display-supports-face-attributes-p question

display-supports-face-attributes-p question

[Po Lu]

I can't seem to make `display-supports-face-attribute-p' return non-nil
for '(:slant italic) on window-system frames in the Haiku terminal
backend I'm working on.

If my understanding is correct, it should return t if a font driver is able
to find a font matching the spec (:slant italic), but it's not working.

Does anyone know what could be wrong here?  Thanks.

Re: display-supports-face-attributes-p question

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Date: Sun, 29 Aug 2021 17:09:48 +0800
> 
> 
> I can't seem to make `display-supports-face-attribute-p' return non-nil
> for '(:slant italic) on window-system frames in the Haiku terminal
> backend I'm working on.
> 
> If my understanding is correct, it should return t if a font driver is able
> to find a font matching the spec (:slant italic), but it's not working.
> 
> Does anyone know what could be wrong here?  Thanks.

Not working in what sense?  Does it fail in
gui_supports_face_attributes_p or does it fail elsewhere?  If it's in
gui_supports_face_attributes_p, can you step through the code there
with a debugger and describe what exactly doesn't work?

Thanks.

Re: display-supports-face-attributes-p question

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> Not working in what sense?  Does it fail in
> gui_supports_face_attributes_p or does it fail elsewhere?  If it's in
> gui_supports_face_attributes_p, can you step through the code there
> with a debugger and describe what exactly doesn't work?

It appears that gui_face_attributes_p can't find properties differing
between the :slant italic font and the default font; the EQ comparison
at xfaces.c:5163 always evaluates to true.  What confuses me is how this
is possible, as, if I understand correctly, (struct font *)->props
should be populated by `make_font_object' from the contents of the
entity, and if the font driver finds a font with :slant italic, the
entity should also have it's slant set to italic.

TIA

Re: display-supports-face-attributes-p question

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Sun, 29 Aug 2021 19:22:44 +0800
> 
> It appears that gui_face_attributes_p can't find properties differing
> between the :slant italic font and the default font; the EQ comparison
> at xfaces.c:5163 always evaluates to true.  What confuses me is how this
> is possible, as, if I understand correctly, (struct font *)->props
> should be populated by `make_font_object' from the contents of the
> entity, and if the font driver finds a font with :slant italic, the
> entity should also have it's slant set to italic.

When the call to lookup_face in gui_supports_face_attributes_p gets
into realize_gui_face, please show the results of

  (gdb) pp attrs[LFACE_FONT_INDEX]

before and after the call to font_load_for_lface in this snippet:

      if (! FONT_OBJECT_P (attrs[LFACE_FONT_INDEX]))
	attrs[LFACE_FONT_INDEX]
	  = font_load_for_lface (f, attrs, Ffont_spec (0, NULL));

Re: display-supports-face-attributes-p question

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> When the call to lookup_face in gui_supports_face_attributes_p gets
> into realize_gui_face, please show the results of
>
>   (gdb) pp attrs[LFACE_FONT_INDEX]
>
> before and after the call to font_load_for_lface in this snippet:
>
>       if (! FONT_OBJECT_P (attrs[LFACE_FONT_INDEX]))
> 	attrs[LFACE_FONT_INDEX]
> 	  = font_load_for_lface (f, attrs, Ffont_spec (0, NULL));

Before:
    #<font-spec nil haiku Noto\ Sans\ Mono nil iso8859-1 normal nil normal 12 nil 100 0 ((:user-spec . "monospace"))>

And after:
    #<font-object "-haiku-Noto Sans Mono-normal-normal-normal-*-12-*-*-*-m-0-iso8859-1">

I don't exactly understand what the code does, but that seems wrong.  I
think both the spec and the object should be italic?

Re: display-supports-face-attributes-p question

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Sun, 29 Aug 2021 21:54:38 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >       if (! FONT_OBJECT_P (attrs[LFACE_FONT_INDEX]))
> > 	attrs[LFACE_FONT_INDEX]
> > 	  = font_load_for_lface (f, attrs, Ffont_spec (0, NULL));
> 
> Before:
>     #<font-spec nil haiku Noto\ Sans\ Mono nil iso8859-1 normal nil normal 12 nil 100 0 ((:user-spec . "monospace"))>
> 
> And after:
>     #<font-object "-haiku-Noto Sans Mono-normal-normal-normal-*-12-*-*-*-m-0-iso8859-1">
> 
> I don't exactly understand what the code does, but that seems wrong.  I
> think both the spec and the object should be italic?

No, only the second one.  The first one is generic, with the :slant
slot unspecified.

So it looks like the problem is inside font_load_for_lface.  Please
step into it, then into font_find_for_lface that one calls, and tell
what is the value of 'entities' here:

	      for (l = 0; SYMBOLP (adstyle[l]); l++)
		{
		  ASET (work, FONT_ADSTYLE_INDEX, adstyle[l]);
		  entities = font_list_entities (f, work); <<<<<<<<<<<<<<<
		  if (! NILP (entities))

It should be a list of font-entity objects, at least some of which
have the 'italic' value in the :slant slot.

Or maybe the code behaves differently in your case, and doesn't even
get to that place?  If so, please try to figure out why.

Re: display-supports-face-attributes-p question

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> It should be a list of font-entity objects, at least some of which
> have the 'italic' value in the :slant slot.

Thanks for the help, I think I've found the problem: my code doesn't
have problems detecting italic styles, but it can't detect italic styles
when monospace is explictly specified.  There are indeed no
font-entities with :slant italic.

Re: display-supports-face-attributes-p question

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Mon, 30 Aug 2021 09:11:46 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > It should be a list of font-entity objects, at least some of which
> > have the 'italic' value in the :slant slot.
> 
> Thanks for the help, I think I've found the problem: my code doesn't
> have problems detecting italic styles, but it can't detect italic styles
> when monospace is explictly specified.  There are indeed no
> font-entities with :slant italic.

Glad I was able to help.  This area (font selection) of the Emacs code
is notoriously under-documented, and I don't consider myself an expert
on its inner workings.