[appl] add example with polygons

appl/CMakeLists.txt

 add_subdirectory(example1)
 add_subdirectory(example2)
 add_subdirectory(example3)
+add_subdirectory(example4)

appl/example4/CMakeLists.txt

+frackit_add_application(NAME example4
+                        SOURCES example4.cc
+                        LABELS example)
+
+set(CMAKE_BUILD_TYPE Release)

appl/example4/README.md

+<!--- Example picture --->
+<p align="center">
+    <img src="../../doc/img/example4_overview.png" alt="frackit example 4" width="100%"/>
+</p>
+
+Example 4
+=========
+
+__As in the previous examples, this description focuses on the C++ implementation
+given in the main file `example4.cc`, but in `example4.py` you can find how to
+realize this example using the Frackit python bindings.__
+
+<b> _In this example you will learn how to:_ </b>
+
+* create a domain geometry by intersection/cut operations on basic geometry types
+* generate random polygons using the `PolygonSampler` class
+* design your own incremental network generation algorithm
+
+Let us consider some idealized, hexahedral geological layer with an embedded
+tunnel structure. The excavation of tunnels can have an effect on the stress
+state in the surrounding rock and can lead to the generation of fractures, in
+particular in the vicinity of the tunnels. This example aims at an illustration
+of how such situations can modeled using `Frackit`.
+
+### Step 1: domain generation
+
+We want to focus on a spherical region around the center of the hexahedral
+layer. To this end, we construct instances of the `Box` and `Sphere` classes
+to represent these two geometries:
+
+```cpp
+// the layer has a thickness of 40m, and initially we consider 100m lateral extent
+const Box layer(-50, -50, -20.0, 50, 50, 20.0);
+const Sphere domainSphere(Point(0.0, 0.0, 0.0), /*radius*/25.0);
+
+// a tunnel with a radius of 5m crosses the domain
+const ctype tunnelRadius = 5.0;
+const Direction tunnelDirection(Vector(1.0, 0.0, 0.0));
+const Circle tunnelBase(Point(-25.0, 0.0, 0.0), tunnelDirection, tunnelRadius);
+const Cylinder tunnel(tunnelBase, /*height*/50.0);
+```
+
+To construct the domain in which we want to place the entities of our network,
+we use the `intersect` function in the `OCCUtilities` namespace to
+compute the geometry of the volume that is the common part of `domainSphere` and `layer`.
+The OpenCascade algorithms work on OpenCascade shapes, and therefore, we convert
+the internal representations into shapes when passing them to the algorithm:
+
+```cpp
+const auto layerSphereCut = OCCUtilities::intersect(OCCUtilities::getShape(layer),
+                                                    OCCUtilities::getShape(domainSphere),
+                                                    /*eps*/1e-6);
+auto solids = OCCUtilities::getSolids(layerSphereCut);
+if (solids.size() != 1) throw std::runtime_error("Intersection operation with domainSphere should yield a single solid");
+```
+
+The result of `intersect` is a generic shape object. However, here we know that the
+result is a single three-dimensional volume, and thus, we obtain the solid shapes
+of the intersection result, of which we know there should only be a single one.
+The function `getSolids` returns a vector of all solids contained in a shape, and
+here we check if the result contains a single one, as is expected. Subsequently,
+we cut the tunnel out of the intersection result (with the `cut` function in the
+`OCCUtilities` namespace) in order to obtain the actual model domain:
+
+```cpp
+const auto tunnelCut = OCCUtilities::cut(solids[0], OCCUtilities::getShape(tunnel), /*eps*/1e-6);
+solids = OCCUtilities::getSolids(tunnelCut);
+if (solids.size() != 1) throw std::runtime_error("Cut operation with tunnel should yield a single solid");
+
+// get the domain shape
+const auto& domain = solids[0];
+```
+
+## Step 2: entity sampler definitions
+
+Instead of randomly sample entities within the domain volume, in this example we
+want to use a different approach. Here, the goal is to generate a network in the
+vicinity of the tunnel. To this end, we use a three-stage algorithm to construct
+the network, where in each stage we generate entities of a different orientation.
