GDI+ Pixbuf Loader: The art of getting native

One of the biggest problems that I've run into during the work of getting a sane building/release process for Gtk+ on windows, is the fact that it has a very Unix tied dependency tree:

• Gtk+ depends on libjpeg, libpng, libtiff, pango, cairo and Glib.
• Glib depends on gettext.
• libtiff depends on libjpeg
• libjpeg and libpng depends on zlib
• pango depends on cairo

Now, zlib and libjpeg are specially problematic to build for a Win32 target, and even more problematic if they're built on a cross compilation environment. So we wrote a few patches to workaround the issue, but the patches are ugly and not suitable for mainstream inclusion at all. So we were kind of screwed.

Until dom and tml had a great idea: why not write a pixbuf loader using the native win32 api to load images?

GDI+ is the imaging API for Windows XP (installable on 2000 and 98/Me), its antecessor, GDI,
would only load jpeg and non transparent png (remember the IE transparency problem? blame old GDI).

Now, GDI+ has some shortcomings, the most important being that it's not included on MinGW 32, so cross compilation is not an option at a first glance. Another one is that it's not included on Windows 2000 or below, not a big deal since there is an installation package for 2000 and 98/Me, and Gtk 2.12 is not supporting 98 anymore.

So, what do we do? Dom enlightened me with the wonders of LoadLibrary (dlload) and the work he did with Jordi Mas on GDI+ support for AbiWord. So we can load the functions pointers with their prototypes on runtime by doing LoadLibrary("gdiplus.dll"); take that lack of linking information!!! I borrowed some code from wine (which is LGPL as well) so I could figure out the structs and the function prototypes.

Some msdn research, some help from dom and there we go:

io-gdip_alpha1.tar.gz

$ git clone http://www.gnome.org/~aruiz/gtk+/io-gdip.git

It doesn't work yet really, all the Win32 api pieces are already there (I get the information and all the ARGB pixels of the images), but for some reason the Gdk.Pixbuf I create is not properly received by the application, I'm gonna need some help from dom or other pixbuf hero to finish it.

The result so far (there is supposed to go a background)

If I get this into a good shape, doing something similar for Mac OS X wouldn't be a big issue I guess.

If you want to help or just support the cause morally, just hang around #win32 at irc.gimp.org, where all the cross platform lovers are!

Update: I forgot Cairo and Pango dependencies, oops.

Comments

[Jeff Waugh]

So much awesome. :-)

[Dom Lachowicz]

I think that the bug is in the ARGB->RGBA conversion. For that, you'll want to borrow the unpremultiply_data() function from cairo:

http://cgit.freedesktop.org/cairo/tree/src/cairo-png.c

Also, you're displaying a multi-frame GIF. After you get the colorspace conversion stuff fixed, we'll want to work on getting animations to load.

Nice work so far :)

[asdasf]

I wonder if there are any license implications by linking directly against closed source software.

[triton]

Nice job.

@asdasf:

No license implications. Think of all the (open-source?) Windows applications that use the Windows API.

[faraox]

Awasome!!! Keep going with this work!

[troll]

Talking about native.. Where's WPF support - for the CURRENT Windows versions, for being native?

[Mikael Hallendal]

This is really great news!

Richard is considering a similar approach on Mac OS X as well.

[kenjiru]

keep up the good work!