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Posted 20 May 2014

# A Convex Hull Algorithm and its implementation in O(n log h)

, 5 Sep 2017
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A very fast Convex Hull algorithm and its implementation

A very fast implementation of Convex Hull algorithm in O(n log h)...

Introduction

According to Wikipedia: In mathematics, the convex hull or convex envelope of a set X of points in the Euclidean plane or Euclidean space is the smallest convex set that contains X. For instance, when X is a bounded subset of the plane, the convex hull may be visualized as the shape formed by a rubber band stretched around X.

You can see a sample of a simple Convex Hull in Diagram 1. The red line is the Convex Hull.

Diagram 1 : Example of a Convex Hull in red

There are many algorithms that exists to find the convex hull in 2 dimensions or 3 dimensions. Wikipedia enumerate many of those: http://en.wikipedia.org/wiki/Convex_hull_algorithms.

A lot of people already tries to find fast algorithm for many years now. Many have found excellent ones. Lately performance reached O(n log h) where h is the number of points forming the convex hull. Today, I propose an algorithm in that order, in fact I re-explain in other words what was found in 2007 by Liu and Chen. I also present its implementation and compare it with few other algorithms. I will explain here, how it works and why it is very fast.

### Motivation

I recently needed to find the convex hull for a large set of point. The simple implementation found in C# was very slow and crashes for a lots of data. There was also some code not written in C# that I could have used. But I wanted one in C# that was quick and reliable. I did not understand every aspects of algorithms found on the web or was too lazy to try to understand them. I thought that I had a good idea and wanted to verify it. I decided to write my own algorithm. Also, I had few other requirements that I would like to meet at the same time: keep original points in the same order without having to make a copy of them and being able to multithreading it.

## Background

This article involve some basic knowledge in mathematics-geometry like quadrant and slope calculation. But you can understand the main concept although you don't remember any of those.

Knowledge of “dichotomy search” could also help understand the concept and the code provided.

There is also a multithreaded portion that could be skipped without compromising main comprehension.

## Liu and Chen Convex Hull Algorithm

Another Convex Hull Algorithm. It has no initial sorting, no tangent calculation, it works differently. According to tests, it seems to be extremely fast with few more tricks/advantages...

### Speed

Until today, "Chan" algorithm was the latest O(n log h) Convex Hull algorithm, where h is the number of vertices forming the convex hull. I thought that its implementation was recognized as the fastest one. But I thing that "Liu and Chen" algorithm would be either faster or very close to Chan. Actually I would have to test both algorithm in the same language to ensure proper comparison but I'm missing time. I will show results in different language, Chan in "C" and "Liu and Chen" in C#.

### Other requirements

Existing implementations found of “Chan” do in place permutation of points to quickly find the Convex Hull. That technic modify the original points set order. The requirement of my project was to keep the original set ordered. In order to do so with algorithm like “Chan” would be to make a copy of the full set of point before calling it. The time required is very fast but it takes 2x the amount of points in memory which it could be undesirable for very large set of points in low memory system.

Also, my requirements also has the possibilities to find the Convex Hull for a list of list of points, which was not directly implemented in Chan and Hull. I implemented that possibility but also still keeping the usual way of feeding the algorithm with a list, or array of points.

### Characteristics of Liu and Chen Convex Hull

Advantages of this Liu and Chen Convex Hull algorithm implementation

• O(n log h), same order as the best actual algorithm
• Results set is separate from the original set of points. Input stay unchanged.
• Maintain small memory usage.
• Could easily be multithreaded (first pass on every threads and second pass on at least 4 threads). Code provided uses both ways (single and multi-threaded).
• No expensive calculations (no sine, no cosine, no tangency, etc). Only slope calculation (subtraction and division) and its usage is highly minimized.
• Could be called from any IReadOnlyList<Point> (Point[], list<Point>, …) or from any IReadOnlyList< IReadOnlyList<Point>>. Example: Array of points or Array of Array of points.

Other advantages that result from the previous ones

• Output sensitive algorithm. I mentioned here only because other author as already wrote it in their own article. To my opinion, I think that any algorithm that mention "h" in its order (ex: "O (n log h)") should be, by default, "output sensitive".

