How to find the RED color regions using OpenCV? [duplicate]

RGBis not a good color space for specific color detection. HSV will be a good choice.

For RED, you can choose the HSV range (0,50,20) ~ (5,255,255) and (175,50,20)~(180,255,255)using the following colormap. Of course, the RED range is not that precise, but it is just ok.

The code taken from my another answer: Detect whether a pixel is red or not

#!/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()

Related answers:

1. Choosing the correct upper and lower HSV boundaries for color detection with`cv::inRange` (OpenCV)
2. How to define a threshold value to detect only green colour objects in an image :Opencv
3. How to detect two different colors using `cv2.inRange` in Python-OpenCV?
4. Detect whether a pixel is red or not

Of course, for the specific question, maybe other color space is also OK.

How to read utility meter needle with opencv?