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 = 20;
sourceImage = imread('peppers.png');
subplot(2, 2, 1);
imshow(sourceImage);
title('Original Image', 'FontSize', 20);
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% 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 ImageAnalyst', 'NumberTitle', 'Off')
uiwait(msgbox('Draw a polygon. Double click to accept/finish it'));
hPoly = impoly;
polyPoints = hPoly.getPosition
mask = hPoly.createMask;
subplot(2, 2, 2);
imshow(mask);
title('Mask', 'FontSize', 20);
croppedImage = sourceImage; % Initialize
% Set outside of polygon to black
croppedImage(~repmat(mask, 1, 1, 3)) = 0;
x1 = round(min(polyPoints(:, 1)))
x2 = round(max(polyPoints(:, 1)))
y1 = round(min(polyPoints(:, 2)))
y2 = round(max(polyPoints(:, 2)))
% Do a rectangular crop
croppedImage = croppedImage(y1:y2, x1:x2, :);
subplot(2, 2, 3);
imshow(croppedImage);
title('Cropped Image', 'FontSize', 20);