Disadvantages of this implementation of Liu and Chen Convex Hull algorithm:

• Not in place. Results set is separate from the original points which in cases where points re-ordering is not an issue, it will take unnecessary additional memory

## Validation of Liu and Chen Algorithm (Results 5 circles)

I verified my algorithm against an implementation of the Chan Hull found on the internet from Pat Morin from the University of Carleton. That code helped me out a lot to fix my bugs. I want to say a very warm “Thank you” to Pat Morin for having shared its code. At the end of tests, I realized that Pat Morin algorithm implementation had a little bug that occurs when the number of points is small.

All those tests were conducted with algorithm implemented in single thread. But, I also implemented my algorithm as multithreaded, in fixed 4 threads, (Ouellet4) which is around 4x faster that my single thread algorithm. I also did another implementation where I do the first pass with all threads and the second pass with 4 threads (OuelletAll).

Diagram2 : Comparison chart of the 4 implementations of 2 algorithms (Chan - “Liu and Chen”)

Diagram 2 show a comparison charts of 5 implementations of 3 different algorithms. Points used are calculated from 5 circles with random center and with random quantity of points. Each circle has a fixed but random radius. I preferred to use that test instead of a single circle because I thought my algorithm could be a little bit less efficient in those circumstances. All the code used is provided in the download.

## Liu and Chen algorithm principle

“Liu and Chen” algorithm is developed for 2D coordinated. It could possibly work also for more dimensions but I’m not sure that it would be possible and efficient as it is in 2D.

Note: looking at the example section will help understand the following text.

The algorithm is split in 3 mains parts that occurs sequentially:

• First pass - Quadrant definition (find left, top, right and bottom)
• Second pass - Set every hull point per quadrant
• Combination - Combine each hull points quadrant

After each virtual quadrant is determined, I do another pass, the second and final one over each points, to try to insert each point in its appropriate quadrant at their appropriate place. Appropriate quadrant is easily determined by verifying against each quadrant root point. Then I use dichotomy to seek for the right place to insert the new candidate. The search is done comparing only the X coordinate, but I could have used Y axis. To know if the new candidate should be inserted or not we have to compare 2 slopes:

• The slope of the candidate point to insert and its predecessor
• The slope of the candidate point predecessor and the candidate point successor.

The comparison of both slopes determine if the point should be inserted or not. The decision to include it or not depends on 2 things: 1- the comparison result (greater than or less than), 2- the quadrant itself.

To insert a point between 2, we only have to verify that its slope fits between the slopes of its 2 neighbors. The insertion of a new point could also invalidate 1 or more existing ones which we have to verify for each new point added. The verification of potential new invalid points should start from that newly inserted point and going up and down until we found a point that is still valid as part of the Convex Hull.

Finally, I combine every quadrant convex hull point vector into only one vector in order to form the complete convex hull result set.

Quadrant offer a great advantage. We could know in advance the slope orientation expected. Knowing that, we could easily and quickly: discard a candidate, change an existing one or add a new one. When points become very large, the quantity of points to discard become very important. Being able to discard them quickly help improve the efficiency. Because all points are organized by quadrant and sorted, we can verify and potentially reject quickly bad candidate at the same time we are doing the dichotomy search.

## Example

The example is defined for those 12 points:

(1,2) (5,3) (3,6) (2,2) (3,4) (6,1) (1,7) (-3,5) (-5, -2) (-4,-3) (2, -6) (-1, 7), (0, 7)

### Step 1 - First pass

We iterate over every points and find 4 points: right, top, left and rightmost points. We determine quadrant by those points. You can see them on Diagram 3. Here we have a special case that should be considered, 3 points with same Y: (1,7), (0,7) and (-1,7). Those points have all same Y that also define the top. In that case we keep only further points. Here we keep (-1,7) and (1,7). (0,7) will not be used to define Quadrant limits.

Diagram 3: First pass result: Quadrant definition

### Step 2 - Pass 2

Iterate over every points a second time and insert or discard point as a convex hull point. Here, on diagram 4, point “A” and point “B” has been found in step 1 as StartPoint and LastPoint of Quadrant1 respectively. We have already inserted Point “C”. We want to add point “D”. The slope of CD (-3/2) is smaller than slope of BC (-1) => we should keep “D”. We insert “D” as a convex hull point. We now should iterate from “D” on each side of it to discard any invalidated point cause by the insertion of “D”. In our case, “C” stay because is still part of the convex hull => slope of AC is smaller than the slope of CD. If we would like to insert a new point (4,6), this insertion would have invalidate “C” and “D” and they would have been removed in order to stay convex.

