Histogram of ROI of an image

Hello , can any one suggest best for , how to find histogram of ROI of an image

Answer 1

[Image Analyst]

I use imhist(). Is your ROI rectangular, or free form? Let me know if you want a demo.

[Inara]

Hello Image Analyst,

I am not the OP but I also have the same problem. My ROIs are free form, do you know how I can calculate their histogram or even how to use them as the parameters of a function? For example, I would like to extract some features from the ROIs (not the whole images).

Thank you very much! Kind Regards, Inara

[Image Analyst]

Inara: See this demo:

% Demo to have the user freehand draw an irregular shape over
% a gray scale image, have it extract only that part to a new image,
% and to calculate the histogram of the image within that shape.

% Change the current folder to the folder of this m-file.
if(~isdeployed)
  cd(fileparts(which(mfilename)));
end
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 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);
imshow(grayImage, []);
axis on;
title('Original Grayscale Image', 'FontSize', fontSize);
set(gcf, 'Position', get(0,'Screensize')); % Maximize figure.
message = sprintf('Left click and hold to begin drawing.\nSimply lift the mouse button to finish');
uiwait(msgbox(message));
hFH = imfreehand();
% Create a binary image ("mask") from the ROI object.
binaryImage = hFH.createMask();
xy = hFH.getPosition;

% Now make it smaller so we can show more images.
subplot(2, 2, 1);
imshow(grayImage, []);
axis on;
drawnow;
title('Original Grayscale Image', 'FontSize', fontSize);

% Display the freehand mask.
subplot(2, 2, 2);
imshow(binaryImage);
axis on;
title('Binary mask of the region', 'FontSize', fontSize);

% Get coordinates of the boundary of the freehand drawn region.
structBoundaries = bwboundaries(binaryImage);
xy=structBoundaries{1}; % Get n by 2 array of x,y coordinates.
x = xy(:, 2); % Columns.
y = xy(:, 1); % Rows.
subplot(2, 2, 3); % Plot over original image.
imshow(grayImage, []);
axis on;
hold on; % Don't blow away the image.
plot(x, y, 'r-', 'LineWidth', 2);
drawnow; % Force it to draw immediately.
title('Grayscale Image with your mask overlaid', 'FontSize', fontSize);

% Get the histogram.
pixelsWithinMask = grayImage(binaryImage);
[pixelCounts grayLevels] = imhist(pixelsWithinMask);
subplot(2, 2, 4);
bar(grayLevels, pixelCounts);
title('Histogram of Grayscale Image within mask', 'FontSize', fontSize);
xlim([0 255]);
grid on;

[Inara]

Hi Image Analyst, It works great. Thank you very much! Kind Regards, Inara

[Andrew Choo]

Hi Image Analyst. Can I have a linear ROI instead of a freehand one for this code? And also, is there a way to compare the intensity values of 2 similar roi with a histogram at the same time? Thank you! Your code has helped me greatly.

Answer 2

[Matt J]

If ROImask is a logical mask for the ROI, you can do

histc(yourImage(ROImask),...)