.TIFF File Format

Introduction

BigTIFF is a variant of TIFF with 64-bit offsets and counts, so a file can be far larger than the 4 GB limit of classic TIFF. It is used for large scans, whole-slide microscope images, satellite and aerial images and other very large rasters. The tags, data types and compression methods are those of classic TIFF; only the header, the directories and the entries are wider.

BigTIFF files normally keep the .tif or .tiff extension. LibTIFF has read and written them since version 4.0.

Structure

The header is 16 bytes: the byte order mark, II or MM, the number 43, the size of an offset in bytes (always 8), two zero bytes, and the 8-byte offset of the first image file directory (IFD).

An IFD starts with an 8-byte entry count. Each entry is 20 bytes: a 2-byte tag, a 2-byte data type, an 8-byte count and an 8-byte value or offset, so values of up to 8 bytes are stored in the entry itself. The directory ends with the 8-byte offset of the next IFD, or zero. Three data types are added for 64-bit values: LONG8 (16), SLONG8 (17) and IFD8 (18).

Telling it apart

The file starts with 49 49 2B 00 (II, 43) or 4D 4D 00 2B (MM, 43), followed by the offset size 8. A classic TIFF file has 42 in place of 43.

Also used for

• GeoTIFF – A TIFF image with tags that place it on the Earth: maps, satellite images, heights.
• GeoTIFF – A TIFF image with tags that place it on the Earth: maps, satellite images, heights.

Variants

• Tagged Image File Format (classic) – the usual TIFF layout, with 32-bit offsets and files up to 4 GB
• BigTIFF – TIFF with 64-bit offsets, for files larger than 4 GB

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