Diagram 4: Second pass, insertion of point into appropriate quadrant

Next, there is the results of the completion of step 2. Diagram 5 show the result of the second pass. We have 4 different vectors of points forming each quadrant portion of the full Convex Hull. They are shown as different colors.

Diagram 5: result of second pass: Find every points forming the convex hull by quadrant

### Step 3 - Combination

Combine each quadrant results into one single result. The diagram 6 show the final result where each Quadrant results has been combined together. You can see that point (-1,7) and point (1,7) has been linked. Each point that are the same for each adjacent quadrant had been merged into only one point.

Diagram 6: Final result: combining quadrant results

## Optimization

### Pre-allocate memory

On start I calculated the number of points and allocates initial memory with this formula: square roots (point count). It’s only an estimation. In my test it was pretty close of the results count. But it is not necessarily fits every needs. If this optimization does not meet your request, nothing prevent you to calculate the initial size of memory results or let it grow by itself as required. But keep in mind that when a list need to grow its internal array, it has a cost. In my test case it was a pretty good estimate and I prefer that instead of either let the list grow itself or reserving the total amount of points.

Due to quadrant usage, in most cases (when there is a lots of random points), I could discard a point without making any calculation only by comparing its Y coordinate with its predecessor or successor Y coordinate. This single quick test is due to the fact that we already used dichotomy on X and then we know that a new candidate should be located into its 2 ‘X’ coordinate neighbors. As shown in the next example, diagram 7, if “B” and “C” are already inserted as a convex hull point. Having “E” and “F” X coordinate between X coordinate of “B” and “C” would make “E” and “F” potential new candidate as neighbors of “B” and “C”. But we don’t need to verify the slope of those 2 points because both points has a Y coordinate less than Y coordinate of “C”. We can discard them immediately without having to calculate the slope.

## Special cases

There is some special cases that either need attention or at least verification to make sure proper results.

### No point in the input collection

When there is no point in the collection we should do nothing and return an empty collection.

### Only one point

When there is only one point in the collection we return a collection with that one point only. Although we ask to close the graph, we should return only that single point.

### 2 points

When there is 2 point in the collection we return either a collection with 2 points or a collection with one point when the 2 points are the same. We also have to verify that closing graph will work as expected. We should also verify to have proper behavior when: 2 points are aligned with X axis or Y axis.

### Any points in line

2 or more points as the same top, left, right or bottom

It could only occur on 2 opposites quadrants at the same time. This is a very special case that need attention. It would not (highly not probable) occurs when testing with high quantity of points randomly positioned in a circle. But it could occurs in any case where points are positioned along a diagonals shape, or when points takes a triangle shape. On diagram 8, for example, point “A” is part of red quadrant (Q1) and also of the orange quadrant (Q3). That could be a problem if we only verify point to the first quadrant that it fits in. In this case, point “A” is part of Q1 but does not participate in Q1 convex values. It could only participate as a convex value for Q3. But if we have tested “A” against Q1 first and switch immediately on the next point for evaluation, then we would miss that point as a convex hull point. When there is overlapping quadrant, we should verify with its opposite quadrant to ensure to not lose any possible convex hull point.

Also, as a note, depending on the implementation, when points set are shaped as a triangle or a diagonal then the algorithm could loses performance. It will still be in O(n log h) but could take up close to twice as long in those worst cases here a single point would be validated against 2 different opposites quadrants.

## The code provided

The code provided give a good approximation of time taken to find the convex hull in few algorithms. Algorithms used for testing are Chan and "Liu and Chen". Be sure to be in “Release”, not in “Debug” to have proper performance comparison results. Comparisons are shown on the output window and saved in temp folder. There is few thing you need to know and take in account when evaluating performance results:

• “Chan” and algorithms are done in “C” while all “Liu and Chen” implementation are done in “C#” (C sharp).
• “Chan” are in place algorithm while “Liu and Chen” create another set of points as results.
• Actual “Chan” implementation that I have has been compiled in a DLL in 32 bits. Being in 32 bits, that forces me to run a maximum set of around 4000000 points. Note that I can’t find the project used to create the DLL. I have source code but I'm not good enough in "C" the recompile them in 64 bits.
• Details of the code from Carleton University is included in zip file.
• I also included another small app to test in 64 bits only. It test each implementation of “Liu and Chen” Convex Hull with very large point set. It shows that it could become interesting to use every CPU for the first pass with amount of points become enormous. But in general I would not mind and use the forced 4 independents threads which is excellent.
• Performance comparison are done in single-threaded but also include multi-threaded time.

