This module requires a robot with a 3D sensor.
What it does ¶
ALSegmentation3D extracts the objects present in the field of view of the robot by doing a segmentation of the depth image (returned by the 3D sensor) in blobs of similar depth.
It also allows you to track, with the head of the robot, a blob at a specified distance or simply track the nearest blob with respect to the camera.
How it works ¶
Segmentation of the depth image ¶
In order to segment the depth image into blobs of similar depth, a region growing based algorithm: pixels in the depth image can be linked to their neighbors to form blobs if the difference of depth between them is below a threshold (DepthThreshold). DepthThreshold can be changed with the function ALSegmentation3DProxy::setDeltaDepthThreshold() and the curent value of DepthThreshold can be retrieved, using function ALSegmentation3DProxy::getDeltaDepthThreshold().
TopOfBlob property ¶
As some of the extracted blobs are people, a specific property, called TopOfBlob, is computed for each of the segmented blobs. It is constructed in order to fit the best with the position of the top of the potential “head” of each blob. This property has real meaning when the blob corresponds with a person. When it does not correspond to a person, it is simply the highest pixel in the center of the blob.
Blob tracking ¶
ALSegmentation3D also provides an API to track, with the head of the robot, a blob present in the center of the image at a specified distance.
From the list of blobs, it is possible to look for blobs that are at a distance close to the given distance and keep only the one which is the most in the center of the image. As long as this center blob is tracked with the head of the robot, it will stay in the center of the image, which makes sure to track always the same blob.
Blob tracking can be turned on and off using function ALSegmentation3DProxy::setBlobTrackingEnabled(). In order to know if blob tracking is running, use function ALSegmentation3DProxy::isBlobTrackingEnabled(). The distance of the blob to track (BlobTrackingDistance) must be set before turning on the blob tracking, with the function ALSegmentation3DProxy::setBlobTrackingDistance(). This distance can be set to -1 to track the nearest blob to the camera. Each time the corresponding blob is found, the value of BlobTrackingDistance is updated with the real distance between the blob and the camera. At any moment, this distance can be retrieved with the function ALSegmentation3DProxy::getBlobTrackingDistance().
The position returned by the blob tracking relies on the position of the TopOfBlob so that the robot will track the head of the person (when the blob corresponds to a person). However, as TopOfBlob corresponds to the potential position of the top of the head, tracking TopOfBlob would make the robot not look at person in his/her eyes. Therefore, a vertical offset (VerticalOffset) is substracted to the tracked position, when it does not belong to the top border of the depth image. This VerticalOffset value (in meters) can be set with ALSegmentation3DProxy::setVerticalOffset(). 0 disables the use of this offset.
Each time the tracked blob is detected, the event Segmentation3D/BlobTrackerUpdated() is raised and the memory key Segmentation3D/TopOfTrackedBlob, containing the information about the top of the blob after applying the offset, is updated. On the opposite, each time the tracked blob is not detected, the event Segmentation3D/TrackedBlobNotFound() is raised.
The event ALTracker/BlobDetected() is also raised when the tracked blob is detected. It contains information similar to Segmentation3D/TopOfTrackedBlob, but with a structure that can be used with function ALTrackerProxy::trackEvent() of the module ALTracker.
Enabling the blob tracking only raises the events and updates the memory keys. It does not make the robot’s head move. Thus after enabling the blob tracking in module ALSegmentation3D, it is still needed to make the robot look in the direction of the tracked blob each time its position is updated, for example using function ALTrackerProxy::trackEvent() of the module ALTracker.