Using an image, how do I find the coordinates of pixels of a certain colors?

Hello,

From the image pasted below, I want to make 2 different calculations.

1. Caclulate the center of mass of the white pebble. To do this, I want to find all the coordinates of the white pebble and the black dot. Once I get the total number of rows and columns, I want to average this to find center of mass. I am trying to write a script that tells matlab to store all the values that are not red pixels.
2. Caclulate the center of mass of only the black dot. I find all the coordinates of the black dot, and then average the total number of rows and columns to find the center of mass.

I have also pasted the code I have so far. The loop I made is not working and when i tried other variations, I had difficulty writing the code so that it only read the pixels of the colors I want.

image = imread('red_backpebble1.png');
redChannel = image(:,:,1);
greenChannel = image(:,:,2);
blueChannel = image(:,:,3);

% black pixels are where red channel and green channel and blue channel in an rgb image are 0;
blackpixelsmask = redChannel == 0 & greenChannel == 0 & blueChannel == 0;

[rows, columns, number] = size(image);
[x, y] = meshgrid(1: columns, 1:rows);

while k == blackpixelsmask 
    fprintf('Pixel #%d is at (x, y) = (%d, %d)\n',...
    k, x(k), y(k));
end

com_x = rows/2;
com_y = columns/2;

disp("The Center of Mass of the black dot is ("+com_x+","+com_y+")")

Answer 1

[Mahesh Taparia]

Hi

You can use regionprops function to find the pixel list and centroid of the white and black part. For more information, you can refer this documentation. For illustration, consider the below code:

I=imread('image.png');
whitePart=(I(:,:,1)==255)&(I(:,:,2)==255)&(I(:,:,3)==255);
blackPart=(I(:,:,1)==0)&(I(:,:,2)==0)&(I(:,:,3)==0);
figure;imshow(whitePart)
figure;imshow(blackPart)
blachRegion=regionprops(blackPart,'all');
whiteRegion=regionprops(whitePart,'all');

Hope it will help!