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Posted
25 May 2007

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I want to use this algorithm to reduce data points on a plot where data is fetched from database to plot. Can someone give an efficient way to do this?
My table has records in millions, and I would like to fetch only a few thousand to plot the chart.





I have not found the check if point is projected to line which contains segment, but actual projection lies behind segment.
Do you perform such check?
In such case tolerance should be calculated as a distance between point and one of the ends of segment, and should _not_ be considered as a length of the projection to the line.
Without such check wrong behaviour could appear if point is close to the line but far behind segment at the same time.
In such case point could be removed by algorithms, but in fact it should be preserved.





can you just tell me value about tolerance as default for google maps data and jsut explain about x and y of point? is it lattitude and longitude value?





In .net there's already System.Drawing.PointF, which I think is precise enough for you demo.





Great work you have here!
I was deeply interested on it, so as much as that I would like to know if, perhaps, you would be willing to share your work under different licenses. For example, would you like to share your work under the LGPL? In this way I would be able to use it under other applications I have been working lately!
Thanks!
Diego





First of all: GREAT WORK!
1 bug:
In the public DouglasPeuckerReduction method should have the loop like this:
while (Points[firstPoint].Equals(Points[lastPoint]))
{
lastPoint;
pointIndexsToKeep.Add(lastPoint);
}
If you do not add the very last point again (and it is a significant point), you loose it.
+1 potential bug:
In the PerpendicaularDistance calculation, System.Drawing.Point is used, where X and Y are Int32. Should X and Y be a not that big number (e.g. 1 million), during calculation one might get easily a bit overflow. It is nicer if X and Y are treated as Int64.
modified 24Apr12 5:52am.





I think you are mistaken on both counts.
1) The loop was eliminating identical points. The fact that your modification adds them into the list of indexes to keep is working against the very purpose of reducing the number of points. Nothing is lost because the original value of lastIndex was already added to the list of indexes to keep, before this loop.
2) I don't see where System.Drawing.Point is being used at all. This code is using a custom class named Point whose X and Y are of type Double.





I really need it,thank you very much!





I implemented this in a pet silverlight project. It works well.





thank you very much for the douglaspeucker algorithm, i have been working on a project that uses that. i have a question, is there way to calculate an optimum epsilon value within the algorithm? or does it have to be user defined?





I have found some issues with your implementation of this code. here are the changes that i have made.
Double maxDistance = 0;
Int32 indexFarthest = 0;
if (lastPointIndex  firstPointIndex > 1)
{
for (Int32 index = firstPointIndex; index < lastPointIndex; index++)
{
Double distance = PerpendicularDistance(points[firstPointIndex], points[lastPointIndex], points[index]);
if (distance > maxDistance)
{
maxDistance = distance;
indexFarthest = index;
}
}
if (maxDistance > Tolerance && indexFarthest != firstPointIndex)
{
pointIndexsToKeep.Add(indexFarthest);
Reduction(points, firstPointIndex, indexFarthest, ref pointIndexsToKeep);
Reduction(points, indexFarthest, lastPointIndex, ref pointIndexsToKeep);
}
}





Maybe some god modification ?? check that out
tolerance in % of Maximum distance in plot
private static void DouglasPeuckerReduction(List<PointD> points, Int32 firstPoint, Int32 lastPoint, Double tolerance, Double recursionMaxDistance, ref List<Int32> pointIndexsToKeep)
{
Double maxDistance = 0;
Int32 indexFarthest = 0;
List<PointD> indexError = new List<PointD>();
// count error for new points  take lower error index
List<double> distanceList = new List<double>();
for (Int32 index = firstPoint; index < lastPoint; index++)
{
Double distance = PerpendicularDistance(points[firstPoint], points[lastPoint], points[index]);
distanceList.Add(distance);
if (distance > maxDistance)
{
maxDistance = distance;
indexFarthest = index;
}
}
// check if current recursion distance is greater than passed through args
if (maxDistance > recursionMaxDistance) recursionMaxDistance = maxDistance;
// check relevant tolerance to recursionMaxDistance
if (maxDistance > (recursionMaxDistance * tolerance) / 100 && indexFarthest != 0)
{
//Add the largest point that exceeds the tolerance
pointIndexsToKeep.Add(indexFarthest);
DouglasPeuckerReduction(points, firstPoint, indexFarthest, tolerance, recursionMaxDistance, ref pointIndexsToKeep);
DouglasPeuckerReduction(points, indexFarthest, lastPoint, tolerance, recursionMaxDistance, ref pointIndexsToKeep);
}
}
public static List<PointD> DouglasPeuckerReduction(this List<PointD> Points, Double Tolerance)
{
if (Points == null  Points.Count < 3)
return Points;
Int32 firstPoint = 0;
Int32 lastPoint = Points.Count  1;
List<Int32> pointIndexsToKeep = new List<Int32>();
//Add the first and last index to the keepers
pointIndexsToKeep.Add(firstPoint);
pointIndexsToKeep.Add(lastPoint);
//The first and the last point can not be the same
while (Points[firstPoint].Equals(Points[lastPoint]))
{
lastPoint;
}
// start Approx with 0 recursionMaxDistance
DouglasPeuckerReduction(Points, firstPoint, lastPoint, Tolerance,0, ref pointIndexsToKeep);
List<PointD> returnPoints = new List<PointD>();
pointIndexsToKeep.Sort();
foreach (Int32 index in pointIndexsToKeep)
{
returnPoints.Add(Points[index]);
}
return returnPoints;modified on Thursday, March 4, 2010 12:32 PM





