C# WPF calculate similarity between two polylines

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I have an first primary polyline with a large number of point.
Then I have a second polyline with a smaller number of point. The second polyline derives from the first through an optimization (es. Ramer-Douglas-Peucker algorithm or something other).
I need to evaluate how good is the optimization (that is to know how similar is the second polyline in comparison with the first) in the fastest way possible.
By doing this I'll be able to compare different optimization algorithms but I need to do it without taking too much time.
Thanx for any help

Posted 3-Sep-15 5:25am

Patrick70__

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Patrice T 3-Sep-15 12:11pm

What have you tried so far ?

Patrice T 3-Sep-15 12:14pm

How do you define similarity ?
How do you compare 2 algorithms ?

Patrick70__ 4-Sep-15 3:43am

I've been thinking about comparing the areas of the two polyline but that's not very smart.
Obviously you are not to compare two algorithms but the results so the two polylines.

Patrice T 4-Sep-15 6:08am

Try to draw a square as primary polyline and a star as a secondary polyline and try to device a method to compare the 2 and to size the differences.

Sergey Alexandrovich Kryukov 3-Sep-15 13:00pm

First of all, it has nothing to do with WPF, this is pure mathematical calculation over the line data. Give the strict mathematical definition of "difference" (there is no any default meaning of such thing), and then the implementation won't be any difficult.
—SA

Patrick70__ 4-Sep-15 3:45am

I'm not sure about that but we might define "difference" how much each point of the optimized curve is far from the original curve.

Sergey Alexandrovich Kryukov 4-Sep-15 10:07am

Okay, the point difference is defined, now, what's the difference between lines? what if they have different number of points? ;-)
—SA

Patrick70__ 4-Sep-15 10:20am

Well all what I think of at the moment is calculate the area between the original and the optimized curve. The smaller the better is the algorithm. But there are several problems. 1. calculating the area might be very difficult. 2. It might be necessary to iterate on all points and that might be slow. 3. I guess there has to be something far better than this....

Sergey Alexandrovich Kryukov 4-Sep-15 10:33am

1) It's not difficult; 2) What to do, depends on how you define it. 3) The algorithm is not the goal of calculations.
Let's stop this useless discussion. You are not ready to solve such problems, because you have no clue on how to define the problems.
—SA

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Patrice T · Answer 1 · 2015-09-04T08:43:00

There is no solution to your question the way you ask it.

Sergei is right when he says

Quote:
You are not ready to solve such problems, because you have no clue on how to define the problems.

First rule of programming: You have to master the problem well enough to explain the computer what to do. It is mandatory because the computer is fast but stupid, he will never guest something.

Advice: Try to solve the problem by hand.
Take a set of arbitrary polylines, some regular, others non regular.
Take a sqare, a rectangle, a cross, a triangle, a star, a pentagon, an hexagon, a horse shoe ... , and you can play with sizes.
Pick one shape and check it against the others graphically and choose the best one. repeat many time, until you have defined the rules for the best answer.

Your definition of best choice will make a new set of problems to solve.