S.J. Bailey, J.G. Cramer, D.J. Prindle, T.A. Trainor, P. Venable and D. Weerasundara
The NA49 event display QCDisplay1 was originally written to provide a visual verification that our main TPC tracking code worked. Because of the large number of tracks (about 300 per TPC) and the large number of measured space points (about 45,000 per TPC) it is very important to be able to easily choose and modify the viewpoint, scale and direction. By looking nearly along the direction of the tracks it is quite easy to see which points belong to tracks. By drawing the tracks at the same time we can see which tracks were missed, merged or suffered some other problem.
Briefly, QCDisplay can draw points and tracks reconstructed from raw data, and GEANT points and tracks. Each of these types of objects can be selected from and the selections can be assigned different colors. Movement, rotation and magnification can be done in a simple way, using the mouse for control. Individual points and tracks can also be selected by clicking on them with the mouse. QCDisplay knows the relationship between points and tracks and highlights the points on a selected track or the track of a selected point. Optionally, the point or track parameters can be displayed in a pop-up window. Pointers from the track or point to related objects can be followed and their parameters displayed in pop-ups.
During the past year we have made a number of enhancements to QCDisplay that make it a more generally useful tool. First, we added the capability of drawing curved tracks for particles bent by the magnetic field. This has made QCDisplay useful to persons working on the vertex TPC tracking code as well as to persons working to match vertex and main TPC tracks. A number of improvements have been made to make it easier to highlight tracking difficulties. With up to 1000 tracks per event and over 90% of the tracks fit very well it is inefficient to look for bad tracks by trial and error. If the event reconstruction program classifies a point or track as having a distinguishing characteristic it can set a bit in a variable. QCDisplay can map these bits to different colors. Optionally, each color can be displayed or not. This can be used to display only those points not assigned to tracks for example.
A more general method to select points or tracks has been provided by a tcl interface to QCDisplay. Within a tcl script one can get access to all information available to QCDisplay. This can be used to assign different colors to tracks depending on the track momentum or the number of points assigned to the track for example. The big advantage of the tcl interface is that it makes it possible for the user to extend the capabilities of QCDisplay without having to modify and relink the source code.
A few other additions have made QCDisplay more convenient to use. One of these is a color editor. This lets the user modify the colors interactively, useful to highlight certain features or just make the display more visually pleasing. The color palette can be saved and reloaded at a later time, making it possible to create sets of colors for different purposes. It is also possible to save the position, direction and magnification of the viewpoint and return to this view later. This is useful when one finds an interesting feature in an event and wants to be able to return. The option of saving the current display to a postscript file has also been added. This is more useful than a screen capture if one wants to enlarge or shrink the image later, for example when including the image within another postscript document.