Remove black pixels from RGB image.

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Luke G.
Luke G. on 15 Jan 2021
Commented: DGM on 31 Jan 2023
I know that variations of this question get asked a lot on here, but I haven't found a solution that works for my problem very well.
How do I remove the black border from the image above programmatically, but not by using imcrop!? I would prefer to find & remove all of the black pixels in the image, by effectively deleting that data--leaving me with a (yes) cropped image that contains only the color region of this image. My attempt is as follows:
% define scale bar:
firstFrame = read(vidObj,1);
imshow(firstFrame,'InitialMagnification',300);
title('Define the scale bar region:','FontSize',16);
scaleBar = drawrectangle('Color',[1 1 0]);
roi_Bar = scaleBar.Position;
% cropped and screwed:
I = imcrop(firstFrame, roi_Bar);
figure;
imshow(I);
% remove black border:
mask = (I(:, :, 1) == 0) & (I(:, :, 2) == 0) & (I(:, :, 3) == 0);
I(mask) = [];
figure;
imshow(I); % this returns a horrific result
Many thanks in advance!
  2 Comments
Mario Malic
Mario Malic on 15 Jan 2021
Hi Luke,
I'll chip in my suggestions in here, while someone else with more knowledge comes by.
The border that you want to remove consists of different colors, not only of RGB [0, 0, 0], but RGB [0, 1, 0], and many other different colors. So, I guess the solution is to find all the colors that border consists of. Unfortunatelly, I can't help with this.
Luke G.
Luke G. on 15 Jan 2021
Thanks - this was a good information! Since all of the values might not be RGB [0 0 0], I could delete all the pixels around the border of the image (shown below), but if the user didn't select exactly the border of the scale bar correctly, this might crop part of the scale bar & lead to erroneous temperature readings.
% cropped and screwed:
I = imcrop(firstFrame, roi_Bar);
figure;
imshow(I);
figure;
bW = 3; % border width
iwant = I(bW:end-bW,bW:end-bW,:);
imshow(iwant);
It would be great if there was a robust edge detection method to use here. Thanks again for your suggestion!

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Accepted Answer

Image Analyst
Image Analyst on 16 Jan 2021
All black pixels cannot be removed since the image must remain rectangular. You can remove whole rows or whole columns, but not randomly scattered isolated groups of black pixels. With that colorbar image you have, you could delete everything up to and including the black outline of the colormap. Is that what you want to do?
  7 Comments
Image Analyst
Image Analyst on 17 Jan 2021
See my attached demo. Make the necessary adaptations.
But the best solution is to just get a thermal camera that outputs the temperature image. I think it's only the very cheapest FLIR model that gives you only a pseudocolored image. Pay a few bucks more and get one that has the image directly in degrees Celsius.
Luke G.
Luke G. on 17 Jan 2021
Thanks Image Analyst! Appreciate the info & support.

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More Answers (1)

Jeffrey Daniels
Jeffrey Daniels on 31 Jan 2023
img_2d = reshape(img,[numrows*numcols],3);
% Find the indices of the rows that are remove_color
remove_color = [0,0,0];
color_idx = ~ismember(img_2d,remove_color, 'rows');
% Replace remove_color with white
img_2d(color_idx) = [255,255,255];
img = reshape(img_2d,[numrows,numcols,3]);
% Remove the rows that are remove_color (Optional, if you really want to "remove" them", but as stated it will no longer be rectanglar. This only works if you want statistics on your colors.)
img_2d_nw = img_2d(~color_idx, :);
  1 Comment
DGM
DGM on 31 Jan 2023
Improvements to make a better answer:
  • Format the code
  • Make the example complete enough that it can be demonstrated in the editor
  • Make sure the example does what you say it does
% a uint8 RGB image with black border
img = imread('peppers.png');
img = padarray(img,[10 10],0,'both');
imshow(img)
% get the size
[numrows,numcols,nchan,~] = size(img);
% reshape the image into a 3-column matrix
img_2d = reshape(img,numrows*numcols,3);
% Find the indices of the rows that are remove_color
remove_color = [0,0,0];
color_idx = ismember(img_2d,remove_color,'rows'); % select the correct region
% Either replace remove_color with a new color ...
new_color = [255 255 0]; % white would be invisible against the webpage background
img_2d(color_idx,:) = repmat(new_color,[nnz(color_idx) 1]); % make sure addressing works
img = reshape(img_2d,[numrows,numcols,3]);
imshow(img)
% ... or remove the rows that are remove_color
% (Optional, if you really want to "remove" them", but as stated it will no longer be rectanglar.
% This only works if you want statistics on your colors.)
img_2d_nw = img_2d(~color_idx, :);

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