Page Image Access Mechanisms


This document describes the mechanisms and programs used by DLXS for accessing and viewing images of pages that correspond to pages in TextClass documents (and possibly other classes). It also explains the particular metadata requirements that exist for this functionality to be possible and shows a sample pageview.dat file. The pageview.dat mechanisms are now deprecated, but still useful for importing information into the Pageview table (see instructions below in Populating the Pageview Table). Also discussed is the mechanism for linking to ImageClass to view Pageviewer images in more detail.

General Information

For collections where the middleware delivers page images rather than or in addition to the text content of the pages, the main mechanism for viewing the pages is the pageviewer-idx CGI program. In order to link from the XML text to the corresponding image, this CGI expects that page images are stored in directories based on (1) the DLXSROOT value, (2) the object directory recorded in the collection manager, and (3) the unique identifier assigned to the XML text and stored in the IDNO element, and that there are page break elements in the document referencing the images. In addition, there must be a Pageview table in the dlxs metadata database that should contain a column for each page image, listing the image file name, its sequence in the XML text, the page number (if any) specified on the page, the OCR confidence value (if available), and a three-letter code for any special features of the page (the default value for no special feature is UNS; see below for more information).

pageviewer-idx connects to the database and retrieves the name and location of the page image file from the Pageview table. pageviewer-idx then decides how to deliver the page. If the stored file format is different from the requested format as recorded in the collection manager (e.g., stored as tiff and requested as gif), a separate program, tif2web, is started to convert the image on the fly. For more information about how pageviewer-idx does its work in deciding how to deliver the page image, see Itemviewer Image Conversion .

The Page Break Element and Page Image Metadata

The PB tag in the XML data, representing a page break, has this form in Text Class:

<PB REF="00000009.tif" SEQ="00000009" RES="600dpi" FMT="TIFF6.0" FTR="TPG" CNF="856" N=iiii"/>

The attributes are:

The information in this PB tag allows the Text Class middleware to create a URL to call the pageviewer-idx program with the parameters necessary to retrieve and display the corresponding page image. pageviewer-idx uses the Pageview table of the dlxs metadata database to do so.

The following are examples of feature codes and their expanded definitions that have been used in various collections mounted by DLPS. You may redefine these or use other codes, but will need to add or change the values in the PageView.cfg file found in the Text Class cgi directory. To see an example of these codes in use in the Text Class interface, go to this page from The Use of the Barometer in the Making of America and note the variety of features in the upper right hand pull down menu labeled "go to." If UNS is the sole feature recorded, no special features will be listed in this menu. Note that the FTR value becomes part of a lookup key in the langmap.en.xml to be referred to from the the pageviewer XSL.

Here is an example:

        <Lookup id="viewer">
          <Item key="">First Page</Item>
          <Item key="">Acknowledgement</Item>
          <Item key="">Advertisement</Item>
          <Item key="">Appendix</Item>
          <Item key="">Bibliography</Item>
          <Item key="">Blank Page</Item>
          <Item key="">Cover Title Page</Item>
          <Item key="">Digest</Item>
          <Item key="">Errata</Item>
          <Item key="">Front Matter</Item>
          <Item key="">History</Item>
          <Item key="">Comprehensive Index</Item>
          <Item key="">List of Illustrations</Item>
          <Item key="">List of Tables</Item>
          <Item key="">Map</Item>
          <Item key="">Miscellaneous</Item>
          <Item key="">Manuscript</Item>
          <Item key="">Notes</Item>
          <Item key="">[n/a]</Item>
          <Item key="">Ordinances</Item>
          <Item key="">Author or Name Index</Item>
          <Item key="">Production Note</Item>
          <Item key="">Preface</Item>
          <Item key="">References</Item>
          <Item key="">Regulations</Item>
          <Item key="">Rules</Item>
          <Item key="">Special Index</Item>
          <Item key="">Subject Index</Item>
          <Item key="">Supplement</Item>
          <Item key="">Table</Item>
          <Item key="">Table of Contents</Item>
          <Item key="">Title Page</Item>
          <Item key=""></Item>
          <Item key="">Volume End Sheets</Item>
          <Item key="">Volume List of Illus</Item>
          <Item key="">Volume Index</Item>
          <Item key="">Various Pagination</Item>
          <Item key="">Volume Title Page</Item>
          <Item key="">Volume Title Page Verso</Item>
          <Item key="viewer.nopagenum">[n/a]</Item>

Populating the Pageview Table

In DLXS releases prior to CD-ROM 8, the pageview.dat, a tab-delimited ASCII file used to locate page images associated with a text, was stored in the directory with the page images for a particular collection. If you have created pageview.dat files and would like to migrate them to the Pageview table, instructions can be found here. Otherwise, metadata about page images for a collection should be entered directly into the Pageview table.

