How can I isolate a faint gradient in an image using matlab?

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Chris 2014-1-8

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评论： Image Analyst 2021-11-17

采纳的回答： Image Analyst

I am trying to trace the border of a shockwave in a series of images as it advances in time.

For reference watch the video below at the 2:30 mark.

http://www.military.com/video/ammunition-and-explosives/explosives/shockwave-captured-on-high-speed/763995636001/

There is a thin almost translucent shock that moves faster than the debris. I have been converting the image to black and white so that I can use edge finding algorithms in matlab, the only trouble that I have is that I can't seem to isolate the shockwave. I have been using the graythresh() function, but it doesn't capture the wave. It captures the more obvious dirt, smoke, and debris instead of the more subtle wave.

I'm thinking that some sort of analysis of the gradient might be effective. Assuming a relatively static background would it be possible to examine two frames and detect the changes in them to isolate the shockwave?

I'm relatively new to image processing, so I'm open to any suggestions.

Thanks, Chris N.

Here are two sample images for reference. They are screenshots of the video separated by a few seconds. The motion and location of the wave becomes apparent when comparing two images. Otherwise it is hard to find the shock location. That is why I feel that some sort of comparison between two frames may work.

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Answer 1

Image Analyst 2014-1-8

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编辑：Image Analyst 2014-1-8

在 MATLAB Online 中打开

What if you simply subtract the two frames?

diffImage = abs(double(frame1) - double(frame2));
imshow(diffImage);

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Chris 2014-1-13

Thank you for the additional replies Sean and Image Analyst!

Sorry for the delay. I did not receive an email about additional posts on this thread. The image subtraction of my images created the image below. The image below has not had any filters applied to it yet.

I noticed that if I use graythresh() on the image using the automatically computed value for the level parameter, the shock completely disappears. The image becomes mostly black. To get the image below I used im2bw(Image,0) where I randomly set the value of the threshold second parameters in the function to 0. It seemed to work.

I will try your suggestions to see if I can clean it up. My end goal is to be able to take a portion and compute the average distance from the center of the expansion.

This might be off topic, but one thing that I was wondering was if the wave shown in the image below represents the path traveled by the actual wave during two frames. Because I am subtracting two frames that represent two different times wouldn't that be interpreted as the distance traveled by the wave between the frames? Just a question. It's more physics instead of matlab.

Image Analyst 2014-1-18

在 MATLAB Online 中打开

Chris: Here is some code to do what I said. I also added a Savitzy-Golay filter (in case you have the Signal Processing Toolbox and want to smooth it some before you try to find the anomoloies).

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 = 20;
% Check that user has the Image Processing Toolbox installed.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
  % User does not have the toolbox installed.
  message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
  reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
  if strcmpi(reply, 'No')
    % User said No, so exit.
    return;
  end
end
% Read in a standard MATLAB gray scale demo image.
folder = 'C:\Users\Chris\Documents\Temporary';
baseFileName = 'capture.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, '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
grayImage = imread(fullFileName);
% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(grayImage);
if numberOfColorBands > 1
  % It's not really gray scale like we expected - it's color.
  % Convert it to gray scale by taking only the green channel.
  grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the original gray scale image.
subplot(2, 1, 1);
imshow(grayImage, []);
axis on;
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 
promptMessage = sprintf('Locate the center of the shock wave,\nor Cancel to abort processing?');
titleBarCaption = 'Continue?';
button = questdlg(promptMessage, titleBarCaption, 'Continue', 'Cancel', 'Continue');
if strcmpi(button, 'Cancel')
  return;
end
[x, y] = ginput(1);
hold on;
plot(x, y, 'r+', 'LineWidth', 3, 'MarkerSize', 15);
% Scan every pixel finding the distance
maxDistance = ceil(sqrt((columns)^2 + (rows^2)))
profile = zeros(1, maxDistance);
for col = 1 : columns
  for row = 1 : rows
    distance = sqrt((col - x)^2 + (row - y)^2) + 1; % Add 1 so you don't get zero.
    index = int32(distance);
    profile(index) = profile(index) + double(grayImage(row, col));
  end
end
% Crop it to actual data
lastIndex = find(profile > 0, 1, 'last');
profile = profile(1:lastIndex);
subplot(2,1,2);
plot(profile, 'LineWidth', 2);
grid on;
title('Average Radial Profile', 'FontSize', fontSize);
% Now smooth with Savitzky-golay filter
k = 1; % Order of the polynomial
windowSize = 7;
smoothedProfile = sgolayfilt(profile, k, windowSize, [], 2);
hold on;
plot(smoothedProfile, 'r-', 'LineWidth', 5);
legend('Original Profile', 'Smoothed Profile');

Sampada J 2021-11-17

@Chris, @Image Analyst I'm Having the same problem. Even after amplifying the subtraction results, the exact boundary is not visible and visible only as distant dots (would seem noise to a layman). Would be great if you guys can share the code.

Image Analyst 2021-11-17

Yes, because not much changed between those two frames. You might try a larger time difference between frames if you want to see a bigger difference.

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Answer 2

Rui Yang 2017-5-23

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Hello Chris, I have encountered a similar problem. I have read your discussion, but can not understand. Could I have a copy of your code and pictures? My email is hypershan92@Gmail.com.

Thank you very much for your sharing.

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Sampada J 2021-11-17

@Rui Yang Please share the code or your own. Would be a great help.

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How can I isolate a faint gradient in an image using matlab?

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