From b2759175b6cd6b8d697541214134bb37d1cbb47d Mon Sep 17 00:00:00 2001
From: Andreas Torebring <andreas.torebring@gmail.com>
Date: Fri, 26 Aug 2022 13:40:31 +0200
Subject: [PATCH] Rectangle extents are rotated correctly

---
 .../Math/PrimitivesHelper.cs                  | 12 ++---
 .../Primitives/RectangleFTests.cs             | 53 +++++++++++++++++++
 2 files changed, 59 insertions(+), 6 deletions(-)

diff --git a/src/cs/MonoGame.Extended/Math/PrimitivesHelper.cs b/src/cs/MonoGame.Extended/Math/PrimitivesHelper.cs
index 543d23d0..ef151bcb 100644
--- a/src/cs/MonoGame.Extended/Math/PrimitivesHelper.cs
+++ b/src/cs/MonoGame.Extended/Math/PrimitivesHelper.cs
@@ -65,10 +65,10 @@ namespace MonoGame.Extended
             // Real-Time Collision Detection, Christer Ericson, 2005. Chapter 4.2; Bounding Volumes - Axis-aligned Bounding Boxes (AABBs). pg 86-87
 
             center = transformMatrix.Transform(center);
-            halfExtents.X = halfExtents.X * Math.Abs(transformMatrix.M11) + halfExtents.X * Math.Abs(transformMatrix.M12) +
-                      halfExtents.X * Math.Abs(transformMatrix.M31);
-            halfExtents.Y = halfExtents.Y * Math.Abs(transformMatrix.M21) + halfExtents.Y * Math.Abs(transformMatrix.M22) +
-                      halfExtents.Y * Math.Abs(transformMatrix.M32);
+            var xRadius = halfExtents.X;
+            var yRadius = halfExtents.Y;
+            halfExtents.X = xRadius * Math.Abs(transformMatrix.M11) + yRadius * Math.Abs(transformMatrix.M12);
+            halfExtents.Y = xRadius * Math.Abs(transformMatrix.M21) + yRadius * Math.Abs(transformMatrix.M22);
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -109,7 +109,7 @@ namespace MonoGame.Extended
         internal static void ClosestPointToPointFromRectangle(Point2 minimum, Point2 maximum, Point2 point, out Point2 result)
         {
             // Real-Time Collision Detection, Christer Ericson, 2005. Chapter 5.1.2; Basic Primitive Tests - Closest-point Computations. pg 130-131
-           
+
             result = point;
 
             // For each coordinate axis, if the point coordinate value is outside box, clamp it to the box, else keep it as is
@@ -125,4 +125,4 @@ namespace MonoGame.Extended
         }
 
     }
-}
\ No newline at end of file
+}
diff --git a/src/cs/Tests/MonoGame.Extended.Tests/Primitives/RectangleFTests.cs b/src/cs/Tests/MonoGame.Extended.Tests/Primitives/RectangleFTests.cs
index 825a685e..9121d6ad 100644
--- a/src/cs/Tests/MonoGame.Extended.Tests/Primitives/RectangleFTests.cs
+++ b/src/cs/Tests/MonoGame.Extended.Tests/Primitives/RectangleFTests.cs
@@ -77,6 +77,59 @@ namespace MonoGame.Extended.Tests.Primitives
 
                 Assert.Equal(new Size2(20, 30), result.Size);
             }
+
+            [Fact]
+            public void Applies_rotation_and_translation()
+            {
+                /*   Rectangle with center point aligned in coordinate system with origin 0.
+                 *
+                 *                    :
+                 *                    :
+                 *                    +-+
+                 *                    | |
+                 *                    |p|
+                 *                    | |
+                 *     ...............0-+............
+                 *                    :
+                 *                    :
+                 *                    :
+                 *
+                 *    Rotate center point p, 90 degrees around origin 0.
+                 *
+                 *                    :
+                 *                    :
+                 *                +---+
+                 *                | p |
+                 *     ...........+---0..............
+                 *                    :
+                 *                    :
+                 *                    :
+                 *
+                 *    Then translate rectangle by x=10 and y=20.
+                 *                    :
+                 *                    :     +---+
+                 *                    :     | p |
+                 *    y=21 - - - - - - - -> +---+
+                 *                    .
+                 *                    :
+                 *    ...............0..............
+                 *                    :
+                 *                    :
+                 *                    :
+                 */
+                var rectangle = new RectangleF(new Point2(0, 0), new Size2(2, 4));
+                var transform =
+                    Matrix2.CreateRotationZ(MathHelper.PiOver2)
+                    *
+                    Matrix2.CreateTranslation(10, 20);
+
+                var result = RectangleF.Transform(rectangle, ref transform);
+
+                Assert.Equal(-2 + 10, result.Center.X, 6);
+                Assert.Equal(1 + 20, result.Center.Y, 6);
+                Assert.Equal(4, result.Size.Width, 6);
+                Assert.Equal(2, result.Size.Height, 6);
+            }
         }
     }
 }