Change specific colour in an image

Hi everybody

I have to change the colors of an object in a picture but when i change red to black, it also change the orange colour. Can you help me solve this please ?

a = imread('coloredChips.png');

figure(1), imshow(a);

% Creat menu of color

disp('Color selection')

disp('r for red')

disp('b for blue')

disp('y for yellow')

% Now we want input from user to tell us which color to change

x = input(' Please select the color you want to change: ','s');

copy = a;

dimension = size(a);

row_n = dimension (1);

col_n = dimension (2);

switch x

case 'r'

for row = 1:row_n

for col = 1:col_n

pixel_red = a(row,col,1);

pixel_green = a(row,col,2);

pixel_blue = a(row,col,3);

if (pixel_red >= 200) && (pixel_green <= 50) && (pixel_green >= 5) && (pixel_blue <= 80) && (pixel_blue >=15)

copy(row,col,1) = 0;

copy(row,col,2) = 0;

copy(row,col,3) = 0;

end

end

end

otherwise

disp('Incorrect selection, please choose from the menu');

end

% display the modified image

figure(2),imshow(copy);

[Dyuman Joshi]

I don't get the logic of comparison here - The red color is the one where the blue and green values are zero.

The comparison you have made is quite different.

There is some information missing in the question - Please clarify.

[Phat]

In this image the blue and green values are not zero. When I compare the RGB value of red, somehow the value of orange is the same and I don't know how to solve this

[Saurabh]

Hi Phat,

Try changing the the format of the image from 'png' to 'jpg' that might solve the problem. I tried this on my end and got this result:

[Dyuman Joshi]

The image is actually an indexed image with a colormap. Use the map to convert it to an rgb image via ind2rgb -

%Read the file accordingly
[a, map] = imread('coloredChips.png');
size(a)

ans = 1×2

494 698

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%showing image w/o map
%imshow(a)
%showing image w map
%imshow(a, map)


%Change the indexed image to a rgb utilising the colormap
img = ind2rgb(a, map);
%show the image
imshow(img)

[DGM]

coloredChips.png is a standard image that comes with MATLAB.

which coloredChips.png

/MATLAB/toolbox/images/imdata/coloredChips.png

The copy that OP attached is a screenshot of the original, which has been resized, padded, and converted to a degraded indexed-color copy.

The proposal to use JPG will only make the damage worse. To expand upon the comment I gave below:

% read the original image
goodpict = im2double(imread('coloredChips.png'));

% read the screenshot and try to fix it
[indpict map] = imread('screenshot.png');
badpict = ind2rgb(indpict,map); % convert to RGB
imwrite(badpict,'worsescreenshot.jpg'); % convert to a trash-quality JPG
badpict = im2double(imread('worsescreenshot.jpg'));
badpict = imcrop(badpict,[90.51 29.51 517.98 390.98]); % crop off the padding
badpict = imresize(badpict,size(goodpict,1:2)); % return to original size

% the error is strongest near edges
err = normalize(abs(goodpict-badpict),'range');
imshow(err,'border','tight')

I'm not showing it here, but MSE only tripled by using JPG. That might almost seem like a small change, but look at the apparent resolution of the error map. The image saved by MATLAB is a 75% 4:2:0 JPG. You're throwing away 75% of your chroma information, when your task is critically dependent on using that chroma information.

Using JPG as part of an image processing routine is never the answer.

[Dyuman Joshi]

"coloredChips.png is a standard image that comes with MATLAB."

I was not aware about this. Thanks for the info!

Answer 1

[DGM]

See also:

https://www.mathworks.com/matlabcentral/answers/52814-switch-between-colors-on-image

https://www.mathworks.com/matlabcentral/answers/1706905-how-can-i-change-the-red-chips-to-black-as-the-red

While this same assignment has been answered before, this is probably the first time that someone tried doing it with an indexed-color screenshot of the test image. The version of the file that comes with MATLAB is already RGB.

inpict = imread('coloredChips.png');
size(inpict) % you should already have a clean RGB copy

ans = 1×3

391 518 3

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This is worth pointing out, since I see no point in using a degraded screenshot when you already have the original image.

% read the original image
goodpict = im2double(imread('coloredChips.png'));

% read the screenshot and try to fix it
[indpict map] = imread('screenshot.png');
badpict = ind2rgb(indpict,map); % convert to RGB
badpict = imcrop(badpict,[90.51 29.51 517.98 390.98]); % crop off the padding
badpict = imresize(badpict,size(goodpict,1:2)); % return to original size

% the error is strongest near edges
err = normalize(abs(goodpict-badpict),'range');
imshow(err,'border','tight')

Answer 2

[Pramil]

Hi Phat,

What you are doing is correct, but in MATLAB pixel values are normalized that means each pixel ranges from “0” to “1” and not from “0” to “255” as you can see in the following link :

https://www.mathworks.com/help/images/pixel-values.html

When extracting individual pixel, multiply each value with “255” and your code works just fine.

Here is an updated code that works fine in MATLAB R2023b:

[img, map] = imread('coloredChips.png'); 
a = ind2rgb(img, map); 
if size(a, 3) ~= 3 
    error('The image must be an RGB image.'); 
end 
figure(1), imshow(a); 

% Creat menu of color 
disp('Color selection')

Color selection

disp('r for red') 

r for red

disp('b for blue') 

b for blue

disp('y for yellow') 

y for yellow

% Now we want input from user to tell us which color to change 
%x = input(' Please select the color you want to change: ','s'); 
x = 'r';
copy = a; 
dimension = size(a); 
row_n = dimension (1); 
col_n = dimension (2); 
switch x 
    
    case 'r' 
        for row = 1:row_n 
            for col = 1:col_n 
                pixel_red = a(row,col,1)*255; 
                pixel_green = a(row,col,2)*255; 
                pixel_blue = a(row,col,3)*255;
                
                if (pixel_red >= 200) && (pixel_green <= 50) && (pixel_green >= 5)  && (pixel_blue <= 80) && (pixel_blue >=15) 
                    copy(row,col,1) = 0; 
                    copy(row,col,2) = 0; 
                    copy(row,col,3) = 0; 
                end 
            end 
        end 
    otherwise 
        disp('Incorrect selection, please choose from the menu'); 
end 
% display the modified image 
figure(2),imshow(copy); 

Hope this helps.