[cairo] PangoCairo plans requiring cairo API

Behdad Esfahbod behdad at behdad.org
Tue May 27 17:49:59 PDT 2008


Thought I write about my near-term PangoCairo plans that need api
changes in cairo.

Bug 527005 – Add pango_cairo_show_text()?

The idea here is to add a pangocairo drop-in replacement for
cairo_show_text.  It will take font options, font matrix, ctm, and font
face cairo_t.  The hard part is how to use the font face.  When I fix
pango to use a given cairo_font_face_t (next item), that would work.
But when the current font face on cairo_t is a toy font face, it makes
no sense to make pango use it, as this would provide no added value to
using cairo_show_text() directly.  What would be useful however, is if
Pango could get the family, slant, and weight values passed to
cairo_select_font() and use those to choose fonts.  That's why I
proposed cairo_toy_font_face_get_* API recently:


The cairo_toy_font_face_create() proposed there is also useful when
Pango wants to use cairo's toy text api as a failsafe fallback means to
render text.

Bug 347237 – Making Pango use a given cairo_font_face_t

This is the most wanted pangocairo feature at this time.  To make it
really useful, we need generic font-face constrcutors like
cairo_font_face_create_from_file/stream/memory() too.  It would then be
trivial to use application-provided fonts with pangocairo.

Bug 469091 – Get rid of pango_cairo_update_context() and pango_cairo_update_layout()

This requires a single API:

int cairo_serial (cairo_t *cr);

The serial number is part of the gstate and will be increased everytime
a property changes on cairo_t.  Serial number is pushed/popped in
gstate, but the "next serial" value lives in the cairo_t itself.  The
idea is that Pango can compare serial number of a cairo_t to
shortcircuit detecting changes.  Rendering to a different cairo_t still
needs calling _update_().


Other text-related suggestions:

- With user-fonts wanting to act as proxies using other fonts, we need
to access text_to_glyphs() functionality of arbitrary fonts.  That calls
for a cairo_scaled_font_text_to_glyphs().  Perhaps an advanced version
accommodating cairo_show_text_glyphs() semantics.

- A way to appoint a function to create toy font faces.  This can be
used to plug pango into the toy text api, making it actually a useful

- Add the twin stroke font an internal failsafe font (implemented as a
user-font) that can be selected by passing NULL to

- When the above changes is done, we can let cairo be usefully compiled
with no font backends at all.



"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
        -- Benjamin Franklin, 1759

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