HAPTIX Simulation World API


This tutorial gives an overview of the haptix-comm simulation-specific API.

We assume that you have already completed the installation step.


The full world API documentation can be found here.

Conventions and terminology

A "transform" is a 3D position and rotation, also known as a pose. In the world API, we represent rotations as quaternions.

Unless otherwise stated, all vectors and transforms are relative to a fixed world frame.

Through the documentation, we refer to "links", "joints", and "models".

A "link" is a rigid body that moves as a whole.

A "joint" holds together two links, constraining their motion. A joint may have limits.

A "model" is a named object that may consist of zero or more links and joints. Most objects in this example will only have one link and zero joints, such as the wood cube and the cricket ball. However, the robotic prosthetic arm is an example of a model with many links and joints.

For example, a typical doorway has three links: the frame, which is fixed, the door itself, and the handle. The hinges that connect the frame and the door are modeled as a joint that allows the door to swing. Likewise, the door and the handle are connected by a joint that allows the handle to turn. The doorway as a whole would be considered a model.


In our example, we will demonstrate how to use the world API to manipulate objects in the ARAT world.

Run the code: MATLAB

The HAPTIX MATLAB SDK should already contain the hxs_example.m script used in this tutorial. If it was moved or deleted, you can download it here If you need to redownload it, make sure to place the script into the folder containing the HAPTIX MATLAB SDK, or on your MATLAB path.

To run the example, first make sure Gazebo is running (double-click on the desktop icon or use the terminal).

Open MATLAB and navigate to the HAPTIX SDK folder. Then type hxs_example into the command prompt.

Watch the Gazebo window as the script runs through each example API call.

Run the code: Octave on Linux

Installing the haptix-comm package on Linux will install an octave folder to /usr/lib/x86_64-linux-gnu/haptix-comm. This folder contains hxs_example script used in this tutorial. If it was moved or deleted, you can download it here

To run the example, first make sure Gazebo is running (double-click on the desktop icon or use the terminal).

In a terminal, navigate to the aforementioned haptix-comm/octave folder and type octave hxs_example.m.

Watch the Gazebo window as the script runs through each example API call.

The code explained


Retrieves simulation information, which includes the current camera transform and a list of all models in the simulation. This API call is a good starting point for iterating through every model in the simulation.

info = hxs_sim_info();

In this example, we use the result info struct to iterate through every model in the world, displaying the names of all models, links, and joints. In this loop, we could also query the links and joints for their current dynamic state, such as position, velocity, and internal/external forces/torques.

hxs_camera_transform and hxs_set_camera_transform

Get and set the position and orientation of the simulation camera angle.

% Get the user camera pose
tx = hxs_camera_transform();
% Move and rotate the user camera pose
new_tx = tx;
new_tx.pos(3) = new_tx.pos(3) + 1;
% assign equvalent of Euler angles rpy(0, 0, M_PI)
new_tx.orient = [0 1 0 0]';

Here, we store the current camera transform in a struct tx, translate and rotate it, and set the new camera transform.

hxs_model_color and hxs_set_model_color

Get and set the color of an object as a (red, green, blue, alpha) 4-tuple, where alpha represents the transparency of the object. An object with an alpha value of 0 is fully transparent (invisible), while an alpha value of 1 means the object is fully opaque (solid).

% Change the table color.
hxs_set_model_color('table', [1;0;0;1])
% Row vectors work, too
hxs_set_model_color('table', [0,1,0,1])
hxs_set_model_color('table', [0;0;1;1])
% Get the color
color = hxs_model_color('table');

Here, we set the color of the table from red to green to blue.


Get points at which one model is contacting other models.

% Get contact information for the cube after it collides with the table
% Applying a force here induces a collision
hxs_apply_force('wood_cube_5cm', 'link', [0, 0, -1], 0.1);
contacts = hxs_contacts('wood_cube_5cm');

This example retrieves the contacts struct and then traverses it to print out each contact point for the wooden cube. Expect to see four contact points, one on each corner of the face of the cube that contacts the "lid" link of the wooden case.

hxs_linear_velocity and hxs_set_linear_velocity

Get and set the linear velocity of the model. The linear velocity is a 3-vector representing the (x, y, z) components of the model's velocity in meters per second.

vel = hxs_linear_velocity('wood_cube_5cm');

When the velocity is retrieved in this way, it represents the velocity of the model's canonical link. For a single-link object like the wood cube, this is the velocity of the whole model. But for the robotic arm, this is the velocity of the forearm link, which is established as the canonical link in SDF.

