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more codec docs.
author robcast
date Thu, 22 Oct 2015 17:07:52 +0200
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1 # Directory layout for images
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3 In digilib all images are identified by the `fn` and (optional) `pn` parameters.
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4 The value for `fn` can be a directory path or a directory path and a filename,
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5 separated by slashes, e.g. "`fn=books/book1/page0002`".
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7 If `fn` is a directory path without filename `pn` is the index number of the
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8 image files in this directory in alphabetical order, e.g.
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9 "`fn=books/book1&pn=2`". The default for `pn` is 1 i.e. the first image.
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11 If `fn` ends in a filename `pn` is ignored. File extensions are also ignored,
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12 i.e. "`books/book1/page0002`" and "`books/book1/page0002.tif`" identify the same
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13 image. It is recommended to omit the file extension.
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15 The directory path in `fn` is relative to the base directory in the `basedir-list`
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16 parameter of the `digilib-config.xml` file, e.g. if
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18 <parameter name="basedir-list" value="/docuserver/images" />
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20 and
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22 fn=books/book1/page0002
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23
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24 then digilib will try to load the file
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25
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26 /docuserver/images/books/book1/page0002.tif
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28 (automatically finding the right file extension)
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31 ## Prescaled images
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32
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33 You can provide any number of scaled-down versions of your images that
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34 digilib can use when a smaller version of an image is requested. Since less data
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35 has to be read and processed this can speed up digilib's performance considerably.
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36
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37 The actual process is that the client requests a certain target size,
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38 digilib scans all available scaled-down versions of the same image, selects the
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39 smallest image that is larger than the requested size and scales it down to the
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40 requested size.
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41
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42 There is another optimization in digilib: if the requested image is *exactly*
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43 the same size and type as the pre-scaled image then the pre-scaled image is sent
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44 unmodified to the client which is a lot faster. So it makes sense to produce
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45 thumbnails of exactly 90 pixel width when they are used in an HTML page where
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46 all images are 90 pixel wide.
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48 The scaled-down versions of the image have to have the same file name as
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49 the original hi-res file. They can have a different type and extension (e.g.
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50 `img002.jpg` for `img002.TIFF`)
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52 The scaled down images have to have the same directory path (the part that
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53 shows up in digilib's "fn" parameter) as the hi-res file wile the first part of each
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54 directory tree is configured by the `basedir-list` parameter in
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55 `digilib-config.xml`.
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57 The sequence of directories in `basedir-list` is from high-res to low-res.
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58 Images must be present in the hires directory but they need not be present in
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59 all lower-res directories.
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61 e.g. if digilib-config.xml contains
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63 <parameter name="basedir-list" value="/images:/scaled:/thumb" />
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65 and a user requests the image `books/book1/page0002` digilib looks for
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67 1. `/thumb/books/book1/page0002.jpg`
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68 2. `/scaled/books/book1/page002.jpg`
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69 3. `/images/books/book1/page002.tif`
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71 (automatically finding the right file extension)
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72 and uses the first image that is bigger than or equal to the requested size.
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73
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74 For batch-prescaling our images we use a very old Perl script called "scale-o-mat"
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75 that uses a lot of freely available imaging libraries (ImageMagick, libtiff, netpbm)
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76 and is available in our [public CVS](https://it-dev.mpiwg-berlin.mpg.de/cvs/cgi-bin/cvsweb/scaleomat/).
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77 The script is given a
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78 hi-res base directory, a destination base directory, a destination size and a
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79 starting directory. It then processes all files in the starting directory and
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80 all its subdirectories and creates scaled images in corresponding directories
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81 under the destination base directory.
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83 We currently use prescaled thumbnails of 100 pixels and images for browser
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84 display of 1500 pixels. Remember that the prescaled image has to be larger (or
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85 the same size) than the requested image size!