[HarfBuzz] MS/Symbol cmap subtables
Eric Muller
emuller at amazon.com
Sun Jan 21 02:22:23 UTC 2018
> The easiest would be to add a new API analogous to
> hb_ot_font_set_funcs(), that does NOT have the symbol shift in it
That works.
Thanks,
Eric.
On 1/19/18 4:43 PM, Behdad Esfahbod wrote:
> Ok, let's see how we can address this...
>
> I don't like a setting on the buffer as currently the get_glyph()
> callback has no way of accessing that information. The easiest would
> be to add a new API analogous to hb_ot_font_set_funcs(), that does NOT
> have the symbol shift in it. It's not the most elegant solution but
> easiest. Would that work for you?
>
> That said, this issue is also related, as it pertains another
> non-Unicode encoding, though, in the font not the buffer:
>
> https://github.com/harfbuzz/harfbuzz/issues/681
>
> On Thu, Jan 18, 2018 at 11:27 PM, Eric Muller <emuller at amazon.com
> <mailto:emuller at amazon.com>> wrote:
>
> I want to build a rendering system where U+0041 renders as an "A",
> regardless of the selected font.
>
> Eric.
>
>
>
> On 1/17/18 3:48 PM, Behdad Esfahbod wrote:
>> What's the actual problem you are facing?
>>
>> On Mon, Jan 15, 2018 at 9:58 AM, Eric Muller <emuller at amazon.com
>> <mailto: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'>A</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'>A</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'></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 <mailto: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/
>
>
>
>
> --
> behdad
> http://behdad.org/
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