Contents
The iap_common package provides basic functions and classes for the interactive perception library (IAP).
Core Concepts
In the following we describe some of the core classes in the iap_common framework.
Features, Sets, Trajectories
Visual features are a central part of the library to define rigid bodies and joints. The following terminology (and corresponding classes) facilitates dealing with features.
Feature
A feature is a point with an ID, either a 3D point in Euclidean space or a 2D point in image space. Every different feature must have unique ID - it Feature class has an internal feature counter and takes care of giving you a unique feature ID, when you instantiate an object.
Note that there is no notion of time in the feature class - it can appear in a sequence of frames, but the feature class does not know anything about this.
FeatureSet
A FeatureSet is a set of different features at some time $frame$. In particular, a feature set has the notion of time: you can assign a frame number $frame$ to it (which corresponds to the frame in your sequence). It can be viewed as a snapshot of different features at some time.
It can be constructed from and exported to a PCL point cloud.
FeatureTrajectory
A FeatureTrajectory contains the same feature at different time frames. It can be seen as the trajectory of one feature over time in a sequence.
FeatureTrajectorySet
As the name suggests, a FeatureTrajectorySet is the combination of a FeatureSet and FeatureTrajectory: It contains a set of features at different points in time.
Clusters
Segmentation algorithms provide a clustering of the features. The following classes allow dealing with them in an easy manner.
Cluster
A Cluster is a specialization of FeatureSet: It contains a set of different features at some time $frame$, with an additional cluster id. It usually appears in the context of segmenting a feature set.
ClusterSet
A ClusterSet is a set of clusters at a fixed time frame. It has no correspondance to any of the Feature*** classes.
ClusterTrajectorySet
A ClusterTrajectorySet is a specialization of FeatureTrajectorySet. It is a set of FeatureTrajectories all belonging to the group (cluster) at a fixed time frame.
Body Motion
Sometimes you want to abstract away from feature cloud motion and express the motion of bodies using transformations. The following classes allow to easily generate and manipulate these transformations.
RigidTransformation
A rigid transformation is a special case of linear transformations, comprising rotations and translations. The class provides different convience functions for calculating rotation axis/angle, chaining transformations etc. (Under the hood it uses Eigen for representing transformations)
BodyTrajectory
A body trajectory can be regarded as a timed sequence of rigid transformations. It provides additional methods for characterizing the trajectory, like the accumulated translation distance, rotation angle, etc.
BodyTrajectoryPair
In particular for the analysis of joints, we are interested in the motion of two bodies relative to each other. This relative motion is represented using the BodyTrajectoryPair class which allows for the extraction of the local trajectory, i.e. a BodyTrajectory expressing the movement of one body with respect to the other.
Kinematic Structure
iap_common provides messages for sending around kinematic structures. A KinematicStructureMsg consists of JointEstimatorResultMsgs, each containing a hypothesis per joint type, encoded in JointMsg. This means a KinematicStructureMsg does not only contain the "most probable" kinematic structure, but for every pair of links an estimation of all possible joint types.
Package Description
ROS Messages
/iap_common/KinematicStructureMsg
description: The kinematic structure. Contains a list of JointEstimatorResultMsg, i.e. the estimation of all joints
/iap_common/JointEstimatorResultMsg
- description: A set of joint hypothesis with according probabilities
/iap_common/JointMsg
- description: A joint with axis information, goodness of fit etc.
/iap_common/AxisMsg
- description: An axis definition for prismatic or revolute joints
/iap_common/BodyTrajectoryMsg
description: Message to encode a BodyTrajectory
/iap_common/BodyTrajectoryPairMsg
description: Message to encode a BodyTrajectoryPair
/iap_common/ClusteredFeatureSetMsg
description: Message to encode ClusteredFeatureSet