Changes the method to return a factor instead of a percentage

Former-commit-id: 0e668b5b5f0b4321eb3386656f53693f6189d51a
Former-commit-id: b1bc22cb28de7e3d6ff33d018886b67b38625715

src/ImageProcessor.UnitTests/Imaging/RotationUnitTests.cs

@@ -18,5 +18,16 @@
             result.Width.Should().Be(expectedWidth, "because the rotated width should have been calculated");
             result.Height.Should().Be(expectedHeight, "because the rotated height should have been calculated");
         }
+
+        [Test]
+        [TestCase(100, 100, 45, 1.41f)]
+        [TestCase(100, 100, 15, 1.22f)]
+        [TestCase(100, 200, 45, 2.12f)]
+        public void RotationZoomIsCalculated(int imageWidth, int imageHeight, float angle, float expected)
+        {
+            float result = Rotation.ZoomAfterRotation(imageWidth, imageHeight, angle);
+
+            result.Should().BeApproximately(expected, 0.01f, "because the zoom level after rotation should have been calculated");
+        }
     }
 }

src/ImageProcessor/Imaging/Rotation.cs

@@ -39,5 +39,24 @@
 
             return result;
         }
+
+        /// <summary>
+        /// Calculates the zoom needed after the rotation.
+        /// </summary>
+        /// <param name="imageWidth">Width of the image.</param>
+        /// <param name="imageHeight">Height of the image.</param>
+        /// <param name="angle">The angle.</param>
+        /// <returns>The zoom needed</returns>
+        public static float ZoomAfterRotation(int imageWidth, int imageHeight, float angle)
+        {
+            double radians = angle * Math.PI / 180d;
+            double radiansSin = Math.Sin(radians);
+            double radiansCos = Math.Cos(radians);
+
+            double widthRotated = (imageWidth * radiansCos) + (imageHeight * radiansSin);
+            double heightRotated = (imageWidth * radiansSin) + (imageHeight * radiansCos);
+
+            return (float)(Math.Max(widthRotated, heightRotated) / 100);
+        }
     }
 }