Solution to exercises week 6

Contents

Exercise 1. Matlab exercise for recognition of round objects

This weeks exercises in on feature extraction, espacially extrancting features from segmented objects. We will look at both some pre processing tecniques and using some built in functions in matlab to extract different features

clear all
close all

% Step 1: Read Image
% Read in pills_etc.png.
RGB = imread('pillsetc.png');
figure(1)
imshow(RGB);
title('Original image');
drawnow

% Step 2: Threshold the Image
% Convert the image to black and white in order to prepare for boundary
% tracing using bwboundaries.
I = rgb2gray(RGB);
threshold = graythresh(I);
bw = im2bw(I,threshold);
figure(2);
imshow(bw)
title('Tresholded image');
drawnow


Step 3: Remove the Noise

Using morphology functions, remove pixels which do not belong to the objects of interest. Remove all object containing fewer than 30 pixels

bw = bwareaopen(bw,30);
figure(3)
imshow(bw)
title('Tresholded image, removed noise');
drawnow

% fill a gap in the pen's cap
se = strel('disk',2);
bw = imclose(bw,se);
figure(4)
imshow(bw)
title('Filled the gap in the cap');
drawnow

% fill any holes, so that regionprops can be used to estimate
% the area enclosed by each of the boundaries
bw = imfill(bw,'holes');
figure(5)
imshow(bw);
title('Filled the holes');


Step 4: Find the Boundaries

Concentrate only on the exterior boundaries. Option 'noholes' will accelerate the processing by preventing bwboundaries from searching for inner contours.

[B,L] = bwboundaries(bw,'noholes');
% Display the label matrix and draw each boundary
figure(5)
imshow(label2rgb(L, @jet, [.5 .5 .5]))
hold on
for k = 1:length(B)
    boundary = B{k};
    plot(boundary(:,2), boundary(:,1), 'w', 'LineWidth', 2)
end
title('Image with indicated boundaries');
drawnow
h5 = gcf;


Step 5: Determine which Objects are Round

Estimate each object's area and perimeter. Use these results to form a simple metric indicating the roundness of an object: metric = 4*pi*area/perimeter^2. This metric is equal to one only for a circle and it is less than one for any other shape. The discrimination process can be controlled by setting an appropriate threshold. In this example use a threshold of 0.94 so that only the pills will be classified as round. Use regionprops to obtain estimates of the area for all of the objects. Notice that the label matrix returned by bwboundaries can be reused by regionprops.

stats = regionprops(L,'Area','Centroid');
threshold = 0.94;
h2=figure(6);
objects=allchild(h5);
copyobj(get(h5,'children'),h2);

% loop over the boundaries
for k = 1:length(B)
    % obtain (X,Y) boundary coordinates corresponding to label 'k'
    boundary = B{k};
    % compute a simple estimate of the object's perimeter
    delta_sq = diff(boundary).^2;
    perimeter = sum(sqrt(sum(delta_sq,2)));
    % obtain the area calculation corresponding to label 'k'
    area = stats(k).Area;
    % compute the roundness metric
    metric = 4*pi*area/perimeter^2;
    % display the results
    metric_string = sprintf('%2.2f',metric);
    % mark objects above the threshold with a black circle
    if metric > threshold
        centroid = stats(k).Centroid;
        plot(centroid(1),centroid(2),'ko');
    end
    %Writing the value of our metric
    text(boundary(1,2)-35,boundary(1,1)+13,metric_string,'Color','y',...
    'FontSize',14,'FontWeight','bold');
end
title(['Metrics closer to 1 indicate that ',...
'the object is approximately round']);
drawnow


Exercise 2. Matlab exercise for selecting grains with a given orientation

We will continue with an example of a image of rice. And we want to find some grains with a certain orientation.

% Step 1: Read Image
clear all
close all
I = imread('rice.png');
figure(7)
imshow(I)
title('Original image');
drawnow


Step 2: Use Morphological Opening to Estimate the Background

Notice that the background illumination is brighter in the center of the image than at the bottom. Use the IMOPEN function to estimate the background illumination.

background = imopen(I,strel('disk',15));
% Display the Background Approximation as a Surface
figure(8)
subplot(211)
surf(double(background(1:8:end,1:8:end))),zlim([0 255]);
set(gca,'ydir','reverse');
title('Background');
subplot(212)
imagesc(background)
colormap gray
drawnow


Step 3: Subtract the Backround Image from the Original Image

Since the image and background are of class uint8, use the function IMSUBTRACT to subtract the background.

figure(9)
I2 = imsubtract(I,background);
imshow(I2)
title('Image with background subtracted');
drawnow


Step 4: Increase the Image Contrast

figure(10)
I3 = imadjust(I2);
imshow(I3);
title('Contrast increased');
drawnow


Step 5: Threshold the Image

Create a new binary image by thresholding the adjusted image.

figure(11)
level = graythresh(I3);
bw = im2bw(I3,level);
imshow(bw)
title('Image tresholded');
drawnow


Step 6: Label Objects in the Image

The function BWLABEL labels all connected component in the binary image. The accuracy of your results depend on: the size of the objects, the accuracy of your approximated background, whether you set the connectivity parameter to 4 or 8, whether or not any objects are touching (in which case they may be labeled as one object). Some of the

%rice grains are touching.
[labeled,numObjects] = bwlabel(bw,4); % 4-connected
numObjects % Count all distinct objects in the image.

