crop greyscale image using matlab

hello. I have a greyscale image (512x512x3) which is attached below. i want to crop it. it has 512 x 512 pixels however the data i am interested in is the middle of the image (non-white area). I tried the imcrop command but it only allowed me to crop in rectangular areas. How can i crop such that all pixels with intensity of 255 are cropped from the image? I think doing so will crop the image such along the border of the non-white area of the image. I have verified using impixelinfo that the intensities of the region of interest are all less that 255.

Answer 1

[Image Analyst]

Try this:

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 long g;
format compact;
fontSize = 20;

%===============================================================================
% Read in a standard MATLAB gray scale demo image.
folder = pwd;
baseFileName = 'PAR1_1.jpg';
% Get the full filename, with path prepended.
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
rgbImage = 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(rgbImage);

% Display the image.
subplot(2, 2, 1);
imshow(rgbImage, []);
title('Original Color Image', 'FontSize', fontSize, 'Interpreter', 'None');

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0, 1, 1]);
% 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') 

grayImage = rgb2gray(rgbImage);
binaryImage = imfill(grayImage ~= 255, 'holes');
binaryImage = bwareafilt(binaryImage, 1); % Extract only the largest.
% Display the image.
subplot(2, 2, 2);
imshow(binaryImage, []);
title('Binary Image', 'FontSize', fontSize, 'Interpreter', 'None');

labeledImage = bwlabel(binaryImage);
props = regionprops(labeledImage, 'BoundingBox');
% Crop the image
croppedImage = imcrop(grayImage, props.BoundingBox);
% Display the image.
subplot(2, 2, 3:4);
imshow(croppedImage, []);
title('Cropped Image', 'FontSize', fontSize, 'Interpreter', 'None');

Answer 2

[Image Analyst]

Images have to be rectangular, so even if you crop it down, there still be white in some of the image. What do you want to do with it? You can leave it as white, or change it to black or any other gray level.

[Parth P]

thanks for the swift response. I guess I will keep it white because then it is completely different than what I need to analyze.