MATLAB: How to extract a portion of an image specified by mask generated from a ROI in Image Processing Toolbox 7.3 (R2011b)

binarycropcroppingextractionimageImage Processing Toolboxprocessing

I have an image where I have specified a region of interest (ROI) using functions such as IMELLIPSE or IMFREEHAND. I would like to extract the part of the image contained within the region specified by such a function.

Best Answer

We first create a binary mask from a specified ROI, which returns a matrix containing only 1 or 0. To return the selected portion of the image, we perform a logical AND of the binary mask matrix with the image using logical indexing. The resulting image will contain the extracted portion.
The steps below provide an example of this process. Here, we use a sample image provided by the Image Processing Toolbox.
% 1) Open the image file as a workspace variable:
img = imread('peppers.png');
% 2) Display the image and store its handle:
h_im = imshow(img);
% 3) Create an ellipse defining a ROI:
e = imellipse(gca,[55 10 120 120]);
% 4) Create a mask from the ellipse:
BW = createMask(e,h_im);
% 4a) (For color images only) Augment the mask to three channels:
BW(:,:,2) = BW;
BW(:,:,3) = BW(:,:,1);
% 5) Use logical indexing to set area outside of ROI to zero:
ROI = img;
ROI(BW == 0) = 0;
% 6) Display extracted portion:
figure, imshow(ROI);
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