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Translating curves in paint to matlab

  1. Jun 9, 2010 #1
    Hi,
    I am currently using matlab to do some work on simple closed curves in the plane. In order to make this work quite general I have opted to use a matrix which stores (x,y) coordinates of points on my curve (rather than use a function).

    I want to feed some interesting curves into my code but currently this relies on me finding a parametric representation and this tends to mean they are not so exciting.

    I would like to be able to draw a closed curve in paint or something and then translate that into a set of coordinates which I can store as a matrix in matlab.

    Does anyone know how I might go about this??

    Thanks,
    Tom
     
    Last edited: Jun 9, 2010
  2. jcsd
  3. Jun 9, 2010 #2
    You can read an image into matlab using the imread command. Check the docs for details and examples.
     
  4. Jun 10, 2010 #3
    Thanks for the tip, however after spending some time trawling the help pages, I'm still not sure how to proceed.

    So at the moment I have drawn a circle in paint and saved it as a greyscale bmp. imread will then turn this into a matrix of 1's and 0's. I take it these refer to the pixels a 1 meaning there is something drawn there and a 0 meaning nothing drawn.

    How can I convert this matrix into an n x 2 matrix which just describes the x and y coordinate of each point on the curve? Am I going to have to run some kind of for loop?

    Thanks,
    Tom
     
  5. Jun 10, 2010 #4

    f95toli

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    Gold Member

    Yes, just step through the matrix and whenever the element==1 you store that position (indices) in your nx2 matrix.
     
  6. Jun 10, 2010 #5
    How am I going to make sure that the points remain next to each other?

    I can see that it would be easy for a line, but it gets more complex for a circle. Then my aim is to make quite complex snake like shapes. Since I will be integrating this data, it would be best if the data points which were close in the picture remained close in the nx2 matrix.

    The good thing is the data points I end up with don't have to accurately represent the curve I drew in paint, so would I be able to interpolate the 'messed up' data to make a smooth curve?

    Also, more fundamentally, I think I must be making a mistake somewhere, here is what I got for my circle:

    A = imread('circle.bmp', 'bmp');

    Then I run this:

    function [x,y] = coordinate_generator(A)
    z = size(A);
    k = 1;
    for i = 1:z(2)
    for j = 1:z(1)
    if A(j,i)==0
    x(k) = i;
    y(k) = j;
    k = k+1;
    end
    end
    end

    Attached is the start picture and the picture I got from plotting [x,y]

    What am I doing wrong?
     

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