(URGENT) PDF accessibility: math not showing up in PDF content text

[ronaldtse]

In PDF diffs it is important that the content does not introduce systematic changes from the original PDFs.

In the Metanorma-generated PDFs, the math content is missing from the content text. This is a sample from ISO 10303-50:
(https://github.com/metanorma/iso-10303-detached-docs/tree/main/sources/iso-10303-50)

In the generated PDFs, all the formula contents are "missing" from the content text. Given that we can insert AsciiMath for it, we would reduce a lot of these false positives.

[Intelligent2013]

In the content tree there is text for x and y:

Copy-pasted text also contains them: arguments y and x, which.

Currently, mn2pdf inserts hidden math as the transparency text (metanorma/metanorma-bipm#188).
Looks like, Acrobat doesn't see such text in the comparison feature.

I'll try to remove transparency mode temporarily locally and test the comparing result.

[ronaldtse]

Thank you for investigating! Indeed this is strange. I can verify in Preview that I can copy and paste this text.

[Intelligent2013]

I've tested with different combination of color + transparency for hidden text:

• color FFFFFF + transparency mode (currently) - there are differences in the Acrobat comparison results (i.e. Acrobat doesn't see the hidden text)
• color FFFFFF without transparency - there are differences in the Acrobat comparison results (i.e. Acrobat doesn't see the hidden text)
• color FEFEFE (almost 'white') without transparency - no differences in the Acrobat comparison results for y and x
  
• color FEFEFE (almost 'white') + transparency mode - there are differences in the Acrobat comparison results (i.e. Acrobat doesn't see the hidden text)

I.e. if the text in white color or/and transparent, then Acrobat ignore it in the compare feature. But it's available for copy-paste feature.
I don't figure out which is workaround solution can be applied...

[ronaldtse]

Then let's just keep it as it for now. I don't think we do comparisons too often...

[ronaldtse]

This is an interesting topic that @stuartgalt would be interested in as the PDF guru...

[ronaldtse]

Similar to #870 this is a problem with Adobe Acrobat's Compare PDF feature. Letting @stuartgalt know in case the PDF TC has (or plan to have) specs for Compare PDF.