Assisted Teleoperation with Augmented Reality

This repository contains the source code for the ATAR package. This ROS package can be used to design interactive augmented reality or virtual reality tasks using a stereo camera and display, master and slave manipulators. I have developed and tested this on a da Vinci Research Kit (DVRK) that comprises 5 manipulators (2 masters, 2 slaves and a camera arm), a stereo endoscope and stereo vision console. I also have tested it with a single display and a Sigma master device interacting with a VR task. I have tried to make the package as modular and generic as possible, so that it can be used for different purposes such as creating a VR or AR interactive simulation with embedded physics or simply overlaying 3D graphics on top of a single camera images. The camera images and the manipulator cartesian measurements are read through ros topics, so if you want to create a simulation that includes one of these you should have a node that publishes them. The package includes the code for the calibrations involved (Camera intrinsics, extrinsics using charuco boards and manipulator base to camera base) and a qt graphical user interface. Bullet Physics library is used for the rigid and soft body dynamics simulation and graphics are generated using the Visualization toolkit (VTK).

Prerequisites

Apart from ROS (Tested with Kinetic), this package needs the following libraries:

• VTK - For the graphics
• Bullet - For the physics

and the following packages from my github page:

• custom_conversions
• custom_msgs
• active_constraints

Opencv and qt libraries are needed too but these should come with ROS.

To build

You should be able to use catkin_make, but I suggest to use catkin tools:

sudo apt-get install python-catkin-tools

Clone the following packages in your catkin workspace src directory and compile them.

git clone https://github.com/neemoh/custom_msgs.git
git clone https://github.com/neemoh/custom_conversions.git
git clone https://github.com/neemoh/active_constraints.git
catkin build

Next download VTK from https://www.vtk.org/download/ , compile it with VTK_RENDERING_BACKEND=OpenGL and install it. In case you are not familiar with building libraries: copy and extract the downloaded library somewhere (I personally put it in /opt). then inside the extracted vtk folder:

mkdir build  && cd build
cmake  -DVTK_RENDERING_BACKEND=OpenGL ..
make -j8
sudo make install

Next we download and compile bullet physics:

cd /opt
sudo mkdir bullet_physics
sudo chown <YOUR USERNAME> bullet_physics
cd bullet_physics
git clone https://github.com/bulletphysics/bullet3.git
cd bullet3
mkdir build && cd build
cmake -DUSE_DOUBLE_PRECISION=OFF -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON -DBUILD_EXTRAS=ON DINSTALL_EXTRA_LIBS=ON ..
make -j8
sudo make install

Assuming everything went well, you can finally download the atar repository in your catkin workspace and compile it:

git clone https://github.com/neemoh/ATAR.git
catkin build

To test it, run the test_vr.launch file and click on task 1. Wait for the compound convex mesh to be generated (you can see the progress in the terminal) and the task should run after that. You should see some spheres and mesh objects falling on a floor, while the camera rotates, which would mean you have successfully built the libs. Congrats! In case you have an nvidia card and its driver has benn installed, you can have faster graphics and also shadows. To test the shadows set the with_shadows flag to true in the lanuch file.

TaskHandler

This is the class that subscribes to a control events topics (published by the GUI) and loads and unloads tasks. When you want to add a new task, you can make a class with a name that starts with Task (because all the files starting with Task will be automatically added to the CMakeLists) and include your task's header in the TaskHandler and add its allocation to the StartTask method. The task class must be a child of SimTask class (Check TaskDemo1).

• Note that you need to reload your CMakeLists project for the automatic addition of your Task file to the cmake project. If you don't know how to do that, just modify a line in the CMakeLists.txt file like add a space or something before compiling the project!

Creating tasks

In my first version of the code the Task class used to have only the simulated objects in it. Later I decided to take everything (i.e. rendering and communication with the manipulators etc) into the task class so that we can have tasks with different rendering configs (ar or vr, different number of rendering windows, resolutions...) and different manipulators. This of course adds some overhead cost (and some memory leak that I will hopefully fix soon!). The block diagram above shows the main elements of a task class. You can have a look at the TaskDemo1 and TaskDemo2 for minimal examples. The main elements are the Rendering, SimObjects and manipulators that are explained in the following.

The usual flow is that you create SimObjects (or vtk actors or Bullet bodies) and add them to the world in the constructor. Then in one of the following two periodically called functions you write task logic and other things that need to be done on runtime. There are two methods of the Task class that are called periodically:

• TaskLoop: This is called from the main thread at a refresh rate of about 30 Hz which makes it the choice for updating graphics and task related logic.
• HapticsThread: This method is called from a separate thread and you can set its refresh rate directly in the method (creating a loop and so on). This thread exists for haptics related matters where a high refresh rate is needed to provide a stable feedback. Have a look at the TaskSteadyHand (haptics thread running at 500Hz or 1KHz) for an example of how to use this thread.

