Visualisation
This Chapter is intended to be read after Chapter
on Visualisation object oriented design in Part II. Many of the concepts
used here are defined there, and it strongly recommended that a writer
of a new visualisation driver or trajectory drawer reads Chapter
first.
The class structure described there is
summarised in .
Creating a new graphics driver
To create a new graphics driver for Geant4, it is necessary to implement a
new set of three classes derived from the three base classes,
G4VGraphicsSystem, G4VSceneHandler
and G4VViewer.
A useful place to start
A skeleton set of classes is included in the code distribution in the
visualisation category under subdirectory visualisation/XXX
(but they are not default-registered graphics systems
To do this,simply instantiate and register, for example:
visManager->RegisterGraphicsSystem(new G4XXX)
before visManager->Initialise().
There are several sets of classes, described in more detail below.
A recommended approach is to copy the files that best match your graphics
system to a new subdirectory with a name that suits your graphics system .
Then
Change the name of the files (change the code -- XXX or
XXXFile, etc., as chosen -- to something that suits
your graphics system ).
Change XXX similarly in all files.
Change XXX similarly in name := G4XXX
in GNUmakefile.
Add your new subdirectory to SUBDIRS and
SUBLIBS in visualisation/GNUmakefile.
Look at the code and use it to build your visualisation
driver. You might also find it useful to look at
ASCIITree (and VTree) as an example
of a minimal graphics driver . Look at FukuiRenderer
as an example of a driver which implements AddSolid
methods for some solids. Look at OpenGL as an example
of a driver which implements a graphical database (display lists) and the
machinery to decide when to rebuild. (OpenGL is complicated by the
proliferation of combinations of the use or not of display lists for
three window systems, X-windows, X with motif (interactive), Microsoft
Windows (Win32), a total of six combinations, and much use is made of
inheritance to avoid code duplication.)
If it requires external libraries, introduce two new environment
variables G4VIS_BUILD_XXX_DRIVER and
G4VIS_USE_XXX
(where XXX is your choice as above) and make the
modifications to:
source/visualization/management/include/G4VisExecutive.iccconfig/G4VIS_BUILD.gmkconfig/G4VIS_USE.gmk
Graphics driver templates in the XXX sub-category
You may use the following templates to help you get started writing a
graphics driver . (The word ``template'' is used in the ordinary sense of the
word; they are not C++ templates.)
G4XXX, G4XXXSceneHandler, G4XXXViewer Templates for the
simplest possible graphics driver . These would be suitable for an ``immediate''
driver, i.e., one which renders each object immediately to a screen.
Of course, if the view needs re-drawing, as, for example, after a
change of viewpoint, the viewer requests a re-issue of drawn
objects.
G4XXXFile, G4XXXFileSceneHandler, G4XXXFileViewer Templates
for a file-writing graphics driver. The particular features are:
delayed opening of the file on receipt of the first item; rewinding
file on ClearView (to simulate the clearing of views and prevent the
duplication of material in the file); closing of the file on ShowView,
which may also trigger the launch of a browser. There are various
degrees of sophistication in, for example, the allocation of filenames
-- see FukuiRenderer or HepRepFile.
These templates also show the use of a specific AddSolid
function whereby the specific parameters, for example, the dimensions of a
G4Box, can be accessed.
G4XXXStored, G4XXXStoredSceneHandler, G4XXXStoredViewer
Templates for a graphics driver with a store/database. The advantage
of a store is that the view can be refreshed, for example, from a
different viewpoint, without a need to recompute. It is up to the
viewer to decide when a re-computation is necessary. They also show how
to distinguish between permanent and transient objects -- see also
Section .
G4XXXSG, G4XXXSGSceneHandler, G4XXXSGViewer Templates for a
sophisticated graphics driver with a scene graph. The scene graph, following Open
Inventor parlance, is a tree of objects that dictates the order in
which the objects are rendered. It obviously lends itself to the
rendering of the Geant4 geometry hierarchy. For example, the Open
Inventor driver draws only the top level volumes unless made invisible
by picking. Thus the user can unwrap a branch of the geometry level
by level. This has performance benefits and gives the user significant
and useful control over the view. These classes show how to make a
scene graph of drawn volumes, i.e.,
the set of volumes that have
not been culled. (Normally, volumes marked invisible are culled,
i.e., not drawn. Also, the user may wish to limit the number of drawn
volumes for performance reasons.) The drivers also have to process
non-geometry items and distinguish between transient and permanent
objects as above.
Important Command Actions
To help understand how the Geant4 Visualization System works, here are a
few important function invocation sequences that follow user commands.
For an explanation of the commands themselves, see command guidance or
the Control section of the Application Developers Guide. For a
fuller explanation of the functions, see appropriate base class head
files or Software Reference Manual.
