[HarfBuzz] MS/Symbol cmap subtables

Wed Jan 17 23:48:02 UTC 2018

What's the actual problem you are facing?

On Mon, Jan 15, 2018 at 9:58 AM, Eric Muller <emuller at amazon.com> wrote:

>
> It's clear that if the symbol font is asked by name, we should do the
> shift.
>
> I think I disagree, in the sense that HB should not impose that behavior
> on it's clients. HB is clearly the right place to implement the behavior,
> but the choice of having that behavior or not should be with the client.
>
> For any document format, rendering the moral equivalent of <p
> font-family='symbol'>&#x0041;</p> with something else that an "A" implies
> that all ASCII is PUA. That's a choice Word, InDesign, Notepad may make if
> they want, but it should not be imposed on all users of HB.
>
> Personally, I think it is a very bad choice for HTML, and Firefox seems to
> agree. It seems nice and user friendly at first, but this makes the
> document ambiguous. What about <p font-family='minion,
> symbol'>&#x0041;</p>? It's an A or not an A depending on the presence of
> "minion" in the client. What does the document mean?
>
> Of course, <p font-family='symbol'>&#xF041;</p> should render with the
> glyph symbol.cmap(F041). So even if the shift is never done, the glyph is
> usable. It's just that you don't have the convenience of an IME-like
> mechanism provided by the shaping engine, but you gain a reliable semantic
> for the text.
>
> That's good behavior [in Word], but beyond what HarfBuzz can do.
>
> Yes, which is why the shift may be acceptable or even desirable for some
> clients, and so hopefully the client could choose.
>
> What would clients do with that control then? How would they set it?
>
> If I build an app that is meant to work like other GDI apps, I allow the
> shift (and may be add mitigating measures like Word). If I build an app
> such as Firefox, I don't allow it. The choice is entirely driven by the
> type application I want to build, and how I want to define my document
> format.
>
>
> If you were to implement this choice, I can see it either in the
> construction of the HB unicode functions, or in the hb_buffer (either
> globally, or one a character by character basis). I have a preference for
> the latter: this choice could be passed down to the cmap lookup functions,
> HB or not; it could also be different on different parts of a document, may
> be reacting to markup.
>
> Eric.
>
>
>
> On 1/15/18 6:46 AM, Behdad Esfahbod wrote:
>
> Hi Eric,
>
> On Mon, Jan 15, 2018 at 2:25 AM, Eric Muller <emuller at amazon.com> wrote:
>
>> It seems that with a font that has only a 3, 0 cmap subtable (and may be
>> some macintosh subtables), then HB will automatically do the shift by F000
>> (in the function get_glyph_from_symbol) for code points below U+00FF that
>> are not mapped by the subtable.
>>
>
> Right. Only in hb-ot-func though. Client font funcs can do otherwise.
>
>
>
>> It is clear that when U+0041 A is set with a symbol font, then that
>> U+0041 has actually the semantics of a PUA code point, and certainly should
>> not be treated as an "A". That's the whole point of a 3,0 cmap subtable.
>>
>
> Correct.
>
>
>> Consider an HTML page. The font-family is only a request and there is no
>> guarantee that the actual font will or will not be a symbol font. Thus the
>> semantic of the HTML page can change depending on the browser environment.
>> Outside a browser, it seems that the safe treatment is therefore to
>> consider all code points below U+00FF as PUA, which is clearly not tenable.
>> So in that environment, I think that the shift should not be done. Of
>> course, U+F041 should work.
>>
>
> My take on this is that it's a bug of the font fallback logic if it falls
> back to a symbol font.  I changed fontconfig to never do that.
>
>
>> Note that behavior of Word 2016 on Windows is actually more elaborate:
>> enter U+0041, and set it with a non-symbol font; copy/paste or save to a
>> text file, and the result is U+0041; but set this A in a symbol font, and
>> copy/paste or save to a text file, and the result is U+F041.
>>
>
> That's good behavior, but beyond what HarfBuzz can do.
>
>
>> I think that the shift should be controllable by the client, rather than
>> systematically applied. I don't have a strong opinion about the default
>> behavior (i.e. when HB's client does not specify whether the shift should
>> be done or not).
>>
>
> What would clients do with that control then? How would they set it?
>
> I implemented this shift in fontconfig and then harfbuzz because in
> LibreOffice and other software, there were existing documents that referred
> to windings or other symbol fonts and encoding characters in the ASCII
> range. It's clear that if the symbol font is asked by name, we should do
> the shift. If it's NOT, then it should not be chosen to render text to
> begin with, which means the shift can be applied unconditionally.
>
> How does that sound?
> behdad
>
>
>> Thoughts?
>>
>> Thanks,
>> Eric.
>>
>
> --
> behdad
> http://behdad.org/
>
>
>

-- 
behdad
http://behdad.org/
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