Zones are among the most powerful concepts in Fade. A zone defines an area in a (Constrained) Delaunay triangulation. Zones can be combined using boolean set operations such as union and difference, a quality mesh can be constructed inside a zone and the triangles of a zone can be extracted. Zones can be defined in several ways:

- Area inside (outside) a polygon
- Area grown from a seed point
- Area of the whole triangulation (global zone)
- Area of arbitrary triangles

The code examples below demonstrate that.

## Preparing the initial situation

Two polygons (ConstraintGraph2 objects) are made, then applyConstraintsAndZones() is called and the triangulation is drawn.

Fade_2D dt; dt.insert(Point2(-100,-100)); dt.insert(Point2(+100,-100)); dt.insert(Point2(+100,+270)); dt.insert(Point2(-100,+270)); // 2) Prepare and insert two constraint graphs std::vector<Point2> vConstraintPoints; vConstraintPoints.push_back(Point2(80,0)); vConstraintPoints.push_back(Point2(49.8,62.5)); vConstraintPoints.push_back(Point2(-17.8,78)); vConstraintPoints.push_back(Point2(-72,34.7)); vConstraintPoints.push_back(Point2(-72,-34.7)); vConstraintPoints.push_back(Point2(-17.8,-78)); vConstraintPoints.push_back(Point2(49.8,-62.5)); std::vector<Segment2> vSegments1; std::vector<Segment2> vSegments2; for(size_t i=0;i<vConstraintPoints.size();++i) { Point2& p0(vConstraintPoints[i]); Point2& p1(vConstraintPoints[(i+1)%vConstraintPoints.size()]); vSegments1.push_back(Segment2(p0,p1)); Point2 p0a(p0.x(),p0.y()+170); Point2 p1a(p1.x(),p1.y()+170); vSegments2.push_back(Segment2(p0a,p1a)); } ConstraintGraph2* pCG1(NULL); ConstraintGraph2* pCG2(NULL); pCG1=dt.createConstraint(vSegments1,CIS_CONSTRAINED_DELAUNAY); pCG2=dt.createConstraint(vSegments2,CIS_CONSTRAINED_DELAUNAY); // 3) Apply and show the triangulation and constraints dt.applyConstraintsAndZones(); dt.show("example4_constraints.ps"); // Verify that the ConstraintGraph2 objects pCG1 and pCG2 are closed if(!pCG1->isPolygon() || !pCG2->isPolygon() ) { cout<<"pCG1 and pCG2 must be closed polygons"<<endl; return 1; }

## Creating an inside and outside zone

ConstraintGraph pCG1 is the lower polygon in the above image. The two code lines below code create one zone inside this circle and one zone outside.

Zone2* pZoneInside(dt.createZone(pCG1,ZL_INSIDE)); Zone2* pZoneOutside(dt.createZone(pCG1,ZL_OUTSIDE));

## Zone growing from a seed point

The code below uses **vCG** (a vector of polygons) and a **seedPoint** near the lower left corner of the below image. Then it grows a zone from this **seedPoint**. The edges of the polygons in **vCG** act as fence which stops the growing process.

vector<ConstraintGraph2*> vCG; vCG.push_back(pCG1); vCG.push_back(pCG2); Point2 seedPoint(-99,-99); // near the lower left corner Zone2* pZoneGrow(dt.createZone(vCG,ZL_GROW,seedPoint));

## The global zone

The global zone is a zone consisting of all existing triangles.

Zone2* pZoneGlobal(dt.createZone(NULL,ZL_GLOBAL));

## Zone creation from arbitrary triangles

A zone can be created from an arbitrary set of triangles. This set does not need to be connected. The example code below first fetches all triangles from the triangulation. Then it chooses a random sample of them and uses it to create a zone.

vector<Triangle2*> vT; dt.getTrianglePointers(vT); random_shuffle(vT.begin(),vT.end()); vT.resize(vT.size()/2); Zone2* pZoneRandomTriangles(dt.createZone(vT));

This article covered creation of zones. The next post Zone Operations shows how the union, the intersection, the difference and symmetric difference of two zones can be computed.