The almost same “Liu and Chen” algorithm could be used as 3 different configurations

• SingleThread: Only one and the same thread for each 3 steps of the algorithm
• “All threads: Use every core to evaluate Quadrant limits (first step), then the same as “4 threads” for second step and for the combination of each quadrant results. in the same thread.

Abstract:

• Be careful: « Bad image format » Error could happen if you try to compile project “ConvexHullTest” in 64bits. But “ConvexHullPerfTestOnly” project is made to be compiled and run in 64 bits.
• The graphic control used is DynamicDataDisplay. It is free and nice but slow. Avoid using it with more than 10000 points.
• You could also call my algorithm with a Collection of Collection of Points which it was a requirement in our project. The interface uses IReadOnlyList which I think it is the most open interface that my algorithm required. An array or a List would works perfectly.
• Code is made to run in Framework 4.5 or higher. Note that “IReadOnlyList<T>” started in .Net 4.5.

## Future – potential development

There is some tracks of potential development for the future… where fun begin.

### Performance of the algorithm

That implementation left spaces for improvements in single thread in the algorithm itself. Like possibly exploiting the angle of the candidate to its quadrant root to improve performance of proper insertion point (~dichotomy search vs very close to exact insertion point). Could it be in O(n) or closer to it?

There is also many multithreading performance possibilities. I should have make the second pass fully multithreaded but I was missing time. I thought that a master thread by quadrant with slaves that would extract only valid possible candidate would have been nice. The master would be the only one which would insert point in its quadrant. With a concurrent stack of only probable candidate between the master and their slaves. The master would clean the stack in priority and then do the same as its slaves. Having that would still continue the advantage of using the full potential of the first pass (determining left, top, right and bottom).

Also, adding a separation of points into buckets would have enable every thread to be fully independent (except for the concurrent stack) and would maximize the time to fully use all threads. Buckets separation could also be done not equally, few big buckets for the beginning and many smaller one for the end. Another thing that could be done is to quickly verify that a quadrant has no point (if its 2 points forming its limit are the same) and them skip that quadrant completely.

### 3 or more Dimensions

It could also probably be implemented in 3 or more dimensions but the complexity could become potentially an issue. To remove that potential complexity we could separate exactly the particularities of each quadrant and then make only and only one generic implementation. But it could decrease performance due to calls required for each specificity of quadrant, perhaps template could solve that problem?

## Important notes

### 2014-08-11

After trying to publish my article to an algorithm journal, I had to improve my test and make them in the same language and compiled both in "release", which was not the case. Initial tests were conducted with Chan algorithm code in "C" 32 bits -"Debug" against "Liu and Chen" in C# 64bits - "Release". I took the time to recompile Chan implementation in "C" 64 bits - "Release". Following results invalidate every statistics show in original article. Chan in "C" become twice as fast as my implementation in C# instead of the opposite. Both were compiled in 64 bits release.

I updated the graphics and also some parts of the text. It took me some times because I was trying to turn my code in C++ which is taking longer than I would have tough initially. I did not complete but I wanted to update the article to ensure to not leave wrong results floating around for too long time.

Apologies: I'd like to apologize to everyone about inducing you in error. It is still possible that my algorithm is the fastest but it is not sure and I think it is probably not. If I have ever have enough time to finalize conversion in C++, I will let you know results.

### 2014-07-25

In 2014-07-25, 14h00, I just found an old article with main concept of my algorithm, I mean of the algorithm I was presenting here.

The article is from Guang-hui Liu and Chuan-bo Chen: A new algorithm for computing the convex hull of a planar point set. It was published at Springer in 2007.

Because it would have been unfair to catch credit for an algorithm I was not the first to create, I renamed the Convex Hull algorithm from "Ouellet" Convex Hull algorithm to "Liu and Chen" Convex Hull algorithm. But be sure, I entirely created myself. Everything you see here is from me... I was not the first.

You can see this article as an implementation of Liu and Chen Convex Hull algorithm.

