Re: Pango device units

[Owen Taylor <otaylor redhat com>]

On Thu, 2003-08-28 at 16:20, Pawel S. Veselov wrote:
> Owen,
>
> On Thu, Aug 28, 2003 at 04:02:52PM -0400, Owen Taylor wrote:
> > On Thu, 2003-08-28 at 15:43, Pawel S. Veselov wrote:
> > > Owen,
> > > Yeah, I'm aware of PANGO_SCALE being a multiplier between device
> > > units and pango units, but it doesn't seem to me that the device
> > > units are actually pixels. If I set a font, say "Sans 10" for
> > > pango layout, or call the pango_font_description_set_size(desc, 10),
> > > the pango_layout_get_pixel_size() returns like 14 for the height.
> > Oh, you mean "it doesn't seem to me that the device units are
> > actually points". The points <=> device units conversion isn't 
> > something that is available at the current time.,
>
> Hmm, from pango_font_description_set_size() doc:
>
> size : the size for the font description in pango units. There are PANGO_SCALE
> Pango units in one device unit (device unit is a point, for font sizes).

That's sort of confusing. What it should say is something like:

 The size is a fixed point number, with a scale of PANGO_SCALE.
 size / PANGO_SCALE gives the font size in points. The conversion
 between points and device units.

I'd like to reserve the term "device units" to the dimensions that
refer to distances on a device.

> from pango_layout_get_pixel_size() doc:
>
> Determines the logical width and height of a PangoLayout in device units.
> (pango_layout_get_size() returns the width and height in thousandths of a
> device unit.) This is simply a convenience function around
> pango_layout_get_extents().
>
> Either doc is wrong, or the question still should sound "what's the
> device unit (point) <=> pixel conversion ?"

The font doc is a bit confused. Feel free to file a bug,

> pango_font_description() also says that SIZE is size in points...
>
> > http://mail.gnome.org/archives/gtk-i18n-list/2003-August/msg00001.html
> > (Includes how to get it if you know you are using Xft2, and
> > are willing to have a bunch of hacky code)
> >
> > http://bugzilla.gnome.org/show_bug.cgi?id=119081
>
> Yeah, that sounds like a work around. I wasn't aware that scaling will
> be different for X/Xft backends... Was the 119081 implemented in the
> pango that just got out ?

No. 1.2.5 is a maintainence release, no new API. (If it was implemented,
it would be closed! :-)

> > > Basically, what I'm trying to do is to figure the largest font size
> > > while fitting text in a box... If I keep scaling down until
> > > pango_layout_get_pixel_size() returns the height I need, the
> > > text becomes extremely small and unreadable...
> > No, I don't understand that. pango_layout_get_pixel_size()
> > returns how big the layout is in pixels.
>
> Well, doc says it still device units, but it looks like those are actually
> pixels. Anyway, am I right in the assumption that if a single line is rendered
> into the layout, the height of the layout will be the height of the font ?
> Does pango add any paddings ?

Generally, yes. For the right definition of "the height of the font". 
There is an inter-line pad (pango_layout_set_spacing) but no padding
around the whole layout.

Regards,
                                                Owen