The first point in 3D is used to propagate Z dimension

python/core/geometry/qgsgeometryutils.sip.in

@@ -388,6 +388,17 @@ Return the coefficients (a, b, c for equation "ax + by + c = 0") of a line defin
 :return: A line (segment) from p to perpendicular point on segment [s1, s2]
 %End
 
+    static bool setZValueFromPoints( const QgsPointSequence &points, QgsPoint &point );
+%Docstring
+A Z dimension is added to ``point`` if one of the point in the list
+``points`` is in 3D. Moreover, the Z value of ``point`` is updated with.
+
+:param points: List of points in which a 3D point is searched.
+:param point: The point to update with Z dimension and value.
+
+:return: true if the point is updated, false otherwise
+%End
+
 
 };
 

src/core/geometry/qgscircle.cpp

@@ -40,6 +40,8 @@ QgsCircle QgsCircle::from2Points( const QgsPoint &pt1, const QgsPoint &pt2 )
   double azimuth = QgsGeometryUtils::lineAngle( pt1.x(), pt1.y(), pt2.x(), pt2.y() ) * 180.0 / M_PI;
   double radius = pt1.distance( pt2 ) / 2.0;
 
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1 << pt2, center );
+
   return QgsCircle( center, radius, azimuth );
 }
 
@@ -138,26 +140,8 @@ QgsCircle QgsCircle::from3Points( const QgsPoint &pt1, const QgsPoint &pt2, cons
   double aSlope = yDelta_a / xDelta_a;
   double bSlope = yDelta_b / xDelta_b;
 
-  // set z cooridnate for center
-  double z = std::numeric_limits<double>::quiet_NaN();
-  if ( p1.is3D() )
-  {
-    z = p1.z();
-  }
-  else if ( p2.is3D() )
-  {
-    z = p2.z();
-  }
-  else if ( p3.is3D() )
-  {
-    z = p3.z();
-  }
-
-  if ( ! std::isnan( z ) )
-  {
-    center.convertTo( QgsWkbTypes::addZ( center.wkbType() ) );
-    center.setZ( z );
-  }
+  // set z coordinate for center
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << p1 << p2 << p3, center );
 
   if ( ( std::fabs( xDelta_a ) <= epsilon ) && ( std::fabs( yDelta_b ) <= epsilon ) )
   {
@@ -197,7 +181,11 @@ QgsCircle QgsCircle::fromCenterDiameter( const QgsPoint &center, double diameter
 QgsCircle QgsCircle::fromCenterPoint( const QgsPoint &center, const QgsPoint &pt1 )
 {
   double azimuth = QgsGeometryUtils::lineAngle( center.x(), center.y(), pt1.x(), pt1.y() ) * 180.0 / M_PI;
-  return QgsCircle( center, center.distance( pt1 ), azimuth );
+
+  QgsPoint centerPt( center );
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1, centerPt );
+
+  return QgsCircle( centerPt, centerPt.distance( pt1 ), azimuth );
 }
 
 QgsCircle QgsCircle::from3Tangents( const QgsPoint &pt1_tg1, const QgsPoint &pt2_tg1, const QgsPoint &pt1_tg2, const QgsPoint &pt2_tg2, const QgsPoint &pt1_tg3, const QgsPoint &pt2_tg3, double epsilon )
@@ -242,7 +230,10 @@ QgsCircle QgsCircle::fromExtent( const QgsPoint &pt1, const QgsPoint &pt2 )
     return QgsCircle();
   }
 
-  return QgsCircle( QgsGeometryUtils::midpoint( pt1, pt2 ), delta_x / 2.0, 0 );
+  QgsPoint center = QgsGeometryUtils::midpoint( pt1, pt2 );
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1 << pt2, center );
+
+  return QgsCircle( center, delta_x / 2.0, 0 );
 }
 
 double QgsCircle::area() const

src/core/geometry/qgsellipse.cpp

@@ -57,6 +57,8 @@ QgsEllipse QgsEllipse::fromFoci( const QgsPoint &pt1, const QgsPoint &pt2, const
   double axis_a = dist / 2.0;
   double axis_b = std::sqrt( std::pow( axis_a, 2.0 ) - std::pow( dist_p1p2 / 2.0, 2.0 ) );
 
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1 << pt2 << pt3, center );
+
   return QgsEllipse( center, axis_a, axis_b, azimuth );
 }
 
@@ -67,6 +69,8 @@ QgsEllipse QgsEllipse::fromExtent( const QgsPoint &pt1, const QgsPoint &pt2 )
   double axis_b = std::fabs( pt2.y() - pt1.y() ) / 2.0;
   double azimuth = 90.0;
 
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1 << pt2, center );
+
   return QgsEllipse( center, axis_a, axis_b, azimuth );
 }
 
@@ -76,7 +80,10 @@ QgsEllipse QgsEllipse::fromCenterPoint( const QgsPoint &center, const QgsPoint &
   double axis_b = std::fabs( pt1.y() - center.y() );
   double azimuth = 90.0;
 
-  return QgsEllipse( center, axis_a, axis_b, azimuth );
+  QgsPoint centerPt( center );
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1, centerPt );
+
+  return QgsEllipse( centerPt, axis_a, axis_b, azimuth );
 }
 
