how to tell whether a shape has a sharp angle

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.

Answer 1

[Image Analyst]

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).

[sty2004]

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]

If you upload a sample image it will be easier to help you.

[Image Analyst]

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]

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]

MATLAB deals with "inf" just fine. I think you can do

if slope1 - slope2 > 0.5

or whatever...