[HarfBuzz] MS/Symbol cmap subtables

[Eric Muller]

> 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 
> <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/

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