bug#59347: 29.0.50; `:family` face setting ignored

[Gregory Heytings <gregory@heytings.org>]

>
> I will note right here that Emacs has no way of knowing whether the 
> fonts returned by the font driver are or aren't variable-pitch.  In 
> fact, AFAIR it is a tricky and not very reliable to try deducing that 
> from the font data Emacs records about each font (see 'font-info').  We 
> just blindly trust the font driver to give us the appropriate list of 
> fonts.  IOW, for Emacs the family is just a meaningless string.
>

Indeed, Emacs trusts the font driver


>> If the specification passed to font_list_entities contains a non-nil 
>> width, weight or slant, that list is immediately filtered and all fonts 
>> that do not match these width, weight or slant exactly are removed from 
>> the list.
>
> And you are saying that this filtering is wrong, yes?
>

No, that filtering in font_list_entities is not wrong, because 
font_list_entities has another caller besides font_find_for_lface: 
Flist_fonts.  What is wrong is to call font_list_entities with these 
attributes non-nil in


>>> why do you consider the family attribute of a face be more important
>>> than other attributes?  if not all the attributes of a spec are "equal"
>>> in their importance, which attributes are more important, and why?
>>
>> Indeed, the attributes are not equal, in fact none of the attributes are
>> ever equal in their importance.  The family is the most important one,
>> followed by the foundry, the registry, the additional style (in that
>> order, see the loop at the end of font_find_for_lface in which Emacs tries
>> to make each of these attributes less specific in turn, starting with the
>> least important one, namely the additional style), followed by the width,
>> height (or size), weight, slant (in the order specified by the variable
>> face-font-selection-order).
>
> That is not the relative importance of interest in the context of this
> discussion, because Emacs already does look for a suitable font in the order
> of the importance you describe.
>
> My question was not about this basic relative importance, it was about
> something else: when none of the fonts of the given FAMILY fits the font
> spec, why do you consider keeping the family to be more important than
> keeping the weight?
>
> And another question: if we are to follow face-font-selection-order, to
> observe the relative importance of the attributes as set by the user, then
> why did your patch only consider relaxing the weight (which is in the
> penultimate place in the order of importance), and not the slant (which is
> the least important attribute, in the default order we use)?
>
>> It is also in that loop (at the end of font_find_for_lface) that
>> face-font-family-alternatives are used.  If the generic "Sans Serif",
>> "Monospace" and "Monospace Serif" families that Emacs uses are not a
>> recognized by the font driver (IOW, if font_list_entities returns an empty
>> result for these families), Emacs falls back to some hard-coded, less
>> generic, family names.
>
> I'm not sure I agree with this part of your description.  The code looks up
> face-font-family-alternatives _before_ the loop in font_find_for_lface,
> i.e., _before_ font_list_entities is called.  Where exactly do you see what
> you describe above?
>
>>> and if bold is fine when semi-bold was requested, what about other
>>> weights, like ultra-light -- are they also okay? if not, why not?
>>
>> Yes, ultra-light is also okay.  If a program requests a font in the Sans
>> Serif family with a semi-bold weight, and the only available font on a
>> given system in that family is a ultra-light one, it's the best possible
>> match for that font specification.  It's up to the user to install a font
>> in the Sans Serif family which has a semi-bold variant on their system, if
>> they need a font in the Sans Serif family with a semi-bold variant (or to
>> install another font that is closer to semi-bold than ultra-light, e.g.
>> one with a bold variant).
>>
>>>
>>> what are the criteria here and with other similar attributes?
>>>
>>
>> The family, foundry, registry and additional style attributes are passed
>> "as is" to the font driver, which returns a list of fonts matching these
>> attributes.  The width, weight and/or slant are converted to numerical
>> values (with font-{width,weight,slant}-table), and font_score, called by
>> font_sort_entities, called by font_select_entity, which is applied on the
>> list of fonts returned by font_list_entities, selects the best match in
>> that list (according the the preferences in face-font-selection-order).
>> If the width, weight and/or slant were already passed to
>> font_list_entities, the list of fonts passed to font_select_entity
>> contains only fonts that match these width, weight and/or slant, and that
>> mechanism is bypassed.
>
> IOW, you want to disable the filtering of candidate fonts in
> font_list_entities, and instead consider _all_ the candidates, selecting the
> best match for the numerical attributes: width, height, weight, and slant.
> And you don't want to relax the non-numerical attributes (family, foundry,
> registry, adstyle) unless there's really no font, of any
> width/height/weight/slant, installed for the specified
> family/foundry/registry/adstyle.  Is that right?
>
> If that is what you want us to do, then I must ask at least about the
> height: is it really reasonable to prefer _any_ height from the given
> family, even if it's radically different from what was requested?
>
> Also, the patch you suggested to install in
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=59347#77 doesn't do the above,
> it is basically a minor semi-kludgey change of the existing code, which only
> considers 'normal' weight when 'medium' was requested.  Why didn't you
> submit a patch that follows your description and your critique to the
> logical conclusion?
>
> The rest of what I write below is based on the assumption that my
> understanding of your critique of the current code is as I describe above;
> if it is wrong, please ignore what's below, and please help me understand
> what is it that you are actually proposing and I misunderstood.
>
> So I see the following issues with your proposal (which AFAIU is different
> from the patch you actually posted):
>
> . we will examine much more fonts than we do now: the current code only
>   examines matching fonts and returns the first one that satisfies the
>   spec; your proposal will require us to examine all of them, in order to
>   find the best match out of many
> . your logic, which says that the family is so much more important than the
>   other attributes is not necessarily correct in all the cases where this
>   code is executed: I can easily imagine cases where the requested weight
>   is so important that no other "close" weight will do, and the caller
>   really wants to get an empty list rather than a deviant font
>
> So I can only agree to installing the patch along the lines of the above
> logic, i.e. to make the code relax the numerical attributes trying to keep
> the family, on the following conditions:
>
> . we add an additional loop, like the one in font_find_for_lface, after the
>   original one, and in that additional loop implement the examination of
>   candidates without filtering then by numerical attributes up front; that
>   additional loop will run only if the one before it came up with no fonts
>   that match the family
> . whether the additional loop will actually run should be controlled by a
>   variable exposed to Lisp, so that if this change causes regressions, we
>   could easily find out this is the reason, and users could work around the
>   regressions without rebuilding Emacs
>
> OK?  And note that my agreement is not to the patch you posted, but to a
> more general change in the logic of examining the candidate fonts.  This is
> how I understand what you think Emacs should do; if I misunderstood, please
> correct me.
>
> Thanks.
>
>
>
>