## Polygon Clipping, Boolean Set Operations

Fade provides polygon clipping and functions to combine polygons through the boolean operations:

- Union (A OR B)
- Intersection (A AND B)
- Difference (A NOT B)
- Symmetric Difference (A XOR B)

You can find the C++ source code for the polygon clipping demo in *examples_2D/ex5_booleanOps.cpp* in the Fade download.

## Creating two Shapes

We need two shapes for this demo. For this purpose we use code that is similar to Polygons and Zones – Example4. But for the sake of completeness we repeat it here briefly:

// * 1 * Insert 4 bounding box points Fade_2D dt; dt.insert(Point2(0,0)); dt.insert(Point2(+100,0)); dt.insert(Point2(100,50)); dt.insert(Point2(0,50)); // * 2 * Create points on two circles std::vector<Point2> vCircle0; std::vector<Point2> vCircle1; int numPoints(8); double radius(22); //generateCircle( num,centerX,centerY,radiusX,radiusY,vOut); generateCircle(numPoints,45.0,25.0,radius,radius,vCircle0); generateCircle(numPoints,55.0,25.0,radius,radius,vCircle1); // * 3 * Create segments std::vector<Segment2> vSegments0; std::vector<Segment2> vSegments1; for(int i=0;i<numPoints;++i) { Segment2 seg0(vCircle0[i],vCircle0[(i+1)%numPoints]); Segment2 seg1(vCircle1[i],vCircle1[(i+1)%numPoints]); vSegments0.push_back(seg0); vSegments1.push_back(seg1); } // * 4 * Insert the segments as constraint graphs. ConstraintGraph2* pCG0=dt.createConstraint(vSegments0,CIS_CONSTRAINED_DELAUNAY); ConstraintGraph2* pCG1=dt.createConstraint(vSegments1,CIS_CONSTRAINED_DELAUNAY); // * 5 * Create two Zone2 objects using ZL_INSIDE: // Note: Boolean operations require that the two zones // belong to the same Fade_2D object! Zone2* pZone0(dt.createZone(pCG0,ZL_INSIDE)); pZone0->show("example5_zone0.ps",true,true); Zone2* pZone1(dt.createZone(pCG1,ZL_INSIDE)); pZone1->show("example5_zone1.ps",true,true);

- Step 1 creates a Fade_2D object and inserts 4 points.
- Next, Step 2 generates circle points in the two vectors
`vCircle0`

and`vCircle1`

- Step 3 creates line segments in
`vSegment0`

and`vSegment1`

- Step 4 creates the two
`ConstraintGraph2`

objects`pCG0`

and`pCG1`

using the segments from Step 3 as input. - Finally Step 5 creates two
`Zone2`

objects inside`pCG0`

and`pCG1`

and draws them.

Observe that `pCG0`

and `pCG1`

intersect each other and thus Fade has subdivided the intersecting constraint-segments automatically.

“

Fade computes the intersection point of two segments using multiple-precision arithmetic. Thus it isnumerically robust against glancing intersections. But be aware that theexactintersection point of two lines may not be representable in double precision arithmetic and thus very tiny rounding errors can occur.”

## Boolean operations with polygons (zones)

Once we have made two Zones in the same `Fade_2D`

instance we can carry out a boolean operation with just one more line of code. Four such operations exist and the result images are shown below:

// Union operation Zone2* pZoneUnion(zoneUnion(pZone0,pZone1)); pZoneUnion->show("example5_zoneUnion.ps",true,true); // Intersection operation Zone2* pZoneIntersection(zoneIntersection(pZone0,pZone1)); pZoneIntersection->show("example5_zoneIntersection.ps",true,true); // Difference operation Zone2* pZoneDifference(zoneDifference(pZone0,pZone1)); pZoneDifference->show("example5_zoneDifference.ps",true,true); // Symmetric Difference operation Zone2* pZoneSymmetricDifference(zoneSymmetricDifference(pZone0,pZone1)); pZoneSymmetricDifference->show("example5_zoneSymmetricDifference.ps", true,true);

## Extracting triangles and Border Polygons

You can use `Zone2::getTriangles()`

to conveniently extract the triangles of a zone. However, if you are rather interested in its boundary, use `Zone2::getBorderEdges()`

to retrieve a vector of `Edge2`

objects.

“You may find it helpful to know that an`Edge2`

object consists of a`Triangle2`

and an`intra-triangle-index`

that identifies the edge. The edge is always counterclockwise oriented around its own triangle.”