% Move by setting linear velocity
hxs_set_linear_velocity('wood_cube_5cm', [-0.5; 0; 0]);

When we set the velocity of the model in this way, the API will set the velocity of each link in the model.

hxs_angular_velocity and hxs_set_angular_velocity

Get and set the angular velocity of a model. The angular velocity is a 3-vector representing the velocity about the (x, y, z) axes in radians per second.

vel = hxs_angular_velocity('wood_cube_5cm');

This function will return the angular velocity of the model's canonical link.

% Move by setting angular velocity
hxs_set_angular_velocity('wood_cube_5cm', [0; 0; 100]);

For single-link models, hxs_set_angular_velocity will set the angular velocity of the model relative to the center of mass of the link.

hxs_apply_force, hxs_apply_torque, and hxs_apply_wrench

The hxs_apply_X functions take a model, a link, a vector or a struct representing the force, torque, or wrench to apply, and a duration to apply the force in seconds.

If duration is "0", then the force, torque, or wrench will have a persistent duration.

hxs_apply_force('wood_cube_5cm', 'link', [-1.0; 0; 0], 0.2);

This function applies a force of 1 Newton in the negative X direction on the wood cube model for 0.2 seconds.

hxs_apply_torque('wood_cube_5cm', 'link', [0; 0; 0.1], 0.1)

This function applies a torque of 0.1 Newton-meters about the Z axis on the wood cube model for 0.1 seconds.

% Apply force and torque at the same time.
wrench = struct('force', [0; 0; 1], 'torque', [0; 0; 0.1]);
hxs_apply_wrench('wood_cube_5cm', 'link', wrench, 0.1);

This function applies a wrench, which is a simultaneous force and torque. The wrench has a force component of 1 Newton in the positive Z direction, and a torque component of 0.1 Newton-meters about the Z axis.

hxs_model_gravity_mode and hxs_set_model_gravity_mode

Get and set the gravity mode of the model. If the gravity mode is 1, then the model will experience gravity (by default, 9.81 meters/second^2 in the negative Z direction). If the gravity mode is 0, the model will not experience gravity and will float in the air unless disturbed by external forces.

% Check gravity mode on wooden cube
gravity_mode = hxs_model_gravity_mode('wood_cube_5cm');
% Turn off gravity for cube, then nudge it upward
hxs_set_model_gravity_mode('wood_cube_5cm', 0);
% Row vectors work, too.
hxs_apply_force('wood_cube_5cm', 'link', [0, 0, 0.1], 0.1);
% Let it fly
% Bring it back down
hxs_set_model_gravity_mode('wood_cube_5cm', gravity_mode);

This example retrieves the current gravity mode, which is 1 by default. It then turns gravity off and nudges it upward. If gravity acted on the cube, it would fall back towards the table, but since gravity was turned off it will float.

hxs_model_transform and hxs_set_model_transform

Get and set the transform of the model. This is defined as the current transform of the canonical link (usually the center of mass for single-link models).

% Get the pose of the cube
tx = hxs_model_transform('wood_cube_5cm');
disp('Cube position:');
disp('Cube orientation:');
% Modify and set the pose
tx.pos(2) = tx.pos(2) + 0.25;
% define a 45 deg rotation about yaw (z) axis
tx.orient = [cos(pi/8) 0 0 sin(pi/8)]';
hxs_set_model_transform('wood_cube_5cm', tx);

This example gets the transform of the cube and then modifies it to a different location and orientation.

Setting the transform of the arm

You can also set the position and orientation of the robotic arm:

% Set the position of the arm. Note that if the motion tracking device is
% active and unpaused, this change will be transient.
arm_tx = struct('pos', [1.0, 0, 1.5], 'orient', [1, 0, 0, 0]);
hxs_set_model_transform(hand, arm_tx)

These commands will be overridden by motion tracking technology such as the Optitrack. If you want them to take effect, make sure motion tracking is paused or disabled.

hxs_model_collide_mode and hxs_set_model_collide_mode

Get and set the collide mode of the model. There are three possible collide modes:

  • hxsCOLLIDE: The default collide mode. The model will collide with other models set to this collide mode.
  • hxsNOCOLLIDE: The model will not collide with anything. It will pass through other models, even if the other models are set to hxsCOLLIDE. hxs_contacts will not detect contacts when this model pass through other models.
  • hxsDETECTIONONLY: The model will pass through other models and contact forces will be ignored, like hxsNOCOLLIDE. However, collisions will still be detected by hxs_contacts. This means that if the hxsDETECTIONONLY model passes through another model, hxs_contacts will have contact information for the models at that timestep (unless the model has hxsNOCOLLIDE set).

