Create narrow band of pixels under gray region

I want to create a narrow band of pixels (white pixels) neighbouring the lower side of gray region. Please see the attached image.

The expected output would be like:

How can I do this? Please help. Thanks.

Answer 1

[Image Analyst]

Threshold for black.

mask = grayImage == 0;

Extract the two largest regions

mask = bwareafilt(mask, 2);

Then find the centroid or bounding box

props = regionprops(mask, 'Centroid');

and extract the lower black region. See if you can do this yourself.

Call imerode() to shrink it. Then XOR the shrunken region with the original region to get just the edges.

mask = imerode(lowerMask, true(9)); % Increase from 9 to get a wider white region.
mask = xor(mask, lowerMask);

Then set those pixels in the original image to white:

grayImage(mask) = 255;