+We use polygonal entities, and at this point we define a convenience lambda that,
+provided a point sampler, returns an instance of `PolygonSampler` of the given
+orientation:
+
+```cpp
+// we use the default sampler types, thus, default distributions (see traits classes)
+// This means, normal distributions for angles and uniform distributions for the size
+using PolygonSampler = Frackit::PolygonSampler</*spaceDimension*/3, ctype>;
+
+// lambda function to construct polygon sampler with given point sampler & orientation
+auto getSampler = [] (const auto& pointSampler, unsigned int orientation) -> PolygonSampler
+{
+    using NormalDistro = std::normal_distribution<ctype>;
+    auto sizeDistro = std::uniform_real_distribution<ctype>(3.0, 6.0);
+    auto numCornerDistro = std::uniform_int_distribution<int>(4, 9);
+
+    switch (orientation)
+    {
+        case 1:
+            return PolygonSampler(pointSampler,                                   // sampler for polygon centers
+                                  NormalDistro(toRadians(15.0), toRadians(10.0)), // strike angle: mean value & standard deviation
+                                  NormalDistro(toRadians(90.0), toRadians(10.0)), // dip angle: mean value & standard deviation
+                                  /*strikeLength/*/sizeDistro, /*dipLength*/sizeDistro, numCornerDistro);
+        case 2:
+            return PolygonSampler(pointSampler,                                  // sampler for polygon centers
+                                  NormalDistro(toRadians(0.0), toRadians(10.0)), // strike angle: mean value & standard deviation
+                                  NormalDistro(toRadians(0.0), toRadians(10.0)), // dip angle: mean value & standard deviation
+                                  /*strikeLength/*/sizeDistro, /*dipLength*/sizeDistro, numCornerDistro);
+        case 3:
+            return PolygonSampler(pointSampler,                                    // sampler for polygon centers
+                                  NormalDistro(toRadians(-75.0), toRadians(10.0)), // strike angle: mean value & standard deviation
+                                  NormalDistro(toRadians(90.0), toRadians(10.0)),  // dip angle: mean value & standard deviation
+                                  /*strikeLength/*/sizeDistro, /*dipLength*/sizeDistro, numCornerDistro);
+        default:
+            throw std::runtime_error("Unsupported orientation index");
+    }
+};
+```
+
+As can be seen, an instance of the `PolygonSampler` class is constructed with
+statistical distributions on the strike and dip angles (polygon orientations),
+the size of the polygon in strike and dip direction (polygon sizes) and the
+number of corners. We use the default distribution types here, which means
+normal distributions for the orientation angles and uniform distributions for
+the sizes and number of corners. As for the other samplers, one can choose
+different distributions by implementing and passing a traits class as template
+argument to `PolygonSampler`. For more details we refer to the corrensponding
+[class documentation](https://git.iws.uni-stuttgart.de/tools/frackit/tree/master/frackit/sampling/polygonsampler.hh)
+
+### Step 3: constraints definitions
+
+The first three examples already explained how to define and use the `Constraints`
+class of `Frackit`. As in those examples, here we also use several instances of
+that class to define constraints to be fulfilled between entities of different
+orientations, or between entities and the domain boundary. For more details on
+this, please look at the previous examples. Here, we want to focus on additional
+custom constraints that we would like to be fulfilled. That is, we want to ensure
+that the intersections between different entities do not produce very small length
+scales on the polygon boundaries as a result of being very close to a polygon
+corner. To this end, we define the following lambda function to be used during
+network generation:
+
+```cpp
+// When polygons intersect, the intersection might be very close to
+// one of the corner points. We want to avoid small length scales caused
+// by this, and this lambda provides the check for it.
