How to Turn your Pi into a motion sensor with SimpleCV OpenCv

How to Turn your Pi into a motion sensor with SimpleCV OpenCv

How to Turn your Pi into a motion sensor with SimpleCV "OpenCv"
How to Turn your Pi into a motion sensor with SimpleCV "OpenCv"

Learn how to implement facial recognition into your Raspberry Pi using Python and a webcam

The Kinect has proven a popular piece of tech to use with the Raspberry Pi. But not everyone has access to this kind of hardware. Another class of project that is popular with Raspberry Pis is using USB cameras to create monitors of one form or another. A lot of these projects use command line applications to talk to the USB camera and generate images or movies that are used as part of the system. But what if you are writing your own program in Python and you want to add some form of image system to your code? Luckily, there are several modules available for you to choose from. In this article, we will take a look at using SimpleCV to get your program to talk with the USB camera. SimpleCV is built on top of OpenCV, making it easier to use for common tasks. Assuming you are using Raspbian, you can go to the main page for SimpleCV ( and download a DEB file. To install it, you can simply run:

sudo dpkg -i SimpleCV-1.31.deb 

Before you do, however, you will want to install all of the dependencies. You can do that with the command:

sudo apt-get install python python-support python-numpy python-scipy ipython pythonopencv python-pygame pythonsetuptools

 You can check that everything worked by running the command ‘simplecv’ at the command line. This will start Python up and run the interactive shell that is provided by the SimpleCV module. You can then try connecting to your USB camera and pulling images from it. Now that everything should be up and running, how do you actually use it in yourown code? You can load all of the available functions and objects into the global scope with the command:

from SimpleCV import *

Making sure that you have your USB camera plugged in, you can now create a camera object with: cam = Camera() This will load the required drivers, and initialise the camera so that it is ready to start taking pictures. Once this object creation returns, you can grab an image from the camera with:

img = cam.getImage()

 At least in the beginning, when you are experimenting, you may want to see what this image looks like. You can do this with:

You will, of course, need to have a GUI up and running in order to actually see the movie. Otherwise, you will get an error when you try and call ‘’.
Don’t forget that you can always pull up documentation with commands like:


SimpleCV is built on top of OpenCV, making it easier to use for common tasks

With the ‘Image’ object, you can do some basic processing tasks right away. You can scale an image by some percentage, say 90%, with ‘img.scale(90,90)’. You
can also crop an image by giving it a start location and saying how many pixels across and how many up and down you want to crop to. This looks like ‘img.crop(100,100,50,50)’. SimpleCV has the location (0,0) as the top-left corner of an image. 

The really interesting functionality in SimpleCV is the ability to find features within an image and to work with them. One of the clearest features you can look for is blobs, where blobs are defined as continuous light regions. The function ‘img.findBlobs()’ will search the captured image for all blobs and return them as a FeatureSet. You can set the minimum number of pixels to consider a single blob, the maximum number of pixels, as well as a threshold value. If you are looking at a region that has some hard edges, you can use the function ‘img. findCorners()’. This function will return a FeatureSet of all of the corners within the captured image. A very simple monitor program could use one of these
functions to see if there is any motion happening. If there is, then the set of blobs or corners will change from one frame to another. Of course, a little more reading will lead you to the ‘img. findMotion()’ function. This function will take two subsequent images and see if any motion can be detected going from one to the other. The default method is to use a block matching algorithm, but you can also use either the Lucas-Kanade method or the Horn-Schunck method. 

The above methods will let you know some features of the captured images, and if any kind of motion has occurred. But what if you are more interested in identifying whether people have been moving around? Maybe you have an area you need to secure from espionage.

You can look for blobs – continuous light regions

In this case, you can use the function ‘ ndSkintoneBlobs()’. You can use a binarise fi lter threshold to set what constitutes a skin tone. If you need to do more, you have access to all of the underlying OpenCV functionality. One of these more advanced functions is face recognition. You can use the function ‘img. fi ndHaarFeatures()’ to look for a known type of object. If you wanted to look for faces, you could use something like:

faces = HaarCascade(“./SimpleCV/

When you start developing these types of programs, one thing that might come into play is timing issues. You want to be sure that your code is fast enough to catch everyone that may be moving through the fi eld of the camera. In order to fi gure out what is costing time, you need to be able to profi le your code. The shell in SimpleCV provides a feature called ‘timeit’ that will give you a quick and dirty profi ling tool that you can use while you are experimenting with different algorithms. So, as an example, you can see how long the ‘fi ndBlobs()’ function takes on your Raspberry Pi with something like: 

img = cam.getImage() timeit img.findBlobs()

 Once you fi nd and fi x the bottlenecks in your code, you can create the end product for your fi nal version. With this article, you should now have enough to start using cameras from within your own programs. We have only been able to cover the bare essentials, however, so don’t forget to go check out the documentation covering all of the other functionality that is available in the SimpleCV module

Full code listing

# SimpleCV provides a simple interface to OpenCV
# First, we will import everything into the local
# namespace
from SimpleCV import *
# Make sure your USB camera is plugged in,
# then you can create a camera object
cam = Camera()
# Getting an image from the camera is straightforward
img = cam.getImage()
# You can rescale this image to half its original size
img2 = img.scale(50,50)
# There are several features that you may want to look at
# You can extract a list of blobs
blobs = img.findBlobs()
# You can draw these blobs and see where they are on
# the image
# or a list of corners
corners = img.findCorners()
# If you want to identify motion, you will need two
# frames
img2 = cam.getImage()
# You can get a FeatureSet of motion vectors with
motion = img2.findMotion(img)
# Face recognition is possible too. You can get a list of
# the types of features you can look for with
# For faces, you can generate a Haar Cascade
faces = HaarCascade(‘face.xml’)
# Now you can search for faces
found_faces = img.findHaarFeatures(faces)
# You can load image files with the Image class
my_img = Image(‘my_image.jpg’)
# You can save images to the hard drive, too‘camera.png’)


SimpleCV is built on top of OpenCV and provides a simplifi ed set of functions. But what can you do if you have more complicated work to do? You always have
the option of using OpenCV directly to gain access to the full set of functions. You can import the module into the local namespace with:

from cv2 import *

Not only do you have the usual image manipulation functions and the feature recognition tools, but you also have the ability to process video. You can use meanshift and camshift to do colour based motion detection. There are functions to look at optical fl ow. These look at apparent motions in a video, from one frame to the next, that are caused by either the object moving or the camera moving. You can even subtract the background from a moving foreground object. This is a common preprocessing step in vision systems. You can even construct 3D information from a set of stereo images gathered by a pair of cameras. With OpenCV, you really can
deal with almost any vision problem you might be tackling.

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