how to remove areas of an image containing a single color?

I have a video that I saved each frame from. The entire video has static text in big white font and it doesnt move so each frame looks very similar. Each frame also has a black background like I want to remove. Is there a way for me to remove any pixle that is (0,0,0) or (1, 1, 1) RGB? Thanks!

heres an example image that I want to crop. I want to crop all of the black and all of the white.

[Cris LaPierre]

What do you mean by remove?

It sounds like you might be interested in background subtraction. If so, check out imsubtract

For videos, you may find this answers helpful: How to use image background subtraction in video

[Walter Roberson]

It is not possible to create a matrix that has "holes" in it. You could detect 0,0,0 and 1,1,1 and have a logical mask that is true everywhere there is background -- but you cannot delete the background leaving only foreground. You can fill the background with something.

[Bradley]

Thanks for this!

In the imsubtract examples this use this line of code that im not fimilar with:

strel('disk',15)

I looked up strel but I still dont really get it. The example says it uses that line of code to estimate the background, how does that work? How do I define the background as being black or anything other than the sonar image? Thanks again!

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 = 16;
markerSize = 20;

%--------------------------------------------------------------------------------------------------------
%    READ IN TEST IMAGE
folder = [];
baseFileName = 'bradley.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)
% Maximize window.
g = gcf;
g.WindowState = 'maximized';
g.Name = 'Demo by Image Analyst';
g.NumberTitle = 'off';
drawnow;

%--------------------------------------------------------------------------------------------
% SPLIT INTO INDIVIDUAL COLOR CHANNELS SO WE CAN FIND WHITE.
[redImage, greenImage, blueImage] = imsplit(rgbImage);
threshold = 200;  % Adjust as necessary.
whitePixels = (redImage >= threshold) & (greenImage >= threshold) & (blueImage >= threshold);
subplot(2, 2, 2);
imshow(whitePixels)
impixelinfo;
axis('on', 'image');
title('White Pixels', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;

%--------------------------------------------------------------------------------------------
% GET THE SECTOR SHAPED MASK.
% Fill holes.
mask = imfill(whitePixels, 'holes');
% Take largest blob only.
mask = bwareafilt(mask, 1);
% Erode away one pixel layer to get rid of the white outline.
mask = imerode(mask, ones(3));
% Erase outside the mask by setting those pixels to zero.
redImage(~mask) = 0;
greenImage(~mask) = 0;
blueImage(~mask) = 0;
% Reconstruct the RGB image.
rgbImage2 = cat(3, redImage, greenImage, blueImage);
% Display the image
subplot(2, 2, 3);
imshow(mask)
impixelinfo;
axis('on', 'image');
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;

%--------------------------------------------------------------------------------------------
% Find bounding box
props = regionprops(mask, 'BoundingBox')
bb = props.BoundingBox;
% Crop original image
rgbImage2 = imcrop(rgbImage2, bb);
subplot(2, 2, 4);
imshow(rgbImage2)
impixelinfo;
axis('on', 'image');
title('Cropped RGBImage', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;

[Bradley]

This is super helpful! I was able to clean it up quite a bit to keep what I wanted. Thank you very much!

in these lines where you set the pixels outside the mask to 0, is there anyway to make them transparent? I intend on importing this in GIS and only want the sonar visible, not the black box around it. Thanks again.

% Erase outside the mask by setting those pixels to zero.
redImage(~mask) = 0;
greenImage(~mask) = 0;
blueImage(~mask) = 0;

[Walter Roberson]

AlphaImage = double(~mask);
imwrite(rgbImage2, 'FileNameGoesHere.png', 'Alpha', AlphaImage)

There are a limited number of image formats that support transparency -- mostly PNG and TIFF.

[Bradley]

I tried adding this to my code, but got errors. Specifically about the size of the image and how alpha is invalid. Heres the error I got:

Error using writepng>parseInputs
The value of 'alpha' is invalid. Expected input to be of size 514x928, but it is of size 536x1030.
Error in writepng (line 20)
[results, unmatched] = parseInputs(data,map,filename,varargin{:});
Error in imwrite (line 566)
        feval(fmt_s.write, data, map, filename, paramPairs{:});
Error in untitled2 (line 27)
imwrite(rgbImage2, 'figure1.png', 'Alpha', AlphaImage) 

Im not exactly sure what this means or how to fix it. This is my first time working with images.

I tried adding this line but it didnt work?

K = imresize(rgbImage2, [514 928]);

Thanks again.

[Image Analyst]

That's because only the RGB image was cropped. The mask is still for the original image. If you want to crop that too you need to call imcrop on it

mask2 = imcrop(mask, bb); % Crop mask.
AlphaImage = double(~mask2);
imwrite(rgbImage2, 'FileNameGoesHere.png', 'Alpha', AlphaImage)