Image Server, version #6 ( #5 < #7 > ... #12)

Created by georg. Last edited by georg, 3 years and 327 days ago.. Viewed 59,782 times. #6

General

The Image Server is a CGI-script that provides pictures in various dimensions.

Features

request with arbitrary bounding box
original picture ratio is maintained
delivering and caching of pictures in dimensions of a certain raster.
picture fits into bounding box (no dimension will be larger, but can be smaller)
picture is just scaled down
allow browser side caching, even for different bounding boxes (using redirection)
support extern storage of scaled pictures to allow mirroring. (redirection map)
support explicit precaching (to be copied to a mirror)

Specification

Behaviour

Request

A picture is requested with an picture name and a bounding box. The bounding box specifies the maximal size of the image. The Image Server has a configured raster in which it provides pictures. Lets assume the raster is set to 50 pixel. The picture size is scaled down to fit in the bounding box and then the width is rounded down multiple of 50. (The height is obviously scaled down as well) The needed version of the picture is looked up in the cache and is generated if needed. The file name contains the dimensions and the request is answered using redirection to that file. That allows browser side caching even for slightly different bounding boxes.

Cache

The cache is an important part of the Image Server since it can reduce the server side load and the latency time. The cache has a space limit and the caching stategy is LRU (least recently used).

Mirrors

A mirror should keep pictures in various sizes to save bandwidth or to gain faster access. It is also handy to be able to store the pictures on cheap webspace without CGI support. The Image Server has a mirror map where every mirror has a list of files it has. Furthermore there could be some information about the bandwith the mirror. If a file is mirrored on more than one mirror is redirected with a probability ratio that correspond to the bandwith ratio.

Interface

CGI

The request URL is:

http://<host>/<path>/imgserv.cgi?src=<image src>&width=<width>&height=<height>

Where <image src> is a complete URL or relative path identifier of the image to request. If the normal request URL of an image would be "images/1.jpg" than the <image src> is "images/1.jpg" too. <width> and <height> are integers greater zero and denote the dimension of the requested image in pixel. Other units are not planed.

Examples:

http://<host>/<path>/imgserv.cgi?src=images/1.jpg&width=800&height=600
http://<host>/<path>/imgserv.cgi?src=http://<otherhost>/album1/slides/image1.jpg&width=800&height=600

Configuration

TODO: describe the configuration file syntax and options

Design

Raster

We introduced a raster in which the images are scaled. This prevends the cache of filling up with various fine grained scaled versions of the same image. The raster width is an configuration option which belongs to the caching group.

This option must be fixed over an reasonable lifetime of the server. The client shouldn't get different scaled images for the same requested dimension. In particular, the server is not allowed to serve a "good matching" version of an images which is a cache hit.

The dimension to serv is calculeted this way:

/*pseudo java code*/int calcWidth;  //bounding box fitting width
int calcHeight; //bounding box fitting height
int servWidth;  //width to serve
int servHeight; //height to serve
int raster;     //width of the raster
servWidth = (calcWidth / raster) * raster; //integer division
servHeight = (calcHeight / raster) * raster;