Introduction to the Inkscape 0.39 Pango Rearchitecting Effort
Wondering what the heck Pango is, why we want it in the codebase, what needs to be done, and/or what's been done so far? Then this document is for you. We'll try to cover the basics to get you oriented, and provide some links to more info.
- What does Inkscape really need for font rendering (perfect world)?
- What is Pango and what does it do?
- What does Inkscape currently have that it'd replace?
- What will Inkscape be able to get from Pango?
- How would Pango fit into Inkscape?
- Who's done what so far with the old code, and with Pango?
- Okay, what do we need to do?
- Pangoification Discussion 2004-06-05
What does Inkscape really need for font rendering (perfect world)?
What Inkscape needs from its font rendering subsystem is:
- Regular font rendering, including a wide range of different font metrics.
- Extraordinarily good support for internationalized fonts (many of the bugs left from the 0.38 Bug Hunt were due to inadequacies in font handling.)
- Great support for generating outlines of text objects
- Good support for us to build text layout algorithms (like text-along-a-path) onto.
- Provided by a well-documented, well-supported, widely-available library that is actively maintained and extended by people other than us, who are very responsive to Inkscape's needs. Ideally the library would add no additional dependencies on us.
What is Pango and what does it do?
Pango is a library that provides layout and rendering of internationalized text, that uses Unicode for its encoding and aims for supporting output in all the world's major languages.
Pango's principle use has been for internationalizing the text in Gnome GUI's. See the gallery: http://www.pango.org/gallery.shtml
From the Pango 1.4 release announcement:
Pango is designed to be modular; the core Pango layout can be used with three different font backends:
- Client-side fonts on X using the Xft2 library - Direct rendering of scalable fonts using the FreeType library - Native fonts on Microsoft platforms
Pango-1.4.0 ships with a wide selection of modules, including modules for Hebrew, Arabic, Hangul, Thai, and a number of Indic scripts. Virtually all of the world's major scripts are supported.
As well as the low level layout rendering routines, Pango includes PangoLayout, a high level driver for laying out entire blocks of text, and routines to assist in editing internationalized text.
More information about Pango is available from http://www.pango.org/"
In other words, Pango is a glue between platform-specific font subsystems and application-specific renderers, with strong internationalization support.
What does Inkscape currently have that it'd replace?
Inkscape currently uses a custom-written library called libnrtype for doing typeface rendering. This library provides wrappers around other platform-specific font subsystems, and allows us to do things like convert text to curves by getting the outlines of individual glyphs, look up typeface metrics, and convert CSS style representations to internal values for font weight, style, etc.
The library includes hooks to Xft, Gnome-Print, and Win32. It has dependencies into libnr. As part of the Inkscape renderer work, livarot has also been added as a dependency for this.
We aren't sure if anyone other than Inkscape and Sodipodi use this library. It was originally written by Lauris for Sodipodi. We've been maintaining it as things like livarot have been introduced, but the Inkscape team are not actively developing it anymore.
In other words, libnrtype is an efficient internal library that enables us to work with fonted text as first-class drawing objects, but unfortunately it does not have the complex support for internationalization that we need.
What will Inkscape be able to get from Pango?
Pango looks like it will bring significant value to us by allowing us to provide much better Internationalized font support, and in doing so, enable us to close a huge chunk of old bug reports.
It also replaces a sizeable chunk of code in Inkscape and instead of the extra overhead of maintaining our own specific code we get all the benefits of sharing a standard, stable, well maintained codebase that is pivotal to the Gnome desktop and has many more resources being put into it. This will result in a net reduction in the size of our codebase, which is a _good_ thing; less code means fewer places where bugs can appear and suck our time away from more important things.
Specifically, beyond what we currently have, Pango gives us:
- Internationalization support
- An actively maintained library (last release 16 Mar 2004)
- Documentation (Ref Man: http://developer.gnome.org/doc/API/2.0/pango/)
- Modular system for renderers gives us a clean interface for hooking Inkscape's renderer to.
- A text layout system (left/right align, tab stops, etc.) - may not be powerful enough to meet our needs, though.
- Already exists as a dependency for Inkscape due to Gtk, so adds no additional dependencies for our users.
How would Pango fit into Inkscape?
While Pango provides a lot that libnrtype does not, there are some things we'll need to consider:
- Text-to-path: Does Pango allow us to create outlines of text?
- Different function calls / changed API
- Any other unknown unknowns
These things aside, though, it could be as simple as just switching out the libnrtype calls with the corresponding ones from pango, and then test the heck out of it.
One approach to doing the conversion is to simply start going through libnrtype, function by function, and changing the routines to become pass-thru calls to the corresponding Pango calls, where feasible. Then, whatever is left-over we a) place in another location in Inkscape, and b) package up and submit as a patch to help improve Pango in the long-term.
Who's done what so far with the old code, and with Pango?
