**Edit: original title "convert numpy array to cvmat" was a mistake - OpenCV's less than useful error message and my not reading the docs!**

With OpenCV 2, IPython now uses NumPy arrays by default.

```
cvimage = cv2.imread("image.png") #using OpenCV 2
type(cvimage)
Out: numpy.ndarray #dtype is uint8
pltimage = plt.imread("image.png") #using Matplotlib
type(pltimage)
Out: numpy.ndarray #dtype is float
plt.imshow(cvimage) # works great
cv2.imshow(cvimage)
TypeError: Required argument 'mat' (pos 2) not found
```

Since `cv2`

uses NumPy arrays by default, there is no longer any cv::Mat constructor and NumPy has no functions to convert to a `cv::Mat`

array.

Any ideas?

The function has the following docstring: `imshow(winname, mat) -> None`

.
You can see the doc string by typing `cv2.imshow.__doc__`

in the interpreter.

Try `cv2.imshow('Image', cvimage)`

.

tl;dr : In original question, first argument of "window name" was missing. "imshow" takes two parameters and only one was supplied.

The question technically asks how to convert a NumPy Array (analogous to CV2 array) into a Mat object (CV). For anyone who is interested, this can be done by:

```
mat_array = cv.fromarray(numpy_array)
```

where mat_array is a Mat object, and numpy_array is a NumPy array or image. I would suggest staying away from older CV structures where possible. Numpy arrays offer much better performance than implemenations in native Python

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