How do I remove the colored area on a part of the map?

How can I remove the colored fill (make it white) outside the selected polygon?

Answer 1

[Image Analyst]

The whole image, even the gray stuff and tick mark labels? Or just the colored parts?

Try this:

% Demo by Image Analyst
% Initialization steps:
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 = 16;

%--------------------------------------------------------------------------------------------------------
%    READ IN TEST IMAGE
folder = [];
baseFileName = 'heatmap.png';
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
rgbImage = imread(fullFileName);

%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(2, 2, 1);
imshow(rgbImage, []);
impixelinfo;
axis('on', 'image');
title('Original RGB Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Update 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(rgbImage);

rows =    687

columns =    507

numberOfColorChannels =      3

% Maximize window.
g = gcf;
g.WindowState = 'maximized';
g.Name = 'Demo by Image Analyst';
g.NumberTitle = 'off';
drawnow;

%--------------------------------------------------------------------------------------------
% Threshold on black to get lines.
[r, g, b] = imsplit(rgbImage);
mask = (r == 0) & (g == 0) & (b == 0);
% Display the mask image.
subplot(2, 2, 2);
imshow(mask, []);
impixelinfo;
axis('on', 'image');
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');

%--------------------------------------------------------------------------------------------
% GET THE AREAS SO WE CAN SEE HOW BIG A LINE WE WANT TO KEEP.
% props = regionprops(mask, 'Area');
% allAreas = sort([props.Area])
% Only keep blobs bigger than 500 pixels.
mask = bwareaopen(mask, 500);
% Get the convex hull.
mask = bwconvhull(mask, "union");
% Display the standard deviation filtered image.
subplot(2, 2, 3);
imshow(mask, []);
impixelinfo;
axis('on', 'image');
title('Filled Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');

%--------------------------------------------------------------------------------------------
% Mask the image to black outside the mask
% Mask image by multiplying each channel by the mask.
maskedRgbImage = rgbImage .* cast(mask, 'like', rgbImage); % R2016b or later. Works for gray scale as well as RGB Color images.
subplot(2, 2, 4);
imshow(maskedRgbImage, [])
impixelinfo;
axis('on', 'image');
caption = sprintf('Masked RGB Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');

●

drawnow;

[Image Analyst]

@Darya, did you see the code I made for you? Is it what you want?