Sorry, I can't do it without using any built-in functions, though the functions I use are pretty low level, like imread(), imshow(), bwboundaries(), etc.
Try this:
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clearvars;
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 16;
%=======================================================================================
% Have user browse for a file, from a specified "starting folder."
% For convenience in browsing, set a starting folder from which to browse.
% startingFolder = pwd; % or 'C:\wherever';
% if ~exist(startingFolder, 'dir')
% % If that folder doesn't exist, just start in the current folder.
% startingFolder = pwd;
% end
% % Get the name of the file that the user wants to use.
% defaultFileName = fullfile(startingFolder, 'n*.*');
% [baseFileName, folder] = uigetfile(defaultFileName, 'Select a file');
% if baseFileName == 0
% % User clicked the Cancel button.
% return;
% end
folder = pwd;
baseFileName = 'traffic-sign-boards-500x500.jpg';
fullFileName = fullfile(folder, baseFileName);
rgbImage = imread(fullFileName);
% rgbImage = decorrstretch(rgbImage,'tol',0.01);
% imwrite(rgbImage, 'test.png');
% Get the dimensions of the image.
[rows1, columns1, numberOfColorChannels1] = size(rgbImage)
% Display the original image.
subplot(2, 2, 1);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Original Color Image\n"%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0.05 1 0.95]);
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by Image Analyst', 'NumberTitle', 'Off')
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
mask = redChannel > 128 & greenChannel < 128 & blueChannel < 128;
% Display the mask image.
subplot(2, 2, 2);
imshow(mask, []);
axis('on', 'image');
caption = sprintf('Color Segmentation');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
% Fill holes, and take the largest blob.
subplot(2, 2, 3);
mask = imfill(mask, 'holes');
mask = bwareafilt(mask, 1);
% Display the image.
subplot(2, 2, 3);
imshow(mask, []);
axis('on', 'image');
title('Final Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Find the border
boundaries = bwboundaries(mask);
boundaries = cell2mat(boundaries); % Convert from cell to double. Format is [rows, columns], which is [y, x]
% Get x and y coordinates.
xBoundary = boundaries(:, 2);
yBoundary = boundaries(:, 1);
% Display the original image with a blue border around it.
subplot(2, 2, 4);
imshow(rgbImage, []);
axis('on', 'image');
caption = sprintf('Original Color Image\nwith Boundary in Blue');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Plot boundary in blue.
darkGreen = [0, 0.5, 0];
hold on;
plot(xBoundary, yBoundary, 'b-', 'LineWidth', 3);
This works just because your image is pure computer graphics, not a real world scene.
See lots of papers on traffic sign detection here: VisionBib