1. Rendering Class

As its name suggests this is where the graphics are produced. I have moved the explanations regarding the augmented reality case to the bottom to simplify the description here. The constructor of the Rendering class let's you specify the configuration of the windows that you want. It is easier to explain this with an example: In our DVRK setup we had two displays in the surgeon console (one for each eye) and we wanted to show what is happening in the console in a third display to viewers. These 3 displays were connected to the same graphics card in a horizontal layout is you can see in the image bellow:

We can have one window with three views in it defined as:

    bool ar_mode = false;
    int n_views = 3;
    bool one_window_per_view = false;
    bool borders_off  = true;
    std::vector<int> view_resolution = {640, 480};
    std::vector<int> window_positions = {1280, 0};

    graphics = new Rendering(view_resolution, ar_mode, n_views,
                             one_window_per_view, borders_off,
                             window_positions);

Having the borders off allows for full screen view in the dvrk displays. You can have multiple windows (up to 3) by setting the one_window_per_view flag as true. However, VTK fails to generate the shadows when there are more than one rendering windows. If you have 2 windows, the window_positions must contain 4 elements(x,y,x,y) and 6 elements if you have 3 windows.

2. SimObjects

You can define objects using the SimObject class which has a graphic actor (actor_) and physics body (rigid_body_). A SimObject can be dynamic (i.e. its pose will be update by physics simulation), static (i.e constant pose), kinematic (i.e. pose is assigned externally, e.g. from a master device). There are some primitive shapes available (Cube, sphere, cylinder, cone and plane) , but more usefully the shape can be from a .obj mesh file. After constructing a SimObject call the AddSimObjectToTask method of the Task to add your object to the simulation.

• Note: You can still use all the functionalities of VTK for creating graphics. When you create a vtk actor, call graphics->AddActorsToScene (my_actor).

For Sphere and Plane shapes you can have a texture image. PNG and JPG formats are supported. To learn more about SimObjects refer to SimObjects.h.

Mesh objects

Meshes are decomposed into approximated compound meshes using the VHACD method. Generating the approximated compound mesh can take up to a few minutes. So we do this only once and save the compound mesh in a separate file that has the same name of the original mesh file with an added _hacd. Next time the application is executed we search for the file with _hacd and if found, it is used and compound mesh generation is not repeated.

• Note: The generated compound meshes are approximate and sometimes the approximation deviates considerably from the original mesh. To check how the generated compound object looks like, you can either open the generated _hacd.obj in blender or set the show_compound_mesh boolean to true in the constructor of SimObject.

Creating mesh objects with Blender:

You can use blender to create mesh objects. When the object is ready, follow these steps to make sure you have the correct units even if your mesh's dimensions are as small as a few millimiters:

• Convert it to mesh (alt-c)
• Go to the Scene tab and in the Units section select Centimeters
• Go to File-> Export and select Wavefront(.obj). In the options set the scale as 0.01 and save.

3. Interact with objects using a haptic device

In order to interact with the virtual environment you would need to have an input device of some sort. The Manipulator class helps you to read the cartesian pose and twist of that device (assuming some other node is publishing them) and transform them to the virtual world reference frame. Check TaskDemo2 for an example of this. You can use the SimMechanism class to make a virtual tool that follows the pose of the real manipulator. I have already created one called:

• SimForceps. It consists of 3 links. first is a small box representing the base. The other two are jaws (mesh objects) that are connected with an elastic link to the base cube. The angle of the jaws is set with a bulletConstraint that is actuated. All these complications is because Kinematic objects interact very aggressively with the dynamic objects, so if the forceps were 3 kinematics links, it would have been impossible to grab something with them. That's why the jaws sometimes get distorted! since are actually dynamic objects and when they hit something their elastic constraint let's them move too far from the base...

Hand-Eye Calibrations

The local to world transformation is found differently in VR and AR cases. The common element in both cases is that we need to know the pose of the camera with respect to the world (camera_to_world_frame_tr). This has to be set from outside of the class by passing the pointer of this Manipulator object to a Rendering object through the SetManipulatorInterestedInCamPose method (check the Demo2Task). After doing that the camera pose will be communicated to the manipulator every time it changes. Now about the rest of the kinematics chain:

VR or MASTER MODE:

The manipulator is interfacing with a master device. Here the local_to_world_frame_tr is calculated as:

local_to_world_frame_tr.M = camera_to_world_frame_tr.M *local_to_image_frame_rot;

where local_to_image_frame_rot is the tr from the base of the master device to the image frame (i.e. the image you see in the display, i.e. the camera!). THe image frame is opencv style: X axis is left to right, y is top to bottom and so z is perpendicular into the image. Note that we are only interested in the rotation from the master base to the image. This tr is set by setting a parameter "/calibrations"+arm_name+"_frame_to_image_frame" with 4 elements representing the quaternion rotation. Check the params_calibrations_test_vr.yaml file to see examples of this.

