OpenCV - Trackbar slider keeps going to zero with video

Question

I'm trying to use the slider to control my lower and upper bounds for HSV masking. I'm able to get the slider but can't get it to hold the position I set; it keeps going back to zero each time a new frame is pulled in.

import numpy as np
import cv2

def nothing(x):
    pass

cap = cv2.VideoCapture(0)

while(True):

    # Make a window for the video feed  
    cv2.namedWindow('frame',cv2.CV_WINDOW_AUTOSIZE)

    # Capture frame-by-frame
    ret, frame = cap.read()

    # Make the trackbar used for HSV masking    
    cv2.createTrackbar('HSV','frame',0,255,nothing)

    # Name the variable used for mask bounds
    j = cv2.getTrackbarPos('HSV','image')

    # Convert BGR to HSV
    hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)

    # define range of color in HSV
    lower = np.array([j-10,100,100])
    upper = np.array([j+10,255,255])

    # Threshold the HSV image to get only selected color
    mask = cv2.inRange(hsv, lower, upper)

    # Bitwise-AND mask the original image
    res = cv2.bitwise_and(frame,frame, mask= mask)

    # Display the resulting frame
    cv2.imshow('frame',res)

    # Press q to quit
    if cv2.waitKey(3) & 0xFF == ord('q'):
        break


# When everything is done, release the capture
cap.release()
cv2.destroyAllWindows()

Answer 1

You are creating track-bar inside while loop, that's why you are getting new track-bar on each frame.

So change your code like,

# Make a window for the video feed  
cv2.namedWindow('frame',cv2.CV_WINDOW_AUTOSIZE)
# Make the trackbar used for HSV masking    
cv2.createTrackbar('HSV','frame',0,255,nothing)

while(True):

    # Capture frame-by-frame
    ret, frame = cap.read()
    ........................
    ........................