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Masking out Image area using Binary Mask

Asked by Dimitris M on 16 May 2012

Hello

I have an image and I also have a binary mask of the same size(ROI in 1, Background 0). How can I create a new image only with the ROI displayed ?

Is there any difference if the image is grayscale plane or RGB ?

Thank you

0 Comments

Dimitris M

3 Answers

Answer by Kye Taylor on 16 May 2012
Accepted answer

Let I be the m-by-n image, and let M be the m-by-n mask.

The new image, Inew, will be m-by-n and have zeros everywhere there is a zero in M and will be the grayscale value wherever M has a one.

Inew = I.*M;

If the image is RGB (i.e. I is m-by-n-by-3), then use the command

Inew = I.*repmat(M,[1,1,3]);

0 Comments

Kye Taylor
Answer by Image Analyst on 16 May 2012

See my demo - it does three different kinds of masking:

% 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.
%
% 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 standard MATLAB gray scale demo image.
grayImage = imread('cameraman.tif');
subplot(2, 3, 1);
imshow(grayImage, []);
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();
% Display the freehand mask.
subplot(2, 3, 2);
imshow(binaryImage);
title('Binary mask of the region', 'FontSize', fontSize);
% 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);
imshow(burnedImage);
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);
imshow(blackMaskedImage);
title('Masked Outside Region', 'FontSize', fontSize);
% Calculate the mean
meanGL = mean(blackMaskedImage(binaryImage));
% Report results.
message = sprintf('Mean value within drawn area = %.3f\nNumber of pixels = %d\nArea in pixels = %.2f', ...
meanGL, numberOfPixels1, numberOfPixels2);
msgbox(message);
% Now do the same but blacken inside the region.
insideMasked = grayImage;
insideMasked(binaryImage) = 0;
subplot(2, 3, 5);
imshow(insideMasked);
title('Masked Inside Region', 'FontSize', fontSize);
% Now crop the image.
topLine = min(x);
bottomLine = max(x);
leftColumn = min(y);
rightColumn = max(y);
width = bottomLine - topLine + 1;
height = rightColumn - leftColumn + 1;
croppedImage = imcrop(blackMaskedImage, [topLine, leftColumn, width, height]);
% Display cropped image.
subplot(2, 3, 6);
imshow(croppedImage);
title('Cropped Image', 'FontSize', fontSize);

4 Comments

Image Analyst on 24 Nov 2012

You've modified my demo somehow and broken it. I know this because line 12 of my demo is not "binaryImage = hFH.createMask();". You probably changed this line for some reason: "hFH = imfreehand();" I can't tell unless you post your code (start your own thread for this).

s on 15 Feb 2013

can u tell me how can i modify above coding if i use colour image instead of gray image?????

Image Analyst on 15 Feb 2013

Please start a new question for this.

Image Analyst
Answer by Dimitris M on 17 May 2012

Thank you both for your nice codes !

1 Comment

Kye Taylor on 17 May 2012

A pleasure!

Dimitris M

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