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how can select a region in image?

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hi i need to select a region in image and then save the value of those pixels by using impixel i can choose some pixel by clicking on picture but it is bothersome and i need to select a lot of pixels some function such as imrect dose not get me the value of pixel can you help me please?

Accepted Answer

Image Analyst
Image Analyst on 2 Feb 2013
I've posted this demo many times before, but here it is again for you. You don't really say what "save the value of those pixels" means to you. They are in a rectangular array - you can use imcrop and imwrite to save out the rectangular array or subarray, or you can get only those pixels selected in some arbitrary region into a 1D list of pixel values and then just leave it saved in the array, or save the 1D list out to a text file or a mat file. You need to be more specific in what "save" means to you.
% 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 mean intensity value of the image within that shape.
% Also calculates the perimeter, centroid, and center of mass (weighted centroid).
% Change the current folder to the folder of this m-file.
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);
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');
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, 3, 1);
imshow(grayImage, []);
axis on;
title('Original Grayscale Image', 'FontSize', fontSize);
% Display the freehand mask.
subplot(2, 3, 2);
axis on;
title('Binary mask of the region', 'FontSize', fontSize);
% Label the binary image and computer the centroid and center of mass.
labeledImage = bwlabel(binaryImage);
measurements = regionprops(binaryImage, grayImage, ...
'area', 'Centroid', 'WeightedCentroid', 'Perimeter');
area = measurements.Area
centroid = measurements.Centroid
centerOfMass = measurements.WeightedCentroid
perimeter = measurements.Perimeter
% Calculate the area, in pixels, that they drew.
numberOfPixels1 = sum(binaryImage(:))
% Another way to calculate it that takes fractional pixels into account.
numberOfPixels2 = bwarea(binaryImage)
% 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, 3, 1); % Plot over original image.
hold on; % Don't blow away the image.
plot(x, y, 'LineWidth', 2);
drawnow; % Force it to draw immediately.
% Burn line into image by setting it to 255 wherever the mask is true.
burnedImage = grayImage;
burnedImage(binaryImage) = 255;
% Display the image with the mask "burned in."
subplot(2, 3, 3);
axis on;
caption = sprintf('New image with\nmask burned into image');
title(caption, 'FontSize', fontSize);
% Mask the image and display it.
% Will keep only the part of the image that's inside the mask, zero outside mask.
blackMaskedImage = grayImage;
blackMaskedImage(~binaryImage) = 0;
subplot(2, 3, 4);
axis on;
title('Masked Outside Region', 'FontSize', fontSize);
% Calculate the mean
meanGL = mean(blackMaskedImage(binaryImage));
% Put up crosses at the centriod and center of mass
hold on;
plot(centroid(1), centroid(2), 'r+', 'MarkerSize', 30, 'LineWidth', 2);
plot(centerOfMass(1), centerOfMass(2), 'g+', 'MarkerSize', 20, 'LineWidth', 2);
% Now do the same but blacken inside the region.
insideMasked = grayImage;
insideMasked(binaryImage) = 0;
subplot(2, 3, 5);
axis on;
title('Masked Inside Region', 'FontSize', fontSize);
% Now crop the image.
leftColumn = min(x);
rightColumn = max(x);
topLine = min(y);
bottomLine = max(y);
width = rightColumn - leftColumn + 1;
height = bottomLine - topLine + 1;
croppedImage = imcrop(blackMaskedImage, [leftColumn, topLine, width, height]);
% Display cropped image.
subplot(2, 3, 6);
axis on;
title('Cropped Image', 'FontSize', fontSize);
% Put up crosses at the centriod and center of mass
hold on;
plot(centroid(1)-leftColumn, centroid(2)-topLine, 'r+', 'MarkerSize', 30, 'LineWidth', 2);
plot(centerOfMass(1)-leftColumn, centerOfMass(2)-topLine, 'g+', 'MarkerSize', 20, 'LineWidth', 2);
% Report results.
message = sprintf('Mean value within drawn area = %.3f\nNumber of pixels = %d\nArea in pixels = %.2f\nperimeter = %.2f\nCentroid at (x,y) = (%.1f, %.1f)\nCenter of Mass at (x,y) = (%.1f, %.1f)\nRed crosshairs at centroid.\nGreen crosshairs at center of mass.', ...
meanGL, numberOfPixels1, numberOfPixels2, perimeter, ...
centroid(1), centroid(2), centerOfMass(1), centerOfMass(2));
David Harrison
David Harrison on 8 Apr 2019
Was this question answered previously? I have tried ginput function, but it only returns the locations of each button click, not the pixel intensity for the area inside the ROI. I have tried impixelregion, which gives me the information I need, but there is no way to save that pixel intensity information for the specified ROI to a matrix to be used later.
Image Analyst
Image Analyst on 8 Apr 2019
If you want the pixels in some boundary/region of an image you can use a binary image that says whether to report that pixel or not.
pixelsInRegion = grayImage(mask);
If you don't have the mask but have a list of boundary coordinates, you can use poly2mask() to get the binary image mask:
mask = poly2mask(x, y, rows, columns);

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More Answers (2)

Daniell Algar
Daniell Algar on 2 Feb 2013
Edited: Daniell Algar on 2 Feb 2013
Sounds like you want the function imfreehand.
nadia naji
nadia naji on 2 Feb 2013
no these function only return position of rectangle but i need the value of pixels in the rectangle.if i also use impixelregion i can not save the value? i need a method that can select an special area in image and save the value of pixels in it
syafienaz anuar
syafienaz anuar on 3 Feb 2017
do you have solution for this problem? cause i need it too

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molly jane fitches
molly jane fitches on 16 May 2017
I am using Maltab R2015a and it crops up with an error caused by the line: burnedImage(binaryImage) = 255;
I believe this is due to the fact that graphic handles are now objects rather than handles. Any advice on how to get around this?
  1 Comment
Image Analyst
Image Analyst on 16 May 2017
No, that's not true. The line of code should work. Anyway, burnedImage is NOT a graphics handle - it's a normal 2-D numerical array. You did something not in my demo. I just ran the demo ( my code)in R2017a and it worked beautifully. I can't fix your code (which is different from mine) unless you attach it.

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