Nathan has the most experience with text layout issues, and has done some work on libnrtype in the past. Several of the core developers have touched on text/font areas of the code, and can provide some guidance around the edges of the code. Several team members have had some limited experience with Pango via its use through the GUI interface, but we don't yet have an experience base built up in it.
Beyond that, while we all are in consensus that this work needs done, that work has not begun in earnest yet. So even someone with no knowledge of fonts or Inkscape internals should be able to catch up to speed with the rest of us pretty darn quick. ;-)
Inkscape already indirectly depends on Pango because <a href="http://sourceforge.net/mailarchive/message.php?msg_id=7809807" title="GTK depends on Pango">Pango it is an internal dependency of GTK</a>
Okay, what do we need to do?
For specific tasks, please refer to the Inkscape Roadmap in the wiki site, but in general here is roughly what will need to be done:
a) Get up to speed in Pango. Get familiar with the reference manual. [Done]
b) Review the libnrtype code.
c) Replace calls of libnrtype routines that have close analogs in Pango with their Pango equivalents. If there are no remaining calls to the libnrtype routines, remove them from the codebase.
d) Analyze the remaining libnrtype routines to see if they can be refactored into Pango calls to get the equivalent behavior, or alternately examine if the calling code could be refactored to use Pango instead of libnrtype.
e) Re-abstract remaining libnrtype code that can't be rewritten as Pango calls and place in a new area of the codebase outside libnrtype. E.g., "nr-type-pango". Consider if it would be appropriate to also submit that chunk of code back to Pango as an added feature; if so, package it as a patch and send it to the Gtk mailing list (or a <a href="http://bugzilla.gnome.org/enter_bug.cgi?product=pango">patch tracker</a> if you can find one). For example, it could be _quite_ handy to refactor out the glyph outlining and text attributes code into some adaptor objects/functions that works with Pango.
f) Start employing Pango for doing text layout for e.g. SPText and friends.
For reference see <a href="http://sourceforge.net/mailarchive/forum.php?thread_id=4172159&forum_id=36054">mental's post about how to do Pango conversion</a> from the <a href="http://inkscape.org/mailing_lists.php">inkscape-devel@ mailing list</a>.
This is a summary of a discussion between mental, bulia, fred, and bryce on Jabber 2004-06-05 regarding adoption of Pango and the issues involved.
The principle issue we are facing at this time is that while many of the Pango issues we've identified seem to have doable work-arounds, it's become a little unclear what our eventual goal design-wise is with it in the long term. Specifically, Pango likes to grid-fit its layouts, which makes it a bit inflexible for things like getting undistorted outlines and layouts.
Pango does well with horizontal text but Fred had a question about how multi-byte characters are handled. E.g., an A with an umlaut requires two bytes. Would this render as a single character entity or as two? Certainly we wouldn't want the system to resort to putting the A and umlaut on separate lines.
Mental explained that Pango provides routines for determining how many "codepoints" makes up a particular character. When you pass this routine a UTF-8 string pointer, it will return the pointer advanced by the next full character. A codepoint can consist of one or more bytes. one or more codepoints define a 'glyph'. One or more glyphs define a character.
A glyph like an umlauts is an example of a "composing character". Fonts combine these composing characters with other base characters to form the various characters particular to a given language.
It looks like pango_layout_move_cursor_visually() would be used to determine the positioning of vertical text. This won't be simple if we don't use the Pango glyph positioning. For more info see: http://developer.gnome.org/doc/API/2.0/pango/pango-Layout-Objects.html#pango-layout-move-cursor-visually
For our own work this suggests we may need to create PangoLayouts to do our own specialized layout.
UPDATE: Fred has successfully tested it out. He says it's only repositioning the text vertically; the tricky part is to get the correct characters.
Control over font kerning is important for Inkscape yet it is uncertain if Pango provides this control. For letter spacing we need to be able to:
- Measure strings
- Count characters
- Handle pairwise kerning
- Glyph width (for things like text-on-path)
Currently, sp-chars creates a NRArenaGlyphGroup for each string, tied to one NRRasterFont, but for things like text-on-path we'll want to have unique transforms for each character. We do not want to have to create a separate GlyphGroup for each glyph though, so need a better solution to this.
The question came up about whether we can eliminate NRRasterFont. This appears to be a necessary step in the pipline, because it is where the transform is taken into account and where glyph caching occurs. There is a question about whether glyph caching is really necessary, or if we could do direct rendering and use performance analysis to determine where such optimizations are really necessary, but since text always has such massive redundancy, caching is likely to prove quite necessary.
We've very concerned that bugs in font handling are going to reproduce quickly on different platforms unless they're closely tracked by developers on those platforms. Is adopting Pango worth enduring those platform issues?
- Definition of character: http://www.w3.org/TR/REC-xml/#dt-character
-- Bryce 4/16/04 -- Alan 18/04/04 UTC, various minor edits