+auto producesSmallLengthScale = [] (const auto& geometry, const auto& is) -> bool
+{
+    // lambda to check if the intersection is too close (< 1mm) to a vertex of the geometry
+    auto isTooClose = [&is] (const auto& v) { return computeDistance(v, is) < 1e-3; };
+
+    const auto shape = OCCUtilities::getShape(geometry);
+    const auto vertices = OCCUtilities::getVertices(shape);
+    return std::any_of(vertices.begin(), vertices.end(), isTooClose);
+};
+```
+
+The lambda `producesSmallLengthScale` checks if the intersection `is` is less than
+1 mm away from any of the vertices of the given geometry. Note that in this lambda
+we again operate on shapes rather than the internal representation of polygons.
+The reason for this is that the above implementation is more generic and works
+for any given geometry type. In this example, it is called both with polygons
+as well as OpenCascade shapes.
+
+### Step 4: entity sampling
+
+As mentioned before, here we want to construct the network in three stages:
+
+* Stage 1: generate entities of orientation 1 close to the tunnel, each entity must intersect the tunnel
+* Stage 2: for each entity `e1` of stage 1, generate an entity of orientation 2 that intersects `e1`
+* Stage 3: for each entity `e2` of stage 2, generate an entity of orientation 3 that intersects `e2`
+
+This ensures that all entities of the network are connected to at least one other,
+and that the overall network remains in the vicinity of the tunnel.
+
+__Stage 1__: In order to efficiently sample entities that intersect the tunnel,
+we search for new candidates in a small region around it. Recall that our lambda
+function `getSampler` expects a point sampler to be passed, which is used to sample
+the spatial location of a new polygonal entity candidate. In this first stage,
+we use a point sampler that uniformly samples points within a hollow cylinder
+around the tunnel. This is done in following two lines of code.
+
+```cpp
+const HollowCylinder sampleCylinder1(tunnel.bottomFace().center(),
+                                     tunnel.direction(),
+                                     tunnel.radius(),       // inner radius
+                                     tunnel.radius() + 2.0, // outer radius
+                                     tunnel.height());
+auto polygonSampler1 = getSampler(makeUniformPointSampler(sampleCylinder1), /*orientationIdx*/1);
+```
+
+Subsequently, `polygonSampler1` is used to get entity candidates until the desired
+number of entities has been accepted, checking each entity against a number of
+constraints. In particular, entities that do not intersect the tunnel are
+rejected immediately.
+
+__Stage 2__: For each entity `e` of stage 1, we want to generate an entity of orientation 2
+that intersects `e`. Therefore, we loop over all entities of stage 1 and sample new
+candidates in their vicinity. This is achieved by computing the bounding box of `e`,
+which is then used to define a region in which new candidates are sampled:
+
+```cpp
+for (const auto& e : entitiesSet1)
+{
+    using std::sqrt;
+
+    // sample an entity of orientation 2 in the vicinity of e
+    const auto rawBBox = getBoundingBox(e);
+    const auto charLength = sqrt(computeMagnitude(e)); // characteristic length of e
+    const Box sampleBox(rawBBox.xMin()-charLength, rawBBox.yMin()-charLength, rawBBox.zMin()-charLength,
+                        rawBBox.xMax()+charLength, rawBBox.yMax()+charLength, rawBBox.zMax()+charLength);
+
+    auto sampler = getSampler(makeUniformPointSampler(sampleBox), /*orientationIdx*/2);
+```
+
+The variable `sampler` now holds an instance of the `PolygonSampler` class, which generates
+polygons of orientation 2 within the box `sampleBox` that describes the vicinity of `e`.
+We then sample candidates until we have found an entity that fulfills all desired constraints.
+After that, it is continued with the next entity of `entitiesSet1` until all have been visited.
+
+
+__Stage 3__: this stage follows the same logic as stage 2, but it samples entities of
+orientation 3 and looks for entity candidates in the vicinity of the entities of stage 2:
+
+```cpp
+for (const auto& e : entitiesSet2)
+{
+    using std::sqrt;
+
+    // sample an entity of orientation 2 in the vicinity of e
+    const auto rawBBox = getBoundingBox(e);
+    const auto charLength = sqrt(computeMagnitude(e));
+    const Box sampleBox(rawBBox.xMin()-charLength, rawBBox.yMin()-charLength, rawBBox.zMin()-charLength,
+                        rawBBox.xMax()+charLength, rawBBox.yMax()+charLength, rawBBox.zMax()+charLength);
+
+    auto sampler = getSampler(makeUniformPointSampler(sampleBox), /*orientationIdx*/3);
+```
+
+Note that due to the restrictive constraints used in this example, the entity
+generation step can take up to a few minutes.
