VP1Doc: Geometry system

Embedded in Athena, VP1 has direct access to the geometrical description of ATLAS in GeoModel, the description used when simulating ATLAS events and on which event reconstruction relies.

The first section below displays a movie, showcasing how to use most features of the geometry system, after which follows the detailed instructions.

Movie

The controller & and how to display volumes

The controller of the geometry system presents the user with check-boxes which can be used to toggle the display of GeoModel volumes from the various subsystems of ATLAS:

Notice that a given checkbox will be greyed out or hidden when the GeoModel geometry of a given subsystem was not built in a given job. At the top of the controller are six buttons which launches dialogs containing further options (which we will discuss in the next sections), while the bottom is used for a summary of how one can change the displayed volumes interactively when the viewer is in pick mode (cf. the 3D viewer documentation):

• Hold down CTRL and click on a volume: Replace the volume with its daughters, if it has any (i.e. open the volume).
• Hold down SHIFT and click on a volume: Replace the volume and any siblings its might have with its mother volume if it has one (i.e. close the mother of the clicked volume).
• Hold down Z and click on a volume: Iconise it (Zap it). This removes the volume from the view. Since it is obviously impossible to click on a volume which is no longer there, the only way to get it back again is to click on its name in the "Icon box", where all currently iconised volumes are shown, with the most recently iconised volume listed at the end of the list.
  
  

  The icon box.

  Not surprisingly, the Icon Box is brought up by clicking the button marked as such in the controller.

Feedback upon selection of volumes

When clicking on a volume (in pick mode), the default behaviour is to print its in the VP1 messagebox. The Interactions dialog allows one to modify this behaviour:

Unchecking the "Geometry is selectable" checkbox means that the geometry volumes won't be pickable, and this for instance allows one to display transparent geometry along with event data while being able to click on event data which is behind or inside the geometry.

When the "Zoom to volumes" option is enabled, the view will zoom closely to the clicked volume. The "Muon Chamber End Views" options only concerns volumes which are part of muon chambers. When it is checked, a click on any part of a muon chamber will zoom to the chamber, and furthermore orient the view so that it is head-on the side of the chamber, and subsequent clicks will rotate the view so that the chamber is aligned either vertically or horizontally with the screen.

Print on selection

The checkboxes in the "Print on selection" box allows one to get more details printed about the selected volume: The "Shape" option gives details about the identity and dimensions of the corresponding GeoVolume, the "Transform" option about the transformation which puts it into it's global position, the "Copy No" identifies the volume to developers implementing the geometry in case of sibling volumes which shares a common shape and name and the "Material" option prints out information about the GeoMaterial of the volume.

The "Mass" option attempts to calculate the mass of a given volume: Both an exclusive mass as given simply by considering the dimensions and density of the volume, and an inclusive mass which also takes into account the dimensions and density of daughter volumes. The inclusive mass is thus what one ideally would measure if taking the real-life detector-part corresponding to the volume and putting it on a weighing scale. The "(unreliable)" refers to the fact that it is not currently not possible to figure out the (spacial) volume of the so-called boolean volumes (see below), so if the volume (or any of its children in case of inclusive mass calculations) is a boolean volume, then the printed masses can not be trusted.

Finally, the "Tree" option prints out the volume tree below the volume, and the "Muon station info" option prints out information which can help identify the station ID of a given chamber (only when the clicked volume is part of a muon chamber obviously).

Display options: Phi-cutaways, transparency and curve-realism

In the display options dialog, one finds a widget with 12 small red circles which, when clicked, hides all volumes in the corresponding phi-sector, or alternatively opens them up if they extend all the way around the beam-axis (NB: For technical reasons boolean volumes can't be opened up like this, so the phi-cutaways won't work for those - see last section below):

This dialog furthermore contains options for changing the transparency setting of all geometry volumes at the same time, and a slider for controlling how realistic curved surfaces are represented (e.g. how many straight edges are used to approximate their curved shape) This is left as an option since higher degrees of realism will lower the rendering speed.

Tree browser

The browser gives a representation of the GeoModel volume tree by putting the volume names into a standard tree browser, similar to the one found in many file-managers. This allows one to quickly get an overview of the available volumes and their relationship to each other. Volumes which are not currently being shown in the 3D view are greyed out.

Misc. dialog: Changing colours, auto-adapting, etc.

This dialog contains everything that didn't really fit anywhere else.

Actions on last selected volume

With the buttons here, one can perform actions on the volume which was last selected. This include changing it's colour by clicking the button to the right of "Edit material:" (in the image below a volume with a purple material was last selected). [NB: As of Nov. 2008, changes to colours are not preserved when saving/restoring configurations to .vp1 files. This is intended to change at some point.]

Pixel/SCT active modules

Use the checkboxes and buttons here to quickly open up the SCT and Pixel geometry, in exactly such a way as to have just the volumes representing the active modules left.

Actions on visible volumes

Sometimes one wishes to for example "remove all volumes whose name ends with Straw", or "open up all volumes with material Air". This can easily be done from here. Note that wild-cards are supported in the input fields, and that case is not considered when matching.

The options for iconising/expanding all visible volumes with non-standard shapes, are there since not all volume shapes used in VP1 (in particular in the muon dead material) can be represented in the .iv (VRML) files that can be output from the context menu in the viewer. Very few users will probably care about these options.

Automatic adaption of shown muon chambers to event data

When displaying hits or in the muon chambers, or muon tracks going through those, the default setting is to display the geometrical volumes of the relevant chambers. To change this behaviour, or to define exactly which parts of the chambers to show, use the Chambers dialog:

The "Reset to all chambers" and "Adapt to event data" buttons do just that. If the "Auto" checkbox is enabled, the chambers will adapt themselves to the event data whenever the event data changes (this checkbox is enabled by default, apart from when in the "Geometry Studies" plugin).

Notes about boolean volumes

Volumes in GeoModel are normally defined as one of several standard types: Boxes, cylinders, etc. These shapes are easily mapped onto 3D representations in VP1. However, it is also possible to define volumes in a boolean manner: Either as a boolean addition (e.g."A box of dimensions XXX combined with a cylinder of dimensions YYY"), or as boolean subtractions (e.g. "A box of dimensions XXX minus a cylinder of dimensions YYY"). Producing in algorithm which reliably can figure out the triangulation of the surfaces of such volume is not a trivial mathematical task. VP1 does contain code which attempts to do this, but it is not working perfectly (offers of help by math prodigies are welcome). Thus, the visualisation of boolean volumes in VP1 will not always be completely reliable, and might also give rise to errors printed to stdout like:

BooleanProcessor::assembleFace(732) : could not find next edge of the contour
BooleanProcessor: boolean operation failed
BooleanProcessor::triangulateContour : could not generate a triangle (infinite loop)
BooleanProcessor: boolean operation failed
BooleanProcessor::caseIE : unimplemented case

Furthermore, phi-cutaways of boolean volumes which extends around the beam-axis are impossible. The display of such volumes is thus accompanied by a warning from VP1:

Geometry/Geo: WARNING: Unknown volume type (boolean?) for volume around Z-axis (type SoPolyhedron). Phi-sector cuts won't work for this!

Finally, as mentioned above, the mass-calculations for boolean volumes is not always completely reliable.

TODO:Update display dialog with outlines. And the colour maps/last selected material. (misc dialog). And add doc for the advanced usage of the PhiSectionWidget