# The reference image

Aim

To provide EthoVision XT an image of the background (that is, the arena without the subject) in such a way it can discriminate between the subject to track and everything else.

Background information

A reference image should not contain the image of the subject. The options below help you create a reference image even when it is not possible to have the subject removed from the video image.

A reference image is used in detection methods Static Subtraction, Dynamic subtraction and Differencing.

Grabbing a reference image is optional. However, if you do not do so, EthoVision XT takes the first video frame available from the camera or video file and considers that as the reference image.

Procedure

1.Click Background in the Method section.

2.In the Reference image window, choose Grab Current (A). Scroll through the video until you find an image without animals. If you track live, make sure that there are no animals in the arena, then click Grab Current (A). This image will be the initial reference image. Skip B and C and click Close.

If your video does not contain images without animals, continue with option B. Also continue with option B if you track live and you cannot start with an empty arena.

3. Grab from other (B). You may have a video with an identical background as the one you use for tracking, but without animals. Or you may have an image of a background without animals. If this is the case, click Grab from Other and select this video file or image file. If you select a video file, the first frame of this file will be used as an initial reference image. If you select an image file, this has to have the same resolution as the video file you use for tracking. Browse to this file and click Open. Skip option C and click Close. If you do not have such video or image, proceed with option C.

4. Start learning (C). With this option EthoVision XT makes an average image of the entire video. If the animals are moving, this process will average out the pixels of the animals, resulting in a reference image without animals.

In the figure below you see an effect of using this option. In A, the rat is in the view at all times. In B, after applying Start learning, the animal is removed from the background.

If you track live, you have to click Start Learning, and subsequently click Stop Learning as soon as you have obtained an initial reference image without animals. Click Close if you have a good reference image without animals. If you use Dynamic subtraction or Differencing and the reference image is still not good, continue with step 5.

5. Acquisition settings: If you run a number of consecutive trials, you may want to choose which image to use as initial reference image. Pay particular attention here, since the video image may not be exactly the same between the end of a trial and the start of the next one.

▪Use saved reference image. Choose this option if the background remains constant between the different trials. For example, when there are no moving objects, bedding material, shadows etc.

▪Use dynamic reference image. Choose this option if the background changes between the different trials, and you assume that the video image at the end of a trial is the same as that of the start of the next trial. For example, when a trial starts a few seconds or minutes after the end of the previous trial.

▪Use first frame of each trial. Choose this option whenever the video image at the end of a trial is likely to differ from that at the start of the next trial. This may be the case for example when you do batch acquisition from a number of video files, where shadows or reflections appear in the arena in the new video. EthoVision XT takes the first video frame available from the camera (or video file) and considers it as the background. Note that if you choose this option and the subject is already in the arena the moment that you start a new trial, EthoVision XT misses the subject in the first frames and finds it only when it moves away from the starting location. To avoid this, release the subject after you have started the trial.

6.Click Close.

Notes

▪By default, the reference images are stored in the folder Bitmap Files of your experiment. If the background has not changed, you can use these images as reference images in other experiments.

▪If you are tracking from video files, you must play the video forward whilst making dynamic subtraction settings. This is because the program needs to update the reference image. Do not skip through the file, since the reference image will then not be correctly made.

▪The option Acquisition settings is available only in Dynamic subtraction and Differencing, since in these methods the background is continually updated during the trial.

See also

▪How the reference image is updated in Dynamic subtraction

▪How the reference image is updated in Differencing