+
+### Step 5: network generation
+
+As in the previous example, we use the `ContainedEntityNetworkBuilder` class
+to construct the network and embed it in our domain:
+
+```cpp
+// construct and write a contained network, i.e. write out both network and domain.
+std::cout << "Building and writing network" << std::endl;
+ContainedEntityNetworkBuilder<ctype> builder;
+
+// add sub-domains
+builder.addConfiningSubDomain(domain,     Id(1));
+builder.addSubDomainEntities(entitiesSet1, Id(1));
+builder.addSubDomainEntities(entitiesSet2, Id(1));
+builder.addSubDomainEntities(entitiesSet3, Id(1));
+```
+
+Here, we give the domain the `Id` "1" and define all entities to be embedded within it.
+The following lines of code then perform the actual construction of the network
+in the call `builer.build()`, and write out both the network and the domain to
+Gmsh file format:
+
+```cpp
+// now we can build and write out the network in Gmsh file format
+GmshWriter gmshWriter(builder.build());
+gmshWriter.write("network", /*sizeAtEntities*/0.5, /*sizeInDomain*/5.0);
+```

appl/example4/example4.py

+import sys
+from time import process_time
+startTime = process_time()
+
+# print welcome message
+print("\n\n"
+      "#########################################################################\n"
+      "## Example 4: Creation of polygonal entities around a tunnel structure ##\n"
+      "#########################################################################\n"
+      "\n\n")
+
+###########################
+# 1.Construct the domain ##
+###########################
+
+# In this example, we consider a fraction of a geological layer
+# around a tunnel structure. To this end, we construct the geological
+# layer as box, and intersect it with a sphere in order to restrict
+# the model domain to a smaller region.
+from frackit.geometry import Box, Sphere, Cylinder, Direction, Vector, Circle, Point
+
+# the layer has a thickness of 40m, and initially we consider 100m lateral extent
+layer = Box(-50.0, -50.0, -20.0, 50.0, 50.0, 20.0)
+domainSphere = Sphere(Point(0.0, 0.0, 0.0), 25.0)
+
+# a tunnel with a radius of 5m crosses the domain
+tunnelRadius = 5.0
+tunnelDirection = Direction(Vector(1.0, 0.0, 0.0))
+tunnelBase = Circle(Point(-25.0, 0.0, 0.0), tunnelDirection, tunnelRadius)
+tunnel = Cylinder(tunnelBase, 50.0)
+
+# the domain is the layer constrained to the domainSphere and cut by the tunnel
+from frackit.occutilities import getShape, getSolids, getFaces, cut
+from frackit.occutilities import intersect as intersectShapes
+solids = getSolids( intersectShapes(getShape(layer), getShape(domainSphere), 1e-6) )
+if len(solids) != 1: sys.exit("Intersection operation with domainSphere should yield a single solid")
+
+solids = getSolids( cut(solids[0], getShape(tunnel), 1e-6) )
+if len(solids) != 1: sys.exit("Cut operation with tunnel should yield a single solid")
+
+domain = solids[0]
+tunnelSurfaceShape = getShape(tunnel.lateralFace())
+sphereSurfaceShape = getFaces(getShape(domainSphere))
+if len(sphereSurfaceShape) != 1: sys.exit("Sphere is expected to contain a single face")
+sphereSurfaceShape = sphereSurfaceShape[0]
+
+########################################################################
+## 2. Make sampler classes to randomly sample points (entity centers) ##
+##    and polygons as entities (using the sampled center points)      ##
+########################################################################
+
+# creates a uniform sampler within an interval
+import random
+def uniformSampler(a, b):
+    def sample(): return random.uniform(a, b)
+    return sample
+
+# creates a sampler from a normal distribution with the given mean and std deviation
+def normalSampler(mean, stdDev):
+    def sample(): return random.gauss(mean, stdDev)
+    return sample
+
+# creates a sampler from an integer uniform distribution
+def choiceSampler(a, b):
+    def sample(): return random.choice([v for v in range(a, b+1)])
+    return sample
+
+# function to create polygon samplers for the different orientations
+from frackit.common import toRadians
+from frackit.sampling import PolygonSampler
+
+sizeDistro = uniformSampler(3.0, 6.0)
+numCornersDistro = choiceSampler(4, 9)
