General video conditions¶
It is advisable to adhere to some guidelines during the realisation of videos of freely-moving animals. Here follows a list of conditions that allow to maximise the probability of success and the accuracy of the tracking.
The higher the number of pixels per individual, the more information to distinguish it from the rest. Notice that, on the downside, the additional information makes the algorithm less time-efficient.
The frame rate must be high enough for the blobs associated with the same individual to overlap in consecutive frames, when moving at average speed. A low frame rate—with respect to the average speed of the animals can cause a bad fragmentation of the video: An essential process in the tracking pipeline, that allows to collect images belonging to the same individual and organise them in fragments. On the contrary, excessively high frame rates will make the information coming from the analysis of the fragments highly redundant. This will increase the computational time necessary to track the video, without guaranteeing an improvement of the identification of the individuals. In the examples provided in this paper, the frame rate ranges from \(25 fps\) to \(50 fps\).
The length of the video for which the system works depends on the number of animals, the distribution of images per individual fragment and the number of pixels per animal. For few animals (8 zebrafish) we can track videos as short as \(\approx 18\) sec (\(\approx 500\) frames at \(28fps\). For large groups we can track videos as short as \(1 min\) (\(\approx 1950\) frames at \(32 fps\)). The system works for longer videos as far as the overall conditions do not change abruptly in different parts of the video. Very large videos with many animals will require a high amount of RAM and could block your computer.
The system works with any video format compatible with OpenCV. We recommend uncompressed or lossless video formats: Some compression algorithms work by deleting pieces of information that could be crucial for the identification of the individuals. However, we have successfully tracked videos with compressed formats: .avi (FPM4 video codec) and .MOV (avc1 video codec).
Illumination has to be as uniform as possible, so that the appearance of the animals is homogeneous along the video. We recommend using indirect light either by making the light reflect on the walls of the setup, or by covering the setup with a light diffuse. Although, we have also tracked videos with retroilluminated arenas, recall that the tracking systems relies on visual features of the animals that this type of illumination could hide.
Definition and focus¶
Images of individuals should be as sharp and focused as possible for their features to be clearly displayed along the entire video. When using wide apertures on the camera, the depth of field can be quite narrow. Make sure that the plane of the sensor of the camera is parallel to the plane of the arena so that animals are focused in all parts of it. In addition, exposition time (shutter speed) should be high enough so that animals do not appear blurred when moving at average speed. Blurred and out of focus images are more difficult to be identified correctly.
The background should be as uniform as possible. To facilitate the detection of the animals during the segmentation process, the background colour has to be chosen in order to maximise the contrast with the animals. Small background inhomogeneity or noise are acceptable and can be removed by the user during the selection of the preprocessing parameters:
- Static or moving objects much smaller or much larger than the animals can be removed by setting the appropriate maximum and minimum pixels size thresholds.
- Static objects of the same size and intensity of the animals can be removed by selecting the option “subtract background” in the preprocessing tab.
- Regions of the frame can be also excluded by selecting a region of interest.
Shadows projected by the individuals on the background can lead to a bad segmentation and hence, to a bad identification. Shadows can be diffused by using a transparent base separated from an opaque background or by using a retroilluminated arena.
Reflections of individuals on the walls of the arena should be avoided: They could be mistaken for an actual individual during the segmentation process. Reflections in opaque walls can be reduced by using either very diffused light or matte walls. For aquatic arenas with transparent walls, reflections can be softened by having water at both sides of the walls. Furthermore, reflections can be removed by selecting an appropriate ROI.
Variability in number of pixels per animal¶
The number of pixels in a blob is one of the criteria used to distinguish individual fish from crossings. An optimal video should fulfil the two following conditions. First, the number of pixels associated with each individual should vary as little as possible along the video. Second, the size an individual should vary as little as possible depending on its position in the arena. In any case, strategies to avoid misidentification are put in place, even in case of variable animal sizes.