Monday, August 10, 2015

Checking your OpenCV version using Python

It was unavoidable — the OpenCV 3 release was bound to break backwards compatibility with some OpenCV 2.4.X functions:

cv2.findContours

and

cv2.normalize

come to mind right off the top of my head.

So how do you ensure that your code will work no matter which version of OpenCV your production environment is using?

Well, the short answer is that you’ll need to create

if

statements around each of the offending functions (or abstract the functions away to a separate method that handles calling the appropriate function based on your OpenCV version).

In order to do this, you’ll need to be able to check your OpenCV version from within your using Python — and that’s exactly what the rest of this blog will show you how to do!

Looking for the source code to this post?
Jump right to the downloads section.

Checking your OpenCV version using Python

The OpenCV version is contained within a special

cv2.__version__

variable, which you can access like this:

$ python
>>> import cv2
>>> cv2.__version__
'3.0.0'

The

cv2.__version__

variable is simply a string which you can split into the major and minor versions:

>>> (major, minor, _) = cv2.__version__.split(".")
>>> major
'3'
>>> minor
'0'

Of course, having to perform this operation every time you need to check your OpenCV version is a bit of pain. To resolve this problem, I have added three new functions to my imutils package, a series of convenience functions to make basic image processing functions with OpenCV and Python easier.

You can see my

is_cv2

is_cv3

, and

check_opencv_version

functions below:

def is_cv2():
    # if we are using OpenCV 2, then our cv2.__version__ will start
    # with '2.'
    return check_opencv_version("2.")

def is_cv3():
    # if we are using OpenCV 3.X, then our cv2.__version__ will start
    # with '3.'
    return check_opencv_version("3.")

def check_opencv_version(major, lib=None):
    # if the supplied library is None, import OpenCV
    if lib is None:
        import cv2 as lib
        
    # return whether or not the current OpenCV version matches the
    # major version number
    return lib.__version__.startswith(major)

The code here is fairly straightforward — I’m simply checking if the

cv2.__version__

string starts with a

, to indicate that we are using OpenCV 3, or if it starts with a

, indicating we are using OpenCV 2.X.

Again, these functions have already been included in the imutils package, which you can install using pip:

$ pip install imutils

If you already have

imutils

installed, you can upgrade to the latest version using:

$ pip install --upgrade imutils

Checking your OpenCV version: a real-world example

Now that we know how to check our OpenCV version using Python as well as defined a couple convenience functions to make the version check easier, let’s see how we can use these functions in a real-world example.

Our goal here is to detect contours in the following image:

Figure 1: We are going to utilize OpenCV 2.4.X and OpenCV 3 to detect the contours of the Tetris blocks.

Figure 1: We are going to utilize OpenCV 2.4.X and OpenCV 3 to detect the contours (i.e. outlines) of the Tetris blocks.

In order to detect contours in an image, we’ll need to use the

cv2.findContours

function. However, as we know, the return signature of

cv2.findContours

has changed slightly between version 3 and 2.4 of OpenCV (the OpenCV 3 version of

cv2.findContours

returns an extra value in the tuple) — thus we’ll need to perform a check to our OpenCV version prior to making a call to

cv2.findContours

to ensure our script does not error out. Let’s take a look at how we can make this check:

# import the necessary packages
from __future__ import print_function
import imutils
import cv2

# load the Tetris block image, convert it to grayscale, and threshold
# the image
print("OpenCV Version: {}".format(cv2.__version__))
image = cv2.imread("tetris_blocks.png")
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 225, 255, cv2.THRESH_BINARY_INV)[1]

# check to see if we are using OpenCV 2.X
if imutils.is_cv2():
        (cnts, _) = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,
                cv2.CHAIN_APPROX_SIMPLE)

# check to see if we are using OpenCV 3
elif imutils.is_cv3():
        (_, cnts, _) = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,
                cv2.CHAIN_APPROX_SIMPLE)

# draw the contours on the image
cv2.drawContours(image, cnts, -1, (240, 0, 159), 3)
cv2.imshow("Image", image)
cv2.waitKey(0)

As you can see, all we need to do is make a call to

is_cv2

and

is_cv3

and then wrap our version specific code inside the

if

statement blocks — that’s it!

Now when I go to execute my script using OpenCV 2.4, it works without a problem:

Figure 2: Our call to cv2.findContours is working in OpenCV 2.4.X.

And the same is true for OpenCV 3:

Figure 3: And the same is true for OpenCV 3 since we are using the is_cv2 and is_cv3 functions to detect OpenCV versions with Python.

Summary

In this blog post we learned how to check our OpenCV version using Python. The OpenCV version is included in a special string variable named

cv2.__version__

. All we need to do is check this variable and we’ll be able to determine our OpenCV version.

Finally, I have defined a few convenience methods inside the imutils package to make checking your OpenCV version easier and more Pythonic. Consider checking the library out if you find yourself needing to consistently check OpenCV versions.