A basic yet very useful application of OpenCV is to track color.

One major challenge while doing color detection is to include all the shades of color.

To solve this problem of shades instead of using RGB color space we use HSV color space.

Don’t know “How to create trackbar in OpenCV Python’ Read ->”Trackbars in OpenCV python“

HSV trackbar code

#import opencv and numpy
import cv2  
import numpy as np

#trackbar callback fucntion to update HSV value
def callback(x):
	global H_low,H_high,S_low,S_high,V_low,V_high
	#assign trackbar position value to H,S,V High and low variable
	H_low = cv2.getTrackbarPos('low H','controls')
	H_high = cv2.getTrackbarPos('high H','controls')
	S_low = cv2.getTrackbarPos('low S','controls')
	S_high = cv2.getTrackbarPos('high S','controls')
	V_low = cv2.getTrackbarPos('low V','controls')
	V_high = cv2.getTrackbarPos('high V','controls')


#create a seperate window named 'controls' for trackbar
cv2.namedWindow('controls',2)
cv2.resizeWindow("controls", 550,10);


#global variable
H_low = 0
H_high = 179
S_low= 0
S_high = 255
V_low= 0
V_high = 255

#create trackbars for high,low H,S,V 
cv2.createTrackbar('low H','controls',0,179,callback)
cv2.createTrackbar('high H','controls',179,179,callback)

cv2.createTrackbar('low S','controls',0,255,callback)
cv2.createTrackbar('high S','controls',255,255,callback)

cv2.createTrackbar('low V','controls',0,255,callback)
cv2.createTrackbar('high V','controls',255,255,callback)


while(1):
	#read source image
	img=cv2.imread("ballimage.jpg")
	#convert sourece image to HSC color mode
	hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

	#
	hsv_low = np.array([H_low, S_low, V_low], np.uint8)
	hsv_high = np.array([H_high, S_high, V_high], np.uint8)

	#making mask for hsv range
	mask = cv2.inRange(hsv, hsv_low, hsv_high)
	print (mask)
	#masking HSV value selected color becomes black
	res = cv2.bitwise_and(img, img, mask=mask)



	#show image
	cv2.imshow('mask',mask)
	cv2.imshow('res',res)
	
	#waitfor the user to press escape and break the while loop 
	k = cv2.waitKey(1) & 0xFF
	if k == 27:
		break
		
#destroys all window
cv2.destroyAllWindows()

Read this great article on Colorspaces and know why convert RGB to HSV for color detection.

Read More about Anatomy of a digital Image

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Om Singh

blog.electroica.com