TECHNICAL FEATURES OF THE TIFF, PNG, GIF
AND JPEG/SPIFF IMAGE FILE FORMATS
Peter Dowdell
6th December 2001
http://www.stridebird.com
INTRODUCTION
Digital image files have developed from an application-specific realm to one where they
are expected to be shareable across applications and platforms. This requirement for
robust data interchange has led to the development of various universally agreed
standards.
In creating an image file format, two key issues that need to be addressed are file size
and inclusion of associated information.
File size is significant both to reduce storage requirements for a large number of images
and to reduce bandwidth requirements for transmitting images. File size reduction is
achieved by using a suitable compression routine and by controlling the depth of colour
information stored per pixel.
Associated information greatly improves the administration of a large set of images.
Images on their own are very difficult to categorise as their identification generally
demands human intervention. By storing fields such as author, creation date,
description and category within the image file itself, processes for filing and indexing an
image library become possible. In an information distribution system, such as proposed
by the RDN, the categorisation of the library is the key to creating a workable, indexable
system.
Several formats have become well established and each have their particular features
and short-comings in a digital distribution scheme.
JPEG/SPIFF
JPEG (Joint Photographic Experts Group) is a compression process for dealing with
continuous-tone images such as found in real-life photography which was finalised
during the early 1990s. JPEG compression is inherently lossy by nature. This means
that image information is permanently lost upon conversion due to reduction processes
which average and normalise the data. This makes the format unsuitable for storing
original images. The JPEG compression process does not specify a file format, and
needs a wrapper of some sort to make it a workable format. At first, the JFIF (JPEG File
Interchange Format) format was released as a basic way to encapsulate a JPEG
encoded data stream, and this format has become prevalent, particularly on the web
where it is natively supported on all popular browsers but also in application such as
digital photography and image scanning. However, JFIF is a very simple structure that
does not allow for the storage of meaningful associated information. The necessity for a
more flexible storage format for JPEG led to the development of the SPIFF format.
SPIFF (Still Picture Interchange File Format) can store images using many different
compression methods and colour space schemes. When using a JPEG compression,
the SPIFF should be readable by a JPEG compliant reader, gracefully ignoring the
extended features of the SPIFF file.
The SPIFF internal layout contains a directory structure. Standard Entries are defined
for this directory, such as creator, time stamp, gamma correction and much more, and
the concept is extensible with application-specific tags allowed.
GIF
GIF (Graphics Interchange Format) was developed by CompuServe, as a simple format
for storing graphical images. It was created with the web in mind, and is natively
supported by all popular browsers. It is possibly the most prevalent of all in terms of
numbers of files in existence. At its heart is the lossless LZW compression algorithm,
and it is here that one of the shortcomings of the format is found. The algorithm is
patented by the Unisys Corporation, and requires applications that implement the codec
(compression / decompression algorithm) apply for a license.
The original GIF87a header contained the minimal information necessary for displaying
of the image, and did not include comments and associated information. The later
GIF89a version extends the functionality and introduces control extensions for including
associated image information and application-specific settings. Its maximum image size
of 64k x 64k, whilst large enough for most present day applications, could be a
drawback in a library system where larger images are conceivable. And its constraint of
a palette of 256 RGB colours is also a drawback for storing more complex imagery such
as photography, where JPEG is superior.
GIF’s support for interlacing, whereby progressive display improves the user experience
over low-bandwidth connections, and broad support for animation and looping on web
browsers provides a simple way to add interest to a web page.
In the long-term, the GIF format is likely to fall to the wayside because of the licensing
issue. The PNG format has been developed to provide an alternative and uses a nonproprietary algorithm.
PNG
PNG (Portable Network Graphic) format has been developed to provide an alternative to
the GIF, and whilst based upon a similar process it avoids the use of a proprietary
compression algorithm. It is unofficially known as “PNG’s Not GIF”, in the tradition of
recursive acronyms in the Open Source community responsible for its creation. It has
been designed with a far greater range of functionality compared to GIF, for example it
can support images to a massive 2G x 2G size, various colour spaces with up to 48-bit
colour depth and support for alpha channels, built-in data integrity checks and a more
sophisticated progressive display function using the Adam7 technique.
Its drawback compared to GIF is that it does not support animation and looping because
it allows only a single image to be stored in the file. There is development underway to
produce the MNG (Multiple Image Network Graphic) to provide this in a non-proprietary
format.
TIFF (TIFF 6.0)
TIFF (Tagged Interchange File Format), developed since 1986, is a widely-supported,
highly versatile format for storing and sharing images. It is utilised in many fax
applications and is widespread as a scanning output format. Its versatility makes it a
great choice for use as a general purpose library and archiving format, but lack of native
support for TIFF in web browsers makes it less suitable for use on the web, requiring the
use of a plug-in or conversion to a web-friendly file type.
TIFF will store multiple images within one file, and will support many compression
methods (including JPEG) and colourspace schemes. Maximum file size is capped at
4Gb. The tagged structure of the file is very flexible, and allows great scope for adding
all manor of custom information within the file.
Its versatile nature can be problematic, with issues such as large strip sizes (a
contiguous portion of the image) overflowing memory allocation, and byte ordering.
However, these can all be avoided with good, robust programming practice and
awareness of the pitfalls.
SUMMARY
The features of the formats discussed in this document are summarised in the following
tables:
COMPARISON OF MAIN FILE FEATURES
Name
Current version
Common extensions
Colour depth
Colour space
Compression
Progressive display
Maximum size (pixels)
Animation
SPIFF
ISO/IEC 10918-3
.spf, .jpg
Bi-tonal to 32bit
RGB
YcbCr - various types
CMY & CMYK
YCCK & photo YCCK
CIE L*a*b
JPEG
Mod Huffman
MR, MMR
JBIG
Yes, for JPEG
4G x 4G (tiled)
64K x 64K
No
GIF
89a
.gif
24bit
Palette RGB
PNG
1.2
.png
Bi-tonal to 48 bit
Palette
RGB
SRGB
ICC
LZW
LZ77 – deflate
Yes, 8 row interlacing
64K x 64K
Yes, Adam7
2G x 2G
TIFF
6.0
.tif, .tiff
Bi-tonal to 24 bit
Palette
RGB
YCbCr
CMYK
CIE L*a*b
Uncompressed
ITU-T6
LZW
JPEG
Possible (JPEG, LZW)
232 - 1
Yes
No
No
Ranked in increasing order of suitability
Future proof : GIF  SPIFF  PNG  TIFF
Web usability : TIFF  SPIFF  PNG  GIF
Extensibility / programmability : GIF  SPIFF  PNG  TIFF