Python OpenCV - getTrackbarPos() Function

getTrackbarPos() is Function in Python OpenCV that returns the current position of the specified trackbar. It takes two arguments. The first is for the trackbar name and the second one is the window name which is the parent of the trackbar. Returns the trackbar position.

Syntax : cv.getTrackbarPos(trackbarname, winname)

Parameters :

• trackbarname: Name of trackbar
• winname: Name of the window that is the parent of the trackbar.

Return : Current position of the specified trackbar

Note: [Only for Qt Backend] Here, If the trackbar is attached to the control panel then winname can be empty or Null.

Below is the implementation:

# Demo Trackbar
# importing cv2 and numpy
import cv2
import numpy

def nothing(x):
    pass

# Creating a window with black image
img = numpy.zeros((300, 512, 3), numpy.uint8)
cv2.namedWindow('image')

# creating trackbars for red color change
cv2.createTrackbar('R', 'image', 0, 255, nothing)

# creating trackbars for Green color change
cv2.createTrackbar('G', 'image', 0, 255, nothing)

# creating trackbars for Blue color change
cv2.createTrackbar('B', 'image', 0, 255, nothing)

while(True):
    # show image
    cv2.imshow('image', img)

    # for button pressing and changing
    k = cv2.waitKey(1) & 0xFF
    if k == 27:
        break

    # get current positions of all Three trackbars
    r = cv2.getTrackbarPos('R', 'image')
    g = cv2.getTrackbarPos('G', 'image')
    b = cv2.getTrackbarPos('B', 'image')

    # display color mixture
    img[:] = [b, g, r]

# close the window
cv2.destroyAllWindows()

Output:

When we move the slider of any of R, G, or B its corresponding getTrackbarPos() values changes and it returns the position of the specific slider. Through which we change the color of the below box. You can see in the code we pass to the argument to getTrackbarPos(). Let's take r for example, we pass 'R' and 'image' to function. Here 'R' is the Trackbar name that we created and 'image' is the name of the window that opens when we run code. Basically, it is the parent window of the trackbar. This returns the position of the slider as an integer value and we save it in r. We create color box with this return value 'r', 'g' and 'b'.