Various people are working on porting desktop virtualization UIs to GTK4. This typically involves virgl, and the GTK3 solution was to use GtkGLArea.
With GTK4, rendering is happening in GL anyway, so it should be enough to just wrap your content in a GdkTexture and hand it to GTK, either by using it as a paintable with GtkPicture, or with a GskTextureNode in your own snapshot() implementation.
This is a nice theory, but the practice is more complicated – the content is typically available as a dmabuf object, and with 4k rendering, you really want to avoid extra copies if you can help it. So we had to look at the available solutions for importing dmabufs as textures into GL without copies.
This turned into a quick tour through the maze of graphics APIs: OpenGL, EGL, GL ES, GLX, DRI, … the list goes on. In the end, it turns out that you can use EGL to wrap a dmabuf into an EGLImage, and use the GL_OES_EGL_image extension to create a GL texture from it.
GLX to EGL
This works fine with our Wayland backend, which uses EGL. Unfortunately, our much older X11 backend has a GL implementation using GLX, and there doesn’t seem to be a way to get a dmabuf imported into a GLX context.
So we had to do a little bit of extra work, and make our X11 backend use EGL as well. Thankfully Emmanuele had an old unfinished branch with such a conversion from a few years ago, which could be made to work (after some initial head scratching why it would not render anything – as always the case when doing GL work).
It turns out that importing dmabufs with EGL can be done outside of GTK just fine, so we don’t need to add Linux-specific API for it. To save you the trouble of writing such code yourself, here is what I’ve come up with.
After we had already decided to port the X11 backend to EGL, I learned that another possibility for importing dmabufs might be to use DRI3PixmapFromBuffer to create an X11 pixmap from a dmabuf, turn it in a GLXPixmap and use glxBindTexImageEXT to make a texture.
Aren’t graphics APIs wonderful! :-)