Hi!
The error you are describing means that idTracker cannot learn the aspect of each individual in the video (sorry for the totally uninformative error message).
Possible causes are the following:
1) Video too short:
in order to learn the aspect of each individual, idTracker needs a
minimum amount of frames where all individuals are visible and moving
separately. If the video is too short, it may not have enough of these
frames. The adequate length of video depends on the number of
individuals an how they move (you need longer videos if you have many
individuals, if they cross very often, if they tend to be immobile for
long periods or if they are not in view all the time). Usually we recor
at least 30 minutes to be sure, but sometimes less can be enough (for
example, for 5 zebrafish usually it is enough with 5 minutes of video).
2) Low quality video and/or wrong tracking parameters: idTracker
needs to correctly distinguish between animals and background. And for
this you need to adjust the tracking parameters until you see all
animals in the displayed frame colored in green, and only the animals
colored in green. Sometimes you may find that an animal is not detected,
or an animal is detected as several different blobs, or that a
background object is detected as an animal (is covered in green). But
this should be the exception (it can happen in, say, up to 10% of the
frames). Besides looking at the green pixels, you can check the
correctness of the detection by looking at the "number of animals
detected" in the bottom of idTracker's window. This number should match
with the number of separate animals or group of animals you see in the
frame. For example, if you have 5 animals and they are separated, the
number should be 5. If you have 5 animals, 3 are separated and the other
2 are crossing, this number should be 4 (3 individuals + 1 pair).
3) Wrong number of individuals: There
is a box where you need to enter the number of individuals. It is
crucial that this number exactly matches the number of individuals in
your video.
4) Animals not moving or together all the time: To
learn the aspect of each individual, idTracker needs to find it in
different postures. For this, the animal must move. So if some of the
individuals in your video are immobile for long periods, identifications
get complicated. Also, the animals must be moving separately for at
least some portions of the video, the tracking will fail if they don't.
For example, mice that are already familiar with the set-up tend to curl
together in a corner and sleep. This is cute, but very bad news for
idTracker.