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    <p><strong>[Answer edited to reflect updates]</strong></p> <p>In your code, you used</p> <pre><code>C1=centroids(1,1); C2=centroids(1,2); </code></pre> <p>After this step, C1 and C2 become single-element scalars. You can check this with <code>size(C1)</code> and <code>size(C2)</code>, which will return <code>[1 1]</code> as the answer. I'm guessing that you wanted to plot the first C1- and C2-point, then extend it all the way to element pair N. That's not necessary, the <code>plot</code> function can handle vectors (and even matrices, but that will appear as a series of plots).</p> <p>I'm not familiar with the image processing toolbox, and I do not have that toolbox so I can't check the function outputs. But as far as I can tell, what you need is a 30-row 2-column array of centroid position data, from which you will curve-fit its position at unknown x. I've removed some plotting functions; I trust this will make the code clearer.</p> <pre><code>for N=1:30 I = figure.image.(['j' num2str(N)]); bw=(I); ss = bwlabel(bw); s = regionprops(bw,'centroid'); centroids = cat(1, centroids, s.Centroid); %Concatenate s.Centroid below centroids end %At this point, "centroids" should be a 30-by-2 array size(centroids) % check if the output from this is [30 2] x=centroids(:,1); y=centroids(:,2); poly=polyfit(x,y,2); %poly is a vector of curve-fitted polynomial coefficients pval=polyval(poly,x); %pval is a vector of curve-fitted values evaluated at x parabola=plot(x,pval); %plot the parabola </code></pre> <p>To get the y-position of the parabola at a point x_i, use <code>polyval(poly,x_i)</code>.</p> <p>Note particularly the proper syntax for the <code>cat</code> function; cat(A,B) concatenates B below A, you can't use <code>cat</code> with only one argument, as in your original code. This is likely the root cause of your headache, since with only 1 argument MATLAB simply takes "s.Centroid" as your new "centroids" array instead of adding it below the existing "centroids" array.</p> <p><strong>Reply to EDIT #3</strong></p> <p>In this part of your code</p> <pre><code>poly=polyfit(C1,C2,2); %'poly' is a vector of polynomial coefficients g=roots(poly); %g solves for polynomial roots v =max(g) %v is the largest root (assumed to be the expected ground-level destination) plot(xPlot,polyval(poly,xPlot),'y') %plots polynomial at given x-values %here, one expects to plot a point for the expected destination. %the call to the plot function should follow syntax similar to the previous line plot(v,'go') %plot function has syntax plot(x,y,options) %therefore it should look like plot(v,polyval(poly,v),'go') </code></pre> <p>I've added some comments. The problem is with the line where you called <code>plot</code> to plot the expected destination; the syntax seems to be wrong. If only 1 data argument is given, MATLAB assumes the given data to be the y-value, using its array index as the x-value (i.e. plotting y(1) at 1, y(2) at 2, etc). This is not what you want for variable v.</p> <p>You can also use <code>plot(v,0,'go')</code> as mentioned by Jonas, but it never hurts to double-check if the extrapolated polynomial value is actually near 0 ;)</p>
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    1. COThanks for answering.. Im getting an error `Warning: Polynomial is not unique; degree >= number of data points. > In polyfit at 72` ... Why is this happening ?
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    2. COFor which value of N are you getting this error? I'm guessing it would probably be N=1 or N=2. This is simply because you can't do polynomial curve-fitting with only 1 or 2 points, there is insufficient information :)
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    3. COYes at N=1 ... as soon as it starts polyfitting basically ... I think its because the centroids store data in one row only .. it overwrites in every loop..For example, I expect to get 23 rows by the end of the code, but all I get is 1 row with the last coordinates of the centroid .. is this why its not working ???
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