Steve on Image Processing with MATLAB

Image processing concepts, algorithms, and MATLAB


Steve on Image Processing with MATLAB has been archived and will not be updated.

Feature AND

I'm going to take a break from looking at M&Ms this week and instead consider a question that Richard asked yesterday:

"I have two binary images of the same size. I want to select regions from image one, where that region has to partially overlap with a region from image two."

I've seen this called the "Feature-AND" operation, and you can use the Image Processing Toolbox function imreconstruct to do it. I'll use one of the toolbox sample images to show you how.

bw = imread('text.png');

Suppose I've got a "marker" image that has a thick line running down the diagonal, like this:

stripe = logical(eye(256));
stripe = imdilate(stripe, ones(3,3));

Now ask the question, "Which objects in the original image touch the line?" It's easy to use a simple elementwise AND to see which pixels in the original image touch the line:

touching_pixels = bw & stripe;

Now to get the entire objects that are touching the line, apply an operation called morphological reconstruction, which is what imreconstruct does. This function takes two inputs, a marker image and a mask image. Use the touching pixels as the marker image, and use the original image as the mask image.

marker = touching_pixels;
mask = bw;
out = imreconstruct(marker, mask);


Here's another variation: "Which objects in the original image contain a vertical stroke that's at least 11 pixels tall?"

Start by eroding the original image by a vertical line structuring element.

se = strel(ones(11,1));
bw_eroded = imerode(bw, se);

You can see that many characters are gone completely. If we define the pixels that are left as the marker image, then reconstruction gives us back the complete objects:

marker = bw_eroded;
out = imreconstruct(marker, mask);

This is a very useful operation, and I recommend that you add it to your bag of image processing tricks.

Published with MATLAB® 7.11

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