How can I make edges black and background white in an photo with detected edges?

Hey, everyone! I'm studying on edge detection at MATLAB and have a sample code for that.The edges are white and the background is black.However, my teacher wants to see that edges are black and the background is white.How can I do that? .Also, edge should be only between land and sea.The edges inside land should not be shown (my code doesn't show them anyways).'erodedimg = 255 - real(erodedimg)' part does the image all white even tho I made the white edges bigger with imdilate function.

Image is attached.

The code:

clc;

I = imread('island.bmp');

BW1 = edge(I,'Prewitt',0.18);

se = strel('square',3);

erodedimg = imdilate(BW1,se);

erodedimg = 255 - real(erodedimg);

imshow(erodedimg);

Please help me about that. Thanks by now for your attention.

[Eleanor Betton]

do you need to be using the edge function? could you use a differnt method of edge detection? You could achive this with thresholding I believe.

[Ayberk Ay]

No, I don't have to use the edge function.How can I achive that with thresholding? can you give me some help about that?

[Eleanor Betton]

By thresholding you can see the sea is mainly has a value less than 80, if you set all picels with less than 80 to be while and anything higher as island you get a rough island shape - it is pretty rough so you probably want to keep trying edge detection.

[Eleanor Betton]

https://uk.mathworks.com/help/images/detecting-a-cell-using-image-segmentation.html

This gives a similar examples which might help.

Answer 1

[Image Analyst]

Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;

%===============================================================================
% Read in demo image.
folder = pwd;
baseFileName = 'island.bmp';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
	% The file doesn't exist -- didn't find it there in that folder.
	% Check the entire search path (other folders) for the file by stripping off the folder.
	fullFileNameOnSearchPath = baseFileName; % No path this time.
	if ~exist(fullFileNameOnSearchPath, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
% Read in the image from disk.  If storedColorMap is not empty, it's an indexed image with a stored colormap.
[grayImage, storedColorMap] = imread(fullFileName);
if ~isempty(storedColorMap)
	grayImage = ind2rgb(grayImage, storedColorMap);
end
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage);
if numberOfColorChannels > 1
	% It's not really gray scale like we expected - it's color.
	% Use weighted sum of ALL channels to create a gray scale image.
	grayImage = rgb2gray(grayImage);
	% ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
	% which in a typical snapshot will be the least noisy channel.
	% grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the image.
hFig = figure;
subplot(2, 3, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize, 'Interpreter', 'None');
hFig.WindowState = 'maximized'; % May not work in earlier versions of MATLAB.
drawnow;

% Display the histogram
subplot(2, 3, 2);
imhist(grayImage);
grid on;

% Binarize the image
binaryImage = imbinarize(grayImage);
% Display the image.
subplot(2, 3, 3);
imshow(binaryImage, []);
title('Initial Binary Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');

% Fill holes.
binaryImage = imfill(binaryImage, 'holes');
% Display the image.
subplot(2, 3, 4);
imshow(binaryImage, []);
title('Filled Binary Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
hp = impixelinfo();

% Get the boundaries
boundaries = bwboundaries(binaryImage);
% Plot them over the inverted gray scale image.
subplot(2, 3, 5);
imshow(255-grayImage, []);
numBoundaries = length(boundaries);
caption = sprintf('Inverted Grayscale Image with %d boundaries', numBoundaries);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
hold on;
for k = 1 : length(boundaries)
	thisBoundary = boundaries{k};
	x = thisBoundary(:, 2);
	y = thisBoundary(:, 1);
	plot(x, y, 'r-', 'LineWidth', 2);
end

% Now make an image that is all white so we can draw the edges (boundaries) in black.
% Plot them over the inverted gray scale image.
subplot(2, 3, 6);
whiteImage = grayImage; % Initialize to same size as original image.
whiteImage(:) = 255; % Make white.
imshow(whiteImage, []);
axis('on', 'image');
caption = sprintf('White Image with %d boundaries', numBoundaries);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
hold on;
% Draw all boundaries in black.
for k = 1 : length(boundaries)
	thisBoundary = boundaries{k};
	x = thisBoundary(:, 2);
	y = thisBoundary(:, 1);
	plot(x, y, 'k-', 'LineWidth', 2);
end

[Ayberk Ay]

thank you so much! It works perfectly!