How to find the RED color regions using OpenCV?

Question

I am trying to make a program where I detect red. However sometimes it is darker than usual so I can\'t just use one value. What is a good range for detecting different shad

Answer 1

You could check that the red component is the maximum and others are both clearly lower:

def red(r, g, b):
    threshold = max(r, g, b)
    return (
        threshold > 8          # stay away from black
        and r == threshold     # red is biggest component
        and g < threshold*0.5  # green is much smaller
        and b < threshold*0.5  # so is b
    )

This can be implemented very efficiently using numpy.

The "right way" would be doing a full conversion to HSV and check there, but it's going to be slower and somewhat trickier (hue is an angle so you cannot just take the absolute value of the difference, moreover colors like (255, 254, 254) are going to be qualified as "red" even if they're considered white for a human).

Note also that human visual system tends to compensate for average, so something could be seen as "blue" even if indeed the biggest component is red, but everything in the image is red, so that "doesn't count" for our brain.

In the image below if you ask a human what color is the part in the circle area most would say "blue" while indeed the biggest component is red: