在 python 中将 mp4 视频文件转换为灰度并获取 Source is empty 错误

Question

Unable to import video media folder into program to convert to gray-scale.

import cv2 
import numpy as np

# The video feed is read in as a VideoCapture object
cap = cv2.VideoCapture(r'C:\Users\Mark\Desktop\Research\Source\Video\Traffic01.mp4')

# ret = a boolean return value for getting the frame
ret, first_frame = cap.read()

# Converts frame to grayscale
prev_gray = cv2.cvtColor(cv2.UMat(first_frame), cv2.COLOR_RGB2GRAY)

while(cap.isOpened()):

    # ret = a boolean return value from getting the frame
    ret, frame = cap.read()

    # Converts each frame to grayscale 
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Frames are read by intervals of 10 milliseconds. 
    #The programs breaks out of the while loop when the user presses the 'q' key
    if cv2.waitKey(10) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed)._src:empty() in function 'cv::cvtColor'

Answer 1

Why is that UMat there? That would seem to answer the question src.empty() from the docs at openCV UMat is/does:- constructs 2D matrix and fills it with the specified value _s. whereas cvtcolor wants a source image/frame

Also why is the first frame so important it has to be outside the main loop?

My advice comment out that first cap.read() and cv2.cvtcolor() lines just before your while loop. HTH