/vis/viewer/clear
viewer->ClearView(); // Clears buffer or rewinds file.
viewer->FinishView(); // Swaps buffer (double buffer systems).
/vis/viewer/flush
/vis/viewer/refresh
/vis/viewer/update
/vis/viewer/rebuild
viewer->SetNeedKernelVisit(true);
/vis/viewer/refresh If the view is ``auto-refresh'', this
command is also invoked after /vis/viewer/create,
/vis/viewer/rebuild or a change of view parameters
(/vis/viewer/set/..., etc.).
viewer->SetView(); // Sets camera position, etc.
viewer->ClearView(); // Clears buffer or rewinds file.
viewer->DrawView(); // Draws to screen or writes to
// file/socket.
/vis/viewer/update
viewer->ShowView(); // Activates interactive windows or
// closes file and/or triggers
// post-processing.
/vis/scene/notifyHandlers For each viewer of the current
scene, the equivalent of
/vis/viewer/refresh
If ``flush'' is specified on the command line, the equivalent of
/vis/viewer/update
/vis/scene/notifyHandlers is also invoked after a change
of scene (/vis/scene/add/..., etc.).
What happens in DrawView?
This depends on the viewer. Those with their own graphical database,
for example, OpenGL's display lists or Open Inventor's scene graph, do
not need to re-traverse the scene unless there has been a significant
change of view parameters. For example, a mere change of viewpoint
requires only a change of model-view matrix whilst a change of
rendering mode from wireframe to surface might require a rebuild of
the graphical database. A rebuild of the run-duration (persistent)
objects in the scene is called a ``kernel visit''; the viewer prints
``Traversing scene data...''.
Note that end-of-event (transient) objects are only rebuilt at the end
of an event or run, under control of the visualisation manager. Smart
scene handlers keep them in separate display lists so that they can be
rebuilt separately from the run-duration objects - see
.
Integrated viewers with no graphical database
For example, G4OpenGLImmediateXViewer::DrawView().
NeedKernelVisit(); // Always need to visit G4 kernel.
ProcessView();
FinishView();
Integrated viewers with graphical database
For example, G4OpenGLStoredXViewer::DrawView().
KernelVisitDecision(); // Private function containing...
if significant change of view parameters...
NeedKernelVisit();
ProcessView();
FinishView();
File-writing viewers For example,
G4DAWNFILEViewer::DrawView().
NeedKernelVisit();
ProcessView();
Note that viewers needing to invoke FinishView must do it in
DrawView.
What happens in ProcessView?
ProcessView is inherited from G4VViewer:
void G4VViewer::ProcessView() {
// If ClearStore has been requested, e.g., if the scene has changed,
// of if the concrete viewer has decided that it necessary to visit
// the kernel, perhaps because the view parameters have changed
// drastically (this should be done in the concrete viewer's
// DrawView)...
if (fNeedKernelVisit) {
fSceneHandler.ProcessScene(*this);
fNeedKernelVisit = false;
}
}
What happens in ProcessScene?
ProcessScene is inherited from G4VSceneHandler}.
It causes a traversal of the run-duration models in the scene.
For drivers with graphical databases, it causes a rebuild
(ClearStore). Then for the run-duration models:
fReadyForTransients = false;
BeginModeling();
for each run-duration model...
pModel -> DescribeYourselfTo(*this);
EndModeling();
fReadyForTransients = true;
(A second pass is made on request -- see
G4VSceneHandler::ProcessScene.)
The use of fReadyForTransients
is described in .
What happens then depends on the type of model:
G4AxesModelG4AxesModel::DescribeYourselfTo
simply calls sceneHandler.AddPrimitive methods directly.
sceneHandler.BeginPrimitives();
sceneHandler.AddPrimitive(x_axis); // etc.
sceneHandler.EndPrimitives();
Most other models are like this, except for the following...
G4PhysicalVolumeModel The geometry is descended
recursively, culling policy is enacted, and for each accepted (and
possibly, clipped) solid:
sceneHandler.PreAddSolid(theAT, *pVisAttribs);
pSol->DescribeYourselfTo(sceneHandler);
// For example, if pSol points to a G4Box...
|-->G4Box::DescribeYourselfTo(G4VGraphicsScene& scene){
scene.AddSolid(*this);
}
sceneHandler.PostAddSolid();
The scene handler may implement the virtual function {
AddSolid(const G4Box&)}, or inherit:
void G4VSceneHandler::AddSolid(const G4Box& box) {
RequestPrimitives(box);
}
RequestPrimitives converts the solid into primitives
(G4Polyhedron) and invokes AddPrimitive:
BeginPrimitives(*fpObjectTransformation);
pPolyhedron = solid.GetPolyhedron();
AddPrimitive(*pPolyhedron);
EndPrimitives();
The resulting default sequence for a G4PhysicalVolumeModel
is shown in .
