Fix duplicated IDs in ALTO XML when multiple pages are present

[jankal]

When running tesseract on a list of images and validating the alto output using the schema (https://www.loc.gov/alto/v3/alto-3-0.xsd), I had the following error:

 Element '{http://www.loc.gov/standards/alto/ns-v3#}ComposedBlock', attribute 'ID': 'cblock_0' is not a valid value of the atomic type '{http://www.loc.gov/standards/alto/ns-v3#}BlockTypeID'.

Digging into the type of BlockTypeID, I found a xsd:ID restriction - meaning the BlockTypeID can only exist once in the whole document / is unique per document.

I incorporated the current page_number into the IDs thus resolving the issue.

[stweil]

Thanks for the fix. I just wonder whether we should try to keep the current IDs for the most common use case where only a single page is processed. This could be achieved for example by

• conditional code
• using counters (bcnt, ...) which are not reset for each additonal image
• using an index of (page_number - 1) * 1000 + bcnt ... in the ID strings

[jankal]

Thanks for your suggestion @stweil!

I added a function GetID to generate the ID. The new GetID function keeps the current ID structure for the first page and starts to add the page_number from the second page on.

Disclosure: I haven't written CPP before, so pls check for rookie mistakes :)