How to convert specific pixel value to pseudocolor?

I have a uint8 image. If i want to convert all pixel with value smaller than 40 to red color. How can i do? Whould you please give me some hints.

[Walter Roberson]

Is the uint8 image RGB or pseudocolor?

Is the desired output RGB or pseudocolor?

When you say "with value smaller than 40", are you referring to intensity in a particular colorplane, or are you referring to brightness, or are you referring to a pseudocolor map index (which is arbitrary and has no inherent meaning) ?

[wang1031 yang]

Sorry, I did not represent my question clearly. I have an uint8 grayscal image. I want to replace all pixels with pixel intensity smaller than 40 by red color. How can i do?

Answer 1

[Image Analyst]

Is one of these what you are looking for? Walter's right about the ambiguity so I picked a couple of possibilities (grayscale changing the image and not using a colormap, and RGB thresholding each channel individually) and programmed up a demo for you. If neither case is what you want, then you must clarify. Don't worry about the length, just copy, paste, and run, then study the code later.

clc;  % Clear command window.
clear;  % Delete all variables.
close all;  % Close all figure windows except those created by imtool.
imtool close all;  % Close all figure windows created by imtool.
workspace;  % Make sure the workspace panel is showing.
fontSize = 16;

% Read in a standard MATLAB color demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'football.jpg';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % Didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
rgbImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows columns numberOfColorBands] = size(rgbImage);
% Display the original color image.
subplot(2, 2, 1);
imshow(rgbImage, []);
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

% Threshold each color channel to get pixels darker than 40.
redMask = redChannel < 40;
greenMask = greenChannel < 40;
blueMask = blueChannel < 40;

% Apply the mask
redChannel(redMask) = 255;
greenChannel(greenMask) = 0;
blueChannel(blueMask) = 0;

% Recombine into RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
% Display
subplot(2, 2, 2);
imshow(rgbImage, []);
title('Masked Color Image', 'FontSize', fontSize);

promptMessage = sprintf('Now we will do it for a gray scale image?');
titleBarCaption = 'Continue with gray scale example?';
button = questdlg(promptMessage, titleBarCaption, 'Continue', 'Cancel', 'Continue');
if strcmp(button, 'Cancel')
  return;
end

% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'cameraman.tif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, '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
grayImage = imread(fullFileName);
% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows columns numberOfColorBands] = size(grayImage);
% Display the original gray scale image.
subplot(2, 2, 3);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);

% Get red, green, and blue channels.
redChannel = grayImage;
greenChannel = grayImage;
blueChannel = grayImage;

% Threshold each color channel to get pixels darker than 40.
mask = grayImage < 40;

% Apply the mask
redChannel(mask) = 255;
greenChannel(mask) = 0;
blueChannel(mask) = 0;

% Recombine into RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
% Display
subplot(2, 2, 4);
imshow(rgbImage, []);
title('Masked Color Image', 'FontSize', fontSize);

[wang1031 yang]

thanks for your help, I'll try it. thank you

[wang1031 yang]

it's what i need, thanks alot.

Answer 2

[DGM]

IPT imoverlay is convenient here, and it will work fine for either I/RGB inputs. While imoverlay() wasn't available at the time, alternatives are not complicated either.

% read an image
inpict = imread('cameraman.tif');

% create a mask
mask = inpict<40;

% apply the mask
outpict = imoverlay(inpict,mask,[1 0 0]);

% that's it.
imshow(outpict,'border','tight')