+
+def getSampler(pointSampler, orientation):
+    if orientation == 1:
+        return PolygonSampler(pointSampler,                                    # sampler for polygon centers
+                              normalSampler(toRadians(15.0), toRadians(10.0)), # strike angle: mean value & standard deviation
+                              normalSampler(toRadians(90.0), toRadians(10.0)), # dip angle: mean value & standard deviation
+                              sizeDistro, sizeDistro, numCornersDistro)        # strike & dip length, number of corners
+    elif orientation == 2:
+        return PolygonSampler(pointSampler,                                   # sampler for polygon centers
+                              normalSampler(toRadians(0.0), toRadians(10.0)), # strike angle: mean value & standard deviation
+                              normalSampler(toRadians(0.0), toRadians(10.0)), # dip angle: mean value & standard deviation
+                              sizeDistro, sizeDistro, numCornersDistro)       # strike & dip length, number of corners
+    elif orientation == 3:
+        return PolygonSampler(pointSampler,                                     # sampler for polygon centers
+                              normalSampler(toRadians(-75.0), toRadians(10.0)), # strike angle: mean value & standard deviation
+                              normalSampler(toRadians(90.0), toRadians(10.0)),  # dip angle: mean value & standard deviation
+                              sizeDistro, sizeDistro, numCornersDistro)         # strike & dip length, number of corners
+    else:
+        import sys
+        sys.exit("Unsupported orientation index")
+
+
+#######################################################################
+## 3. Define constraints that should be fulfilled among the entities ##
+##    of different orientations.                                     ##
+#######################################################################
+
+from frackit.entitynetwork import EntityNetworkConstraints, EntityNetworkConstraintsMatrix
+
+# constructs default constraints for this test to be modified after
+def makeDefaultConstraints():
+    c = EntityNetworkConstraints()
+    c.setMinDistance(1.0)
+    c.setMinIntersectingAngle(toRadians(25.0))
+    c.setMinIntersectionMagnitude(0.1)
+    c.setMinIntersectionDistance(0.1)
+    return c
+
+# Define constraints between entities of orientation 1
+constraints1 = makeDefaultConstraints()
+
+# Define constraints between entities of orientation 2
+# we want to enforce larger spacing between those entities
+constraints2 = makeDefaultConstraints()
+constraints2.setMinDistance(0.5)
+
+# Define constraints between entities of orientation 3
+constraints3 = makeDefaultConstraints()
+constraints3.setMinDistance(0.15)
+
+# Define constraints between entities of different sets
+constraintsOnOther = makeDefaultConstraints()
+constraintsOnOther.setMinDistance(0.25)
+constraintsOnOther.setMinIntersectingAngle(toRadians(30.0))
+
+# Moreover, we define constraints w.r.t. the domain boundary
+constraintsOnBoundary = makeDefaultConstraints()
+constraintsOnBoundary.setMinIntersectingAngle(toRadians(10.0))
+
+# When polygons intersect, the intersection might be very close to
+# one of the corner points. We want to avoid small length scales caused
+# by this, and this lambda provides the check for it.
+def producesSmallLengthScale(geometry, isection):
+
+    from frackit.geometry import computeDistance
+    from frackit.occutilities import getVertices
+    verts = getVertices(getShape(geometry))
+    if any((computeDistance(v, isection) < 1e-3 for v in verts)): return True
+    return False
+
+###########################
+## 4. Network generation ##
+###########################
+
+# dictionary to store entities of the three orientations in
+from frackit.common import Id
+setId1 = Id(1); setId2 = Id(2); setId3 = Id(3);
+entitySets = {setId1: [], setId2: [], setId3: []}
+
+# In a first step, we want to populate entities of orientation 1 around
+# the tunnel, and require each entity to be intersecting with it. To this end,
+# we sample center points within a hollow cylinder around the tunnel.
