What is output from OpenCV's Dense optical flow (Farneback) function? How can this be used to build an optical flow map in Python?

Question

I am trying to use the output of Opencv's dense optical flow function to draw a quiver plot of the motion vectors but have not been able to find what the function actually outputs. Here is the code:

import cv2
import numpy as np

cap = cv2.VideoCapture('GOPR1745.avi')

ret, frame1 = cap.read()
prvs = cv2.cvtColor(frame1,cv2.COLOR_BGR2GRAY)
hsv = np.zeros_like(frame1)

hsv[...,1] = 255
count=0

while(1):
    ret, frame2 = cap.read()
    next = cv2.cvtColor(frame2,cv2.COLOR_BGR2GRAY)
    flow = cv2.calcOpticalFlowFarneback(prvs,next,None, 0.5, 3, 15, 3, 10, 1.2, 0)
    mag, ang = cv2.cartToPolar(flow[...,0], flow[...,1])

    hsv[...,0] = ang*180/np.pi/2
    hsv[...,2] = cv2.normalize(mag,None,0,255,cv2.NORM_MINMAX)
    rgb = cv2.cvtColor(hsv,cv2.COLOR_HSV2BGR)
    if count==10:
        count=0

        print "flow",flow

    cv2.imshow('frame2',rgb)
    count=count+1
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break
    elif k == ord('s'):
    prvs = next

cap.release()
cv2.destroyAllWindows()

This is effectively the same code as given in the OpenCv tutorial on dense optical flow. I receive the following output from the print function:

flow [[[  0.00000000e+00   0.00000000e+00]
  [  0.00000000e+00   0.00000000e+00]
  [  0.00000000e+00   0.00000000e+00]
  ..., 
  [  0.00000000e+00   0.00000000e+00]
  [  0.00000000e+00   0.00000000e+00]
  [  0.00000000e+00   0.00000000e+00]]

 ..., 
 [[ -3.54891084e-14  -1.38642463e-14]
  [ -2.58058853e-14  -1.54020863e-14]
  [ -5.56561768e-14  -1.88019359e-14]
  ..., 
  [ -7.59403916e-15   1.16633225e-13]
  [  7.22156371e-14  -1.61951507e-13]
  [ -4.30715618e-15  -4.39530987e-14]]

 [[ -3.54891084e-14  -1.38642463e-14]
  [ -2.58058853e-14  -1.54020863e-14]
  [ -5.56561768e-14  -1.88019359e-14]
  ..., 
  [ -7.59403916e-15   1.16633225e-13]
  [  7.22156371e-14  -1.61951507e-13]
  [ -4.30715618e-15  -4.39530987e-14]]

I would like to know what exactly these values are? Original X,Y coordinates? Final X,Y coordinates? Distance moved?

I plan to try and find the initial and final coordinates to make a quiver plot using code from the following page: https://www.getdatajoy.com/examples/python-plots/vector-fields This is because in python there is no function that i am aware of that plots an optical flow map for you.

Thank you in advance!

Answer 1

You were almost there. Lets first take a look at the calcOpticalFlowFarneback Documentation it says there:

flow – computed flow image that has the same size as prev and type CV_32FC2.

So what you are actually getting is a matrix that has the same size as your input frame.
Each element in that flow matrix is a point that represents the displacement of that pixel from the prev frame. Meaning that you get a point with x and y values (in pixel units) that gives you the delta x and delta y from the last frame.