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Remove black pixels from RGB image.

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Luke G.
Luke G. on 15 Jan 2021
Commented: Luke G. on 17 Jan 2021
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);
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);
% remove black border:
mask = (I(:, :, 1) == 0) & (I(:, :, 2) == 0) & (I(:, :, 3) == 0);
I(mask) = [];
imshow(I); % this returns a horrific result
Many thanks in advance!


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);
bW = 3; % border width
iwant = I(bW:end-bW,bW:end-bW,:);
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?


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Luke G.
Luke G. on 16 Jan 2021
Yes, that's my current solution (code below).
img_scale = rgb2gray(imcrop(firstFrame, roi_Bar));
bW = 3; % border width
img_scale = img_scale(bW:end-bW,bW:end-bW,:); % trim border
img_scale = img_scale(:,1,:); % keep vertical strip
scaleRows = length(img_scale);
However, the border isn't necessarily fixed width: I'm designing this so that the user draws a rectangle over the scale bar in the image (using drawrectangle), however if the rectangle drawn by the user isn't precisely at the border of the scale bar, then removing a fix width border of pixels might leave some black pixels in the frame OR it might cut off part of the scale bar (more serious issue).
Stepping back from this^ problem, the larger problem is that I want to find the hottest point in each frame of infrared video & calculate the temperature based on a changing scale bar. Instead of the method above, I'm wondering if it makes sense to just analyze the grayscale image and find the hottest (i.e., "whitest") point in the grayscale region of interest. I've sketched some code for this (see below), but so far it's not picking up on the hottest region... I'm afraid a more accurate solution might require some statistics/probability. Thoughts? Suggestions?
%% Setup:
clc; close all; clear;
folder = pwd; fontSize = 14;
%% Get movie data:
% select .mp4 files:
uiwait(msgbox({'Select your video on the following page.'},'WELCOME','modal'));
mviFileName = uigetfile('*.mp4','Multiselect','off');
movieFullFileName = fullfile(folder, mviFileName);
% video object, pre-allocation:
vidObj = VideoReader(mviFileName);
numFrames = floor((vidObj.Duration)*(vidObj.FrameRate))-1;
tempData = cell(numFrames,1);
frameData = cell(numFrames,1);
frameDataText = cell(numFrames,1);
%% Set up for analysis:
prompt1 = sprintf('Use your mouse to outline the requested info on the next few plots.');
% define max & min temp read-outs:
firstFrame = read(vidObj,1);
title('Define lower scale limit:','FontSize',16);
T_lo = drawrectangle('Color',[1 1 0]);
roi_T_lo = T_lo.Position;
title('Define upper scale limit:','FontSize',16);
T_hi = drawrectangle('Color',[1 1 0]);
roi_T_hi = T_hi.Position;
% define region for analysis:
title('Select region for analysis:','FontSize',16)
find = drawrectangle('Color',[1 1 0]);
roi_find = find.Position;
close all
%% Analysis:
% preallocation:
T_hottest = zeros(1,numFrames);
t = zeros(1,numFrames);
hot_row = zeros(1,numFrames); hot_col = zeros(1,numFrames);
% execution:
for k = 1:numFrames
img = rgb2gray(imcrop(readFrame(vidObj),roi_find));
S = sum(img,3); %
[~,idx] = max(S(:));
[hot_row(k), hot_col(k)] = ind2sub(size(S),idx);
% scaling:
T_min = ocr(readFrame(vidObj),roi_T_lo); % obj. char. recognition
T_min = sscanf(T_min.Text,'%d')/10; % lower temp
T_max = ocr(readFrame(vidObj),roi_T_hi); % obj. char. recognition
T_max = sscanf(T_max.Text,'%d')/10; % upper temp
% ADD CODE: *if we get special characters, ask for help*
% calculating the max temp:
T_hottest(k) = T_min + ((T_max-T_min)/255)*img(hot_row(k),hot_col(k));
t(k) = vidObj.CurrentTime;
hold on; plot(hot_row(k),hot_col(k),'ro')
tite = sprintf('t = %.3f s',t(k)); title(tite);
txt = sprintf(' %.1f %cC',T_hottest(k),char(176));
hText = text(hot_row(k),hot_col(k),txt,'Color','red','FontSize',12);
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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