In this article, you will learn about a contour plot class, which is an improved version of the Level Curve Tracing Algorithm. It was designed to draw iso-contour of a user-defined function f(x,y).

## Introduction

This article presents a contour plot class. It is designed to draw iso-contour of a user-defined function f(x,y). I wrote it to integrate it in a graphic library: Plot Graphic Library

The class is based on the algorithm presented in [1] (Check References section). It is basically an improved version of the Level Curve Tracing Algorithm.

## Understanding the Algorithm

The algorithm uses several tuning parameters that the user must choose in order to have the best quality/performance ratio of the algorithm:

- Domain of x,y:
double pLimits[4]={0,1,2,3,4};
CContour contour;
contour.SetLimits(pLimits);

- Size of the primary grid: The grid on which evaluate the function
`f(x,y)`

. See `SetFirGrid, GetColFir, GetRowFir`

. The parameter influences greatly the quality of the contour. - Size of the secondary grid: The grid where the function is going to be evaluated. This grid can be much finer that the first grid. See
`SetSecGrid, GetColSec, GetRowSec`

.

## Classes

### CContour

Main contour class. This class cannot be directly but has to be inherited. The inherited class must implement the `ExportLine`

function. To generate contours, use

void Generate()

Make to have set the field function (`f(x,y)`

) before calling this function. The function will call `ExportLine`

for each new segment.

### CGLContour

Use this class to draw contours to an OpenGL device context.

### CListContour

Use this class to generate contour and store them as line strip. The user can retrieve each contour and use it as he wills. This function uses two sub-classes:

`CLineStrip`

, a `list < int > `

containing the index of the points `CLineStripList`

, a `list <CLineStrip*>`

The line strip can be accessed by:

CLineStripList* GetList(iPane);

where `iPane`

is the index of the contour.

## How To...

### Set Up a Contour Object

Suppose that we have inherited a class from `CContour`

and overridden `ExportLine`

function.

class CMyContour : CContour
{
void ExportLine(...);
}

Now, first set the function `f(x,y)`

:

double myF(double x, double y)
{ [...] return ... };
CMyContour contour;
contour.SetFieldFcn(myF);

Then set the iso-contour values, i.e.:

int n;
CMyContour contour;
vector<double> vIso(n);
for (int i=0;i<n;i++)
{ ... }
contour.SetPlanes(vIso);

The contour is ready to be used.

### Draw Contours using OpenGL

Use `CGLContour`

as inherited function of `CContour`

.

CGLContour contour;
[...]
contour.Generate();

### Retrieve Contours in Line Strip

Use `CListContour`

as inherited function of `CContour`

. Only the index of the points with respect to second grid are stored in the list. You can access their real value by using `GetXi()`

and `GetYi()`

functions.

CGLContour contour;
[...]
contour.Generate();
CLineStripList* pStripList;
pStripList=contour.GetLines(0);
ASSERT(pStripList);
CLineStrip::iterator pos;
for (pos=pStripList->begin();
pos != pStripList->end() ; pos++)
{
pStrip=(*pos);
ASSERT(pStrip);
if (pStrip->empty())
continue;
[...]
}

## Updates

- 31
^{st} August, 2002: Added contribution from Chenggang Zhou: better strip compression, threshold merging, area of strip, boundary detection, also some minor changes I don't remember... - 4
^{th} March, 2002: All the code is now using STL. :)

## References

## License

This article has no explicit license attached to it, but may contain usage terms in the article text or the download files themselves. If in doubt, please contact the author via the discussion board below.

A list of licenses authors might use can be found here.

Jonathan de Halleux is Civil Engineer in Applied Mathematics. He finished his PhD in 2004 in the rainy country of Belgium. After 2 years in the Common Language Runtime (i.e. .net), he is now working at Microsoft Research on Pex (http://research.microsoft.com/pex).