[HarfBuzz] MS/Symbol cmap subtables
Eric Muller
emuller at amazon.com
Fri Jan 19 07:27:34 UTC 2018
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/
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