The `Zone2::getBorderEdges()`

command returns edges of one or more contours in arbitrary order. Sometimes you’ll want to turn such an edge-soup into a set of one or more polygons. And you can do that using the `edgesToPolygons()`

function, see below:

// * 7 * Retrieve the (oriented but unordered) border edges of // pZoneSymmetricDifference vector<Edge2> vBorders; pZoneSymmetricDifference->getBorderEdges(vBorders); Visualizer2 v("example5_symDiffBorders.ps"); dt.show(&v,false); // show only the triangles v.addObject(vBorders,Color(CGREEN,0.5,true)); v.writeFile(); // * 8 * Turn the border edges into polygons vector<Edge2> vRemainingEdges; vector<vector<Edge2> > vvEdges; edgesToPolygons(vBorders,vvEdges,vRemainingEdges); // * 9 * Draw Visualizer2 polyVis("example5_polygons.ps"); pZoneSymmetricDifference->show(&polyVis,false,false); // show only the triangles for(size_t poly=0;poly<vvEdges.size();++poly) { const vector<Edge2>& vPolygon(vvEdges[poly]); Color c(Color::getNextColorName(),1.0,false); // Next color from a (repeating) sequence for(size_t j=0;j<vPolygon.size();++j) { const Edge2& edge(vPolygon[j]); polyVis.addObject(edge,c); Point2* pSourcePoint(edge.getSrc()); Label lab(*pSourcePoint,toString(j),true,16); polyVis.addObject(lab,c); } } polyVis.writeFile();

“Above it was said that Edge2 objects are oriented counterclockwise with respect to their own triangle. Observe that this results in the outer polygon in this example being oriented counterclockwise, while the inner one is oriented clockwise. This is sometimes a useful property!”

This post has shown how to combine polygonal areas through set operations and how to extract the boundary polygons of the resulting area. But Zone2 objects can also be defined in order to create high quality triangular meshes, see Example7 – Mesh Generation.

## 6 replies on “Polygon Clipping, Boolean Operations – Example5”

Good day. My name is Boris, I am a student from Moscow Institute of Physics and Technology. It would be really cool if you could help me with the following situation:

In your example above you define coordinates of points of one polygon like coordinates of points of another polygon plus 90 and as a result, we have two intersecting polygons where all intersecting segments intersect only at one point. And everything is going fine. But if I change 90 to 60 everything is change and two leftmost segments start to intersect in infinite amount of points. Triangulation is still going fine but problem occurs when I try to call getBorderEdges() method of an pZoneUnion object. It writes values to std::vector in a wrong order and resulting figure doesn’t have any similarity with one I am waiting for. So the question is: Did I miss some functionality to solve this situation? And if I didn’t is there some way to solve it?

Dear Boris,

The documentation of getBorderEdges() says that the edges are CCW oriented, i.e. they point in counterclockwise direction but they are NOT necessarily ordered. I admit, this is misleading. Next week a new version will be released and then this issue will be solved. However, visualization works when you add this code to example5.cpp:

It shows the the correct border of the union of the two shapes. If you need the segments in the right order, please use the sortRing function from freeFunctions.h.

hth

Dear Bernhard,

thank you! Your answer helped me very much. Now I have another problem and I try to find a way of solving it. I want to find a union of two nonconvex polygons and it’s possible that some number of holes will appear inside this union. Awesome getBoundarySegments() method in combination with sortRing() method properly return outer circuit. But is there some way to find circuits of holes inside this union?

Thank you,

Boris

Dear Boris,

Yes. A new release was due anyway and I have added that feature. Please download Fade2D v1.58. Extract the edges of the union zone using Zone2::getBoundaryEdges() and then use the edgesToPolygons() function to organize these edges as polygons. I have added demo code to example5.cpp, see also the description above (added right now) and the Documentation of edgesToPolygons

Hi,

My name is Bastien and I am currently working on a 3D engine. I’m interested in adding Boolean operations in it. I tried to do a simple case by reusing your example5, but I’m told that my shape is not a closed polygon. I really don’t understand why because it’s a simple square. Am I doing something wrong ?

Here is my code :

Fade_2D dt;

// * 2 * Prepare two vectors of Segments

std::vector vSegments1;

// square

Point2 p0(0,0);

Point2 p1(2,0);

Point2 p2(2,2);

Point2 p3(0,2);

vSegments1.push_back(Segment2(p0,p1));

vSegments1.push_back(Segment2(p1,p2));

vSegments1.push_back(Segment2(p2,p3));

vSegments1.push_back(Segment2(p3,p1));

// inner hole

std::vector vSegments2;

Point2 h0(0.5,0.5);

Point2 h1(1.5,0.5);

Point2 h2(1.5,1.5);

Point2 h3(0.5,1.5);

vSegments2.push_back(Segment2(h0,h1));

vSegments2.push_back(Segment2(h1,h2));

vSegments2.push_back(Segment2(h2,h3));

vSegments2.push_back(Segment2(h3,h1));

// * 3 * Use the segments to create two ConstraintGraphs. Then show what we have

ConstraintGraph2* pCG1=dt.createConstraint(vSegments1,CIS_CONSTRAINED_DELAUNAY);

ConstraintGraph2* pCG2=dt.createConstraint(vSegments2,CIS_CONSTRAINED_DELAUNAY);

dt.show(“example5_constraints.ps”);

// Make sure pCG1 and pCG2 are closed

if(!pCG1->isPolygon() || !pCG2->isPolygon() )

{

std::cout<<"pCG1 and pCG2 must be closed polygons, stop"<show(“example5_zone0.ps”,true,true); // all triangles=true, show constraints=true

Hi Bastien,

this is not a polygon:

vSegments1.push_back(Segment2(p0,p1));

vSegments1.push_back(Segment2(p1,p2));

vSegments1.push_back(Segment2(p2,p3));

vSegments1.push_back(Segment2(p3,p1));

The last line should contain a segment from p3 to p0.

Best regards

Bernhard