User:Abhijit

ESA Summer of Code Project Proposal

Non-vacuum gravity simulator for the BRL-CAD Solid Modeling tool

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About Myself

• Name: Abhijit Nandy
• Mailing List ID: abhijit.nandy@gmail.com
• IRC ID: abhi2011

I am an international student (nationality Indian) in the second year of the Computer Engineering programme at TU Delft, The Netherlands. I have a computer science background and was working in the software industry for three years from 2006 to 2009. After that I decided to pursue a course in Computer Engineering to get acquainted with hardware development. More details are present in my CV. Currently I have finished my courses and thesis work and so I have free time on my hand to devote 40 hours a week to the development of BRL-CAD.

Brief project summary

Currently BRL-CAD provides facilities for representing geometry, but has limited capability for simulating physics effects on that geometry. The facilities currently present were designed with animations in mind. The proposal is to integrate rigid body dynamics using the Bullet Physics Engine or the Open Dynamics Engine (ODE). Bullet is provided under the zlib license while ODE is provided under both the BSD license and the LGPL. Both engines provide a collision detection engine and a Newtonian physics model. The exact physics engine to be used will be researched in the first week (see Development timeline). The chosen engine’s collision detection mechanism will be used to check for overlap of axis aligned bounding boxes or the bounding sphere. If an overlap is detected more accurate collision detection will be delegated to a custom collision handler that may use the BRL-CAD gqa and rtcheck tools for precise detection. Other collision handlers can be added if required in case they are absent for any of the BRL-CAD primitives. After the collision handling is tested to work correctly with the rest of the system, support will be added for user specified forces apart from ambient gravity. If time permits I intend to work on integrating and improving the system for joining objects together as well(see Detailed project description).

Detailed project description

The physics will be implemented as a command in the BRL-CAD Archer GUI. This will be of the form:

Archer> runphysics N

The parameter N is the number of simulation steps to run the physics. After the user invokes this command, the Tcl code for the command will call a C++ function containing the logic for doing the physics simulation. The function can be of the form:

int ged_runphysics(struct ged *gedp, int argc, const char *argv[])

The following occurs in the C++ function which will be central to the implementation:

• The struct ged object that is typically passed to plugins will be used to build up the physics world in a btDiscreteDynamicsWorld provided by Bullet.
• A Bullet collision shape will be inserted into the physics world corresponding to each object present in the struct ged. These are based on convex primitives such as btBoxShape, btSphereShape, btCylinderShape and they can approximate the BRL-CAD objects. The collision shapes will be constructed from BRL-CAD geometry information.
• After this the force vectors requested by the user will be read either from the struct ged or using other LIBGED commands. These will be applied on the collision shapes in the physics world.
• The physics world will then be stepped just once and the ged object positions will be set to the new positions using information from the Bullet btDiscreteDynamicsWorld.
• The repositioning of objects in Archer happens automatically by the command wrapper, once the objects are modified using LIBGED commands.
• The C++ function will then exit and control will return to Tcl.

The Tcl command wrapper will call the C++ function N times to obtain the simulation result after N steps. As the object positions will be updated on each call to ged_runphysics(), the user will be able to see the scene’s objects animate in Archer.

Particularly with regard to Bullet, as that is the only physics engine I have used so far, I am aware that it provides facilities for contact tests [2] and callbacks using the contactTest and contactPairTest queries. These can be accelerated using faster broadphase algorithms such as btDbvtBroadphase or btAxisSweep3 which affects the speed of test for overlapping AABBs [3]. Therefore real time updates should be possible. Moreover there are facilities for more fine grained control over the collision pipeline of Bullet, utilizing a custom class derived from btCollisionDispatcher [3]. Custom collision detection algorithms can be specified using btDispatcher::registerCollisionAlgorithm [3].

Physics engines also typically provide constraints such as point to point, hinge and 6 degree of freedom constraints which can be used to join objects together and improve the rudimentary joint rigging systems currently present.

More details will be added to this section as my research proceeds.

