MATLAB: How to define the red, green, and blue Threshold value

color extractionimage segmentation

% Demo macro to very, very simple color detection in RGB color space
% by ImageAnalyst
clc;
close all;
% Read standard MATLAB demo image.
rgbImage = imread('onion.png');
% Display the original image.
subplot(3, 4, 1);
imshow(rgbImage);
title('Original RGB Image');
% Maximize figure.
set(gcf, 'Position', get(0, 'ScreenSize'));
% Split the original image into color bands.
redBand = rgbImage(:,:, 1);
greenBand = rgbImage(:,:, 2);
blueBand = rgbImage(:,:, 3);
% Display them.

subplot(3, 4, 2);
imshow(redBand);
title('Red band');
subplot(3, 4, 3);
imshow(greenBand);
title('Green band');
subplot(3, 4, 4);
imshow(blueBand);
title('Blue Band');
% Threshold each color band.
redthreshold = 68;
greenThreshold = 70;
blueThreshold = 72;
redMask = (redBand > redthreshold);
greenMask = (greenBand < greenThreshold);
blueMask = (blueBand < blueThreshold);
% Display them.
subplot(3, 4, 6);
imshow(redMask, []);
title('Red Mask');
subplot(3, 4, 7);
imshow(greenMask, []);
title('Green Mask');
subplot(3, 4, 8);
imshow(blueMask, []);
title('Blue Mask');
% Combine the masks to find where all 3 are "true."
redObjectsMask = uint8(redMask & greenMask & blueMask);
subplot(3, 4, 9);
imshow(redObjectsMask, []);
title('Red Objects Mask');
maskedrgbImage = uint8(zeros(size(redObjectsMask))); % Initialize
maskedrgbImage(:,:,1) = rgbImage(:,:,1) .* redObjectsMask;
maskedrgbImage(:,:,2) = rgbImage(:,:,2) .* redObjectsMask;
maskedrgbImage(:,:,3) = rgbImage(:,:,3) .* redObjectsMask;
subplot(3, 4, 10);
imshow(maskedrgbImage);
title('Masked Original Image');

hi. i found above coding from imageAnalyst. my question : 1.) how can i define the red, green, and blue Threshold value?any formula to find the value?for example in my image i want to extract the gold color, and how can i know the exact pixel for gold in RGB colorspace? 2.) can someone explain to me how the coding above works?

tq so much

Best Answer

Use impixelinfo() to have it display the RGB values as you mouse around the image. See updated demo, attached. for the red, green, and blue threshold for the yellow pepper, try these (that I got from the Color Thresholder App):

% Define thresholds for channel 1 (Red) based on histogram settings
channel1Min = 244.000;
channel1Max = 255.000;
% Define thresholds for channel 2 (Green) based on histogram settings
channel2Min = 131.000;
channel2Max = 247.000;
% Define thresholds for channel 3 (Blue) based on histogram settings
channel3Min = 0.000;
channel3Max = 164.000;

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MATLAB: How to make backgound of image black

This script will do it. Adjust as needed.

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures if you have the Image Processing Toolbox.
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 13;
% Check that user has the Image Processing Toolbox installed.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
  % User does not have the toolbox installed.
  message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
  reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
  if strcmpi(reply, 'No')
    % User said No, so exit.
    return;
  end
end
% Read in a standard MATLAB color demo image.
folder = 'D:\Temporary stuff';
baseFileName = 'moogseed5.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, 3, 1);
imshow(rgbImage);
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.

set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Get the dimensions of the image.  
% numberOfColorBands should be = 3 for an RGB image.
[rows, columns, numberOfColorBands] = size(rgbImage);
if numberOfColorBands > 1
  % Convert it to gray scale by taking only the green channel.
  % The green channel will have the highest contrast for these green seeds.
  grayImage = rgbImage(:, :, 2); % Take green channel.
end
% Display the original green channel gray scale image.
subplot(2, 3, 2);
imshow(grayImage, []);
title('Green Channel Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 
% Let's compute and display the histogram.
[pixelCount, grayLevels] = imhist(grayImage);
subplot(2, 3, 3); 
bar(grayLevels, pixelCount);
grid on;
title('Histogram of Green Channel', 'FontSize', fontSize);
xlim([0 grayLevels(end)]); % Scale x axis manually.
% Create a binary image via thresholding.
binaryImage = grayImage > 55;
% Display the image.


subplot(2, 3, 4);
imshow(binaryImage, []);
title('Binary Image', 'FontSize', fontSize);
% Clean up.
binaryImage = imclearborder(binaryImage);
binaryImage = imfill(binaryImage, 'holes');
binaryImage = bwareaopen(binaryImage, 500);
% Display the image.
subplot(2, 3, 5);
imshow(binaryImage, []);
title('Cleaned Binary Image', 'FontSize', fontSize);
% Mask the RGB image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(binaryImage, class(rgbImage)));
% Display the image.
subplot(2, 3, 6);
imshow(maskedRgbImage, []);
title('Masked RGB Image', 'FontSize', fontSize);

MATLAB: Imwrite within a function

For some reason you initialized maskedrgbImage to a single color channel. Not sure if that would cause a problem, or even what problem you observed (but didn't share) but I think this untested code should work:

maskedrgbImage = zeros(size(rgbImage), 'uint8');
maskedrgbImage(:,:,1) = rgbImage(:,:,1) .* redObjectsMask;
maskedrgbImage(:,:,2) = rgbImage(:,:,2);
maskedrgbImage(:,:,3) = rgbImage(:,:,3) .* redObjectsMask;
baseFileName = 'IA2_MaskFiltered.tiff';
yourFolder = 'D:\'; % or whatever...
fullFileName = fullfile(yourFolder, baseFileName);
imwrite(maskedrgbImage, fullFileName);

Make sure that maskedrgbImage is still a uint8 or uint16 image when you call imwrite().

Best Answer

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MATLAB: How to make backgound of image black

MATLAB: Imwrite within a function

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