Steve on Image Processing with MATLAB

Image processing concepts, algorithms, and MATLAB

Image visualization using transparency

Transparent graphics objects can be used effectively to visualize image processing concepts. Two particularly useful techniques are:

  • Highlighting image regions with transparent patches
  • Displaying one image transparently over another

Today I'll show how to highlight image regions with patches. For this example I'll use the 'Extrema' measurement returned by regionprops. The extrema for a given object are eight points: the left-most pixel on the bottom, the right-most pixel on the bottom, the top-most pixel on the right, the bottom-most pixel on the right, and so on.

I'll start with the rice image, segmenting it using techniques I've shown before.

I = imread('rice.png');

Even out the illumination with the tophat operator, threshold, and then clean up the thresholded image a bit.

I2 = imtophat(I, ones(15, 15));
bw = im2bw(I2, graythresh(I2));
bw2 = bwareaopen(bw, 5);
bw3 = imclearborder(bw2);

Label the binary objects and compute the extrema.

L = bwlabel(bw3);
s = regionprops(L, 'Extrema');

Each object has 8 extrema points associated with it.

ans =

   11.5000   86.5000
   12.5000   86.5000
   34.5000  100.5000
   34.5000  102.5000
   33.5000  103.5000
   27.5000  103.5000
    9.5000   89.5000
    9.5000   87.5000

We can superimpose the extrema-bounded shapes on top of the original rice image by using patch objects.


hold on
for k = 1:numel(s)
    x = s(k).Extrema(:,1);
    y = s(k).Extrema(:,2);
    patch(x, y, 'g')
hold off

The above visualization is pretty clear. If you zoom in on some of the odd, larger shapes, though, you can't really tell what's going on.

axis([120 200 1 75])

We solve that by displaying the patches transparently.


hold on
for k = 1:numel(s)
    x = s(k).Extrema(:,1);
    y = s(k).Extrema(:,2);
    patch(x, y, 'g', 'FaceAlpha', 0.3)
hold off

Now if you zoom in on the same region, we can see exactly what caused the unusual region.

axis([120 200 1 75])

Two of the rice grains were touching.

Next time I'll show a couple of techniques for visualizing one image transparently superimposed on another.

Published with MATLAB® 7.6

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