Re: [xmlroff] Re: xmlroff on Windows

[Tony G. <Ton...@Su...>]

Jesper Stigberg wrote at 25 Apr 2003 16:50:25 +0200:
 > Sorry for the late reply.

That's okay.  I've only just got back from the second half of my
vacation.

 > Tony Graham wrote:
 > > Are you able to contribute your code to xmlroff or PangoPDF?
 > > 
 > 
 > Yes, as soon as I have some more time and the code is in reasonable shape I will.

Great.

 > Perhaps the best thing would be to write the 'screen renderer' in a cross 
 > platform tool kit (wxWindows,QT,GTK+) and use a pango backend based on
 > FT2/fontconfig.

There is a QT example in the core Pango examples that may be useful to
you.

...
 > > Now that none of area-to-pdf uses PDFlib, it is also possible to break
 > > up area-to-pdf and put each area type's code into the area type's
 > > implementation of the currently unused 'draw' function of FoArea.
 > 
 > Is this something you are planning todo in the nearest future?

It's actually something that I've wanted to do for a long time but haven't

 > And now ower to something completely different.
 > 
 > *** If I understand xmlroff correctly you are using pt as 'device-unit'. This 
 > works ok when using pangopdflib since it too (I assume) use pt as
 > unit, but my pango-win32 backend use pixel as unit. The result of
 > this is that all lengths/widths/heights are sometimes measured in
 > pixel (when they are 'received' from pango) and sometimes in
 > pt. It's not possible to know when to convert from pt to px and
 > when not to do it.

Yes, xmlroff uses pt as the 'device-unit'; i.e., it converts all
dimensions in the FO expressions into points.

I haven't looked at the Pango Win32 backend, but Pango uses 1/1024 pt
as a 'Pango Unit', and defines PANGO_SCALE to simplify converting
points to Pango units.  Pango uses Pango units except when it doesn't;
e.g. for font sizes.  Unfortunately, the Pango functions just use
'int' for all dimensions rather than defining a type for Pango units.
(Actually, PangoGlyphUnit is defined in pango-glyph.h but is mostly
only used in doc comments.)

 > I can see two solutions to this:
 > 
 > 1. All pango-backends compatible with xmlroff must use pt as a unit.
 > 
 > 2. Use a 'scale' in xmlroff to convert to pango-device-units, or convert from 
 > pango-device-units to xmlroff-device-units.

Or:

3. Require the child types of FoDoc (wish I had a better name for it)
   to handle the conversion from points (i.e., xmlroff units) to
   Pango backend units.

   For example, the current fo_doc_pdflib_render_layout() code does
   the multiplication by PANGO_SCALE to convert xmlroff units to Pango
   units.

   And, having complained about Pango, it now seems that xmlroff
   should define and use a typedef for xmlroff units to save people
   from repeating your confusion.

 > 1. will loose precision and will not be compatible with:
 > http://developer.gnome.org/doc/API/2.0/pango/pango-Glyph-Storage.html#PangoGlyphUnit
 > 
 > 2. seems the way to do it. I don't know if there is an API in pango that lets 
 > you express the device-unit in a 'known' unit, i.e. 'pango_deviceunit_to_twips' 
 > etc. If not, the 'scale' has to be set explicit by the 'client' of xmlroff/libfo.

In theory, the FT2 backend requires you to specify the resolution when
creating a PangoContext (or a PangoFT2Context), but the call to
FT_Set_Char_Size() in pangoft2.c shows that the resolution isn't
really used.

Also, the XSL Recommendation errata says that 1 px = 1/96 inch.

Regards,


Tony Graham
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