title: OpenCV - Trackbar slider keeps going to zero with video
body: <p>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.  </p>
  
  <pre><code>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) &amp; 0xFF == ord('q'):
          break
  
  
  # When everything is done, release the capture
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>