International Archives of the Photogrammetry, Remote Sensing
into different tiles and we only serve up the tiles that are
necessary to make that part of the image appear.
Base layer
Figure 3.1. Image Pyramid
4. Implementation of the System
Working with the pyramid on the server side, GSN client is
easily to be designed. The easiest and highest performance
solution is to keep an image pyramid which is the same as the
pyramid on the server side.
Firstly, GSN builds an image pyramid on the server side for a
large high-resolution image. Some information is needed to
notice GSN client: the original image resolution, the bounding
box of the image, the maximum pyramid level and the minimal
pyramid level. On the same time, every pyramid image in
different level needs to be cut to small image partitions
(128X128, 256X256 or more) In GSN system, they are
represented by Image Objects.
Secondly, an image pool is needed to store images which have
been downloaded from server. This pool acts as a cache of your
web browsers or windows file systems. In fact, it is not only a
cache which serves GSN client, but also an image server on the
client side. Instead communicating with GSN server, GSN
client send image requests to this image pool. The image pool
will transfer and store image partitions to the client side. While
the client request images, it will search the image pool first, if
image is in pool, it will send the image partitions requested to
display module. Otherwise, it will forward requests from
: Transmit
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and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
display module to the server if the requested images are
unavailable from the pool.
Finally, an overview image needs to be stored in GSN client. It
is important to generate image overviews so that users can
familiarize themselves with large-scale imagery features.
Getting lost in the trees really can happen when viewing
forested images. Having an overview image makes viewing a
large image much the same as viewing a map. Users can
quickly identify roads, landmarks and water bodies in
overviews, then zoom in on regions of interest. Without an
overview, or at least geographically coordinated displays, most
users quickly become frustrated with orientation. Additionally,
an overview image can help image pool response users' request
as soon as possible. Users' operation is usually unpredictable.
Therefore, it will easily happen that the images requested by
users are not in image pool. In order to present map smoothly
without “black” screen, GSN image pool will return images re-
sampled from overview image immediately to display module.
The users’ operation will be seamless.
It looks like two modules are needed for expand GSN client.
Actually, the GSN client should be divided to four modules and
each module itself must be a thread. They run concurrently but
without interrupting and communicating with each other.
e Display Module
This module has four features: response to all users’
operations from both mouse and keyboard, drawing map
or images to users’ screen, all APIs related with map
presentation, and all APIs about map analysis.
e Image Pool Module
This module has three features: process users’ requests,
store images loaded from server and manage pool.
e Draw Image Object Module
It will draw Image Object on users’ screen. In this paper,
Image Object refers to basic elements which consist of
image pyramid.
e Image Streaming Transmission Module
This module communicates with GSN server and gets
GSN client required Image Objects. It can transmit Image
Objects either serially or concurrently.
Operate map
Retum
image from
pool
immediatel
Draw map to
screen
Figure 4.1. Improved GSN client structure
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