+from frackit.sampling import SamplingStatus
+from frackit.geometry import HollowCylinder
+numTargetEntities = 75
+status = SamplingStatus()
+status.setTargetCount(setId1, numTargetEntities)
+sampleCylinder1 = HollowCylinder(tunnel.bottomFace().center(), tunnel.direction(),
+                                 tunnel.radius(), tunnel.radius() + 2.0, tunnel.height())
+
+from frackit.geometry import computeMagnitude, getBoundingBox
+from frackit.geometry import intersect
+from frackit.occutilities import getFaces
+from frackit.sampling import makeUniformPointSampler
+print('## Step 1: generation of entities of orientation 1 around the tunnel\n\n\n')
+
+area = 0.0;
+polygonSampler1 = getSampler(makeUniformPointSampler(sampleCylinder1), 1)
+while not status.finished():
+    candidate = polygonSampler1()
+    isection = intersect(tunnel.lateralFace(), candidate)
+
+    # skip entities that don't intersect the tunnel
+    if all( "Empty" in g.name() for g in isection ):
+        status.increaseRejectedCounter(); continue;
+
+    # avoid small length scales produced by the intersections
+    if producesSmallLengthScale(candidate, isection):
+         status.increaseRejectedCounter(); continue;
+
+    # compute the part contained in the domain
+    containedShape = intersectShapes(getShape(candidate), domain, 1e-6)
+    containedFaces = getFaces(containedShape)
+
+    # skip everything that does not lead to a single contained face
+    if len(containedFaces) != 1: status.increaseRejectedCounter(); continue;
+
+    # if too little of the candidate is inside the domain, reject it
+    containedFace = containedFaces[0];
+    containedFaceArea = computeMagnitude(containedFace);
+    if containedFaceArea < 0.25*candidate.area():
+        status.increaseRejectedCounter(); continue;
+
+    # check constraints to other entities and domain boundary
+    if not constraints1.evaluate(entitySets[setId1], containedFace):
+        status.increaseRejectedCounter(); continue;
+    if not constraintsOnBoundary.evaluate(tunnelSurfaceShape, containedFace):
+        status.increaseRejectedCounter(); continue;
+    if not constraintsOnBoundary.evaluate(sphereSurfaceShape, containedFace):
+        status.increaseRejectedCounter(); continue;
+
+    area += containedFaceArea
+    entitySets[setId1].append(containedFace)
+    status.increaseCounter(setId1)
+    status.print()
+
+print('\n\n\n## Step 2: for each entity so far, try to generate an intersecting entity with orientation 2\n\n\n')
+
+# after rejecting 500 candidates for an entity, we move to the next one
+maxTriesForEntity = 500;
+def printSkipMessage ():
+    print('\t\t -- Skipped search for this entity after ' + \
+          str(maxTriesForEntity) + ' unsuccessful tries. --\n')
+
+from math import sqrt
+status.reset()
+status.setTargetCount(setId2, len(entitySets[setId1]))
+for e in entitySets[setId1]:
+
+    # sample an entity of orientation 2 in the vicinity of e
+    rawBBox = getBoundingBox(e);
+    charLength = sqrt(computeMagnitude(e));
+    sampleBox = Box(rawBBox.xMin()-charLength, rawBBox.yMin()-charLength, rawBBox.zMin()-charLength,
+                    rawBBox.xMax()+charLength, rawBBox.yMax()+charLength, rawBBox.zMax()+charLength);
+
+    tryCount = 0;
+    accepted = False;
+    sampler = getSampler(makeUniformPointSampler(sampleBox), 2);
+
+    while not accepted and tryCount <= maxTriesForEntity:
+        tryCount += 1
+        candidate = sampler()
+        isection = intersect(candidate, e)
+
+        # we only want intersecting entities
+        if all( "Empty" in g.name() for g in isection ):
+            status.increaseRejectedCounter(); continue;
+
+        # avoid small length scales produced by the intersections
+        if producesSmallLengthScale(candidate, isection):
+             status.increaseRejectedCounter(); continue;
