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Image Processing Toolbox

Measuring Regions in Grayscale Images

The function regionprops is very useful for measuring the properties of objects in a binary image. There are documentation examples and product demos showing how to do this.

But sometimes you need to process pixel values in the "original" grayscale image. In other words, suppose your process is something like this:

1. Segment grayscale image to get a binary image of objects.

2. Analyze the original grayscale pixel values corresponding to each object in the binary image.

This demo shows you how to accomplish this workflow using regionprops.

Contents

Step 1: Create Synthetic Image

Create a grayscale image that contains some distinct regions.

I = propsSynthesizeImage;
imshow(I)
title('Synthetic Image')

Step 2: Create a Binary Image

Segment the grayscale image by creating a binary image containing the objects in the image.

BW = I > 0;
imshow(BW)
title('Binary Image')

Step 3: Calculate Object Properties Using Pixel Values of Grayscale Image

The regionprops function supports several properties that can be used with grayscale images, including 'WeightedCentroid', 'MeanIntensity', 'MinIntensity', and 'MaxIntensity'. These properties use the original pixel values of the objects for their calculations.

For example, you can use regionprops to calculate both the centroid and weighted centroid of objects in the image. Note how you pass in the binary image (BW) containing your objects and the original grayscale image (I) as arguments into regionprops.

s = regionprops(BW, I, {'Centroid','WeightedCentroid'});

To compare the weighted centroid locations with the unweighted centroid locations, display the original image and then, using the hold and plot functions, superimpose the centroids on the image.

imshow(I)
title('Weighted (red) and Unweighted (blue) Centroid Locations');
hold on
numObj = numel(s);
for k = 1 : numObj
    plot(s(k).WeightedCentroid(1), s(k).WeightedCentroid(2), 'r*');
    plot(s(k).Centroid(1), s(k).Centroid(2), 'bo');
end
hold off

Step 4: Calculate Custom Pixel Value-Based Properties

You can use the 'PixelValues' property to do custom calculations based on the pixel values of the original grayscale image. The 'PixelValues' property returns a vector containing the grayscale values of pixels in a region.

As an example, calculate the standard deviation of each region.

s = regionprops(BW, I, {'Centroid','PixelValues','BoundingBox'});
imshow(I);
title('Standard Deviation of Regions');
hold on
for k = 1 : numObj
    s(k).StandardDeviation = std(double(s(k).PixelValues));
    text(s(k).Centroid(1),s(k).Centroid(2), ...
        sprintf('%2.1f', s(k).StandardDeviation), ...
        'EdgeColor','b','Color','r');
end
hold off

This figure shows the standard deviation measurement superimposed on each object in the image. You can also view the results in other ways, for example as a bar plot showing the standard deviation by label number.

figure
bar(1:numObj,[s.StandardDeviation]);
xlabel('Region Label Number');
ylabel('Standard Deviation');

You can use the plot to determine how to partition the data. For example, the following code identifies objects with a standard deviation lower than 50.

sStd = [s.StandardDeviation];
lowStd = find(sStd < 50);

imshow(I);
title('Objects Having Standard Deviation < 50');
hold on;
for k = 1 : length(lowStd)
    rectangle('Position', s(lowStd(k)).BoundingBox, ...
        'EdgeColor','y');
end
hold off;