This probably won't be the answer that you wanted, but you could use a
custom writer, e.g., based on the provided sample.lua. See
https://pandoc.org/MANUAL.html#custom-writers.

Here's the relevant code (this isn't all the code relating to footnotes,
but it's the bit that has the special character!):

function Note(s)
    local num = #notes + 1
    -- insert the back reference right before the final closing tag.
    s = string.gsub(
        s, '(.*)</', '%1 <a href="#fnref' .. num ..  '">&#8617;</a></')
    -- add a list item with the note to the note table.
    table.insert(notes, '<li id="fn' .. num .. '">' .. s .. '</li>')
    -- return the footnote reference, linked to the note.
    return '<a id="fnref' .. num .. '" href="#fn' .. num ..
        '"><sup>' .. num .. '</sup></a>'
end

On Tue, 21 Sept 2021 at 13:23, Vin Cent <irakay17-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> Ok, after some more reading, I'm still unsure how to proceed ; and I
> actually don't see how I could to that with a filter.
> I have dumped pandoc's native output format ; and the character I need to
> replace is not part of it. I guess it is added at the time of pandoc's
> writing the output document.
> I don't know how how I can replace that character. Hopefully there is a
> pandoc setting I can tune so that I won't need to replace it at some late
> stage.
>
>
>
> On Tuesday, September 21, 2021 at 10:42:14 AM UTC+2 Vin Cent wrote:
>
>> Sorry for answering myself. I have RTFMed a bit in the while.
>> I actually hint this can be simply done with a LUA filter.
>> I will try to implement it as a self-exercice and report the result here.
>>
>> On Tuesday, September 21, 2021 at 9:34:26 AM UTC+2 Vin Cent wrote:
>>
>>> Hi,
>>>
>>> I am generating epub3 from latex source.
>>> I was wondering why footnotes show a backlink to the original text when
>>> I display the document in calibre, and why they do not when I display the
>>> document on a kobo reader...
>>>
>>> ... until I found the backlink is indeed well present. It is "just" not
>>> displayed by kobo because the character set (by pandoc, I think) has no
>>> rendering on their font.
>>>
>>> The backlink character, copied and pasted from the .epub document
>>> itself, seems to be "↩︎".
>>>
>>> I see two possible tricks here so that the link will appear on kobo :
>>> 1. Change the character to another one that would be rendered by kobo.
>>> 2. Change the default font of the document to one that has rendering for
>>> that character.
>>>
>>> I have tried all available fonts on my device. There are actually three
>>> classes of them.
>>> (Listing them all below, this might be of interest to somebody in the
>>> future)
>>>
>>> Avenir Next ; Georgia ; Kobo Nickel : these fonts render absolutely
>>> nothing for that character, leading the human to believe the backlink does
>>> not exist. It is present, clickable, just not rendered.
>>>
>>> Amasis ; Caecilia ; Gill Sans ; Malabar ; OpenDislexic : these fonts
>>> render "__" for that character. I find it "better" than the first family,
>>> but still not great. It is not obvious for a non-tehnical human that this
>>> is a backlink to the text.
>>>
>>> AR UDJingxihei ; Kobo UD Kakugo ; Kobo Tsukishi Mincho : display
>>> oriental character (I think this is chinese / japanese depending on font)
>>> for the backlink character.
>>>
>>>
>>> Therefore, I tend to favor the first solution. Do you guys know of a way
>>> to customize the "backlink ↩︎" character set by pandoc in epub ?
>>> Or is there a third approach ?
>>>
>>> Thanks,
>>>
>>> Vincent
>>>
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