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Computer Storage of Graphics
Graphics may be stored and manipulated by computer in two main ways: vector-based
and bitmap. In vector-based storage images are described mathematically,
e.g. the description of a circle includes its radius, the co-ordinates
of its centre and its colour. In bitmap (also known as raster) storage
information about every pixel (i.e. dot) forming the image is held.
Vector graphic file sizes are generally much smaller than bitmapped graphics.
Vector graphics may also be resized without degradation. A number of multimedia
authoring products for the web, such as Macromedia's Flash (see
5.3.3) and Director employ vector graphics to produce compact files
for fast download. However, vector graphics are unsuitable for the storage
of complex images such photographs which contain irregular shapes and
subtle variations in colour.
Storage for the Web
At present, web browsers display only bitmapped images by default (i.e.
without need for additional plug-ins). The most common formats found on
the web are GIF and JPG, which are described below along with the more
recent PNG format. The TIF format is also described for its role in offline
archival storage. The following information was obtained from Fulton [39]
and PlanMagic Corporation [85].
Compuserve Graphics Interchange Format (GIF)
The GIF format was developed by Compuserve. Images are compressed to facilitate
fast transmission of graphical media over telephone lines. The standard
compression method is a lossless one, i.e. decompression restores the
original data without degradation. Software such as Adobe Photoshop also
provides the option of lossy compression, resulting in smaller file sizes
but with some irrecoverable loss of original data. GIF files can contain
between 2 and 256 colours. The format is most suited to line art and images
containing blocks of the same colour e.g. company logos. GIF 89a files
may contain animation (animated GIFs).
Joint Photographic Experts Group (JPG)
JPG is widely regarded as the best format for photographs on the web.
JPG uses a form of lossy compression. The JPG compression algorithm offers
a choice of compression level allowing the operator to select an acceptable
compromise between quality and download time. Since some quality is lost
every time a JPG is saved JPG files should not be used for editing, instead
the image should be edited in its original form and only saved as a JPG
as a final stage.
Portable Network Graphics (PNG)
PNG was intended to be a replacement for GIF. PNG boasts an impressive
technical specification and supports 24-bit and 48-bit colour. It uses
an improved form of lossless compression similar to that used in PKZIP
and is seen as a likely replacement for TIF.
Tag Image File Format (TIF)
TIF was developed jointly by Microsoft and Aldus as "a portable method
of storing bitmap images", and is supported across a platform range.
TIF is the format of choice for archival, master copies. TIF files may
be compressed or non-compressed. Compressed TIFs use a form of lossless
compression. TIF files are generally very large.
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