## References

• “Space-Efficient Planar Convex Hull Algorithms” from Hervé Brönnimann, John Iacono, Jyrki Katajainen, Pat Morin, Jason Morrison, Godfield Toussaint. This reference is from a pdf paper “insitu-tcs.pdf” which I can’t find any more on the web but I included in my download. The “C” code of Chan implementation come also from that same place. The author of the code is Pat Morin from the Carleton University. I also included the source code for reference. Although that implementation has a bug, it really helped me a lot in order to find all of my bugs. I suspect it is only a question of “<” instead of “<=” of something similar.
• This is a link to an article that give good explanations of many popular algorithms to evaluate the Convex Hull in 2D: http://www.tcs.fudan.edu.cn/rudolf/Courses/Algorithms/Alg_cs_07w/Webprojects/Zhaobo_hull/ (2017-09-05, this link is obsolete, sorry)

## Acknowledgment

• Thanks to Omar Saad, my project manager, for explaining me what was a convex Hull and having left me the time to write this article and code.
• Thanks to Paul Labbé, a co-worker, for showing me what is the vector multiplication.
• Thanks to Pat Morin for its free “C” code of Chan algorithm implementation.

## History

1. Version 1.0 - Released 2014-05-20, Initial post
2. Version 1.1 - Released 2014-05-21, Fixed some minor bugs in code. Light modification in text.
3. Version 1.2 - Released 2014-05-22, Fixed some image and text according to colleague remarks.
4. Version 1.3 - Released 2014-06-17, Code change: use of StopWatch instead of DateTime and added rectangle performance tests. Both suggestions from Qwertie - Thanks!
5. Version 1.4 - Released 2014-07-24, Slight modification/improvement in text. Correction of Diagram 8: Overlapping quadrant.
6. Version 1.5 - Released 2014-07-25, A very bad day... I realized that I was not first one to discover that algorithm. I modified my algorithm to not take credit for something I wasn't the first one to discover. The content and code is still valid but please read Errata at the begining to find the first article published with this algorithm. I will also update Wikipedia because it wasn't there and that's why I wrote all of this.
7. Version 1.6 - Released 2014-08-11, Another bad day, just after 2014-07-25 I took some time to find/compile the Chan algorithm in C++ from Pat Morin in 64 bits release (it was in 32 bits debug). Results were just awesome. The algorithm was extremely fast, more than twice as fast than in debug. I updated the article accordingly and updated the comparison chart also. Actual results show a clear advantage on performance of Chan Algorithm. But there is something that left to be done to have proper comparison: to make my implementation in C++ instead of C#... I started, but I'm misses time.
8. Version 1.7 - Released 2016-02-04, Smjert brings my attention on an error. Thanks a lots. Error is in Q4.
9. Version 1.8 - Released 2016-02-13, Corrected 2 small bugs in Q2 and Q3 when the new value to check for insertion has the same X value as one that is already present as a Hull values. Added some tests to ensure to detect those cases. I also added the PatMorin implementation of ChanHull in 64 bits Release (it was the 32bits debug).  A special thanks to Smjert: after he asked me the question, I realised that I did not pay enough attention to this case and made some checks to ensure proper behavior.
10. Version 1.9 - Released 2016-02-17, The most major change is a new way to discard point. Instead of using slope, I use vector multiplication. I took the formula from Tryss in http://forums.futura-sciences.com/mathematiques-college-lycee/510583-point-dun-cote-de-lautre-dune.html. The idea is from a co-worker: Paul Labbé. Its idea brings better performance and more clarity to the code because I could transfer specific quadrant code to their base class. It uses 2 multiplications instead of a division but it is still faster. I added tests to proove that. It also uses a little less memory. I fixed few bugs that could happen in rare cases. I reworked everything. It is more simple and easier to follow. I added lot more tests which make me think that I have no more errors. As a side note, I know I have to update all of my article with those related changes, I will do it soon after I will complete some other related works. I just want to share all of my works the sooner. I'm pretty confident this is the fastest algorithm to find a ConvexHull in 2D, ever :). Hope you will like it a lots.
11. Version 1.9.1 - Rearrange paragraphes in order to not start the article with 2 erratas. I move them into "Important notes" section.
12. Version 1.9.2 - Removed the a hyperlink for "Futan University" because it is obsolete and I can't find an alternative. Sorry.

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Hi,

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