Encoded Overlay Planes always have a bit depth of 1, and are encoded separately from the Pixel Data in Overlay Data (60xx,3000). The following two Data Elements shall define the Overlay Plane structure:
Note: There is no Data Element analogous to Bits Stored (0028,0101) since Overlay Planes always have a bit depth of 1.
Note: Restrictions on the allowed values for these Data Elements are defined in PS 3.3. Formerly overlay data stored in unused bits of Pixel Data (7FE0,0010) was described, and these attributes had meaningful values but this usage has been retired. See PS 3.5 2004. For overlays encoded in Overlay Data Element (60xx,3000), Overlay Bits Allocated (60xx,0100) is always 1 and Overlay Bit Position (60xx,0102) is always 0.
For Overlay Data Element (60xx,3000), the Value Representation OW is most often required. The Value Representation OB may also be used for Overlay Data in cases where the Value Representation is explicitly conveyed (see Annex A).
Note: The DICOM default Transfer Syntax (Implicit VR Little Endian) does not explicitly convey Value Representation and therefore the VR of OB may not be used for Pixel Data when using the default Transfer Syntax.
Overlay Data is encoded as the direct concatenation of the bits of a single Overlay Plane, where the first bit of an Overlay Plane is encoded in the least significant bit, immediately followed by the next bit of the Overlay Plane in the next most significant bit. When the Overlay Data crosses a word boundary in the OW case, or a byte boundary in the OB case, it shall continue to be encoded, least significant bit to most significant bit, in the next word, or byte, respectively (see Annex D). For Overlay Data encoded with the Value Representation OW, the byte ordering of the resulting 2-byte words is defined by the Little Endian or Big Endian Transfer Syntaxes negotiated at the Association Establishment (see Annex A).
Note: For Overlay Data encoded with the Value Representation OB, the Overlay Data encoding is unaffected by Little Endian or Big Endian byte ordering.