Detecting colors on a RGB image

Hello, I have a assignment in my class where I need to load a picture of a rubik's cube, and detect the colors of the cube and write them down into a matrix, here is an example of the picture.

The matrix for this image should be:

orange orange blue

blue blue orange

orange green red

My professor told me I should use functions like. RGB2gray(), imfilter(), graythresh(), bwlabel(), regionprops() etc. but I dont know how to do this.

Any help with a code?

[Ameer Hamza]

Since this is an assignment, so it would not be beneficial if we directly write the code. What have you already tried, and what algorithm you propose to detect those colors?

[Maida Kasumovic]

Its not an important assignment for my class since image processing is not my major, its just a small part of one of my subjects so it would be nice if I could get some help. I have tried something with boundingboxes in the regionprops, and got two codes for the picture above.

The first one:

%% Bounding box

close all

clear all

clc

I = imread('sedam.jpg');

I_size = size(I);

threshold = 150;

bw = zeros(I_size(1),I_size(2));

for i = 1:I_size(1)

for j = 1:I_size(2)

pixel(1) = I(i,j,1);

pixel(2) = I(i,j,2);

pixel(3) = I(i,j,3);

if ( abs(norm(double(pixel) - double([0 0 255]))) < threshold)

bw(i,j) = 1;

end

if ( abs(norm(double(pixel) - double([0 255 0]))) < threshold)

bw(i,j) = 1;

end

if ( abs(norm(double(pixel) - double([255 0 0]))) < threshold)

bw(i,j) = 1;

end

end

end

figure,imshow(bw);

stats=regionprops(bw,'BoundingBox','Centroid');

figure

imshow(I)

hold on

for obj=1:(length(stats))

bbox=stats(obj).BoundingBox;

bcentroid=stats(obj).Centroid;

rectangle('Position',bbox,'EdgeColor','r','LineWidth',2)

end

The second one:

rgbImage=imread('sedam.jpg');

% Convert RGB to HSV color space.

hsvImage = rgb2hsv(rgbImage);

% Extract the Saturation channel and threshold it at, say, 0.25

mask = hsvImage(:,:,2) > 0.675;

% extract the 16 largest blobs

mask = bwareafilt(mask, 16);

% Get bounding boxes

stats = regionprops(mask, 'BoundingBox','Centroid')

figure

imshow(rgbImage)

hold on

for obj=1:(length(stats))

bbox=stats(obj).BoundingBox;

bcentroid=stats(obj).Centroid;

rectangle('Position',bbox,'EdgeColor','r','LineWidth',2)

end

Answer 1

[Image Analyst]

Try this:

%-----------------------------------------------------------------------------------------------------------------------------------
% Read in image.
folder = pwd;
baseFileName = 'sedam.jpg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
	% The file doesn't exist -- didn't find it there in that folder.
	% Check the entire search path (other folders) for the file by stripping off the folder.
	fullFileNameOnSearchPath = baseFileName; % No path this time.
	if ~exist(fullFileNameOnSearchPath, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
rgbImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Display the RGB image full size.
subplot(1, 2, 1);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Original Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.


% Set up figure properties:
% Enlarge figure to full screen.
hFig1 = gcf;
hFig1.Units = 'Normalized';
hFig1.WindowState = 'maximized';
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
hFig1.Name = 'Demo by Image Analyst';

% Threshold the pixels that are not gray scale.
hsvImage = rgb2hsv(rgbImage);
threshold = .2;
mask = hsvImage(:, :, 2) > threshold & hsvImage(:, :, 3) > 0.2;
% Get rid of background
mask = imclearborder(mask);
% Fill holes
mask = imfill(mask, 'holes');
% Take the largest 9 blobs only.
mask = bwareafilt(mask, 9);
% Display the binary image.
subplot(1, 2, 2);
imshow(mask, []);
axis('on', 'image');
caption = sprintf('Mask Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.

% Extract the individual red, green, and blue color channels using imsplit() (introduced in R2018b).
[redChannel, greenChannel, blueChannel] = imsplit(rgbImage);

% Measure the colors
propsR = regionprops('table', mask, redChannel, 'MeanIntensity')
propsG = regionprops('table', mask,greenChannel, 'MeanIntensity')
propsB = regionprops('table', mask, blueChannel, 'MeanIntensity')

Here are the colors:

propsR =

9×1 table

MeanIntensity

________________

224.190361623616

2.59752107003672

222.903755176795

10.3684728359575

225.160324612946

2.80965263253615

3.1237139461745

225.969213059489

156.868206509418

propsG =

9×1 table

MeanIntensity

________________

87.6111586715867

89.561960229492

87.2096606050236

152.169745591831

89.2610873563985

92.877315972538

94.3119784382858

94.6842312368222

24.6052271222715

propsB =

9×1 table

MeanIntensity

________________

53.8946715867159

146.488723588315

55.069970309673

31.8296795229311

56.3948326176616

148.606142584341

147.144448383856

61.6532827852725

20.9011979577586

They are not in left-to-right, top-to-bottom order so you'll have to sort them if you want to know which color is where.

