Knowledge Base

Swap Nose-Point and Tail-Base

Aim

To exchange the nose-point and the tail-base within a sample (or range of), after checking that these were assigned incorrectly by EthoVision XT.

In the example below, the nose-point and the tail-base point are assigned incorrectly (left). The Swap action puts the nose-point and the tail-base point back in the correct place.

To check that nose-point and tail-base were mistakenly exchanged, choose Analysis > Results > Integrated Visualization. Play back the tracks with the video on the background. Check that the nose-point and tail-base do correspond to the actual orientation of the subject.

Procedure

Choose Acquisition > Edit Tracks. Then choose one of the two options:

• Swap nose-point and tail-base point manually. Look for the samples with exchanged nose-point and tail-base, and give the Swap command for those samples.
• Swap nose-point and tail-base point automatically. Define a criterion for automatic detection of changes in orientation of the nose-point and tail-base. EthoVision XT searches your tracks for the samples where a sudden change in orientation has occurred. It is up to you to swap nose-point and tail-base for those samples.

Notes

• By selecting the tracking methods Rodents / For Occlusions or Adult Fish / For Occlusions, you can minimize the occurrence of nose-tail swaps. However, this method requires much processor load, so make sure you do not miss samples.
• If a large number of samples contain wrongly-assigned nose-point and tail-base, it may mean that the detection settings for nose-point and tail-base detection are not optimal, or that you need to improve the lighting or other settings of your setup.
• You can click Save on the toolbar so that if you are editing many swaps and make a mistake, you do not lose your edits.
• When you swap nose- and tail-base points, the Head Direction line is recalculated too. This influences the calculation of Head Direction, Head Directed to Zone, and Turn Angle and Angular Velocity when they are based on Head Direction.

Swap Nose-Point and Tail-Base Point Manually

Aim

To correct body point swaps that may have occurred when tracking the nose-point and the tail-base point.

Procedure

1. Play the track until you find samples with wrongly-assigned nose-point and tail-base. Choose Analysis > Results > Integrated Visualization. Play back the tracks with the video on the background. Check that the nose-point and tail-base do correspond to the actual orientation of the subject.
2. Select the sample (or range of) in which you want to swap the nose-point and tail-base. The selected samples are highlighted.
3. Click the Swap Nose and Tail button on the toolbar, or press the W key. The nose-point and tail-base within each sample in the selection are swapped. In the Samples List, the coordinates are swapped between Nose-point and Tail-base for those samples.

Notes

• When you swap nose- and tail-base points, the Head Direction line is recalculated as the prolongation of the segment joining the center-point and the tail-base point. This influences the calculation of Head Direction, Head Directed to Zone, and Turn Angle and Angular Velocity when they are based on Head Direction.

Swap Nose-Point and Tail-Base Point Automatically

With automatic swapping, EthoVision looks for the samples where the subject made a turn exceeding a user-specified threshold relative to the previous sample. These turns are interpreted as nose-point and tail-base being incorrectly assigned. You can then select those samples and let EthoVision exchange the corresponding nose-point and tail-base.

Use this method when a large number of consecutive samples have body points wrongly assigned.

What Does 'Turn' Mean Here?

Turn refers to the change in orientation of the nose-tail base vector. The nose-tail base vector is the vector connecting the nose-point and the tail-base point within a sample. With automatic swapping, if the change in the vector's direction from sample n to n+1 is larger than the angle you specify, the sample n+1 is considered for swapping. Then it is up to you to decide whether to swap the points.

If the nose-point or the tail-base is missing, the nose-tail base vector is the vector connecting the center with the other point. If both nose- and tail-base points are missing, the sample is not considered for swapping.

If samples following n+1 have a vector's turn lower than the specified threshold, they too are assumed to have nose-point and tail-base detected wrongly. Therefore, they are included in the swapping interval.

Example: A - the body points are assigned correctly. B - the nose-tail base vector makes a turn greater than the specified threshold (for example, 150°). This is the start of the swapping interval. C - a new significant turn of the nose-tail base vector is found. Samples between B and C (not included) are swapped. The samples after C are considered correct, until a new significant turn is found.

Procedure

1. Position the track at the beginning and check whether the first sample in your track is orientated correctly. If you have tracked data from a video file, make sure that the video file is on the background. If you have tracked live, make sure that the nose-tail base orientation matched the direction of movement.
2. Click the Auto Swap button on the toolbar, or press Ctrl+F.