Virtual Point 1 provides interactive 3D display of detector and event data

22 July 2008

VP1 is used here to display truth tracks and reconstructed calorimeter clusters, as well as TRT hits projected onto three projection planes (click for larger image)

In the spring of 2007, work started on a new event display for ATLAS. This display was named "Virtual Point 1" (VP1 for short), in reference to the location of the ATLAS detector: Point 1 at the LHC. What sets VP1 apart is its primary focus on providing a hardware accelerated 3D interactive display of detector and event data. But just as importantly, it is integrated directly in the ATLAS offline software framework, Athena. This means that VP1 can use any type of event data supported by ATLAS, and since it uses the native Event Data Model (EDM), it has automatic access to detector alignments and calibrations.

With access to an offline software release, most users will want to launch VP1 on an event file, such as the Event Summary Data (ESD) and Analysis Object Data (AOD) files created as output of reconstruction jobs, simply by typing "vp1 myeventfile.pool.root". While waiting for a new ATLAS stable release, interested users can download updated versions of VP1 for any release from the VP1 website. Advanced users might wish to instead launch VP1 directly from their custom Athena jobs, being able to investigate the contents of each event directly as it is being supplied. The combination of having a 3D display and of being integrated directly inside Athena is very powerful, and allows for very truthful representations of data. A particularly important benefit of this, which has already proven itself again and again over the last year, is that software problems that could otherwise be very hard to track down, can often be instantly recognised and understood once associated data is visualised in VP1.

Reconstructed tracks and jets are shown here along with a more detailed view of the calorimeter data, showing the individual cells in the various calorimeter subsystems (click for larger image)

The tight integration within the offline software means that VP1 has direct access to ATLAS helper tools. An example of where this is being used is in the display of tracks: In order to save on disk space, only a few points on each track are usually stored in data files. To reproduce a truthful representation of those tracks in an event display, one obviously needs to interpolate the trajectories between those points, taking into account in particular the complex magnetic field of ATLAS. In VP1 this interpolation is carried out with a few calls to the extrapolation tools normally used by the reconstruction algorithms. Additionally, it is possible to use ATLAS standard track fitters to refit the tracks in real time.  Another interesting example of the benefits of running inside Athena is the integration with the FatRas fast simulation: Not only can VP1 be used to display the events generated by FatRas, but when run in single particle mode ("vp1 -fatras"), it is even possible to steer the event generation settings from within VP1, click "next event" and immediately see the result of the resulting simulation.

In addition to an advanced display of tracking data, VP1 in particular excels in the visualisation of the ATLAS geometry, and is arguably the most useful tool for understanding and debugging of detector description across all subsystems. The VP1 calorimeter system is designed to display characteristics of ATLAS Calorimeter readout cells. Such characteristics include cell energy, time and pulse shape. The users can interactively select calorimeter subsystems, apply cuts and use an Athena tool for masking bad channels. Currently the VP1 calorimeter functionality is being extended by a new system for Calorimeter Clusters.

A closeup of a MDT chamber in the muon spectrometer, showing the hits along with a reconstructed track. The track was fitted using the hits coloured in blue, and both hits and track are projected onto the chamber side, facilitating track debugging. In the background, TGC hits are shown (click for larger image)

VP1 has been successfully integrated into the on-line monitoring environment at the ATLAS control room. A special client-server architecture for running VP1 in an online mode has been developed: One central server application receives events during data taking, reconstructs them and writes out a separate ESD file for every reconstructed event. These single event ESD files are then picked up and displayed by client applications, which can run VP1 in different configurations on any control room desk. In addition to that, a special 'live' mode for VP1 has been developed, allowing remote users to display these online events on any machine outside of the control room. Running VP1 in this mode is as simple as typing "vp1 -live", and no further special configuration is needed.

Although this is a recent project with only a few dedicated developers, the plugin infrastructure and Athena integration of VP1 makes it straightforward for any Athena developer to contribute to the VP1 functionality. Thus, during the last year, around 15 different persons made various degrees of contributions to the code, often benefiting from 3D building code fragments previously written for the now deprecated HEPVis/v-atlas event display. During this time, the focus of the project has been on providing a stable application with key plugins, and on integration in the online infrastructure at point 1.

Entering the second half of 2008 and the long awaited onset of actual data taking, the focus of VP1 development is shifting towards end users, aiming at completing features while striving to improve its usability. As part of this process it is also intended to arrange several VP1 tutorials, with specific dates to be announced at the VP1 website and hyper-news forum at a later date.

Thomas Kittelmann

University of Pittsburgh, USA

Vakhtang Tsulaia

University of Pittsburgh, USA