TIFFIFD

In TIFF, an Image File Directory (IFD) is a data structure that stores metadata tags describing an image and pointers to the image data. A TIFF file may contain multiple IFDs, each representing an image or a subfile.

An IFD starts with a 2-byte count of directory entries, followed by that many 12-byte directory entries. Each entry has Tag (2 bytes), Type (2 bytes), Count (4 bytes), Value or Offset (4 bytes). After the entries, a 4-byte offset to the next IFD is stored; 0 means none. Endianness is determined by the first two bytes of the file header (II or MM). If a field’s value fits in 4 bytes, the Value field holds it; otherwise the data is stored elsewhere and the Value field contains the offset.

Tags and data: Directory entries specify metadata like image dimensions, color format, compression, tile/strip offsets, and resolution. Common tags include ImageWidth (Tag 256), ImageLength (257), BitsPerSample (258), Compression (259), PhotometricInterpretation (262), StripOffsets (273), RowsPerStrip (278), StripByteCounts (279). Additional strands include XResolution, YResolution, ResolutionUnit, and date or software.

Usage: TIFF readers parse the IFD to obtain image metadata and to locate the image data. Multiple IFDs enable multi-page TIFFs and Exif sub-IFDs in modern metadata sets. BigTIFF extension expands sizes to 64-bit.

Note: "TIFFIFD" is commonly used in libraries to refer to the image file directory data structure, not a separate format. The IFD concept is central to TIFF's flexibility and wide compatibility.