MATLAB: Change specific color in an image to another one

imageimage analysisimage processingMATLAB

Hello,

I am trying to change colors in an image into another color. I have generated some code myself but something seems to be wrong. What I am trying to achieve on the attached file is to change every purple data point into a green data point, or whatever color I would prefer. Currently, the code that I have written is able to change purple to green but the edges of the marked points are still in purple, I believe this has something to do with the intensity at the edges being different or something…

[I,m] = imread('iteration31.png');
%image = imshow(I,'Colormap',m);
rgbImage = ind2rgb(I,m);
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
purplePixels = redChannel == 1 & greenChannel == 0 & blueChannel == 1;
% MAKE THEM GREEN 
redChannel(purplePixels) = 0;
greenChannel(purplePixels) = 1;
blueChannel(purplePixels) = 0;
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
imshow(rgbImage);

Best Answer

This is a tough problem. Since you are using a raster image, objects that look purple are actually many small variations of purple. One thing you could try to do is compute which colors are purple-ish, and then modify those colors in the colormap.

Here is an example that may yield slightly better results than what you were experiencing, but still not perfect:

% Read image
[I,m] = imread('iteration31.png');
% Find purple-ish colors
mask1 = abs(m(:,3)-m(:,1))<0.8;
mask2 = abs(m(:,3)-m(:,2))>0.3;
mask3 = abs(m(:,2)-m(:,1))>0.3;
mask = mask1 & mask2 & mask3;
% Convert purple-ish colors to green-ish colors
m(mask,:) = 1-m(mask,:);
% Create and show newly colored image
rgb = ind2rgb(I,m);
imshow(rgb);

Related Solutions

MATLAB: Read three different RGB band and Swap the Band

Try this:

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Now swap around in any way you want.
newRedChannel = greenChannel;
newGreenChannel = blueChannel; 
newBlueChannel = redChannel;
% Recombine separate color channels into a single, true color RGB image.
NewRgbImage = cat(3, newRedChannel, newGreenChannel, newBlueChannel);

MATLAB: Coloring a river on a grayscale image

Are you interested in a vectorized solution instead of the double for loop? Just threshold to get the mask, then set each color channel using that mask to the proper values (255 or 0 depending on what color channel), then combine into an rgb image and display it. See below.

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 short g;
format compact;
fontSize = 25;  
%===============================================================================

% Get the name of the image the user wants to use.
baseFileName = 'MUGgbnH.png';
% Get the full filename, with path prepended.
folder = pwd
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
%===============================================================================
% Read in demo image.
grayImage = imread(fullFileName);
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
  % It's not really gray scale like we expected - it's color.
  % Use weighted sum of ALL channels to create a gray scale image.
%   grayImage = rgb2gray(grayImage);
  % ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
  % which in a typical snapshot will be the least noisy channel.
  grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the original image.
subplot(2, 2, 1);
imshow(grayImage, []);
axis on;
caption = sprintf('Original Color 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.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0.05 1 0.95]);
% 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.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
% Find the mask.
mask = grayImage <= 48;
% Display the image.

subplot(2, 2, 2);
imshow(mask, []);
axis on;
caption = sprintf('Binary Mask Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
hp = impixelinfo();
drawnow;
% Create an RGB image making it yellow where the mask is.  Yellow is (255,255,0)
redChannel = grayImage; % Initialize


redChannel(mask) = 255;
greenChannel = grayImage; % Initialize
greenChannel(mask) = 255;
blueChannel = grayImage; % Initialize
blueChannel(mask) = 0;
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
% Display the image.
subplot(2, 2, 3);
imshow(rgbImage, []);
axis on;
caption = sprintf('Masked Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
hp = impixelinfo();
drawnow;

Best Answer

Related Solutions

MATLAB: Read three different RGB band and Swap the Band

MATLAB: Coloring a river on a grayscale image

Related Question