Support for striketrough text on write_html function

[viniciustozzi]

Please explain your intent
It would be greate to have support for striketrough text on write_html function. Currently it already works for bold, italic and underline.

Describe the solution you'd like
When using write_html() function it is already possible to use tag_styles argument to map the tag <s> to a TextStyle or FontFace, but unfortunately there is no way to set the emphasis to striketrough. Maybe adding a new Text Emphasis for striketrough ("S") would be enough. However I am not sure how to actually render this on the PDF itself.

Additional context
With the following snippet it's possible to see that <b> tag works just fine but when I try with <s> tag it does not work:

from fpdf import FPDF

pdf = FPDF()
pdf.write_html(
    text="<b>Bold Text works </b> <s> but strikethrough does not </s>.",
)
pdf.output("issue_1322.pdf")

[Lucas-C]

Hi @viniciustozzi

Sure, this an excellent suggestion for a feature!

Pull Requests are welcome to implement this 🙂

First, we could either implement this in HTML2FPDF, or else in the base FPDF class, similarly to how "underlined text" is implemented.

If we chose the second option, "strikethrough" should be part of the GraphicsState.
But another question arise: should it be included in TextEmphasis, that currently combines bold, italics & underline text styling?

IMHO, yes, we should implement strikethrough in exactly the same way that underline is currently implemented.
But it would be very interesting to have other people view on this 🙂
Ping other maintainers @andersonhc & @gmischler

[andersonhc]

I agree it would be a nice addition and we should implement strikethrough like underline.

We load underline thickness and position from the font's "post" table - with hardcoded values for the core fonts.
I believe the equivalent for strikethrough would be yStrikeoutPosition and yStrikeoutSize on the OS/2 table.

[Lucas-C]

I opened PR #1340 as a starting point.

It does not seem that yStrikeoutPosition / yStrikeoutSize are always present in OS/2 tables.
I will investigate further 🙂

[Lucas-C]

I figured it out, I was using the wrong table in the font 😅

The PR is ready for review

[Lucas-C]

This has been implemented!

It can be tested by using the latest, unreleased, development version straight from the development branch of this repository:

pip install git+https://github.com/py-pdf/fpdf2.git@master

The next release will include this new feature.