turn on/off the mask subtraction operation. If Mask Subtraction Sequence (0028, 6100) exists then DicomObjects will sets the default value of Mask to 1, otherwise 0.
This property determines which frame is going to be used for subtraction. The selected frame is subtracted from the chosen display frame (or all frames if a cine mode is in use) before display.
Check the help file for more information about Mask and MaskFrame.
For more information on how to implement shifting of the mask in COM, see Pixel Shifting
Please note that in the .NET version of DicomObjects, Mask Subtraction is far more complex and flexible, being based on the MaskRanges property, which then gives the same degree of flexibility as defined in DICOM Presentation states.