Note the sequence of calls at the core:
sceneHandler.PreAddSolid(theAT, *pVisAttribs);
pSol->DescribeYourselfTo(sceneHandler);
|-->sceneHandler.AddSolid(*this);
|-->RequestPrimitives(solid);
|-->BeginPrimitives (*fpObjectTransformation);
|-->pPolyhedron = solid.GetPolyhedron();
|-->AddPrimitive(*pPolyhedron);
|-->EndPrimitives();
sceneHandler.PostAddSolid();
is reduced to
sceneHandler.PreAddSolid(theAT, *pVisAttribs);
pSol->DescribeYourselfTo(sceneHandler);
|-->sceneHandler.AddSolid(*this);
sceneHandler.PostAddSolid();
if the scene handler implements its own AddSolid.
Moreover, the sequence
BeginPrimitives (*fpObjectTransformation);
AddPrimitive(*pPolyhedron);
EndPrimitives();
can be invoked without a prior PreAddSolid, etc.
The flag fProcessingSolid will be false for the
last case. The possibility of any or all of these three scenarios
occurring, for both permanent and
transient objects, affects the implementation of a scene handler if
there is any attempt to build a graphical database. This is reflected
in the templates XXXStored and
XXXSG described in
.
Transients are discussed in
.
G4TrajectoriesModel At end of event, the trajectory
container is unpacked and, for each trajectory,
sceneHandler.AddCompound called.
The scene handler may implement this virtual function or inherit:
void G4VSceneHandler::AddCompound (const G4VTrajectory& traj) {
traj.DrawTrajectory(((G4TrajectoriesModel*)fpModel)->GetDrawingMode());
}
Similarly, the user may implement DrawTrajectory
or inherit:
void G4VTrajectory::DrawTrajectory(G4int i_mode) const {
G4VVisManager* pVVisManager = G4VVisManager::GetConcreteInstance();
if (0 != pVVisManager) {
pVVisManager->DispatchToModel(*this, i_mode);
}
}
Thence, the Draw method of the current trajectory model
is invoked (see for details
on trajectory models), which in turn, invokes Draw
methods of the visualisation manager.
The resulting default sequence for a G4TrajectoriesModel
is shown in .
Dealing with transient objects
Any visualisable object not defined in the run-duration part of a
scene is treated as ``transient''. This includes trajectories, hits
or anything drawn by the user through the G4VVisManager
user-level interface (unless as part of a run-duration model implementation).
A flag, fReadyForTransients}, is maintained by
the scene handler. In fact, its normal state is true, and
only temporarily, during handling of the run-duration part of the scene, is
it set to false -- see description of ProcessScene,
.
If the driver supports a graphical database, it is smart to
distinguish transient and permanent objects. In this case, every
Add method of the scene handler must be transient-aware.
In some cases, it is enough to open a graphical data base component in
BeginPrimitives, fill it in AddPrimitive
and close it appropriately in EndPrimitives.
In others, initialisation is done in BeginModeling
and consolidation in EndModeling -- see
G4OpenGLStoredSceneHandler.
If any AddSolid method is
implemented, then the graphical data base component should be opened
in PreAddSolid, protecting against double opening,
for example,
void G4XXXStoredSceneHandler::BeginPrimitives
(const G4Transform3D& objectTransformation) {
G4VSceneHandler::BeginPrimitives(objectTransformation);
// If thread of control has already passed through PreAddSolid,
// avoid opening a graphical data base component again.
if (!fProcessingSolid) {
for other solids.
The reason for this distinction is that at end of run the user
typically wants to display trajectories on a view of the detector,
then, at the end of the next event
There is an option to accumulate trajectories across events and runs
-- see commands
/vis/scene/endOfEventAction and
/vis/scene/endOfRunAction.
, erase the old and see new trajectories. The visualisation manager
messages the scene handler with ClearTransientStore
just before drawing the trajectories to achieve this.
If the driver does not have a graphical database or does not
distinguish between transient and persistent objects, it must emulate
ClearTransientStore. Typically, it must erase everything,
including the detector, and re-draw the detector and other run-duration
objects, ready for the transients to be added. File-writing drivers must
rewind the output file. Typically:
void G4HepRepFileSceneHandler::ClearTransientStore() {
G4VSceneHandler::ClearTransientStore();
// This is typically called after an update and before drawing hits
// of the next event. To simulate the clearing of "transients"
// (hits, etc.) the detector is redrawn...
if (fpViewer) {
fpViewer -> SetView();
fpViewer -> ClearView();
fpViewer -> DrawView();
}
}
ClearView rewinds the output file and
DrawView re-draws the detector, etc.
(For smart drivers, DrawView is smart enough to
know not to redraw the detector, etc., unless the view parameters have
changed significantly -- see
)
More about scene models
Scene models conform to the G4VModel abstract interface.
