MyPaint and goats at LGM2012

Already covered in the news from LGM was the release of GIMP 2.8, and that GIMP 2.10 will be fully GEGLified. The goat-invasion branch which has most of that work, the result of 3 weeks of pippin and mitch on a couch hacking together, has already landed in master. This means that GIMP now has support for high bit-depth workflows for most operations. Finally.

Putting the goat in MyPaint

During LGM I started working on using GEGL in MyPaint. I have already mentioned this idea several times, so it was time to stop talking and get hacking.

As a first step in making use of GEGL I wanted to replace the current surface implementation with one based on GeglBuffer. Since GeglBuffer already provides tiling, and can store any buffer data supported by Babl this turned out to be easy. Øyvind (pippin) added the semi-quirky pixel format we currently use* in MyPaint to Babl, and I was able to get a rough working GEGL based Surface implementation the first evening.

The MyPaint brush engine working on top of GeglBuffer

* RGBA premultiplied alpha, in 16 bit unsigned integers with  2^15 being the maximum value.

The next couple of days went to moving to the GeglBufferIterator API instead of gegl_buffer_{get,set} to have zero-copy access to improve performance, and improving GEGL and GEGL-GTK so that some of the hacks in the initial implementation could be removed.

Most of the work is in the gegl branch of MyPaint. A simple test application,, is included, and you can read README.gegl for how to try it out. Warning: only intended for curious developers at this stage.

A lots of work remains to be done for MyPaint to be able to fully use GEGL. The progress is tracked in two bugs, one for MyPaint work and one for GEGL issues. Because one cannot combine PyGObject with PyGTK, it will likely not be possible to fully integrate GEGL in MyPaint before porting to PyGI and GTK+ 3.

Oh, in case the goat references are lost on you – check the GEGL page on wikipedia.

Making GEGL easier to use in graphical applications

So, in the last couple of months I’ve been working a bit on GEGL. Some of the work has already been covered by LWN, so I guess it is time that I blog about it…

GEGL is a generic image processing library which is used by applications like GIMP, (and in the future maybe MyPaint and DarkTable). It provides applications with a graph based image processing backend that can do non-destructive processing of high-bitdepth images, among other things.

One of the problems that I think has been limiting adaptation of GEGL has been the entry barrier to starting to use it in a graphical application. While GEGL provides the image processing backend, it did not provide good and easy ways of displaying the output on screen. Now it does!

GTK+, Clutter and Qt integration libraries

Some code for integrating GEGL in GTK+ based applications has existed in the GEGL tree for a long time, but it was not well maintained and there was no public API. After brushing up the code to use Cairo for rendering and to support both GTK+ 2 and 3, it was split out to a separate library and repository: gegl-gtk. This library now provides a GtkWidget for displaying the output of a node in the GEGL graph, with basic support for scaling and translations. Any change in the GEGL graph will be reflected in the view widget. This makes it trivial for applications using a GTK+ based user interface to get started using GEGL, see for instance the provided examples in C or in Python.

The same functionality is provided for Clutter based user interfaces by gegl-clutter in form of a ClutterActor. This code was previously available as clutter-gegl, but has now been renamed and moved to be a part of the GEGL project, and is maintained by Øyvind Kolsås. Example code in C.

Last but not least, gegl-qt was created to serve the needs of applications using Qt based user interfaces. The different widget systems (QWidget-, QGraphicsWidget- and QML-based) are all supported. In addition to the features currently available in the GTK+ and Clutter versions, the Qt view widgets also support auto-scaling and auto-centering. Python bindings via PySide is planned, but blocking on a PySide issue at the moment.

A pretty boring screenshot showing two QWidget based examples (code: 1, 2) for transformations:

Artwork: “Wanted“, speedpainting by David Revoy

The first stable release of gegl-qt and gegl-gtk will hopefully be available soon. The list of tasks can be found in the README files.

Display operations

In GEGL, image processing is described as a graph of operations. “gegl:display” and “gegl:gtk-display” operations existed in the gegl tree, and by attaching one of these to a node in the graph one could display the output of the graph at the given node in a window . Such display operations are useful for applications that just want to show the output of a graph without having to use a GUI library directly.

The problem was that both of these operations were optional, so applications could not rely on this functionality to be present. This is solved by letting the “gegl:display” operation be a meta-operation, which uses other operations as a handler to actually display the output. Such display handler operations are now provided by gegl (optional, using SDL), gegl-gtk (using GTK+) and gegl-qt (using Qt). In addition a fallback operation that will export a PNG file and launch an external application to display it will be provided in GEGL.

More to GEGL stuff to come soon, hopefully.