For a client project I needed to enable OCR for images inside PDFs. Unfortunately ManifoldCF does not provide configuration options to handle this. It would be nice to have these options for the Tika content extraction:
1. Enable PDF image extraction for OCR: https://tika.apache.org/1.7/api/org/apache/tika/parser/pdf/PDFParserConfig.html#setExtractInlineImages%28boolean%29
2. Set default language for tesseract: https://tika.apache.org/1.7/api/org/apache/tika/parser/ocr/TesseractOCRConfig.html#setLanguage%28java.lang.String%29
Tika OCR is based on tesseract, an Open Source OCR library intially developed by Hewlett-Packard and later continued by Google. It is available from https://github.com/tesseract-ocr/tesseract . It needs to be installed with the tesseract binary available in the PATH environment variable - alternatively it can be set using an Tika API method. Once it is installed and Tika is configured correctly, it works like a charm.
When indexing images or PDFs containing images instead of real text, OCR is necessary for making those documents searchable.