+        if producesSmallLengthScale(e, isection):
+             status.increaseRejectedCounter(); continue;
+
+        # compute the part contained in the domain
+        containedShape = intersectShapes(getShape(candidate), domain, 1e-6)
+        containedFaces = getFaces(containedShape)
+
+        # skip everything that does not lead to a single contained face
+        if len(containedFaces) != 1: status.increaseRejectedCounter(); continue;
+
+        # if too little of the candidate is inside the domain, reject it
+        containedFace = containedFaces[0];
+        containedFaceArea = computeMagnitude(containedFace);
+        if containedFaceArea < 0.25*candidate.area():
+            status.increaseRejectedCounter(); continue;
+
+        # check constraints to other entities and domain boundary
+        if not constraintsOnOther.evaluate(entitySets[setId1], containedFace):
+            status.increaseRejectedCounter(); continue;
+        if not constraints2.evaluate(entitySets[setId2], containedFace):
+            status.increaseRejectedCounter(); continue;
+        if not constraintsOnBoundary.evaluate(sphereSurfaceShape, containedFace):
+            status.increaseRejectedCounter(); continue;
+        if not constraintsOnBoundary.evaluate(tunnelSurfaceShape, containedFace):
+            status.increaseRejectedCounter(); continue;
+
+        accepted = True;
+        area += containedFaceArea
+        entitySets[setId2].append(containedFace)
+        status.increaseCounter(setId2)
+        status.print()
+
+    if tryCount >= maxTriesForEntity: printSkipMessage()
+
+
+print('\n\n\n## Step 3: for each entity of orientation 2, try to generate an intersecting entity with orientation 3\n\n\n')
+status.reset()
+status.setTargetCount(setId3, len(entitySets[setId2]))
+
+for e  in entitySets[setId2]:
+
+    # sample an entity of orientation 2 in the vicinity of e
+    rawBBox = getBoundingBox(e);
+    charLength = sqrt(computeMagnitude(e));
+    sampleBox = Box(rawBBox.xMin()-charLength, rawBBox.yMin()-charLength, rawBBox.zMin()-charLength,
+                    rawBBox.xMax()+charLength, rawBBox.yMax()+charLength, rawBBox.zMax()+charLength);
+
+    tryCount = 0;
+    accepted = False;
+    sampler = getSampler(makeUniformPointSampler(sampleBox), 3);
+
+    while not accepted and tryCount <= maxTriesForEntity:
+        tryCount += 1
+        candidate = sampler()
+        isection = intersect(candidate, e)
+
+        # we only want intersecting entities
+        if all( "Empty" in g.name() for g in isection ):
+            status.increaseRejectedCounter(); continue;
+
+        # avoid small length scales produced by the intersections
+        if producesSmallLengthScale(candidate, isection):
+             status.increaseRejectedCounter(); continue;
+        if producesSmallLengthScale(e, isection):
+             status.increaseRejectedCounter(); continue;
+
+        # compute the part contained in the domain
+        containedShape = intersectShapes(getShape(candidate), domain, 1e-6)
+        containedFaces = getFaces(containedShape)
+
+        # skip everything that does not lead to a single contained face
+        if len(containedFaces) != 1: status.increaseRejectedCounter(); continue;
+
+        # if too little of the candidate is inside the domain, reject it
+        containedFace = containedFaces[0];
+        containedFaceArea = computeMagnitude(containedFace);
+        if containedFaceArea < 0.25*candidate.area():
+            status.increaseRejectedCounter(); continue;
+
+        # check constraints to other entities and domain boundary
+        if not constraintsOnOther.evaluate(entitySets[setId1], containedFace):
+            status.increaseRejectedCounter(); continue;
+        if not constraintsOnOther.evaluate(entitySets[setId2], containedFace):
+            status.increaseRejectedCounter(); continue;