The explosive growth of the Internet and handheld wireless devices
is widening a large gap between mobile clients. Mobile clients vary
in their hardware and software resources such as screen size, color
depth, effective bandwidth, processing power, and the ability to
handle different data formats. This makes it difficult for servers
to support a wide range of client variations. Applicationlevel
adaptation is required to provide a meaningful Internet experience
across the range of client capabilities.
In our research, we introduce an image transcoding proxy server
placed between the generic WWW servers and the heterogeneous mobile
hosts. The main function of this proxy is to reprocess, on the fly,
the most common image formats used on the web (GIF, JPEG) for quick
transmission of reasonable quality. Common transcoding operations
are scaling, color reduction, and reducing the JPEG quality factor.
Unlike most related work, our proxy is designed to be extremely
flexible. The selection of transcoding operations is based on the
mobile device characteristics and can adapt to dynamically changing
bandwidth on the proxy-client link. The policy decisions are used to
select the most appropriate operations in order to achieve the
smallest file size and acceptable image quality. More aggressive
transcoding operations result in greater loss of image quality.
Based on a user survey, we determined minimal acceptable thresholds
for each transcoding operation. As our results show, these
thresholds are dependent on the display quality of the user device.
