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1 # A short introduction to digilib
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2
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3 ## digilib -- how does it work?
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4
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5 The image server digilib is a state-less web-based client-server
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6 application for interactive viewing and manipulation of images.
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7
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8 ## Frontend and Scaler server
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9
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10 digilib consists mainly of two parts, the image server component proper,
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11 called "Scaler" and a client-side part that runs in the users web
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12 browser.
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13
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14 
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15
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16 The users browser sends an HTTP request for a certain (zoomed, scaled,
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17 rotated) image to the Scaler server and the server returns the image
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18 data as HTTP response.
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19
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20 To complete the schematics of figure 1 we must also take into account
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21 that the client-side part consisting of HTML and Javascript code has
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22 also been requested and loaded from a frontend-web server into the users
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23 browser.
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24
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25 
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26
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27 To date there are several frontend implementations for digilib like the
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28 current "jquery" version that only requires static HTML and Javascript and the
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29 older "greyskin" version (grey buttons, implemented in JSP) that come with
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30 the default [digilib distribution][1] or the "[Zogilib][2]" frontend version
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31 implemented in ZOPE.
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32
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33 The frontend-server and the Scaler-server do not have to run
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34 on the same machine and often there are several frontends that use the
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35 same Scaler server.
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36
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37 ### Request formats
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38
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39 The format of the HTTP requests for the frontend and the
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40 Scaler server can be completely different. The HTML and Javascript code
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41 of the frontend just has to generate the correct request for the Scaler
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42 to produce the desired image. Most of the current frontend implementations
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43 share some or all of the parameters with the Scaler API.
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44
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45 ### Scaler
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46
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47 The [Scaler API](scaler-api.html) is documented on the digilib.berlios.de
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48 pages. Here is a minimal version:
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49
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50 The Scaler servlet takes parameters in the HTTP request format:
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51 `Scaler/request\_path/?param1=value1¶m2=value2&...` Unknown parameters
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52 will be silently ignored.
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53
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54 Recognised parameters:
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55
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56 - `request_path` path to file or directory.
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57 - `fn` path to file or directory. This path will be added to the
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58 `request_path` behind the servlet name. Either parameter can be
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59 empty. All paths are relative to the configured base directory
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60 (digilib-config parameter `basedir-list`).
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61 - `pn` page number. Index into the (alphabetically sorted)
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62 directory given by the path. Starts with 1. Ignored if the path
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63 points to a file. Default: 1.
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64 - `dw` destination image width (pixels). If omitted the image is
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65 scaled to fit `dh.`
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66 - `dh` destination image height (pixels). If omitted the image
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67 is scaled to fit `dw`.
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68 - `wx` relative x offset of the image area to be sent (0 <=
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69 `wx` <= 1). Default: 0.
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70 - `wy` relative y offset of the image area to be sent (0 <=
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71 `wy` <= 1). Default: 0.
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72 - `ww` relative width of the image area to be sent (0 <= `ww`
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73 <= 1). Default: 1.
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74 - `wh` relative height of the image area to be sent (0 <= `wh`
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75 <= 1). Default: 1.
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76 - The image to be loaded can be specified by the `request_path`
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77 (deprecated) or the `fn` (preferred) parameter and the optional
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78 index `pn`
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79 - if `fn` points to a directory then the file with the index `pn`
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80 (in alphabetical order according to ASCII) will be loaded
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81 - if `fn` points to a file (with or without extension) then this
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82 file will be loaded
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83
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84 The image will be scaled equally in horizontal and vertical direction
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85 such that the resulting image does not exceed the rectangle [`dw`,`dh`].
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86 If only either height or width is given the image is scaled to match
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87 only the given parameter. The size of the resulting image in the other
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88 parameter is determined by the aspect ratio of the image.
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89
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90 An example for a Scaler URL is:
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91 `http://digilib.mpiwg-berlin.mpg.de/digitallibrary/servlet/Scaler?fn=experimental/digilib-test/images&wh=0.1712&ww=0.1282&wy=0.1681&wx=0.6895&dw=862&dh=904`
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92 such a URL can be used as src attribute to an img element in the
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93 frontend HTML.
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94
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95 [1]: http://hg.berlios.de/repos/digilib/file/default/webapp/src/main/webapp
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96
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97 [2]: http://itgroup.mpiwg-berlin.mpg.de/cgi-bin/cvsweb.cgi/zogiLib/
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