AR or SLAVE MODE:

In the augmented reality case we are interfacing with a slave arm that is seen in the camera images. Here we need to find the transformation from the slave to the world frame by performing a calibration procedure. This calibration is done by pointing at 6 known points on a charuco board and is implemented as a separate temporary thread when the DoArmToWorldFrameCalibration method is called. Here we directly calculate the local_to_world_frame_tr and therefore we don't need to SetWorldToCamTrfrom outside like in the VR case. After the calibration a ros parameter is set called: "/calibrations/world_frame_to_"+arm_name+"_frame" you can set this parameter in the params_calibrations_ar.yaml so that you don't have to repeat the calibration as long as the base of the robot does not move with respect to the world (board) coordinate.

TODO: Augmented Reality

Things to explain: camera. Calibrations (intrinsic, extrinsic)

intrinsic calib files: make a folder named camera_info in the .ros folder in your home directory and copy the default_intrinsics.yaml file from the /ATAR/resources folder in camera_info.

AR Camera

This class used to interface with a camera through ros is similar to what the Manipulator class is for a robot arm. It reads images, intrinsic and extrinsic calibrations and in case nothing is found on topics for the latter two, it calculates them. For each view we create one ARCamera and for each ARCamera you need to provide a cam_name as a ros parameter cam__name where is from 0 to 2 (e.g: cam_0_name). Check out test_ar.launch. Here are some details about each of these 3 tasks:

1- reading images

This simply uses image transport from ros to read images that are published on a topic. We are of course assuming that an external node is publishing the images. For example for a simple usb camera you can use the uvc_camera package of ros. The topic we listen to is:

• "/"+cam_name+ "/image_raw" You can use remap in the lunch file if your camera node publishes the images on a different topic:

    <remap from="/ar_core/left/image_raw" to="/dvrk/left/image_raw"/>
  

2- Intrinsic calibration file

Intrinsic camera parameters are needed for a correct mapping of the 3D world to the camera images. We can get the parameters in two way:

• A yaml file. We search in your home directory under the .ros/camera_info directory for a file named as <camera_name>_intrinsics.yaml
• Subscribe to camera_info topic. This is the ros way of accessing intrinsic parameters that is: the camera node that publishes the images publishes also a topic called camera_info that contains needed parameters about the camera.
• If ARCamera doesn't find the parameters from any of the above two, it starts the intrinsic calibration procedure with a charuco board (explained bellow) in which you need to take at least 15 different poses of the board in front of the camera (instructions are shown on the image). If the calibration is successful we create a yaml file and put it in the camera_info directory. unfortunately this file has a different format than that used by ros and that's why we name it <camera_name>_intrinsics.yaml although ros camera_files are supposed to be called <camera_name>.yaml.

3- Extrinsic calibration

Extrinsic calibration refers basically to the pose of the camera with respect to a world reference frame. In ATAR we need this pose to correctly map the virtual 3D world to the real world of the images. Like intrinsic calibration here we use a charuco board again. To make things simpler we assume that the world_reference frame is on a corner of the charuco board. This means that if both intrinsic and extrinsic calibrations are correct, when you position a small virtual sphere at [0, 0, 0] it should appear on a corner of the charuco board. The extrinsic (i.e. camera pose) is found in two ways in ARCamera

• Read as a PoseStamped message from one of the following topics: "/calibrations/world_frame_to_"+cam_name+"_frame" "/"+cam_name+ "/world_to_camera_transform"
• Calculated at each iteration using the aruco library and the charuco board.

Charuco board

Both for intrinsic and extrinsic calibrations we rely on a charuco board. This is a checker board that has some fiducial markers (called aruco) instead of its white squares. We use the aruco library to detect those markers in the camera image and perform intrinsic or extrinsic calibration based on the markers' spatial information. You need to print a charuco board (for example charuco_d0_h4_w6_sl200_m10_ml150.png that is in the resources folder) on a paper and make it solid by attaching it to a cardboard. Then you'll need to set the parameters of the board in your lunch file so that atar knows what to look for. In the test_ar.launch file you can see what are the elements of board_params. If you want to make a different board run

rosrun atar create_charuco_board <YOUR PARAMS>
 

Miscellaneous

• Grasping:

TODO:

• After Manipulator to world calibration, the calculated transformation is used and set as a param, but not saved in the params_calibrations.YAML file. So if the ros master is restarted the transformation is lost.
• Add the option of directly reading usb camera in the ARCamera class.