Note: The implementation will not rely on any polygonal mesh data as BRL-CAD does not store a mesh for its primitives. Instead physics objects for Bullet’s dynamics world will be created from other geometry information e.g. radius and tire width can be used to make a bounding cylinder for a tire.

Log

• Aug 10th:

1. Finished the code for the bounding box. It works now for basics shapes.Should be extended to work for regions and groups.

• Aug 9th:

1. Mostly reading libwdb and librt code and finishing the function
2. Read some of the tutorial articles on BRL-CAD

• Aug 8th:

1. Mostly reading code and documentation.
2. Started working on function for librt which calculates the bounding RPP when a shapes, region or group is passed in the database internal format struct rt_db_internal.

• Aug 7th:

1. Figured out how to transform objects to their new orientations and positions now in Mged !. The plan is to now move geometry in a model using the output transformation matrices of a physics sim. However mged draws the object only when the command is finished, so the command needs to be repeatedly called for multiple steps, to see the object animate.
2. Changed the runphysics command to "simulate" to imply more general usage based on brlcad's suggestion.
3. Working on a wrapper for the C++ based Bullet physics engine code to be called from the simulate.c file.
4. Started maintaining daily logs

• Aug 5th - Aug 6th:

1. Inserted a runphysics command in Mged.
2. Working on moving objects using matrix transforms inside Mged
3. Reading principles of effective modeling and going through code in librt.
4. Downloaded and compiled Bullet to make sure its installed and working.

• Aug 3rd - Aug 4th:

1. Finished most of the tutorials and articles for BRL-CAD
2. Worked through the Interactive Ray Tracing using NIRT guide to get familiar with ray tracing.
3. Converted the bounding box program to use librt functions only.

• Aug 1st - Aug 2nd:

1. Compiled BRL-CAD on OpenSUSE 11.4 using debugging options
2. Went through code in libged especially get_obj_bounds.c
3. Finished a stand alone program based on the very high level libged for getting the bounding box of arbitrarily shaped geometry.

Development timeline

• Week 1: Determine method for creating the bounding box(bb) for arbitrary geometry in BRL-CAD. Figure out how to move objects inside Mged(using a command wrapper for example). Try to move objects according to the output of a physics simulation. The simulation is of a sphere dropping to a ground plane under gravity.
• Week 2: Insert bb in the physics engine for arbitrarily shaped geometry and test whether custom collision handlers get called when the bb overlaps for 2 such objects(say a sph and an arb8). Improve collision detection for checking for more accurate collisions after a bb overlap is reported. Implement response for an impact.
• Week 3: Add more options to the runphysics command for more accurate simulations.Add support for user specified forces and further improving the custom collision handlers. Allow bodies to be defined as static or dynamic.
• Week 4: Testing of added code on more complex models. Ensure BRL-CAD's primitives are properly supported for collision detection and response.Check if the command wrapper can be moved to Archer.
• Week 5-12: TBD, maybe more testing and code improvements, add commands for easy addition of physics to objects.

Expected Result and eventual benefits for BRL-CAD

The expected result is to allow the user to model an object such as a sphere or a box and apply forces on them. The forces can be gravity, but will also allow the user to specify other additional forces to be applied during the simulation. The user can then “turn on” the physics using a simple command. The command allows the user to specify the number of physics steps as well. After the simulation begins the user will be able to see the object move in the GUI as the physics is updated. As time permits accurate material properties can be allowed to be specified and will be used to affect object collisions during the simulation. The system will allow frames to be captured during the simulation for video playback later on. Adding accurate physics to BRL-CAD will expand the scope of its usage allowing users to simulate and test concepts within the software. It provides an exciting addition to a powerful piece of software.

References

1. Bullet Physics: http://bulletphysics.org/mediawiki-1.5.8/index.php/Bullet_User_Manual_and_API_documentation
2. Bullet custom collision callbacks and triggers : http://bulletphysics.org/mediawiki-1.5.8/index.php/Collision_Callbacks_and_Triggers
3. Bullet User Manual: http://code.google.com/p/bullet/source/browse/trunk/Bullet_User_Manual.pdf
4. Open Dynamics Engine: http://www.ode.org/