Frequently Asked Questions


Why can’t the linker find

In the rare circumstances that you installed UFO from source for the first time by calling make install, the dynamic linker does not know that the library exists. If this is the case issue

$ sudo ldconfig

on Debian systems or

$ su
$ ldconfig

on openSUSE systems.

If this is not working, the library is neither installed into /usr/lib nor /usr/local/lib on 32-bit systems or /usr/lib64 and /usr/local/lib64 on 64-bit systems.


Why do I get a “libfilter<foo>.so not found” message?

Because the UFO core system is unable to locate the filters. By default it looks into ${LIBDIR}/ufo. If you don’t want to install the filters system-wide, you can tell the system to try other paths as well by appending paths to the UFO_PLUGIN_PATH environment variable.

Can I split a linear data stream?

The output data stream of a node can be split by setting the UFO_SEND_SEQUENTIAL mode and adjusting the number of expecting data items on each connected node:

from gi.repository import Ufo

in1_node.set_num_expected(0, 5)  # expect five items on the first input
in2_node.set_num_expected(0, -1) # expect all items

g = Ufo.TaskGraph()
g.connect_nodes(out_node, in1_node)
g.connect_nodes(out_node, in2_node)

The connection order matters here! If it would be reversed, in2_node would receive all items whereas in1_node wouldn’t receive anything.

How can I control the debug output from libufo?

Generally, UFO emits debug messages under the log domain Ufo. If you use a UFO-based tool and cannot see debug messages, you might have to enable them by setting the G_MESSAGES_DEBUG environment variable, i.e.:


To handle these messages from within a script or program, you must set a log handler that decides what to do with the messages. To ignore all messages in Python, you would have to write something like this:

from gi.repository import Ufo, GLib

def ignore_message(domain, level, message, user):

if __name__ == '__main__':
    GLib.log_set_handler("Ufo", GLib.LogLevelFlags.LEVEL_MASK,
        ignore_message, None)

How can I use Numpy output?

Install the ufo-python-tools. You can then use the BufferInput filter to process Numpy arrays data:

from gi.repository import Ufo
import ufo.numpy
import numpy as np

arrays = [ i*np.eye(100, dtype=np.float32) for i in range(1, 10) ]
buffers = [ ufo.numpy.fromarray(a) for a in arrays ]

pm = Ufo.PluginManager()
numpy_input = pm.get_task('bufferinput')

How can I synchronize two properties?

Although this is a general GObject question, synchronizing two properties is particularly important if the receiving filter depends on a changed property. For example, the back-projection should start only if a center-of-rotation is known. In Python you can use the bind_property function from the ufotools module like this:

from gi.repository import Ufo
import ufotools.bind_property

pm = Ufo.PluginManager()
cor = g.get_task('centerofrotation')
bp = g.get_task('backproject')

# Now connect the properties
ufotools.bind_property(cor, 'center', bp, 'axis-pos')

In C, the similar g_object_bind_property function is provided out-of-the-box.