如何在 opencv 中获取视频的前一帧 python

Question

我想根据视频中不断增加的障碍物检测障碍物 size.To 首先我在灰度图像上应用 SIFT 以获得当前帧的特征点。接下来将当前帧的特征点与上一帧进行比较，我想应用蛮力算法。为此，我想获得前一帧中的特征点。如何访问 opencv python 中的上一帧？以及当当前帧是视频的第一帧时如何避免访问上一帧？

下面是python中写的获取当前帧特征点的代码。

import cv2
import numpy as np


cap = cv2.VideoCapture('video3.mov')

  while(cap.isOpened()):

  ret, frame = cap.read()

  gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

  #detect key feature points
  sift = cv2.xfeatures2d.SIFT_create()
  kp, des = sift.detectAndCompute(gray, None)

 #draw key points detected
 img=cv2.drawKeypoints(gray,kp,gray,flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)

 cv2.imshow("grayframe",img)

 if cv2.waitKey(100) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Answer 1

只需将当前帧保存为下一次迭代的前一帧。如果需要超过 1 个，请使用列表。

import cv2
import numpy as np


cap = cv2.VideoCapture('video3.mov')
previousFrame=None

while(cap.isOpened()):

  ret, frame = cap.read()

  if previousFrame is not None:
      #use previous frame here
      pass

  #save current frame
  previousFrame=frame


  gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

  #detect key feature points
  sift = cv2.xfeatures2d.SIFT_create()
  kp, des = sift.detectAndCompute(gray, None)

  #draw key points detected
  img=cv2.drawKeypoints(gray,kp,gray,flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)

  cv2.imshow("grayframe",img)


  if cv2.waitKey(100) & 0xFF == ord('q'):
       break

cap.release()
cv2.destroyAllWindows()

Answer 2

OpenCV 中没有访问前一帧的特定函数。您的问题可以通过在进入 while 循环之前调用一次 cap.read() 来解决。在读取新帧之前使用变量 prev_frame 存储前一帧。最后，作为一种好的做法，您应该在对帧进行计算之前验证帧是否已正确读取。您的代码可能类似于：

import cv2
import numpy as np

cap = cv2.VideoCapture('video3.mov')
ret, frame = cap.read()

while(cap.isOpened()):
    prev_frame=frame[:]
    ret, frame = cap.read()
    if ret:
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

        #detect key feature points
        sift = cv2.xfeatures2d.SIFT_create()
        kp, des = sift.detectAndCompute(gray, None)

        #some magic with prev_frame

        #draw key points detected
        img=cv2.drawKeypoints(gray,kp,gray, flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)

        cv2.imshow("grayframe",img)
    else:
        print('Could not read frame')

    if cv2.waitKey(100) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Answer 3

您还可以 get/set 从零开始的帧索引 (CAP_PROP_POS_FRAMES)，如果您希望灵活地返回多个帧，与特定帧进行比较，这可能会很有用，等。请注意，尽管这会重置下一个 read() 的位置，因此如果您真的只想要前一帧，则根据其他答案将其存储在变量中可能更好。

next_frame = cap.get(cv2.CAP_PROP_POS_FRAMES)
current_frame = next_frame - 1
previous_frame = current_frame - 1

if previous_frame >= 0:
  cap.set(cv2.CAP_PROP_POS_FRAMES, previous_frame)
  ret, frame = cap.read()

如何在 opencv 中获取视频的前一帧 python

How to get previous frame of a video in opencv python

python

opencv

video-processing