.TIFF File Format

Introduction

Classic TIFF is the layout described by the TIFF 6.0 specification of 1992, and the one almost every .tif and .tiff file uses. Its offsets are 32-bit, which limits a file to 4 GB; larger images use BigTIFF.

A file can hold several pages, any of a long list of compression methods, high bit depths and many colour spaces, all described by numbered tags.

Structure

A TIFF file starts with an 8-byte header. The first two bytes give the byte order of the whole file: II for little-endian (Intel) or MM for big-endian (Motorola). The number 42 follows, then the offset of the first image file directory (IFD).

An IFD is a count of entries, then 12 bytes per entry, then the offset of the next IFD (zero for the last). Each entry is a tag number, a data type, a count and either the value itself, when it fits in 4 bytes, or the offset where the value is stored. Tags give the width, height, bits per sample, compression and the offsets and lengths of the strips or tiles that hold the pixels. Each page of a multi-page file has its own IFD.

Telling it apart

The file starts with 49 49 2A 00 (II, 42) or 4D 4D 00 2A (MM, 42). A BigTIFF file has 43 in place of 42.

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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