How to get a rectangular subimage from regionprops(Image,'BoundingBox') in Matlab?

Question

I have some particles that I've identified in a larger image, and need to parse into smaller images for each particle. I've used the regionprops 'BoundingBox' function, but haven't been successful yet. How can I now make a rectangular subimage of image I using BoundingBox? I can use BoundingBox to draw a rectangle on the original image, but the parameters returned by BoundingBox seem not to be of pixel dimension (x,y, width, height), (x1, y1, x2, y2), etc, which I would expect a bounding box to return. I've written some sample code using coins.png to make it easier for anyone to understand. Can you help me with this? Thanks!

figure(1);
I = imread('coins.png');
bw = im2bw(I, graythresh(I));
bw2 = imfill(bw,'holes');
imshow(bw2);


figure(2);
L = bwlabel(bw2);
imshow(label2rgb(L, @jet, [.7 .7 .7]))

figure(3);
imshow(I);
s = regionprops(L, 'BoundingBox');
rectangle('Position', s(1).BoundingBox);

Answer 1

According to the documentation of REGIONPROPS:

BoundingBox is [ul_corner width], where:

• ul_corner: is in the form [x y z ...] and specifies the upper-left corner of the bounding box

• width: is in the form [x_width y_width ...] and specifies the width of the bounding box along each dimension

Now you can use IMCROP functions as imcrop(I, rect) where:

rect is a four-element position vector [xmin ymin width height] that specifies the size and position of the crop rectangle.

Thus:

s = regionprops(L, 'BoundingBox');

subImage = imcrop(I, s(1).BoundingBox);
imshow(subImage)

Answer 2

The parameters returned by regionprops are [y,x,width,height] in matrix coordinates (see also "unexpected Matlab".

Thus, to extract the rectangle, you write:

subImage = I(round(s(1).BoundingBox(2):s(1).BoundingBox(2)+s(1).BoundingBox(4)),...
       round(s(1).BoundingBox(1):s(1).BoundingBox(1)+s(1).BoundingBox(3)));