How to change specific RGB value to another RGB value in a PNG uint8 image?

%I have the following code:

RGB = imread('image.png');

[r c z] = size(RGB);

for i=1:r

for j=1:c

if (RGB(i,j,1)==50 && RGB(i,j,2)==205 && RGB(i,j,3)==50) 

RGB(i,j,1)=255; RGB(i,j,2)=255; RGB(i,j,3)=255;

elseif (RGB(i,j,1)==255 && RGB(i,j,2)==228 && RGB(i,j,3)==181)

RGB(i,j,1)=255; RGB(i,j,2)=255; RGB(i,j,3)=255;

elseif (RGB(i,j,1)==0 && RGB(i,j,2)==0 && RGB(i,j,3)==0) %MATLAB won't run this part

RGB(i,j,1)==255 && RGB(i,j,2)==255 && RGB(i,j,3)==255;

elseif (RGB(i,j,1)==250 && RGB(i,j,2)==250 && RGB(i,j,3)==250) %MATLAB also won't run this part

RGB(i,j,1)==255 && RGB(i,j,2)==255 && RGB(i,j,3)==255;

  else
      RGB(i,j,1)=0; RGB(i,j,2)=0; RGB(i,j,3)=0;
        end 
    end
end

%So that's my code

the original image is here: https://dl.dropboxusercontent.com/u/9183365/image.png

My problem now is why MATLAB does not change the RGB values as indicated in the code comments? Also, is there a way to use operators to simplify the code instead of using so much elseifs? I shall be using this for one of my clustering algorithms. Thank you so much in advance for any help.

[jmmmrd]

also, the imshow(RGB) differs if I use the .jpg version of the file. i'm not sure why:

jpg version of the original image: https://dl.dropboxusercontent.com/u/9183365/image.jpg

thanks.

Answer 1

[Walter Roberson]

You have

elseif (RGB(i,j,1)==0 && RGB(i,j,2)==0 && RGB(i,j,3)==0) %MATLAB won't run this part

RGB(i,j,1)==255 && RGB(i,j,2)==255 && RGB(i,j,3)==255;

You want to do assignments not tests, same kind of structure as you had used already where you had

RGB(i,j,1)=255; RGB(i,j,2)=255; RGB(i,j,3)=255;

[jmmmrd]

Thanks Walter.

I have revised my code to something like this:

RGB = imread('image.png');

[r c z] = size(RGB);

for i=1:r

      for j=1:c
          if ((RGB(i,j,1)==50 && RGB(i,j,2)==205 && RGB(i,j,3)==50)|(RGB(i,j,1)==255 && RGB(i,j,2)==228 && RGB(i,j,3)==181)|(RGB(i,j,1)==0 && RGB(i,j,2)==0 && RGB(i,j,3)==0)|(RGB(i,j,1)==250 && RGB(i,j,2)==250 && RGB(i,j,3)==250)) 

          RGB(i,j,1)=255; RGB(i,j,2)=255; RGB(i,j,3)=255;

      else

          RGB(i,j,1)=0; RGB(i,j,2)=0; RGB(i,j,3)=0;
  end
  end
  end

Next thing I'll try to figure out is how to extract the centroids corresponding to each "black square or rectangle" and store them in a horizontal matrix variable.

[jmmmrd]

i still don't get though why the .jpg version of the file doesn't work as nice as the .png. thanks

[Walter Roberson]

How did you create the .png and .jpg files ?