[Maida Kasumovic]

Thank you so much for your help,

does this sort them in a random order every time or its always the same order just not left to right top to bottom?

[Image Analyst]

It's not random. It goes from top down, then left to right. So whatever blob it hits first as it's going down columns and then moving over to the next column, is blob #1, which may not be the upper left one - it's the one that sticks out most to the left, which in this case is the 3rd one down in the first column. To see the label number, add the code like I did to the bottom of this to stamp the blob number onto the blob.

% Demo by Image Analyst
% Initialization Steps.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 18;
%-----------------------------------------------------------------------------------------------------------------------------------
% Read in image.
folder = pwd;
baseFileName = 'sedam.jpg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
	% The file doesn't exist -- didn't find it there in that folder.
	% Check the entire search path (other folders) for the file by stripping off the folder.
	fullFileNameOnSearchPath = baseFileName; % No path this time.
	if ~exist(fullFileNameOnSearchPath, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
rgbImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Display the RGB image full size.
subplot(1, 2, 1);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Original Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.


% Set up figure properties:
% Enlarge figure to full screen.
hFig1 = gcf;
hFig1.Units = 'Normalized';
hFig1.WindowState = 'maximized';
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
hFig1.Name = 'Demo by Image Analyst';

% Threshold the pixels that are not gray scale.
hsvImage = rgb2hsv(rgbImage);
threshold = .2;
mask = hsvImage(:, :, 2) > threshold & hsvImage(:, :, 3) > 0.2;
% Get rid of background
mask = imclearborder(mask);
% Fill holes
mask = imfill(mask, 'holes');
% Take the largest 9 blobs only.
mask = bwareafilt(mask, 9);
% Display the binary image.
subplot(1, 2, 2);
imshow(mask, []);
axis('on', 'image');
caption = sprintf('Mask Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.

% Extract the individual red, green, and blue color channels using imsplit() (introduced in R2018b).
[redChannel, greenChannel, blueChannel] = imsplit(rgbImage);

% Measure the colors
propsR = regionprops('table', mask, redChannel, 'MeanIntensity', 'Centroid')
propsG = regionprops('table', mask,greenChannel, 'MeanIntensity', 'Centroid')
propsB = regionprops('table', mask, blueChannel, 'MeanIntensity', 'Centroid')

% Label the blobs with the id
hold on;
for k = 1 : size(propsR, 1)
	thisCentroid = propsR{k,"Centroid"};
	x = thisCentroid(1);
	y = thisCentroid(2);
	plot(x, y, 'r.', 'MarkerSize', 30);
	labelString = sprintf('  %d', k);
	text(x, y, labelString, 'FontSize', 15, 'Color', 'r');
end

If you want them in a particular order you can use kmeans(ycentroids, 3) to get the row and column that the centroids are grouped into.

[Sang]

What's wrong with me?

[Image Analyst]

Try this:

% Demo by Image Analyst
% Initialization Steps.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 18;
%-----------------------------------------------------------------------------------------------------------------------------------
% Read in image.
folder = pwd;
baseFileName = 'sedam.jpg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
    % The file doesn't exist -- didn't find it there in that folder.
    % Check the entire search path (other folders) for the file by stripping off the folder.
    fullFileNameOnSearchPath = baseFileName; % No path this time.
    if ~exist(fullFileNameOnSearchPath, 'file')
        % Still didn't find it.  Alert user.
        errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
        uiwait(warndlg(errorMessage));
        return;
    end
end
rgbImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Display the RGB image full size.
subplot(1, 2, 1);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Original Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Set up figure properties:
% Enlarge figure to full screen.
hFig1 = gcf;
hFig1.Units = 'Normalized';
hFig1.WindowState = 'maximized';
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
hFig1.Name = 'Demo by Image Analyst';
% Threshold the pixels that are not gray scale.
hsvImage = rgb2hsv(rgbImage);
threshold = .2;
mask = hsvImage(:, :, 2) > threshold & hsvImage(:, :, 3) > 0.2;
% Get rid of background
mask = imclearborder(mask);
% Fill holes
mask = imfill(mask, 'holes');
% Take the largest 9 blobs only.
mask = bwareafilt(mask, 9);
% Display the binary image.
subplot(1, 2, 2);
imshow(mask, []);
axis('on', 'image');
caption = sprintf('Mask Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Extract the individual red, green, and blue color channels using imsplit() (introduced in R2018b).
[redChannel, greenChannel, blueChannel] = imsplit(rgbImage);
% Measure the colors
propsR = regionprops('table', mask, redChannel, 'MeanIntensity', 'Centroid')
propsG = regionprops('table', mask,greenChannel, 'MeanIntensity', 'Centroid')
propsB = regionprops('table', mask, blueChannel, 'MeanIntensity', 'Centroid')
% Label the blobs with the id
hold on;
for k = 1 : size(propsR, 1)
    thisCentroid = propsR{k,"Centroid"};
    x = thisCentroid(1);
    y = thisCentroid(2);
    plot(x, y, 'r.', 'MarkerSize', 30);
    labelString = sprintf('  %d', k);
    text(x, y, labelString, 'FontSize', 15, 'Color', 'r');
end