Re: Using pango in KMSCON (terminal-emulator)

[David Herrmann <dh herrmann gmail com>]

Hi Behdad

On Tue, Feb 19, 2013 at 8:28 AM, Behdad Esfahbod <behdad behdad org> wrote:
> On 02/16/2013 04:33 PM, David Herrmann wrote:
> > 3) Whenever I have to draw a cell, I ask pango to give me a buffer for
> > the given Unicode char (or better: Unicode string which might contain
> > combining-characters)
>
> The problem is: to correctly handle Unicode text, you need to give Pango
> paragraph at a time.  Anything else will have its shortcomings.  libvte gives
> Pango a cell at a time (including combining characters).  That works fine for
> many scripts, but not for, say, Indic scripts, or for right-to-left text, or
> Arabic shaping, etc.
>
> If you are willing to let go of those, the question becomes, why do you want
> to use Pango to begin with?  Ie. what writing systems do you want to support?
>  You may not need to use Pango to begin with.

I have tried a lot. In fact, I have a basic 256-glyphs 8x16-pixel
static font compiled in as fallback. I have also an optional
GNU-Unifont backend that uses 8x16 / 16x16 pixel glyphs.
I have written a freetype2+font-config backend and a pango_ft2
backend. So I don't particularly depend on pango, but it has provided
the best results of all backends.

That is, it provides exactly what I need. It loads system
configuration automatically, I can tell it the default font properties
and it provides me with the best available glyphs it can find.
I'm particularly talking about the "glyph-substitution" if the font
doesn't provide the glyph. I think I could extend the
freetype2+font-config backend to do that, either. But why bother if
pango can do that by itself? (and also I don't know how to do that
with font-config..)

You're right, pango is probably a bit blown for this simple task. But
pango is installed on most systems, anyway, so I can make use of it.
And users who don't want pango can still use my static built-in fonts.
Maybe sometime I can write a proper freetype2+font-config backend that
does all the parts I want. But until then, I'm pretty happy with
pango.

> > 4) It's important that I get full access to the 2D buffer of the
> > glyph. I sometimes need to change stride/size/etc. and the
> > hardware-blitters have different constraints. Hence, I cannot use any
> > opaque "glyph"/gtk/etc. structures but instead need _direct_ buffer
> > access.
>
> You can get that with pangocairo.  I hear you want to do your own blitting.
> That's fine.

So you mean I should allocate a cairo_image_surface_t, draw to it and
then use the buffer of the image_surface? Yeah, ok, that sounds about
right.

> > What do I currently do?
> >
> > Font-Management:
> > 1) I start with pango_ft2_font_map_new()
> > 2) For each font I use I create a pango-context from this ft2 font-map.
> > 3) I set the user-selected font-description for the pango-context
> > 4) I render the standard ASCII character set to measure the standard
> > width, height and baseline
> > 5) These standard sizes are then used as default cell-size
>
> That sounds about right.  I assume you prefer not to have a cairo dependency
> added in?  If you don't mind, pangocairo is the preferred backend.  pangoft2
> is obsolete.

I don't want the cairo-dependency in the main kmscon program. However,
as long as the pango font-backend uses cairo only internally, this is
fine. I mean, if a user installs the kmscon pango backend, then they
did compile pango with pango-cairo, anyway (at least 99% do that,
right?). So I don't mind as long as I pass the raw data buffers to the
kmscon text-renderers and don't have to pass cairo_t or similar.

But as you said, that's possible and I will have a look and try using
pangocairo with cairo_image_surface_t's.

> > However, I'd appreciate if someone could tell me whether there are
> > ways to improve this? And I have some questions that I couldn't figure
> > out myself:
> >
> > 1) How do I disable AA fonts? How can I enable subpixel rendering?
>
> I don't think pangoft2 supports subpixel rendering.  pangocairo does.  Not
> sure about AA.  You may be able to get away with that by calling a function
> using pango_ft2_font_map_set_default_substitute() and setting FC_ANTIALIAS in
> there.

I just checked it out, I can set all the subpixel/AA/etc. properties
if I pass the right subpixel-layout. So pangocairo is the way to go,
thanks.

> > 2) How can I detect system-defaults? That is, let the user define
> > whether to use AA-fonts, subpixel-rendering, font-name, ... I
> > currently configure this in kmscon-specific configuration but I'd
> > prefer font-config options.
>
> You should already be getting fontconfig going.  Just ask for font named
> "monospace".  There is not "system default" font size.  There *is* X / GNOME
> specific stuff for that, but I assume that's not what you are looking for.

I had a user who complained that their config wasn't correctly read.
Turned out to be a wrong configuration and everything works now. I
never played much with font-config so I wasn't sure. But you're right,
everything works properly.
font-config settings are fine, GNOME/KDE etc. settings don't make much
sense, right.

> > 3) How am I notified about font-updates in the system? If the
> > currently used font is updated I'd like to clear my cache and let
> > pango re-render the glyphs.
>
> For fontconfig changes, you can do something like this:
>
> http://git.gnome.org/browse/gnome-settings-daemon/tree/plugins/xsettings/fontconfig-monitor.c

Awesome! Thanks! dnotify/inotify on the font-config files, few-seconds
timeout to combine multiple notifications and then clear the caches.
One thing is missing though: You don't update the monitor itself if
the configuration changes? Is that on purpose? There might be new
font-directories or configuration files that we should add to the
monitor.

Also, which pango object do I need to recreate on change? Everything
down to the font-map?

> > 4) What should I use as default render-mode? High-DPI (>300?) no AA
> > but subpixel? Low-DPI AA but no subpixel? I have actually no idea what
> > looks best... And what to do if I cannot get DPI or subpixel-layout
> > information for a display?
>
> If anything, you should enable subpixel for low DPI and siable for high.  It's
> tricky though.  Best to not try to out-guess the user.  Just do nothing?

I recently read your article about font-rendering with high DPI and
was wondering what to do. But maybe that decision belongs to
font-config and pango, and I should just let them chose the default
and not care at all. Users can always use font-configurations to
overwrite it, anyway.

Thanks a lot for your help! I will give pangocairo a try.
Regards
David