TIFF Tag NewSubfileType
|Code||254 (hex 0x00FE)|
A general indication of the kind of data contained in this subfile.
Replaces the old SubfileType field, due to limitations in the definition of that field.
NewSubfileType is mainly useful when there are multiple subfiles in a single TIFF file.
This field is made up of a set of 32 flag bits. Unused bits are expected to be 0. Bit 0 is the low-order bit. Currently defined values are:
Bit 0 is 1 if the image is a reduced-resolution version of another image in this TIFF file; else the bit is 0.
Bit 1 is 1 if the image is a single page of a multi-page image (see the PageNumber field description); else the bit is 0.
Bit 2 is 1 if the image defines a transparency mask for another image in this TIFF file. The PhotometricInterpretation value must be 4, designating a transparency mask.
The specification thus says that these values are defined as bit flags because they are independent of each other. Not all combinations make sense though, it is not entirely clear how an image can be e.g. both a reduced-resolution version and a single page of a multi-page image, or any combination of the above.
The Mixed Raster Content part of RFC 2301 adds a definition for this bit:
Bit 4 indicates MRC imaging model as described in ITU-T Recommendation T.44 [T.44], and must be set for Primary IFDs and all MRC-specific child IFDs. (See RFC 2301, and the ImageLayer tag.)
The DNG specification adds a definition for these bits:
Bit 3 is 1 if the image defines a depth map for another image in this TIFF file.
Bit 4 is 1 if the image contains enhanced image data.
The DNG specification mentions that bits 2 (transparency mask) and 3 (depth map) can be combined with bit 0 (reduced-resolution version).
LibTiff defines these values:
FILETYPE_REDUCEDIMAGE = 1;
FILETYPE_PAGE = 2;
FILETYPE_MASK = 4;
RFC 2301 - File Format for Internet Fax