I have used this and it does a nice job of reducing a series of lat/longs for display on maps.
A couple of points for people looking to use the code:
I spent ages trying to hunt down a stack over flow error, before realising that the algo was crashing when passed a tolerance of 0. I'd suggest a simple sanity check of the arguments that get passed is a useful addition.
If you want to reduce the number of points to a useful number for display on Google maps or Bing maps (more than 2000 points in a polyline is slow!), it's quite useful to add a little bit of code to work out a tolerance value to get the right reduction. You could do that with either looking at the point count and picking a relevant tolerance, or interatively running the algo with progressively higher tolerances until the number of points returned was small enough.





This is awesome! I need to simplify some polygons and your work is a great help.





"Plot Graphic Library" has very useful others functions with LineApproximator and class KeyContainer. Do you planning remake this on C#?





Double area = Math.Abs(.5 * (Point1.X * Point2.Y + Point2.X * Point.Y + Point.X * Point1.Y  Point2.X * Point1.Y  Point.X * Point2.Y  Point1.X * Point.Y));
Double bottom = Math.Sqrt(Math.Pow(Point1.X  Point2.X, 2) + Math.Pow(Point1.Y  Point2.Y, 2));
Double height = area / bottom * 2;
//it's only for rectangular triangle
//i think must be someting like this:
Double a = Math.Sqrt(Math.Pow(Point1.X  Point2.X, 2) + Math.Pow(Point1.Y  Point2.Y, 2));
Double b = Math.Sqrt(Math.Pow(Point2.X  Point.X, 2) + Math.Pow(Point2.Y  Point.Y, 2));
Double c = Math.Sqrt(Math.Pow(Point.X  Point1.X, 2) + Math.Pow(Point.Y  Point1.Y, 2));
Double p = (a + b + c) / 2;
Double perpendicular = 2 * (Math.Sqrt(p*(p  a)*(p  b)*(p  c))) / a;





Double area = Math.Abs(.5 * (Point1.X * Point2.Y + Point2.X * Point.Y + Point.X * Point1.Y  Point2.X * Point1.Y  Point.X * Point2.Y  Point1.X * Point.Y));
Double bottom = Math.Sqrt(Math.Pow(Point1.X  Point2.X, 2) + Math.Pow(Point1.Y  Point2.Y, 2));
Double height = area / bottom * 2;
//it's true only for rectangular triangle......
//here must be something like this:
Double a = Math.Sqrt(Math.Pow(Point1.X  Point2.X, 2) + Math.Pow(Point1.Y  Point2.Y, 2));
Double b = Math.Sqrt(Math.Pow(Point2.X  Point.X, 2) + Math.Pow(Point2.Y  Point.Y, 2));
Double c = Math.Sqrt(Math.Pow(Point.X  Point1.X, 2) + Math.Pow(Point.Y  Point1.Y, 2));
Double p = (a + b + c) / 2;
Double perpendicular = 2 * (Math.Sqrt(p*(p  a)*(p  b)*(p  c))) / a;
//i think so... ))





.... from what one would get by projecting a curve via least squares into the piecewise linear space (of parameter n pieces)?





I do not know. I was just wanting to port varius c and c++ versions to c#, being that I could not find one.
From what I just read about your post, I would hazard to say that yea thwy would give the same result.





In this code:
for (Int32 index = firstPoint; index < lastPoint; index++)
{
Double distance = PerpendicularDistance(points[firstPoint], points[lastPoint], points[index]);
if (distance > maxDistance)
{
maxDistance = distance;
indexFarthest = index;
}
index++;
}
Is the second index++ correct? It seems to be skipping half of the points.
Jim





We caught that in unit testing but I forgot to update this post, Been kinda busy. You are right that should not me there. I will update the code sample.





How about some background on the algorithm, how it works, why its used...you know...an ARTICLE not just code!
John Conwell





I could have done that but Jonathan de Halleux did an excellent job of that (Here).
I just wanted to get this out there and searchable by others. If I can not find what I am doing and I can release this type of information I do it.
I placed that link in the original post but it is not so easy to see.





http://nettopologysuite.googlecode.com/svn/trunk/NetTopologySuite/Simplify/





That is nice, I can't believe that after performing google search for a c# implementation of the DouglasPeucker algorith I did not find this.
Thanks for the information.







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