Efficient segment boundary marking after segmentation of an image

Question

One can mark the boundary of a binary image by bwboundaries function of MATLAB.

What should be done for obtaining boundaries of all segments as a binary image?

I have segmented an image and want to know if there is a way to mark boundaries between each neighbouring segment without applying morphological operations on each segment.

I have added images to illustrate what i want to do. Actually i want to obtain a binary image that keeps pink boundary marker pixels between all segments. Thus, I can overlay them with original image by the help of imoverlay function of Steve Eddins.

Random colored labeling of segmentation result:

Roughly-marked pink boundaries between segments:

Answer 1

You can find the region boundaries using a range filter, which finds the intensity range within each pixel's neighborhood. This takes advantage of the fact that the label matrix only has non-zero range at the region boundaries.

im = imread('https://i.stack.imgur.com/qPiA3.png');
boundaries = rangefilt(im,ones(3)) > 0;
imoverlay(label2rgb(im),boundaries,[0 0 0]);

These edges are also two pixels wide. Actually, I think the edges have to be two pixels wide; otherwise the regions will "lose" pixels to the border non-uniformly.

Answer 2

As a round-about way, I thought of making use of edge function of MATLAB. First, I need to apply something like a label2gray operation. labels is the segmentation output (first image provided in the question) in the code below.

grayLabels = mat2gray(255* double(labels) ./ double(max(labels(:)))); %label2gray
bw_boundaries = edge(grayLabels,0.001);

Answer 3

Since erosion and dilation work on non-binary images as well, you can write

img = imread('https://i.stack.imgur.com/qPiA3.png');
ei = imerode(img,ones(3));
di = imdilate(img,ones(3));
boundaries = ei~=img | di~=img;

This results in a bw image that has a boundary at the edge of each colored region (thus, the boundary line will be two pixels wide).

Note that this will not return an ordered list of pixels as bwboundaries, but rather a logical mask like bwperim, which is what imoverlay needs as input.