# Ramer–Douglas–Peucker algorithm

The Ramer–Douglas–Peucker algorithm, also known as the Douglas–Peucker algorithm and iterative end-point fit algorithm, is an algorithm that decimates a curve composed of line segments to a similar curve with fewer points. It was one of the earliest successful algorithms developed for cartographic generalization.

## Idea

The purpose of the algorithm is, given a curve composed of line segments (which is also called a Polyline in some contexts), to find a similar curve with fewer points. The algorithm defines 'dissimilar' based on the maximum distance between the original curve and the simplified curve (i.e., the Hausdorff distance between the curves). The simplified curve consists of a subset of the points that defined the original curve.

## Algorithm

The starting curve is an ordered set of points or lines and the distance dimension ε > 0.

The algorithm recursively divides the line. Initially it is given all the points between the first and last point. It automatically marks the first and last point to be kept. It then finds the point that is farthest from the line segment with the first and last points as end points; this point is obviously farthest on the curve from the approximating line segment between the end points. If the point is closer than ε to the line segment, then any points not currently marked to be kept can be discarded without the simplified curve being worse than ε.

If the point farthest from the line segment is greater than ε from the approximation then that point must be kept. The algorithm recursively calls itself with the first point and the farthest point and then with the farthest point and the last point, which includes the farthest point being marked as kept.

When the recursion is completed a new output curve can be generated consisting of all and only those points that have been marked as kept.

### Non-parametric Ramer–Douglas–Peucker

The choice of ε is usually user-defined. Like most line fitting, polygonal approximation or dominant point detection methods, it can be made non-parametric by using the error bound due to digitization and quantization as a termination condition.

### Pseudocode

Assuming the input is a one-based array:

``` # source: https://karthaus.nl/rdp/
function DouglasPeucker(PointList[], epsilon)
# Find the point with the maximum distance
dmax = 0
index = 0
end = length(PointList)
for i = 2 to (end - 1) {
d = perpendicularDistance(PointList[i], Line(PointList, PointList[end]))
if (d > dmax) {
index = i
dmax = d
}
}

ResultList[] = empty;

# If max distance is greater than epsilon, recursively simplify
if (dmax > epsilon) {
# Recursive call
recResults1[] = DouglasPeucker(PointList[1...index], epsilon)
recResults2[] = DouglasPeucker(PointList[index...end], epsilon)

# Build the result list
ResultList[] = {recResults1[1...length(recResults1) - 1], recResults2[1...length(recResults2)]}
} else {
ResultList[] = {PointList, PointList[end]}
}
# Return the result
return ResultList[]
```

## Application

The algorithm is used for the processing of vector graphics and cartographic generalization. It does not always preserve the property of non-self-intersection for curves which has led to the development of variant algorithms.

The algorithm is widely used in robotics to perform simplification and denoising of range data acquired by a rotating range scanner; in this field it is known as the split-and-merge algorithm and is attributed to Duda and Hart.

## Complexity

The running time of this algorithm when run on a polyline consisting of n – 1 segments and n vertices is given by the recurrence T(n) = T(i + 1) + T(ni) + O(n) where i = 1, 2,..., n − 2 is the value of `index` in the pseudocode. In the worst case, i = 1 or i = n − 2 at each recursive invocation and this algorithm has a running time of Θ(n2). In the best case i = n/2 or i = n ± 1/2 at each recursive invocation in which case the running time has the well-known solution (via the master theorem for divide-and-conquer recurrences) of O(n log n).

Using (fully or semi-) dynamic convex hull data structures, the simplification performed by the algorithm can be accomplished in O(n log n) time.

Under certain circumstances, the complexity can be reduced to O(n) using an iterative approach.

## Similar algorithms

Alternative algorithms for line simplification include: