finding the length of a pendulum in motion

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Judith Amulen
Judith Amulen 2011-2-9
When i write the following code, for count = 1:nFrames lab = bwlabel(seg_pend(:, :, count)); property = regionprops(lab, 'Centroid'); pend_centers(count, :) = property.Centroid; end i get an error of "too many outputs requested" how can i correct it?

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Brett Shoelson
Brett Shoelson 2011-2-9
property = regionprops(lab,'Centroid') returns a struct, the fields of which correspond to each ROI (blob) in lab. To convert the centroids of those ROIs into a vector, you might consider something like:
b = reshape([property.Centroid],2,[])';
and then write them en masse to pend_centers.
Or, inside of your for loop, consider writing:
pend_centers(count,:) = property(count).Centroid;
Cheers,
Brett
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Judith Amulen
Judith Amulen 2011-2-11
with the second option, i get an error of "index exceeds matrix dimensions"
Judith Amulen
Judith Amulen 2011-2-11
i don't clearly understand the first option though. could u please explain a little more if you don't mind! thanks.

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Walter Roberson
Walter Roberson 2011-2-9
You shouldn't try to measure the length of a pendulum in motion. The bottom tip is moving so much faster than the anchor point that you need to take in to account relativistic contraction, but you can't, because you cannot simultaneously measure the position and the velocity (Heisenberg). It is thus advisable to instead wait until the pendulum reaches to top of its arch and thus has a velocity of 0, as you would not need any relativistic correction for the length then.

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