 QgsEllipse QgsEllipse::fromCenter2Points( const QgsPoint &center, const QgsPoint &pt1, const QgsPoint &pt2 )
@@ -88,7 +95,10 @@ QgsEllipse QgsEllipse::fromCenter2Points( const QgsPoint &center, const QgsPoint
   QgsPoint pp = center.project( length, 90 + azimuth );
   double axis_b = center.distance( pp );
 
-  return QgsEllipse( center, axis_a, axis_b, azimuth );
+  QgsPoint centerPt( center );
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << pt1 << pt2, centerPt );
+
+  return QgsEllipse( centerPt, axis_a, axis_b, azimuth );
 }
 
 bool QgsEllipse::operator ==( const QgsEllipse &elp ) const

src/core/geometry/qgsgeometryutils.cpp

@@ -248,22 +248,8 @@ bool QgsGeometryUtils::lineIntersection( const QgsPoint &p1, QgsVector v1, const
 
   intersection = QgsPoint( p1.x() + v1.x() * k, p1.y() + v1.y() * k );
 
-  // z support for inter point
-  double z = std::numeric_limits<double>::quiet_NaN();
-  if ( p1.is3D() )
-  {
-    z = p1.z();
-  }
-  else if ( p2.is3D() )
-  {
-    z = p2.z();
-  }
-
-  if ( ! std::isnan( z ) )
-  {
-    intersection.convertTo( QgsWkbTypes::addZ( intersection.wkbType() ) );
-    intersection.setZ( z );
-  }
+  // z support for intersection point
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << p1 << p2, intersection );
 
   return true;
 }
@@ -623,26 +609,7 @@ bool QgsGeometryUtils::segmentMidPoint( const QgsPoint &p1, const QgsPoint &p2,
   result = possibleMidPoints.at( minDistIndex );
 
   // add z support if necessary
-  double z = std::numeric_limits<double>::quiet_NaN();
-
-  if ( p1.is3D() && p2.is3D() )
-  {
-    z = ( p1.z() + p2.z() ) / 2.;
-  }
-  else if ( p1.is3D() && !p2.is3D() )
-  {
-    z = p1.z();
-  }
-  else if ( !p1.is3D() && p2.is3D() )
-  {
-    z = p2.z();
-  }
-
-  if ( ! std::isnan( z ) )
-  {
-    result.convertTo( QgsWkbTypes::addZ( result.wkbType() ) );
-    result.setZ( z );
-  }
+  QgsGeometryUtils::setZValueFromPoints( QgsPointSequence() << p1 << p2, result );
 
   return true;
 }
@@ -1205,3 +1172,21 @@ double QgsGeometryUtils::averageAngle( double a1, double a2 )
   }
   return normalizedAngle( resultAngle );
 }
+
+bool QgsGeometryUtils::setZValueFromPoints( const QgsPointSequence &points, QgsPoint &point )
+{
+  bool rc = false;
+
+  for ( const QgsPoint &pt : points )
+  {
+    if ( pt.is3D() )
+    {
+      point.convertTo( QgsWkbTypes::addZ( point.wkbType() ) );
+      point.setZ( pt.z() );
+      rc = true;
+      break;
+    }
+  }
+
+  return rc;
+}

src/core/geometry/qgsgeometryutils.h

@@ -410,6 +410,16 @@ class CORE_EXPORT QgsGeometryUtils
      */
     static QgsLineString perpendicularSegment( const QgsPoint &p, const QgsPoint &s1, const QgsPoint &s2 );
 
+    /**
+     * A Z dimension is added to \a point if one of the point in the list
+     * \a points is in 3D. Moreover, the Z value of \a point is updated with.
+     *
+     * \param points List of points in which a 3D point is searched.
+     * \param point The point to update with Z dimension and value.
+     * \returns true if the point is updated, false otherwise
+     */
+    static bool setZValueFromPoints( const QgsPointSequence &points, QgsPoint &point );
+
     //! \note not available in Python bindings
     enum ComponentType SIP_SKIP
     {

src/core/geometry/qgstriangle.cpp

@@ -574,26 +574,11 @@ QgsPoint QgsTriangle::inscribedCenter() const
                l.at( 1 ) * vertexAt( 0 ).y() +
                l.at( 2 ) * vertexAt( 1 ).y() ) / perimeter();
 
-  double z = std::numeric_limits<double>::quiet_NaN();
-  if ( vertexAt( 0 ).is3D() )
-  {
-    z = vertexAt( 0 ).z();
-  }
-  else if ( vertexAt( 1 ).is3D() )
-  {
-    z = vertexAt( 1 ).z();
-  }
-  else if ( vertexAt( 2 ).is3D() )
-  {
-    z = vertexAt( 2 ).z();
-  }
-
   QgsPoint center( x, y );
-  if ( !std::isnan( z ) )
-  {
-    center.convertTo( QgsWkbTypes::PointZ );
-    center.setZ( z );
-  }
+
+  QgsPointSequence points;
+  points << vertexAt( 0 ) << vertexAt( 1 ) << vertexAt( 2 );
+  QgsGeometryUtils::setZValueFromPoints( points, center );
 
   return center;
 }