From 754375f6609f9ddeda62b589ceea1747f2942343 Mon Sep 17 00:00:00 2001
From: Nyall Dawson <nyall.dawson@gmail.com>
Date: Wed, 10 Jun 2020 20:46:37 +1000
Subject: [PATCH] Improve method for calculation of rectangle centers

More numerically stable in the case of massive rectangles.

This is the ultimate cause behind #36898 -- during rendering of the
worldwide layer in Winkel Tripel the inverse transform fails, so
the renderer sets a "maximal" filter extent for the layer (+/- double
max). The previous method for calculating the rectangles center would
fail, because it would overflow. The new method handles this case
and correctly returns 0,0.

Fixes bounding boxes show incorrectly when simplication is enabled
for a layer in certain projections.

Fixes #36898
Refs #30686 (partial fix -- this report includes two different bugs)
---
 src/core/geometry/qgsrectangle.h    |  2 +-
 tests/src/core/testqgsrectangle.cpp | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/core/geometry/qgsrectangle.h b/src/core/geometry/qgsrectangle.h
index 911b72863ff6..fa2edc89f7d0 100644
--- a/src/core/geometry/qgsrectangle.h
+++ b/src/core/geometry/qgsrectangle.h
@@ -227,7 +227,7 @@ class CORE_EXPORT QgsRectangle
     /**
      * Returns the center point of the rectangle.
      */
-    QgsPointXY center() const { return QgsPointXY( mXmin + width() / 2, mYmin + height() / 2 ); }
+    QgsPointXY center() const { return QgsPointXY( mXmax * 0.5 + mXmin * 0.5, mYmin * 0.5 + mYmax * 0.5 ); }
 
     /**
      * Scale the rectangle around its center point.
diff --git a/tests/src/core/testqgsrectangle.cpp b/tests/src/core/testqgsrectangle.cpp
index a5ee0977b079..74e50c151e9f 100644
--- a/tests/src/core/testqgsrectangle.cpp
+++ b/tests/src/core/testqgsrectangle.cpp
@@ -43,6 +43,7 @@ class TestQgsRectangle: public QObject
     void scale();
     void snappedToGrid();
     void distanceToPoint();
+    void center();
 };
 
 void TestQgsRectangle::isEmpty()
@@ -404,5 +405,22 @@ void TestQgsRectangle::distanceToPoint()
   QGSCOMPARENEAR( rect.distance( QgsPointXY( 25, 115 ) ), 7.071068, 0.00001 );
 }
 
+void TestQgsRectangle::center()
+{
+  QgsRectangle rect( 10, 100, 20, 110 );
+  QCOMPARE( rect.center().x(), 15.0 );
+  QCOMPARE( rect.center().y(), 105.0 );
+  rect = QgsRectangle( 10, 100, 10, 100 );
+  QCOMPARE( rect.center().x(), 10.0 );
+  QCOMPARE( rect.center().y(), 100.0 );
+  rect = QgsRectangle( -10, -100, 10, 100 );
+  QCOMPARE( rect.center().x(), 0.0 );
+  QCOMPARE( rect.center().y(), 0.0 );
+  // a "maximal" rect
+  rect = QgsRectangle( std::numeric_limits<double>::lowest(), std::numeric_limits<double>::lowest(), std::numeric_limits<double>::max(), std::numeric_limits<double>::max() );
+  QCOMPARE( rect.center().x(), 0.0 );
+  QCOMPARE( rect.center().y(), 0.0 );
+}
+
 QGSTEST_MAIN( TestQgsRectangle )
 #include "testqgsrectangle.moc"