Programming error?

2011 8 2
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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.

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Sean de Wolski
Sean de Wolski 2011-8-2

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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.

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Lizan
Lizan 2011-8-3
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
Lizan 2011-8-3
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?

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Jan
Jan 2011-8-2

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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.

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