Exporting a Triangulation - Example 8

A frequently asked question is how to export a triangulation as a list of points and indices for use in one’s own software. Users have two options: they can implement this themselves using custom indices, or they can utilize the FadeExport data structure. Using FadeExport is more memory-efficient as it releases memory within Fade while building the FadeExport data structure, thus preventing memory usage peaks during the data export.

The FadeExport Data Structure

FadeExport is a fairly simple struct with a few methods for convenient usage and also for demonstration purposes. It is intentionally implemented entirely in a header file, allowing you to easily adapt the source code for your project.

struct FadeExport
{
 void print() const;
 bool writeObj(const char* filename) const;
 void extractTriangleNeighborships(std::vector<std::pair<int,int> >& vNeigs) const;
 void getCornerIndices(int triIdx,int& vtxIdx0,int& vtxIdx1,int& vtxIdx2) const;
 void getCoordinates(int vtxIdx,double& x,double& y) const;

 // DATA
 double* aCoords; ///< Cartesian coordinates (dim*numPoints)
 int* aCustomIndices; ///< Custom indices of the points (only when exported)
 int* aTriangles; ///< 3 counterclockwise oriented vertex-indices per triangle (3*numTriangles)
};

Data Export from Fade

Step 1 in the code below creates some points. However, the order in which we pass these points to Fade may not be the same as the order in which we receive them later. Therefore, we can optionally use Point2::setCustomIndex() to set our own indices for the points.
Step 2 inserts the points into a Fade_2D object
Step 3 uses Fade_2D::exportTriangulation() to store the geometry data in a FadeExport struct. You can specify whether you also want to export the custom indices and whether you want to release data in the Fade_2D object bit by bit during the export. The latter is particularly useful for handling large data, because it prevents temporary data duplication in memory until the Fade_2D object can be deleted.

	// * 1 *   Create some points
	std::vector<Point2> vPoints;
	generateCircle(6,25.0,25.0,10.0,10.0,vPoints);
	for(size_t i=0;i<vPoints.size();++i)
	{
		// Optional: Set an arbitrary custom index
		vPoints[i].setCustomIndex(int(i));
	}

	// * 2 *   Triangulate
	Fade_2D dt;
	dt.insert(vPoints);

	// * 3 *   Export (clears the Fade object)
	FadeExport fadeExport;
	bool bCustomIndices(true); // To retrieve custom indices also
	bool bClear(true); // Clear memory in $dt
	dt.exportTriangulation(fadeExport,bCustomIndices,bClear);

	//////////////////////////////////////////////////////
	// The Fade object has been cleared to avoid memory //
	// peaks. Now verify if we have received all data...//
	//////////////////////////////////////////////////////
...

For demonstration purposes, i.e., to show you how to access the exported information, the code in Steps 4, 5, and 6 prints and draws the data in the FadeExport struct.

Step 4: We iterate over the vertex indices and retrieve the x,y coordinates (or x,y,z in the 2.5D case). These coordinates are stored in FadeExport::aCoords and you can access them either through indices or conveniently via the getCoordinates(vtxIdx,x,y) method.
Step 5: We output the triangles and draw them. To achieve this we determine the three vertex indices per triangle index and, for each vertex-index, we retrieve the associated coordinates. Although we could calculate the indices and access the arrays directly, using the getCornerIndices() and getCoordinates() methods is a more convenient and user-friendly approach.
Step 6: Fade does not explicitly store information about which triangle is adjacent to another to conserve additional memory in the FadeExport struct. If your data structure requires this information, you can rapidly determine it from the other stored data using the extractTriangleNeighborships() method.

...
	//////////////////////////////////////////////////////
	// The Fade object has been cleared to avoid memory //
	// peaks. Now verify if we have received all data...//
	//////////////////////////////////////////////////////

	// * 4 *   Vertex output
	Visualizer2 visExp("example8_exported.ps");
	for(int vtxIdx=0;vtxIdx<fadeExport.numPoints;++vtxIdx)
	{
		double x,y;
		fadeExport.getCoordinates(vtxIdx,x,y);

		std::string s(" "+toString(vtxIdx));
		// Are there custom indices? Then add
		if(fadeExport.numCustomIndices==fadeExport.numPoints)
		{
			int customIdx(fadeExport.aCustomIndices[vtxIdx]);
			s.append("(customIdx="+toString(customIdx)+")");
		}
		std::cout<<"Vertex"<<s<<": "<<x<<" "<<y<<std::endl;
		Label label(Point2(x,y),s.c_str(),true,15);
		visExp.addObject(label,Color(CRED));
	}

	// * 5 *   Triangle output
	for(int triIdx=0;triIdx<fadeExport.numTriangles;++triIdx)
	{
		int vtxIdx0,vtxIdx1,vtxIdx2;
		fadeExport.getCornerIndices(triIdx,vtxIdx0,vtxIdx1,vtxIdx2);
		std::cout<<"Triangle "<<triIdx<<": "<<vtxIdx0<<" "<<vtxIdx1<<" "<<vtxIdx2<<std::endl;
		// Fetch also the coordinates and draw the edges
		double x0,y0;
		double x1,y1;
		double x2,y2;
		fadeExport.getCoordinates(vtxIdx0,x0,y0);
		fadeExport.getCoordinates(vtxIdx1,x1,y1);
		fadeExport.getCoordinates(vtxIdx2,x2,y2);
		Point2 p0(x0,y0);
		Point2 p1(x1,y1);
		Point2 p2(x2,y2);
		visExp.addObject(Segment2(p0,p1),Color(CBLACK));
		visExp.addObject(Segment2(p1,p2),Color(CBLACK));
		visExp.addObject(Segment2(p2,p0),Color(CBLACK));
		double midX((x0+x1+x2)/3.0);
		double midY((y0+y1+y2)/3.0);
		std::string text("T"+toString(triIdx));
		Label l(Point2(midX,midY),text.c_str(),true,15);
		visExp.addObject(l,Color(CBLUE));
	}

	// * 6 *   Neighbors output
	std::vector<std::pair<int,int> > vNeigs;
	fadeExport.extractTriangleNeighborships(vNeigs);
	for(size_t i=0;i<vNeigs.size();++i)
	{
		std::cout<<"Triangle "<<vNeigs[i].first<<" <-> Triangle "<<vNeigs[i].second<<std::endl;
	}
	visExp.writeFile(); // Write the postscript file

	// The above is for demonstration purposes, the easier way
	// to output FadeExport data is using its print() or writeObj()
	// function:
	//fadeExport.print();
	//fadeExport.writeObj("out.obj");

	return 0;

The output of Example 8 is:



Vertex 0(customIdx=4): 20 16.3398

Vertex 1(customIdx=3): 15 25

Vertex 2(customIdx=2): 20 33.6603

Vertex 3(customIdx=1): 30 33.6602

Vertex 4(customIdx=0): 35 25

Vertex 5(customIdx=5): 30 16.3397
Triangle 0: 0 2 1

Triangle 1: 3 2 0

Triangle 2: 5 3 0

Triangle 3: 5 4 3
Triangle 0 <-> Triangle 1

Triangle 1 <-> Triangle 2

Triangle 2 <-> Triangle 3

The FadeExport Data Structure

Data Export from Fade

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