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Masking out Image area using Binary Mask

Asked by Dimitris M on 16 May 2012
Latest activity Commented on by Image Analyst
on 3 Nov 2018
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

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6 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]);

  2 Comments

function masked = maskout(src,mask)
% mask: binary, same size as src, but does not have to be same data type (int vs logical)
% src: rgb or gray image
masked = bsxfun(@times, src, cast(mask,class(src)));
end
Or the more modern way we've been promoting:
% An alternate method to multiplication channel by channel.
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));
When you use cast now, it nags you to use the 'like' keyword instead of a call to the class() function.

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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);

  14 Comments

Sir, I already have a binary mask to be applied on an RGB image.
how do I apply the binary mask using image 'imoverlay' to extract only remaining RGB parts of the image after applying the binary mask?
Use the mask to index into each color channel:
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Get the pixel values for each color channel but only within the mask.
redPixels = redChannel(imoverlay); % A 1-d vector (list).
greenPixels = greenChannel(imoverlay);
bluePixels = blueChannel(imoverlay);

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Answer by Dimitris M on 17 May 2012

Thank you both for your nice codes !

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Answer by Ashmil Mohammed on 2 Jul 2015

Can I create a mask that can be used in varying illumination ? Or is there any other way I can separate out an object when the intensities change?

  1 Comment

I suggest you start a new discussion and attach a few of your images that illustrate the problem.

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Answer by Pablo Dias on 29 Aug 2018

Considering RGB image:
I(repmat(M,[1,1,3])~=0)=0;

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Answer by Jolene Yang on 3 Nov 2018

any idea how to make the mask align to the RV of the image?

  1 Comment

Have no idea what you're referring to. How is this an Answer to the original question at the top???
And what is RV? A recreational vehicle in an image you have? A random variable? Something else?
First read this link then post your question in a new thread.

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