# Differencing

Aim

To detect the subject, while compensating for spatial and temporal changes in the background.

How Differencing works

Like with Dynamic subtraction, the Differencing method updates the reference image over time. Differencing makes a statistical (probabilistic) comparison between each pixel in the reference image and the pixels of the current image. The statistical comparison uses the variance in the contrast between the current and reference image to calculate the probability that each pixel is the subject.

The Differencing method takes more processor load than the subtraction methods. Therefore, when using Differencing, make sure you computer meets the system requirements.

Procedure

1.In the Method section of the Detection Settings window, select Differencing.

2.Click the Background button. The Reference Image window opens with the image that is currently used as background. The aim is to obtain a reference image that does not contain images of the animals you want to track. To do so, follow the instructions on the screen in consecutive order. If A fails, move on to B, if that fails move on to C. See Optimize the reference image.

3.From the Subject color … list, select one of the options from the list, depending on the color of the subject you want to track.

4. Next, if necessary, adjust the position of the Sensitivity slider and change the option selected in the Background Changes list.

The Sensitivity slider determines what difference in contrast from the background is seen as the animal. For an image with good contrast, there is no need to change the slider. For images with less contrast, adjust the position of the slider to the right or the left until the subject is properly detected.

In the Background Changes list you can select options that reflect how fast the background changes. For example, a cage with bedding might change a lot because of animals kicking around the bedding material. If this case, to prevent changes in the background to interfere with detection, select 'Medium fast' or faster. Usually, 'Medium slow' works just fine.

important As much as possible of the animal's body must be detected for good tracking.