Requirements

• Some familiarity with configuring Solr
• Solr >= 7.5
• OCR documents need to be in hOCR, ALTO or MiniOCR formats, with at least page-, and word-level segmentation

Obtaining the plugin JAR

To use the latest release version, refer to the GitHub Releases list. From there, download the JAR file for the latest version. To make the plugin available to Solr, create a new directory $SOLR_HOME/contrib/ocrsearch/lib and place the JAR you just downloaded there.

Core Configuration

To enable the use of the plugin for your Solr core, you will have to edit both the solrconfig.xml and the schema.xml file in your core’s conf directory.

SolrConfig

In your core’s `solrconfig.xml, you need to:

1. Instruct the core to load the OCR highlighting plugin, so it can find the classes needed to perform OCR indexing and highlighting.
2. Define a search component that will perform the OCR highlighting at query time
3. Add the search component to your request handlers that will trigger the highlighting.

<config>
  <!-- ...other configuration options... -->

  <!--
    Tell Solr to load all JAR files from the directory installed the plugin to. 
    This assumes a directory structure where the cores are in `$SOLR_HOME/server/solr/$CORE`
    and the plugin JAR was installed in `$SOLR_HOME/contrib/ocrsearch/lib`. Adjust the
    path if you setup differs.
  -->
  <lib dir="../../../contrib/ocrsearch/lib" regex=".*\.jar" />

  <!-- Add a new named search component that takes care of highlighting OCR field values. -->
  <searchComponent class="de.digitalcollections.solrocr.solr.OcrHighlightComponent"
                   name="ocrHighlight" />

   <!-- ...other search components... -->

  <!--
    Instruct the request handlers you want to enable OCR highlighting for to include the
    search component you defined above. This example uses the standard /select handler.

    CAUTION: Make sure that the OCR highlight component is listed **before** the standard
    highlighting component, but **after** the query component.
  -->
  <requestHandler name="/select" class="solr.SearchHandler">
      <arr name="components">
          <str>query</str>
          <str>ocrHighlight</str>
          <str>highlight</str>
      </arr>
  </requestHandler>
</config>

If you run into problems, a look into these sections of the Solr user’s guide might be helpful:

• Resource and Plugin Loading
• RequestHandlers and SearchComponents in SolrConfig

Schema

In the core’s schema.xml, you need to:

1. Define a new field type that will hold your indexed OCR text
2. Define which fields are going to hold the indexed OCR text.

The field type for OCR text is usually identical to your regular text field, with the difference that there are one or two extra character filters at the beginning of your index analysis chain: - ExternalUtf8ContentFilterFactory will (optionally) allow you to index and highlight OCR from external sources on the file system. More on this in the Indexing chapter. - OcrCharFilterFactory will retrieve the raw OCR data and extract the plain text that is going to pass through the rest of the analysis chain. It will auto-detect the used OCR formats, which means that you can use different OCR formats alongside each other. After this filter, Solr will treat the field just like a regular text field for purposes of analysis.

<schema>
  <types>
    <fieldtype name="text_ocr" class="solr.TextField" storeOffsetsWithPositions="true" 
               termVectors="true">
      <analyzer type="index">
        <!-- For loading external files as field values during indexing -->
        <charFilter class="de.digitalcollections.solrocr.lucene.filters.ExternalUtf8ContentFilterFactory" />
        <!-- For converting OCR to plaintext -->
        <charFilter class="de.digitalcollections.solrocr.lucene.filters.OcrCharFilterFactory" />
        <!-- ...rest of your index analysis chain... -->
      </analyzer>
      <analyzer type="query">
        <!-- your query analysis chain, should not include the character filters -->
      </analyzer>
    </fieldtype>
  </types>

  <fields>
    <!-- ...your other fields ... -->

    <!-- A field that uses the OCR field type. Has to be `stored`. -->
    <field name="ocr_text" type="text_ocr" multiValued="false" indexed="true" stored="true" />
  </fields>
</schema>

If you struggle with setting up your schema, a look into the Schema Design chapter of the Solr user’s guide might be helpful.