# cv2.optflow.calcOpticalFlowSparseRLOF Giving Assertion Error

**URL:** <https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835>\
**Category:** Python\
**Tags:** optflow\
**Created:** [August 19, 2021, 2:16am UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835 "2021-08-19T02:16:07Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![help](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/help/32/3010_2.png) [@help](https://forum.opencv.org/u/help)\
**Post date:** [August 19, 2021, 2:16am UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/1 "2021-08-19T02:16:07Z")

</div>

Hello, I am trying to use SparseRLOF Optical Flow to calculate displacement of falling particles. I have already done this using Lucas Kanade and it worked fine. I wanted to give this algorithm a shot and see if it does better according to some metrics. Unfortunately, I a facing an error when I call the calcOpticalFlowSparseRLOF() function. I have pasted the error below. I have also pasted my code below. Please let me know if you can help. This is my first time using this forum to post. If I need to reformat, please let me know.

The code:

```auto
#importing the necessary libraries
import cv2
import numpy as np
import matplotlib.pyplot as plt
import math

a = 9.81*1000*2.8

y0 = 140 #v0 = 0 at y0
dy = np.arange(0, 1081-y0, 1)
y_vals = np.arange(y0,1081,1)
v = np.sqrt(2*a*(dy))
zip_iterator = zip(y_vals, v)
velocity_dict = dict(zip_iterator)

fps = 240

# Creating a VideoCapture object to read the video
cap = cv2.VideoCapture('quinoa_short.mp4')
orb = cv2.ORB_create(nfeatures =10000)
ret, old_frame = cap.read()

# Loop untill the end of the video
loop_var = 1
normalized_sum = 0
while (cap.isOpened()):
    # Capture frame-by-frame
    ret, new_frame = cap.read()
    print(loop_var)
    if new_frame is None:
    	break
    kp, des = orb.detectAndCompute(new_frame, None)
    p0 = []
    p_guess = []
    for i in kp:
        #filter points
    	if i.pt[1] >=225 and i.pt[0] <= 1190:
            p0.append(i.pt)
            x = i.pt[0]
            y = i.pt[1] + velocity_dict[math.floor(i.pt[1])]/fps + (a/2)*(1/fps)**2
            point = (x, y)
            p_guess.append(point)
    #optical flow 
    retval	=	cv2.optflow.RLOFOpticalFlowParameter_create()
    retval.setUseInitialFlow(True)
    p1, st, err = cv2.optflow.calcOpticalFlowSparseRLOF(old_frame, new_frame, np.float32(p0), nextPts = np.float32(p_guess), rlofParam = retval)

```

The error:

```auto
Traceback (most recent call last):
  File "/Users/vishnu/Documents/Summer Project/Metrics Folder/RLOF/Quinoa/gravity_sim.py", line 58, in <module>
    p1, st, err = cv2.optflow.calcOpticalFlowSparseRLOF(old_frame, new_frame, np.float32(p0), nextPts = np.float32(p_guess), rlofParam = retval)
cv2.error: OpenCV(4.5.1) /private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/pip-req-build-p3unqtkr/opencv_contrib/modules/optflow/src/rlof/rlof_localflow.cpp:402: error: (-215:Assertion failed) nextPtsMat.checkVector(2, CV_32F, true) == npoints in function 'calcLocalOpticalFlowCore'

```

I used the same exact vector formats for the Lucas Kanade method and am therefore confused as to why this problem is occurring with SparseRLOF. Any input would be appreciated.

---

<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [August 19, 2021, 6:32pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/2 "2021-08-19T18:32:28Z")

</div>

try some wild reshaping, like to `(-1, 1, 2)`

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<div class="post-metadata">

**Author:** ![help](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/help/32/3010_2.png) [@help](https://forum.opencv.org/u/help)\
**Post date:** [August 19, 2021, 7:05pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/3 "2021-08-19T19:05:07Z")

</div>

I tried that. It did not work. Is the function working properly? I’m not sure if it is something I’m doing or if there is something wrong with the function itself.

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<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [August 19, 2021, 8:14pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/4 "2021-08-19T20:14:40Z")

</div>

convert `p_guess` to numpy array

post these:  
`p_guess.shape`  
`p_guess.dtype`  
`p_guess.data.contiguous`

and show how you tried to set shape = (-1, 1, 2)

crosspost:

> <https://stackoverflow.com/questions/68853953/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error>

---

<div class="post-metadata">

**Author:** ![help](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/help/32/3010_2.png) [@help](https://forum.opencv.org/u/help)\
**Post date:** [August 19, 2021, 10:03pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/5 "2021-08-19T22:03:50Z")

</div>

Hello,  
My reshape line is shown below:  
`p_guess = np.array(p_guess).reshape(-1,1,2)`

printing p\_guess.shape returns:  
`(5589, 2)`  
printing p\_guess.data.contiguous returns:  
`True`  
printing p\_guess.dtype returns:  
`float64`  
These outputs are without reshaping. reshaping just changes the shape (5589, 1, 2) where 5589 is the number of key points.

---

<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [August 19, 2021, 10:51pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/6 "2021-08-19T22:51:52Z")

</div>

> [@help](#):
>
> printing p\_guess.dtype returns:  
> `float64`

it needs to be float32, so ask for `.astype(np.float32)` or see about causing that type to begin with

---

<div class="post-metadata">

**Author:** ![help](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/help/32/3010_2.png) [@help](https://forum.opencv.org/u/help)\
**Post date:** [August 19, 2021, 11:33pm UTC](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835/7 "2021-08-19T23:33:14Z")

</div>

Right, I resolved that error when doing the Lucas Kanade algorithm. In this line, I pass in p\_guess by converting it to float32:  
`p1, st, err = cv2.optflow.calcOpticalFlowSparseRLOF(old_frame, new_frame, np.float32(p0), nextPts = np.float32(p_guess), rlofParam = retval)`