How to retrieve a total pixel value above an average-based threshold in Python

Question

Currently, I am practicing with retrieving the total of the pixel values above a threshold based on the mean of the whole image. (I am very new to Python). I am using Python 3.5.2, and the above code was copied from the Atom program I am using to write and experiment with the code.

For the time being, I am just practicing with the red channel - but eventually, I will need to individually analyse all colour channels.

The complete code that I am using so far:

import os  
from skimage import io  
from tkinter import *  
from tkinter.filedialog import askopenfilename  
def callback():  
    M = askopenfilename()       #to select a file  
    image = io.imread(M)        #to read the selected file  
    red = image[:,:,0]          #selecting the red channel  
    red_av = red.mean()         #average pixel value of the red channel  
    threshold = red_av + 100    #setting the threshold value  
    red_val = red > threshold  
    red_sum = sum(red_val)  
    print(red_sum)  
Button(text = 'Select Image', command = callback).pack(fill = X)  
mainloop()

Now, everything works so far, except when I run the program, red_sum comes out to be the number of pixels above the threshold, not the total of the pixels.

What I am I missing? I am thinking that my (possible naive) way of declaring the red_val variable has something to do with it.

But, how do I retrieve the total pixel value above the threshold?

Answer 1

When you did (red > threshold) you got a mask such that all the pixels in red that are above the threshold got the value 1 and 0 otherwise. Now to get the values, you can just multiply the mask with the red channel. The multiplication will zero all the values that are less than the threshold and will leave the values over the threshold unchanged.

The code:

red_val = (red > threshold)*red
red_sum = sum(red_val)