how to tell whether a shape has a sharp angle

2012 6 16
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How to tell whether a shape contains a sharp angle like less than 30 degree. Is there any simple algorithm to check? Thanks in advance.

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Image Analyst
Image Analyst 2012-6-16

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Get the boundaries with bwboundaries() and then calculate the angle between discrete sets of points (more than just one pair of points with another pair of points).

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sty2004
sty2004 2012-6-17
It is still too complicated for me...
Is that I have to pick two nearby lines then calculate the angle between?
Can you further elaborate?
Ryan
Ryan 2012-6-17
If you upload a sample image it will be easier to help you.
Image Analyst
Image Analyst 2012-6-17
Let's say you want to see if there is a corner at perimeter element # 100. So you inspect elements, say, 90 - 100, and 100 - 110. Use polyfit to get the slope of the line and see if the two slopes are sufficiently different to indicate a change in slope. Something like (untested)
coeffs1 = polyfit(90:100, signal(90-100), 1);
slope1 = coeffs1(1);
coeffs2 = polyfit(100:110, signal(100-110), 1);
slope2 = coeffs2(1);
etc.
sty2004
sty2004 2012-6-19
For a circle there are infinte slopes for its vertical slopes on both sides, there will be big difference in slope, how I should deal with this?
Image Analyst
Image Analyst 2012-6-19
MATLAB deals with "inf" just fine. I think you can do
if slope1 - slope2 > 0.5
or whatever...

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