John Hewson created PDFBOX-2805:
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Summary: Incorrect CapHeight calculation and usage in form fields
Key: PDFBOX-2805
URL: https://issues.apache.org/jira/browse/PDFBOX-2805
Project: PDFBox
Issue Type: Bug
Components: AcroForm, PDModel
Affects Versions: 2.0.0
Reporter: John Hewson
I discovered that the CapHeight value was being incorrectly calculated by
TrueTypeEmbedder. Fixing this has a knock-on effect for CreateFormField, which
uses that value for vertical text positioning, after the fix, it no longer
works correctly.
I took a look at this and the problem is that CapHeight isn't the right value
to use when calculating text positioning for form fields. I've never seen the
CapHeight used in a typographical setting and its use is unorthodox. I did an
experiment by modifying the embedded CapHeight and I can confirm that Acrobat
*does not* use it when calculating the position of form field text.
The natural choice for text positioning in form fields would be to use either
the depth of the font's bbox or the descender. Some experiments with Acrobat
show that the bbox's depth yield the best results. There's still a slight
difference of around 0.02pt which is unaccounted for when compared to Acrobat,
but this is still an improvement on the previous, now broken code which used
the CapHeight.
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