使用 OpenCV 保存视频时出现错误消息

Question

我在运行代码时遇到错误，我不明白发生了什么，但我认为是程序完成时，因为我得到了我想要的结果，即将现有视频转换为灰度和保存它。

cv2.error: OpenCV(4.1.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty( ) 在函数'cv::cvtColor'中

cap = cv2.VideoCapture('videos/output.avi')
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('results/output.avi', fourcc, 20.0, (640, 480))

while (cap.isOpened()):
    _, frame = cap.read()
    frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    out.write(frame)

    # Key events
    key = cv2.waitKey(1)
    if key == 27:  # esc
        break

cap.release()
cv2.destroyAllWindows()

谢谢！

Answer 1

在某些情况下，您的视频中至少有一个帧未正确读入。这就是cv2.cvtColor 方法抛出错误的原因，因为您提供的帧数据是空的。

您应该考虑使用cv2.VideoCapture.read() 的第一个输出来确保正确捕获视频帧，然后将其写入文件。第一个输出是一个标志，用于确定当前帧是否已成功读入。此外，您需要处理我们到达视频结尾的结尾。在这种情况下，标志将是False，因此我们应该退出循环。最后，如果您打算编写灰度帧，cv2.VideoWriter 中有一个可选的第五个参数，称为isColor，我们可以将其设置为False，以允许我们直接编写灰度帧。这意味着不再需要调用cv2.cvtColor。

我推荐的另一件事是从视频文件中推断帧的宽度和高度，而不是自己设置。这样输入和输出分辨率是相同的。最后，完成后不要忘记释放cv2.VideoWriter 对象，我已经为视频文件添加了额外的检查以查看它是否已正确打开：

import numpy as np
import cv2
import sys

cap = cv2.VideoCapture('videos/output.avi')

# Check to see if the video has properly opened
if not cap.isOpened():
    print("File could not be opened")
    sys.exit(1)

fourcc = cv2.VideoWriter_fourcc(*'XVID')
frame_width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))  # Get the frame width and height
frame_height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT)) 

# Change
out = cv2.VideoWriter('results/output.avi', fourcc, 20.0, (frame_width, frame_height), isColor=False)

while True:
    ret, frame = cap.read() # New

    if not ret: # New
        break # Get out if we don't read a frame successfully

    frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    out.write(frame)

    # Key events
    key = cv2.waitKey(1)
    if key == 27:  # esc
        break

cap.release()
out.release() # New
cv2.destroyAllWindows()

作为一个小提示，您没有显示任何窗口，因此cv2.destroyAllWindows() 在这里是多余的。考虑将其从您的代码中删除。

Answer 2

所以这个答案有另一种方法（这里，你也可以通过改变B，G，R值前面的相应权重来提取不同的颜色）

import cv2
cap = cv2.VideoCapture('videos/output.avi')
frame_width = int(cap.get(3))  # finds the frame width automatically
frame_height = int(cap.get(4))  # finds the frame height automatically

fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('results/outpy.avi', cv2.VideoWriter_fourcc('M', 'J', 'P', 'G'), 10, (frame_width, frame_height))

while (cap.isOpened()): # value is true if the file is successfully opened.
    ret, frame = cap.read()
    if ret == True:  # checks if the return value is True or False. False means file ended.
        # grey = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) # the grey matrix has a different shape than the frame matrix
        # that's why the output files were blank
        # to circumvent this RGB2GRAY, I manually added the "NORMALIZED" R,G,B values.

        frame[:,:,0] = 0.114*frame[:,:,0]+0.587*frame[:,:,1]+0.299*frame[:,:,2] #multiplying normalized co-efficients to B,G,R
# for extracting red, make 0.299 as 1.0 and others as 0.0; same goes for other colours.
        frame[:, :, 1]=frame[:,:,0] # making the G and R values same as the B.
        frame[:, :, 2]=frame[:,:,0]
        # now frame is a 3d grayscale matrix. Identical to the cv2.cvtColor method....except it is 3d
        # now frame is grey scaled...R,G,B values for each pixel,holds the same number....
        out.write(frame)
    else:
        break
cap.release()
out.release()
cv2.destroyAllWindows()