Detect whether a pixel is red or not

Question

We can define the range of red color in HSV as below. I want to detect that whether a certain pixel is red or not? How can I do that in Python? I spend whole day, but unable to find solution. Please resolve my problem. I'm very new to Python. Code that I'm using is:

img=cv2.imread("img.png")
img_hsv=cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

# lower mask (0-10)
lower_red = np.array([0,50,50])
upper_red = np.array([10,255,255])
mask0 = cv2.inRange(img_hsv, lower_red, upper_red)

# upper mask (170-180)
lower_red = np.array([170,50,50])
upper_red = np.array([180,255,255])
mask1 = cv2.inRange(img_hsv, lower_red, upper_red)
image_height,image_width,_=img.shape    
for i in range(image_height):
   for j in range(image_width):
       if img_hsv[i][j][1]>=lower_red and img_hsv[i][j][1]<=upper_red:
          print("Found red")

Answer 1

You are almost right. You can merge the masks of lower RED and higher RED together to a single mask.

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For this ColorChecker.png:

My Steps to find the RED:

1. Read the image and convert to hsv.

2. I choose the red ranges (lower 0~5, upper 175~180) using this colormap:

3. Then merge the masks, you can judge whether the pixel is red or not by the mask. Or "crop" the region(s) for visualization:

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#!/usr/bin/python3
# 2018.07.08 10:39:15 CST
# 2018.07.08 11:09:44 CST
import cv2
import numpy as np
## Read and merge
img = cv2.imread("ColorChecker.png")
img_hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

## Gen lower mask (0-5) and upper mask (175-180) of RED
mask1 = cv2.inRange(img_hsv, (0,50,20), (5,255,255))
mask2 = cv2.inRange(img_hsv, (175,50,20), (180,255,255))

## Merge the mask and crop the red regions
mask = cv2.bitwise_or(mask1, mask2 )
croped = cv2.bitwise_and(img, img, mask=mask)

## Display
cv2.imshow("mask", mask)
cv2.imshow("croped", croped)
cv2.waitKey()

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1. Choosing the correct upper and lower HSV boundaries for color detection with`cv::inRange` (OpenCV)

2. How to detect two different colors using `cv2.inRange` in Python-OpenCV?