Creating pageview.dat Files (For Information Only)

The pageview.dat file for a particular XML text can be automatically generated provided that the metadata required is stored as attributes in the page break (PB) elements in the text. On the distribution CD-ROM, in the directory /l1/bin/t/text/, you will find a perl script named When run with a directory path as its sole argument, it will work through the subdirectories, creating pageview.dat files for all files with a .sgm* extension. (For XML files, you will need to edit lines 27 and 51 to point the script to files with the extension .xml.) For example,

/l1/bin/t/text/ /l1/obj/a/

will run through all the subdirectories below /l1/obj/a/ and report on the files it finds and work it is doing:

Working on xml files in directory: /l1/obj/a/j/l/ajl7777.0001.001
Working on file: /l1/obj/a/j/l/ajl7777.0001.001/ajl7777.0001.001.xml
Working on PB tag for sequence: 0001
Working on PB tag for sequence: 0002
Working on PB tag for sequence: 0003
Working on PB tag for sequence: 0004

Working with a document containing these four page break tags:

<PB REF="00000001.tif" SEQ="00000001" RES="600dpi" FMT="TIFF6.0" FTR="TPG" CNF="852" N="1"/>
<PB REF="00000002.tif" SEQ="00000002" RES="600dpi" FMT="TIFF6.0" FTR="UNSPEC" CNF="100" N="2"/>
<PB REF="00000003.tif" SEQ="00000003" RES="600dpi" FMT="TIFF6.0" FTR="UNSPEC" CNF="884" N="3"/>
<PB REF="00000004.tif" SEQ="00000004" RES="600dpi" FMT="TIFF6.0" FTR="UNSPEC" CNF="872" N="4"/>

would result in a pageview.dat file that contained this data:

## File:        /l1/obj/b/a/b/bab3633.0001.001/pageview.dat
## Created:     Mon Aug  6 11:32:55 EDT 2001
#filename       seq       pagenum confid  feature
00000001.tif    00000001        00000001        852     TPG
00000002.tif    00000002        00000002        100     UNS
00000003.tif    00000003        00000003        884     UNS
00000004.tif    00000004        00000004        872     UNS

Linking from Pageviewer to ImageClass

Pageviewer can handle both bitonal and continuous tone image formats. The output of a page capture process might be a bitonal scan of a page containing a continuous tone plate and a second continuous scan of the plate. Or the entire page might be captured as a continuous tone image. So there are two cases.

Pageviewer can provide a links to the image if it is placed under ImageCLass management. The bitonal page with thumbnail link is shown in the following illustration.

Detailed Description of the Mechanism


The continuous tone images and optional thumb images are placed under ImageClass management ase described in Image Class Image Loading for Online Delivery.

DLXS idresolver CGI and DLXS database idresolver database table

The DLXS IdResolver CGI script is delivered in DLXSROOT/cgi/i/idresolver/idresolver. It depends on a table in the DLXS database called idresolver. Check DLXSROOT/misc/db/MySQL-dump.sql for the schema. A row consists of an ID and a URL to ImageClass for that ID.

The default format of the ID is ic-collid-idno-ENTITY-[th|rec] where th is for the thumb and rec is for the full sized image. ENTITY is the value of the ENTITY attribute in the <FIGURE> tag in the XML. Simple bitonal page collections use the <PB> tag to indicate pages. If the page has a continuous tone plate or is itself a continuous tone image for which ImageClass viewing is desirable, the <FIGURE> tag is used in addition to the <PB> tag.

The virtual host running the idresolver CGI is assigned to $LibGlobals::gIdResolverHost via an install-time question. It can be changed later by editing DLXSROOT/lib/LibGlobals.cfg.

Population of the idresolver table is accomplished by a variety of local mechanisms which will vary depending upon your data preparation processes.

DLXS databse PageviewFig table

The final component to this mechanism is the DLXS database PageviewFig table.

This table is consulted by Pageviewer to determine whether to present links to ImageClass when a given page is displayed. Check DLXSROOT/misc/db/MySQL-dump.sql for the schema. A row consists of (1) the idno of the item, (2) the page sequence number (seq) from the SEQ attribute of the <PB> tag for the page being viewed and (3) the value of the ENTITY attribute from the <FIGURE> tag accompanying the <PB> tag in the XML.

The PageviewFig table can be populated by the DLXSROOT/bin/t/text/ Perl script. It accepts a single command-line option with values of -a to insert rows and -d to delete rows. The script will prompt for DLXSROOT and for a path relative to DLXSROOT where the XPAT Data Dictionary for the collection resides, e.g. /idx/c/coll/coll.dd. The script will perform XPAT queries and parse the ENTITY attribute value from the <FIGURE> tag and the SEQ attribute of the containing <PB> tag and the idno of the containing item. The result is inserted into the table. Deletion is accomplished by deleting all rows with idno values for the collection in question.

Putting it all together

When Pageviewer displays the image for a given page (seq) for a given item (idno) in some collection it uses these keys to look up the corresponding ENTITY value in the PageviewFig table. It constructs two IDs from the ENTITY for lookup in the idresolver table. It then builds links using the URLs to ImageClass that come back in the query result. If the thumbnail image URL is empty it builds a text link instead of a thumb link.