TypeError: Required argument 'mat' (pos 2) not found

Question

I have the code below with cv2 . This code is downloaded from https://github.com/dipakkr/3d-cnn-action-recognition. I want to use cv2.imshow to visualize the frames of the video it get. But I get the following error. What is the problem? I am doubtful of whether this code is really able to read the video as what is returns as the output is an array of zeros.

def video3d(self, filename, color=False, skip=True):

        cap = cv2.VideoCapture(filename)
        #ret, frame=cap.read()
        #cv2.imshow('frame', frame)
        nframe = cap.get(cv2.CAP_PROP_FRAME_COUNT) #Returns the specified VideoCapture property  ,,Number of frames in the video file

        print (nframe, "nframe")

        if skip:
            frames = [x * nframe / self.depth for x in range(self.depth)]
            print (frames, "frameeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees")

        else:
            frames = [x for x in range(self.depth)]
            print (frames, "frameseeeeeeeeeeeeeeeeeeeeeeeeeeeeeee2")

        framearray = []

        for i in range(self.depth):
            cap.set(cv2.CAP_PROP_POS_FRAMES, frames[i])  #Sets a property in the VideoCapture. ,,0-based index of the frame to be decoded/captured next.
            ret, frame = cap.read()
            cv2.imshow(frame)
            print(ret, "reeeeeeeeeeeeeeeeettttttttt")
            print(frame ,"frame issssssssssss:")
            frame = cv2.resize(frame, (self.height, self.width))
            print(frame, "frame222 isssssssssssssss")
            #cv2.imshow(frame)
            if color:
                framearray.append(frame)
            else:
                framearray.append(cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY))

        cap.release()
        return np.array(framearray)


X.append(vid3d.video3d(v_file_path, color=color, skip=skip))

Error:

    main()
  File "3dcnn.py", line 151, in main
    args.output, args.color, args.skip)
  File "3dcnn.py", line 103, in loaddata
    X.append(vid3d.video3d(v_file_path, color=color, skip=skip))
  File "/home/gxa131/Documents/final_project_computationalintelligence/3d-cnn-action-recognition/videoto3d.py", line 34, in video3d
    cv2.imshow(frame)
TypeError: Required argument 'mat' (pos 2) not found

Answer 1

The first argument to cv2.imshow is the window name, so it's considering the second input mat (the image) as missing. If you don't want to name the window, you can just give an empty string as the first input parameter.

cv2.imshow('', frame) 

Answer 2

cv2 didn't the find the "mat" (matrix), because you passed the image as the first argument, but the first argument is supposed to be the window name.

cv2.imshow(winname, mat)

In most cases, you don't care about the window name, so use this:

cv2.imshow('', frame)
cv2.waitKey(0)

This is the result when you change the window name:

import numpy as np
import cv2

image = np.full((300, 300, 3), 255).astype(np.uint8)

cv2.putText(image, 'some picture', (20, 60),
            cv2.FONT_HERSHEY_SIMPLEX, 1, [0, 0, 0])

cv2.imshow('custom window name', image)
cv2.waitKey(0)

One of the reasons you might want to change window names is to plot many pictures at once, in different windows.