the first step in getting to a modern tool kit with a decent handling of the graphic pipeline is to actually have a decent handling of the graphic pipeline; it turns out that Cairo does some of that for us, but since everyone has decided that the only API to be exposed by GPU drivers is OpenGL (and its variants) we probably need to do something about it.

as promised during GUADEC, I’m going to blog a bit more about the development of GSK — and now that I have some code, it’s actually easier to do.

so, let’s start from the top, and speak about GDK.

in April 2008 I was in Berlin, enjoying the city, the company, and good food, and incidentally attending the first GTK+ hackfest. those were the days of Project Ridley, and when the plan for GTK+ 3.0 was to release without deprecated symbols and with all the instance structures sealed.

in the long discussions about the issue of a “blessed” canvas library to be used by GTK app developers and by the GNOME project, we ended up discussing the support of the OpenGL API in GDK and GTK+. the [original bug][bug-opegl] had been opened by Owen about 5 years prior, and while we had ancillary libraries like GtkGLExt and GtkGLArea, the integration was a pretty sore point. the consensus at the end of the hackfest was to provide wrappers around the platform-specific bits of OpenGL inside GDK, enough to create a GL context and bind it to a specific GdkWindow, to let people draw with OpenGL commands at the right time in the drawing cycle of GTK+ widgets. the consensus was also that I would look at the bug, as a person that at the time was dealing with OpenGL inside tool kits for his day job.

well, that didn’t really work out, because cue to 6 years after that hackfest, the bug is still open.

to be fair, the landscape of GTK and GDK has changed a lot since those days. we actually released GTK+ 3.0, and with a lot more features than just deprecations removal; the whole frame cycle is much better, and the paint sequence is reliable and completely different than before. yet, we still have to rely on poorly integrated external libraries to deal with OpenGL.

right after GUADEC, I started hacking on getting the minimal amount of API necessary to create a GL context, and being able to use it to draw on a GTK widget. it turns out that it wasn’t that big of a job to get something on the screen in a semi-reliable way — after all, we already had libraries like GtkGLExt and GtkGLArea living outside of the GTK git repository that did that, even if they had to use deprecated or broken API. the complex part of this work involved being able to draw GL inside the same infrastructure that we currently use for Cairo. we need to be able to synchronise the frame drawing, and we need to be able to blend the contents of the GL area with both content that was drawn before and after, likely with Cairo — otherwise we would not be able to do things like drawing an overlay notification on top of the usual spinning gears, while keeping the background color of the window:

welcome to the world of tomorrow (for values of tomorrow close to 2005)

luckily, thanks to Alex, the amount of changes in the internals of GDK was kept to a minimum, and we can enjoy GL rendering running natively on X11 and Wayland, using GLX or EGL respectively.

on top of the low level API, we have a GtkGLArea widget that renders all the GL commands you submit to it, and it behaves like any other GTK+ widgets.

today, Matthias merged the topic branch into master , which means that, barring disastrous regressions, GTK+ 3.16 will finally have native OpenGL support — and we’ll be one step closer to GSK as well.