how to get the number of channels from an image, in OpenCV 2?

Question

The answers at Can I determine the number of channels in cv::Mat Opencv answer this question for OpenCV 1: you use the Mat.channels() method of the image.

But in cv2 (I'm using 2.4.6), the image data structure I have doesn't have a channels() method. I'm using Python 2.7.

Code snippet:

cam = cv2.VideoCapture(source) ret, img = cam.read() # Here's where I would like to find the number of channels in img. 

Interactive attempt:

>>> img.channels() Traceback (most recent call last):   File "<interactive input>", line 1, in <module> AttributeError: 'numpy.ndarray' object has no attribute 'channels' >>> type(img) <type 'numpy.ndarray'> >>> img.dtype dtype('uint8') >>> dir(img) ['T',  '__abs__',  '__add__', ...  'transpose',  'var',  'view'] # Nothing obvious that would expose the number of channels. 

Thanks for any help.

Answer 1

Use img.shape

It provides you the shape of img in all directions. ie number of rows, number of columns for a 2D array (grayscale image). For 3D array, it gives you number of channels also.

So if len(img.shape) gives you two, it has a single channel.

If len(img.shape) gives you three, third element gives you number of channels.

For more details, visit here

Answer 2

I'm kind of late but there is another simple way out there:

Use image.ndim Source, will give your right number of channels as below:

 if image.ndim == 2:      channels = 1 #single (grayscale)  if image.ndim == 3:      channels = image.shape[-1] 

Edit: In one-liners:

channels = image.shape[-1] if image.ndim == 3 else 1 

Since a image is a nothing but a numpy array. Checkout OpenCV docs here: docs