Tagged Image File Format

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The Tagged Image File Format (TIFF or TIF for short ) is a file format for storing image data. It was originally (acquired in 1994 by Adobe) by Aldus and Microsoft developed for color separation from scanned raster images. TIFF files have the MIME type image / tiff.

Use

TIFF is, in addition to PDF and EPS, an important format for exchange of data in the pre-press in publishing and printing, because it supports the CMYK color model used by them. In addition, you can TIFF images with high color depth ( up to 32 bits per color component ) store, which is why TIFF is often used to exchange data in the RAW conversion.

On the Internet, TIFF is used to provide users such as publishers, high-resolution images in printable, lossless quality. It is accepted that these files have a multiple of the size of a lossy-compressed JPEG image. TIFF has thus established itself as the de facto standard for high-quality images.

Established itself more and more one provided with additional tags TIFF variant, the so-called GeoTIFF For the area of ​​raster-based geographic information. It allows, for example in card images, aerial photographs and other similar information, specify where the situation depicted in the picture is exactly the coordinates relative to the earth.

TIFF is also used to archive monochrome graphics (eg technical drawings ) as " 4 Fax Group" compression arising in connection with the very compact files.

To facilitate their work the print media, high resolution images in addition to the JPEG format used by some organizations, such as ESA or NASA, sometimes also available in TIFF, while at low resolutions usually comes only the lossy JPEG format for use.

Properties

The coding of numbers ( byte order ) can be carried out either as big endian or little endian. In one file, several images can be stored ( multi-page TIFF). These may be different versions of the same image, for example, as the preview image ( " thumbnail " ) and the original image. TIFF knows different color spaces and algorithms for data compression. Most of them are lossless (eg LZW, Run Length Encoding ), but TIFF can also serve as a container format for JPEG images, which (DCT ) may be lossy compressed. It is also possible to embed IPTC metadata in the TIFF file.

Individual pixels can exist at TIFF from any number of single values ​​(samples ). Samples, in addition to the standard case " a byte equal to a sample " also parts of a byte drugs (eg, 1, 2 or 4 bits) or consist of multiple bytes. In addition to integers, floating point numbers also can be stored as image data. The ability to store transparency information (alpha channel) also exists.

Image data into groups of rows of pixels, called Stripes ( stripes), or as rectangular tiles (tiles ) are stored. Storage is independent of the others, so that parts of the image, depending on the choice of the size of the strips or tiles, can be loaded relatively quickly for each strip or for each tile. Other formats require you to load all of the image data prior to the desired section. The aim of the subdivision in the design was mainly that items can be stored completely in memory. The 1992- specification recommends for eight kilobyte maximum size.

Programs such as Photoshop offer to create TIFF files with separate levels. There is also the possibility to save TIFFs with an image pyramid. This file contained within a multi-resolution of the image. In order to view a small preview of the image faster, since they do not need to load the full resolution of the image, for example layout programs or image viewer.

Limitations

The greatest disadvantage of TIFF is its complexity. The variety of possible valid TIFF files can be supported only with difficulty by individual programs. In the specification of the file format so a subset of valid TIFF files is defined that can process any TIFF -enabled program should be called Baseline TIFF.

For 32-bit offset values ​​are always used. This means that only points can be referenced up to four gigabytes of file beginning. At the time of development of the TIFF introduced in practice is not a limitation noticed lately, however, particularly in some disciplines (eg, astronomy) very large images that TIFF can not save. A streaming of TIFF files is not in all cases without buffering the entire file possible because TIFF does at many points of offset values ​​use, can reference the data that are the point at which they are referenced. Thus, random access buffering in memory or just a necessity. For this reason, TIFF is also unsuitable for the WWW. Adobe has (unlike his document format PDF) never added at TIFF later a method of linearization, in the so converted file ensures an order that makes recourse unnecessary.

Basic structure

The first eight bytes of a TIFF file containing a signature that contains the byte order and the Magic Number 42, also an offset for the first Image File Directory ( IFD).

Such IFD find information about an image in the TIFF file and consists of a list of labels (English: "tags" ), individual information units. Such a marking can for example describe the width of the image in pixels, or the name of the software that the TIFF file has been generated.

Each tag has its own identification number (eg 256 for the image width) and a type ( such as 16- bit integers, 32 -bit floating-point numbers, strings, and more. ). Some of these tags must be present ( for example, the image width), while others are optional (eg the name of the software). A number of applications use proprietary tags. The structure of the data that is stored or referenced in such a tag is then usually not documented. A number of own proprietary tags you can apply at Adobe.

At the end of the IFD is an offset value that the next IFD referenced in the file, or to 0 if the current IFD was the last. In this way, any number of images are stored in the file as long as the total amount of data does not exceed four gigabytes (see restrictions above).

The IFD structure is reused in the Exif part of JFIF files.

Specification

A detailed description of the format for developers is provided free by Adobe as a PDF file. The latest version is 6.0 dated 3 June 1992. It is supplemented by TIFF Technical Notes. These are texts which add TIFF individual skills, including the Deflate method for lossless data compression that is already being used in gzip and PNG.