Available models are listed and described there in varying detail.
describes their
use in some common command actions.
In the design of a new model, care should be taken to handle the
possibility that the G4ModelingParameters pointer is zero.
Currently the only use of the modeling parameters is to communicate
the culling policy. Most models, therefore, have no need for modeling
parameters.
Enhanced Trajectory Drawing
Creating a new trajectory model
New trajectory models must inherit from G4VTrajectoryModel and
implement these pure virtual functions:
virtual void Draw(const G4VTrajectory&, G4int i_mode = 0,
const G4bool& visible = true) const = 0;
virtual void Print(std::ostream& ostr) const = 0;
To use the new model directly in compiled code, simply
register it with the G4VisManager, eg:
G4VisManager* visManager = new G4VisExecutive;
visManager->Initialise();
// Create custom model
MyCustomTrajectoryModel* myModel =
new MyCustomTrajectoryModel("custom");
// Configure it if necessary then register with G4VisManager
...
visManager->RegisterModel(myModel);
Adding interactive functionality
Additional classes need to be written if the new model is to
be created and configured interactively:
Messenger classes
Messengers to configure the model should inherit from
G4VModelCommand. The concrete trajectory model type should be
used for the template parameter, eg:
class G4MyCustomModelCommand
: public G4VModelCommand<G4TrajectoryDrawByParticleID> {
...
};
A number of general use templated commands are available in
G4ModelCommandsT.hh.
Factory class
A factory class responsible for the model and associated messenger
creation must also be written. The factory should inherit from
G4VModelFactory. The abstract model type should be used for the
template parameter, eg:
class G4TrajectoryDrawByChargeFactory
: public G4VModelFactory<G4VTrajectoryModel> {
...
};
The model and associated messengers should be constructed in the Create
method. Optionally, a context object can also be created, with its own
associated messengers. For example:
ModelAndMessengers
G4TrajectoryDrawByParticleIDFactory::
Create(const G4String& placement, const G4String& name)
{
// Create default context and model
G4VisTrajContext* context = new G4VisTrajContext("default");
G4TrajectoryDrawByParticleID* model =
new G4TrajectoryDrawByParticleID(name, context);
// Create messengers for default context configuration
AddContextMsgrs(context, messengers, placement+"/"+name);
// Create messengers for drawer
messengers.push_back(new
G4ModelCmdSetStringColour<G4TrajectoryDrawByParticleID>
(model, placement));
messengers.push_back(new
G4ModelCmdSetDefaultColour<G4TrajectoryDrawByParticleID>
(model, placement));
messengers.push_back(new
G4ModelCmdVerbose<G4TrajectoryDrawByParticleID>
(model, placement));
return ModelAndMessengers(model, messengers);
}
The new factory must then be registered with the visualisation manager.
This should be done by overriding the G4VisManager::RegisterModelFactory
method in a subclass. See, for example, the G4VisManager implementation:
G4VisExecutive::RegisterModelFactories()
{
...
RegisterModelFactory(new G4TrajectoryDrawByParticleIDFactory());
}
Trajectory Filtering
Creating a new trajectory filter model
New trajectory filters must inherit at least from G4VFilter. The
models supplied with the Geant4 distribution inherit from
G4SmartFilter, which implements some specialisations on top of
G4VFilter. The models implement these pure virtual functions:
// Evaluate method implemented in subclass
virtual G4bool Evaluate(const T&) = 0;
// Print subclass configuration
virtual void Print(std::ostream& ostr) const = 0;
To use the new filter model directly in compiled code, simply
register it with the G4VisManager, eg:
G4VisManager* visManager = new G4VisExecutive;
visManager->Initialise();
// Create custom model
MyCustomTrajectoryFilterModel* myModel =
new MyCustomTrajectoryFilterModel("custom");
// Configure it if necessary then register with G4VisManager
...
visManager->RegisterModel(myModel);
Adding interactive functionality
Additional classes need to be written if the new model is to
be created and configured interactively. The mechanism is exactly
the same as that used to create enchanced trajectory drawing
models and associated messengers. See the filter factories in
G4TrajectoryFilterFactories for example implementations.
Other Resources
The following sections contain various information for extending
other class functionalities of Geant4 visualisation:
User's Guide for Application Developers, Chapter 8 - Visualization
User's Guide for Toolkit Developers, Object-oriented Analysis
and Design of Geant4 Classes, .
[Status of this chapter]
03.12.05 ``Enhanced Trajectory Drawing'' added by Jane Tinsley.
03.12.05 ``Creating a new visualisation driver'' (from Part II)
by John Allison.
09.01.06 ``Creating a new visualisation driver'' considerably expanded
by John Allison.
20.06.06 Some sections improved or added from draft vis paper.
John Allison.
Dec. 2006 Conversion from latex to Docbook verson by K. Amako