For most applications, we recommend either hxsCOLLIDE or hxsDETECTIONONLY.

% Check collide mode on the cube
collide_mode = hxs_model_collide_mode('wood_cube_5cm');
% Let it drop through the table
hxs_set_model_collide_mode('wood_cube_5cm', 0);
% Hack: apply a small force to disturb the cube to make it actually fall.
hxs_apply_force('wood_cube_5cm', 'link', [0; 0; 0.1], 0.1);
% Turn collisions back on (won't bring the cube back, of course)
hxs_set_model_collide_mode('wood_cube_5cm', collide_mode);

In this example, we check the cube's collide mode, which is set to the default hxsCOLLIDE. We then set it to 0, which corresponds to hxsNOCOLLIDE, causing it to drop through the table. We then set it back to the original collide mode.

hxs_add_model and hxs_remove_model

hxs_add_model adds a new model to the scene based on a complete SDF description. You can also specify a new name for the model and an initial position and orientation.

To learn how to create a model in SDF, refer to the Gazebo model building tutorials or learn how to use the model editor.

hxs_remove_model removes the model with the matching name from the world. Note that hxs_remove_model will return an error if it tries to remove a model that does not exist.

% Define a new model.  Here, we're taking the cricket_ball model from:
%  https://bitbucket.org/osrf/gazebo_models/src/default/cricket_ball/model.sdf
% and tweaking it slightly (just changing the color from Red to Green).
sdf = '<sdf version="1.5"> <model name="cricket_ball"> <link name="link"> <pose>0 0 0.0375 0 0 0</pose> <inertial> <mass>0.1467</mass> <inertia> <ixx>8.251875e-05</ixx> <ixy>0</ixy> <ixz>0</ixz> <iyy>8.251875e-05</iyy> <iyz>0</iyz> <izz>8.251875e-05</izz> </inertia> </inertial> <collision name="collision"> <geometry> <sphere> <radius>0.0375</radius> </sphere> </geometry> </collision> <visual name="visual"> <geometry> <sphere> <radius>0.0375</radius> </sphere> </geometry> <material> <script> <uri>file://media/materials/scripts/gazebo.material</uri> <name>Gazebo/Green</name> </script> </material> </visual> </link> </model> </sdf>';
% Add the new model to the world, at the world origin, 5m off the ground, with
% gravity enabled.  Then it will drop onto the table.
hxs_add_model(sdf, 'green_cricket_ball', [0; 0; 5], [0; 0; 0], 1);
% Define and add a constraint
constraint_sdf = '<sdf version="1.5"><joint name="test_constraint" type="revolute"><parent>table::link</parent><child>wood_cube_5cm::link</child><axis><xyz>0 1 0</xyz></axis></joint></sdf>';
% Enable when there's an implementation of hxs_add_constraint in Gazebo.
hxs_add_constraint(constraint_sdf, 'wood_cube_5cm');
% Roll the ball to the right
hxs_apply_torque('green_cricket_ball', 'link', [0; 0.05; 0], 0.1)
% Remove constraint
% Enable when there's an implementation of hxs_remove_constraint in Gazebo.
hxs_remove_constraint('test_constraint', 'wood_cube_5cm'); 
% Remove the model

This example takes a hardcoded SDF string and creates a new green_cricket_ball model. The model appears in the global coordinate frame at position (0, 0, 5), which means that it drops onto the wooden case. We apply a torque to show that the model reacts to external disturbances, then remove it from the world.


For models with one or more joints, this function takes a model and the name of one of its joints and two scalar values. It then sets the position and velocity of the specified joint in the model based on the scalar inputs.

% Set the state of a wrist joint.  Note that, because there's a controller
% acting on the wrist, this change will only be transient; the controller will
% restore the wrist back to the current target position.
hxs_set_model_joint_state(hand, hand_joint, 0.5, 0.0);

% Set the position of the arm. Note that if the motion tracking device is
% active and unpaused, this change will be transient.
arm_tx = struct('pos', [1.0, 0, 1.5], 'orient', [1, 0, 0, 0]);
hxs_set_model_transform(hand, arm_tx)

% Move the camera
% Reset the world, which will move the camera back

In this example, we try to set the state of the robotic arm's wrist joint. The joint will snap back to its original position because the motors in the arm are actively controlling the joints. To properly set the wrist joint, use hx_update (described in another tutorial).


Reset the world. If argument "0" is passed to reset, the robotic arm and the viewpoint will not reset. If a non-zero argument is passed to reset, everything in the world will reset to its original position.

% Move the camera
% Reset the world, which will move the camera back