BOOT CAMP 461 (30/01/07)
Multimedia Formats and Codecs part 2
In part two of this short series on multimedia
files we turn our attention to image and graphic formats and codecs. There are
dozens of them but it’s not as bad as it sounds. Most are proprietary formats,
tied in with image editing programs and specialised applications and only a
small handful file types are of interest to PC and digital camera users.
By far the most popular image format is JPEG
(extension .jpg or .jpeg) and this is the one you are most likely to come
across if you own a digital camera or use pictures to illustrate documents,
pdfs, web pages and so on. For the record JPEG stands for Joint Photographic
Expert Group; it was set up by the International Standards Organisation (ISO)
in 1982 and the technical standard it created (ISO 10918-1) was ratified by the ISO
JPEG is a compressed or ‘lossy’ system that
reduces the size of picture files by discarding repetitive data and information
that the eye cannot perceive or doesn’t notice. It is based on the principle
that we are much more sensitive to variations in brightness than colour, so it
is possible to reduce the amount of colour information in an image file without
significant loss of quality. JPEG compression uses 24-bit coding (8 bits for
each primary colour), which basically means it is capable of representing 16.7
million colours (also known as Truecolor’) and is roughly the same number of
colours that the human eye can distinguish. Extra data about the image can also
be embedded in JPEG files, see this week’s Top Tip.
The amount of compression applied to a JPEG
file or image can be varied but as it is increased so too do the number of
defects or ‘artefacts’ and highly compressed images become increasingly
‘blocky’. However, minor artefacts tend
not to show up show up in photographs that contain a lot of non-uniform texture
and fine detail. Image quality also suffers when JPEGs are edited and
repeatedly re-saved and it is not suitable for line drawings and graphic images
as processing artefacts can result in blurring and fuzzy edges.
Bitmap (extension .bmp) is an elderly ‘raster
graphics’ file format, nowadays mostly used by the Windows operating system
for, amongst other things, background desktop images, splash screens and icons.
Some scanners also have it as the default output file type. Bitmaps are
uncompressed so it is a ‘lossless’ format, capable of very high quality images
with 2 (1-bit), 16 (4-bit), 256 (8.bit), 65,535 (16-bit) or 16.7 million
(24-bit) colours but files tend to be very large. Windows Paint and most image
editing programs can handle bitmap files but in the wider world it tends not to
be very popular due to size of the files, which can be slow to copy and transfer.
GIF or the Graphics Interchange Format
(extension .gif) was developed by the pioneering Internet Service Provider
CompuServe back in 1987 and is the standard file format for web page graphics.
It can only handle up 256 (8-bit) colours so it isn’t much use for photographs
but it is very good at rendering drawings, diagrams and animated graphics and
because data is highly compressed, file sizes can be very small. Problems over
patents and licensing dogged the GIF format for many years. Several alternatives
have been developed, the most popular being PNG or Portable Network Graphics
(extension .png). In addition to being more flexible and capable of better
quality PNG files sizes are even smaller than equivalent GIFs and it is rapidly
growing in popularity.
TIFF or Tagged Image File Format or TIFF
(extension .tff), developed in the early 1990s by Aldus (now owned by Adobe) is
mostly used for storing and processing images in high-end picture editing and
publishing applications like PhotoShop and Quark. It’s also used on scanners
and some digital cameras. It’s a flexible and lossless raster graphics format
with file sizes falling somewhere between bitmaps and JPEGs but the key
benefits are that TIFFs contain embedded technical data or ‘tags’ about the image
and files can be repeatedly edited and saved without any loss of quality.
Some mid-range and most high-end digital
cameras have the option to save or export images in ‘RAW’ format. In fact this
isn’t a format as such, or at least not a single format but a way of storing or
copying picture files without any compression, to ensure maximum quality.
However, there is no set standard and each camera manufacturer has its own way
of processing the data so specialist software is needed to view the image and convert
it to one of the mainstream image formats.
Next Week -- Multimedia Formats part 3
(or coder - decoder) file system, software utility or program add-on used to
process digital data
Portable Document Format
- web-friendly, cross-platform format for encoding documents and graphics,
normally viewed using free Adobe Acrobat Reader program
A means of representing an image using digital data, by describing the
colour of each individual picture element or ‘pixel’/
Most digital cameras attach a small data file, called Exif
(Exchangeable Image File), to JPEG and RAW image files when they are saved or
exported to a PC. These are similar to the data ‘tags’ in TIFFs. It is up to the
manufacturer to decide how to use the
‘tags’ but normally they include the make and model of the camera, the
time and date the photograph was shot plus a variety of camera settings, such
as aperture, focal length, film speed, shutter speed, whether or not flash was
used, AE mode a thumbnail or preview image plus titles and copyright details.
Cameras equipped with GPS receivers can use Exif tags to record information
about where the picture was taken.
© R. Maybury 2006, 2401