import cv2
filepath = 'test.mp4'
cap = cv2.VideoCapture(filepath)
if not cap.isOpened():
    pass # エラー処理
for idx in  range(int(cap.get(cv2.CAP_PROP_FRAME_COUNT))):
    ret, frame = cap.read()
    print(frame.shape)
    break

import cv2
import numpy as np

height = 720
width = 1280
img = np.zeros((height, width, 3))
img += [255,0,0][::-1] # 赤

cv2.imwrite('img.png',img)

import cv2
import numpy as np

height = 720
width = 1280
img = np.zeros((height, width))
img += 100 # 灰色

cv2.imwrite('img.png',img)