Motion Detection

6 min read

(For more resources related to this topic, see here.)

Obtaining the frame difference

To begin with, we create a patch with name Frame001.pd. Put in all those elements for displaying the live webcam image in a rectangle. We use a dimen 800 600 message for the gemwin object to show the GEM window in 800 x 600 pixels. We plan to display the video image in the full size of the window.

The aspect ratio of the current GEM window is now 4:3. We use a rectangle of size 5.33 x 4 (4:3 aspect ratio) to cover the whole GEM window:

Now we have one single frame of the video image. To make a comparison with another frame, we have to store that frame in memory. In the following patch, you can click on the bang box to store a copy of the current video frame in the buffer. The latest video frame will compare against the stored copy, as shown in the following screenshot:

The object to compare two frames is pix_diff. It is similar to the Difference layer option in Photoshop. Those pixels that are the same in both frames are black. The color areas are those with changes across the two frames. Here is what you would expect in the GEM window:

To further simplify the image, we can get rid of the color and use only black and white to indicate the changes:

The pix_grey object converts a color image into grey scale. The pix_threshold object will zero out the pixels (black) with color information lower than a threshold value supplied by the horizontal slider that has value between 0 and 1. Refer to the following screenshot:

Note that a default slider has a value between 0 and 127. You have to change the range to 0 and 1 using the Properties window of the slider.

In this case, we can obtain the information about those pixels that are different from the stored image.

Detecting presence

Based on the knowledge about those pixels that have changed between the stored image and the current video image, we can detect the presence of a foreground subject in front of a static background. Point your webcam in front of a relatively static background; click on the bang box, which is next to the Store comment, to store the background image in the pix_buffer object. Anything that appears in front of the background will be shown in the GEM window. Now we can ask the question: how can we know if there is anything present in front of the background? The answer will be in the pix_blob object:

The pix_blob object calculates the centroid of an image.

The centroid ( of an image is its center of mass. Imagine that you cut out the shape of the image in a cardboard. The centroid is the center of mass of that piece of cardboard. You can balance the cardboard by using one finger to hold it as the center of mass.

In our example, the image is mostly a black-grey scale image. The pix_blob object finds out the center of the nonblack pixels and returns its position in the first and second outlets. The third outlet indicates the size of the nonblack pixel group. To detect the presence of a foreground subject in front of the background, the first and second number boxes connected to the corresponding pix_blob outlets will return roughly the center of the foreground subject. The third number box will tell how big that foreground subject is.

If you pay attention to the changes in the three number boxes, you can guess how we will implement the way to detect presence. When you click on the store image bang button, the third number box (size) will turn zero immediately. Once you enter into the frame, in front of the background, the number increases. The bigger the portion you occupy of the frame, the larger the number is. To complete the logic, we can check whether the third number box value is greater than a predefined number. If it is, we conclude that something is present in front of the background. If it is not, there is nothing in front of the background. The following patch Frame002.pd will try to display a warning message when something is present:

A comparison object > 0.002 detects the size of the grey area (blob). If it is true, it sends a value 1 to the gemhead object for the warning text to display. If it is false, it sends a value 0. We’ll use a new technique to turn on/off the text. Each gemhead object can accept a toggle input to turn it on or off. A value 1 enables the rendering of that gemhead path. A value 0 disables the rendering. When you first click on the store image bang button, the third number box value drops to 0. Minor changes in the background will not trigger the text message:

If there is significant change in front of the background, the size number box will have a value larger than 0.002. It thus enables the rendering of the text2d message to display the WARNING message.

After you click on the Store bang box, you can drag the horizontal slider attached to the pix_threshold object. Drag it towards the right-hand side until the image in the GEM window turns completely black. It will roughly be the threshold value. Note also that we use a number in each gemhead object. It is the rendering order. The default one is 50. The larger number will be rendered after the lower number. In this case, the gemhead object for the pix_video object will render first. The gemhead object for the text2d object will render afterwards.

In this case, we can guarantee that the text will always be on top of the video:

Actually, you can replace the previous version with a single pix_background object. A reset message will replace the bang button to store the background image. In the following patch, it will show either the clear or warning message on the screen, depending on the presence of a subject in front of the background image:

The GEM window at this moment shows only a black screen when there isn’t anything in front of the background. For most applications, it would be better to have the live video image on screen. In the following patch, we split the video signal into two – one to the pix_background object for detection and one to the pix_texture object for display:

The patch requires two pix_separator objects to separate the two video streams from pix_video, in order not to let one affect the other. Here is the background image after clicking on the reset message:

The warning message shows up after the subject entered the frame, and is triggered by the comparison object > 0.005 in the patch:

We have been using the pix_blob object to detect presence in front of a static background image. The pix_blob object will also return the position of the subject (blob) in front of the webcam. We are going to look into this in the next section.


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