Image segmentation but 'edge' does not work

i need this result, to outline it in red

but i keep getting this error message.

Error using activecontour>parse_inputs (line 381)
The 'edge' method is not supported for color or vector-valued images.

Error in activecontour (line 266)
[A, mask, N, method, smoothfactor,contractionbias] = parse_inputs(args{:});

i attached the image and my code. Thank you!!

lungct = imread('lungCT.png');
twodimimgg =  im2double(lungct); %coverted to double
imshow(twodimimgg)
r = drawrectangle;
mask = createMask(r);
bw = activecontour(twodimimgg,mask,200,'edge');
hold on;
visboundaries(bw,'Color','r'); 

[Walter Roberson]

lungct = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/809364/lungCT.PNG');
size(lungct)

ans = 1×3

361 512 3

It is a color image.

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Image Analyst]

Try this:

% Demo by Image Analyst
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 = 22;
lungct = imread('LungCT.png');
[rows, columns, numberOfColorChannels] = size(lungct)
fprintf('Original image : %d rows, %d columns, %d color channels.\n', rows, columns, numberOfColorChannels);
if numberOfColorChannels > 1
    fprintf('The original image is true color RGB, 24 bits.\nNow we will convert it to gray scale...\n');
    % Convert to gray scale.
    lungct = rgb2gray(lungct);
    % Update size.
    [rows, columns, numberOfColorChannels] = size(lungct);
    fprintf('Gray scale image : %d rows, %d columns, %d color channels.\n', rows, columns, numberOfColorChannels);
else
    fprintf('The original image is gray scale: %d rows, %d columns, %d color channels.\n', rows, columns, numberOfColorChannels);
end
twodimimgg =  im2double(lungct); % Convert to double
subplot(2, 2, 1);
imshow(twodimimgg, [])
title('Gray Scale Image', 'FontSize',fontSize)
g = gcf;
g.WindowState = 'maximized';
% Have user draw a rectangular ROI.
uiwait(helpdlg('Draw a rectangle over the image.  Simply lift the mouse button to accept it.'))
r = drawrectangle;
mask = createMask(r);
subplot(2, 2, 2);
imshow(mask)
title('The mask you drew', 'FontSize',fontSize)
bw = activecontour(twodimimgg,mask,200,'edge');
subplot(2, 2, 3);
imshow(bw)
hold on;
visboundaries(bw,'Color','r'); 
title('Active Contours', 'FontSize',fontSize)

Tell me what you see printed in the command window.

Answer 2

[DGM]

The image is actually a 3-channel (RGB) image. You can flatten it:

lungct = imread('lungCT.png');
lungct = rgb2gray(lungct);

[DGM]

it works without error in R2019b.

If it's still giving you an error about color inputs, verify the size of the arguments passed to activecontour(). They should be 2D, not 3D.