% Step 7: Examine the Label Matrix
% Each distinct object is labeled with the same integer value. Crop part of a grain that is labeled with 50s.
rect = [105 125 10 10];
grain = imcrop(labeled,rect) % Crop a portion of labeled

% Step 8: View the Whole Label Matrix
% A good way to view a label matrix is to display it as a pseudo-color indexed image. In the pseudo-color
% image, the number that identifies each object in the label matrix maps to a different color in the
% associated colormap matrix. Use LABEL2RGB to choose the colormap, the background color, and how
% objects in the label matrix map to colors in the colormap.
figure(12)
RGB_label = label2rgb(labeled, @spring, 'c', 'shuffle');
imshow(RGB_label)
title('Pseudo color image');
numObjects =

   101


grain =

     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0    50    50    50    50    50    50    50    50     0
     0     0     0    50    50    50    50    50    50    50     0
     0     0     0    50    50    50    50    50    50     0     0
     0     0     0    50    50    50    50    50    50     0     0
     0     0     0     0    50    50    50     0     0     0     0


Step 9: Measure Object Properties in the Image

The REGIONPROPS command measures object or region properties in an image and returns them in a structure array. When applied to an image with labeled components, it creates one structure element for each component. Use regionprops to create a structure array containing some basic properties for the labeled image.

graindata = regionprops(labeled,'basic')
% To find the area of the component labeled with 50s, use dot notation to
% access the Area field in the 50th element in the graindata structure
% array.
graindata(50).Area

% Step 10: Compute Statistical Properties of Objects in the Image
% Create a new vector allgrains which holds the area measurement for each grain
allgrains = [graindata.Area];
max_area = max(allgrains) % Find the maximum area of all the grains.
biggrain = find(allgrains==max_area) % Find the grain number that has this area.
mean(allgrains) % Find the mean of the area of all the grains.

% Step 11: Create a Histogram of the Area of the Grains
nbins = 20;
figure(13)
hist(allgrains,nbins)
title('Histrogram of grain area');
graindata = 

101x1 struct array with fields:

    Area
    Centroid
    BoundingBox


ans =

   203


max_area =

   404


biggrain =

    59


ans =

  175.0396


Step 12: Measure the orientation of the Grains

Now, measure the orientation of each grain Find out how yourself :-)

graindata2 = regionprops(labeled,'Orientation','Centroid')

% Step 13: Select the grains with orientation within 0 and +/-20 degrees
% Do it by yourself!
figure(14)
imshow(I), hold on
for k = 1:length(graindata2)
    metric = graindata2(k).Orientation;
    metric_string = sprintf('%1.1f',metric);
    if -20 <= metric && metric <= 20
        centroid = graindata2(k).Centroid;
        plot(centroid(1),centroid(2),'g+');
        text(centroid(1)+2,centroid(2),metric_string,'Color','g',...
        'FontSize',14,'FontWeight','bold');
    end
end
title('Grains with orientation indicated');

% A challenge for you:
% The grain which has orientation of 0.0 in the image is actually not a
% grain just some noise. Try to remove this noise!
graindata2 = 

101x1 struct array with fields:

    Centroid
    Orientation


Exercise 3

Use the images circle2.png, circle5.png and circle15.png (from http://www.uio.no/studier/emner/matnat/ifi/INF4300/h15/undervisningsmateriale ) to evaluate the accuracy of the different formulas for estimation of area and perimeter given in the lecture notes. Ill simply get you going by using the built in regionprops in matlab

% Loading the images
clear all
close all
circle2 = imread('circle2.png');
circle5 = imread('circle5.png');
circle15 = imread('circle15.png');

% Displaying them
figure(15)
subplot(311)
imshow(circle2)
title('Circle 2');
subplot(312)
imshow(circle5)
title('Circle 5');
subplot(313)
imshow(circle15)
title('Circle 15');

% Filling the circles
circle2_filled = imfill(circle2,'holes');
circle5_filled = imfill(circle5,'holes');
circle15_filled = imfill(circle15,'holes');

% Displaying
figure(16)
subplot(311)
imshow(circle2_filled)
title('Circle 2 filled');
subplot(312)
imshow(circle5_filled)
title('Circle 5 filled');
subplot(313)
imshow(circle15_filled)
title('Circle 15 filled');

% Using regionprops to find the area
circle2_label = bwlabel(circle2_filled,4);
circle2_props = regionprops(circle2_label,'Area')

circle5_label = bwlabel(circle5_filled,4);
circle5_props = regionprops(circle5_label,'Area')

circle15_label = bwlabel(circle15_filled,4);
circle15_props = regionprops(circle15_label,'Area')

%I'm leaving the rest of the fun for you!!!
circle2_props = 

    Area: 21


circle5_props = 

    Area: 101


circle15_props = 

    Area: 761


Exercise 4

See the solution below:

% post_id = 570; %delete this line to force new post;
% permaLink = http://inf4300.olemarius.net/2015/10/15/exercises-m/;

Solution as PDF:
TheSolution

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