Programming error?

My problem is that I have an image which I uploads. I want to obtain the line profile (intensity) for a certain leaning line of my choice.

My error arises when I am obtaining the intensity profile (mode_vecX). See code below..., also I have to round of my y-vector. Is that needed?

      % Uploads the Image to a vector:
             image_vec = imread(image_filename);

       % Creates the coordinate vectors of the image  
              pixlength_vecX = 1:length(image_vec(:,1));
              pixlength_vecY = 1:length(image_vec(1,:));

         % Obtains the points which you want the slope between: 
              disp('Please pick two points along the line profile you want to investigate: '); 
              imshow(image_vec); 
              title('Please pick two points along the line profile you want to investigate: ');
              xlabel('x pixel'); 
              ylabel('y pixel');
              [x,y] = ginput(2); 
              close all           

           % Finds the line's equation y = m*x + k
              p = polyfit(x,y,1);
              yPixel_vec = polyval(p,pixlength_vecY); % 

            % Obtains the intensity at coordinate (x,y)   
             mode_vecX = image_vec(pixlength_vecY,round(yPixel_vec));

The error message I get when I run this is:

??? Index exceeds matrix dimensions.

Error in ==> imageProfile at 69 mode_vecX = image_vec(pixlength_vecY,round(yPixel_vec));

Something is wrong here and I need some other thoughts and eyes. Many thanks for the assistance.

Answer 1

[Sean de Wolski]

In reply to your deleted answer from yesterday, your error was, as Jan suggested, that you were confusing x/y with row/col. For image processing in general I (and others) highly recommend getting in the habit of using the first and second dimension - naming your variables r/c instead of x/y etc.

This is a working solution to your problem yesterday:

% Obtains the points which you want the slope between: 
disp('Please pick the two points of the line profile (pick upper point first): '); 
imshow(image_vec); 
title('Please pick the two points of the line profile (pick upper point first): ');
xlabel('x pixel'); 
ylabel('y pixel');
[x,y] = ginput(2); 
close all

% slope of the line between point A and B
m = diff(x)/diff(y); %note switch
b = x(1)-m*y(1);
% Function coordinates f(x) = y
fline = @(pix)pix.*m+b;

xPixel = 1:size(image_vec,2);
yPixel = fline(xPixel);

figure

imshow(image_vec);
hold on; 
plot(yPixel,xPixel,'g');  %note switch!
title('Uploaded Image');
xlabel('x-pixel'); ylabel('y-pixel');

If you want the value on the line - use round (or fix / ceil) and sub2ind to get the values on the image.

[Lizan]

Thank you. I rewrote my code yesterday, using col and row instead of x and y and I got the same result as above (although I am using polyfit), however...

I have one remaining problem with this code. When I try to obtain the values of my image for these lines (row,col) I keep getting the message "??? Subscript indices must either be real positive integers or logicals.". I have looked into this and I noticed that some of the values of the row coordinate is negative. So I tried to fix this by zero those that are below zero,.. but the problem still remains. I am using round. Then I tried using sub2ind but this is not working. The vectors are in the same size as the image size.

Got any idea what the problem could be?

[Lizan]

Instead of using zero I replaced the negative values with value one.

Also, I replaced all values above the image size with the max. image lengths.

This fixed the problem.

Now when I am plotting image_vec(ypixel, xpixel), from above, I get all values for the whole image. I just want the values along this line.

What is wrong here?

Answer 2

[Jan]

I assume you have confused the 1st and 2nd dimension and the 1st argument must be "pixlength_vecX" in this line:

% Buggy:
mode_vecX = image_vec(pixlength_vecY, round(yPixel_vec))

To solve such problems by your own, let the debugger stop MATLAB, when the error occurs:

dbstop if error

Then inspect, why either "pixlength_vecY" or "round(yPixel_vec)" exceeds the dimensions of image_vec.

This forum cannot replace a debugger and it is up to you to fix your programs as far as possible.