Add UnitConverter tests

src/ImageSharp/Common/Helpers/UnitConverter.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.MetaData.Profiles.Exif;
@@ -19,6 +18,12 @@ namespace SixLabors.ImageSharp.Common.Helpers
         /// </summary>
         private const double CmsInMeter = 1 / 0.01D;
 
+        /// <summary>
+        /// The number of centimeters in an inch.
+        /// 1 inch is equal to exactly 2.54 centimeters.
+        /// </summary>
+        private const double CmsInInch = 2.54D;
+
         /// <summary>
         /// The number of inches in a meter.
         /// 1 inch is equal to exactly 0.0254 meters.
@@ -57,6 +62,22 @@ namespace SixLabors.ImageSharp.Common.Helpers
         [MethodImpl(InliningOptions.ShortMethod)]
         public static double InchToMeter(double x) => x * InchesInMeter;
 
+        /// <summary>
+        /// Scales the value from centimeters to inches.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static double CmToInch(double x) => x / CmsInInch;
+
+        /// <summary>
+        /// Scales the value from inches to centimeters.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static double InchToCm(double x) => x * CmsInInch;
+
         /// <summary>
         /// Converts an <see cref="ExifTag.ResolutionUnit"/> to a <see cref="PixelResolutionUnit"/>.
         /// </summary>

tests/ImageSharp.Tests/Helpers/UnitConverterHelperTests.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Common.Helpers;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Helpers
+{
+    public class UnitConverterHelperTests
+    {
+        [Fact]
+        public void InchToFromMeter()
+        {
+            const double expected = 96D;
+            double actual = UnitConverter.InchToMeter(expected);
+            actual = UnitConverter.MeterToInch(actual);
+
+            Assert.Equal(expected, actual, 15);
+        }
+
+        [Fact]
+        public void InchToFromCm()
+        {
+            const double expected = 96D;
+            double actual = UnitConverter.InchToCm(expected);
+            actual = UnitConverter.CmToInch(actual);
+
+            Assert.Equal(expected, actual, 15);
+        }
+
+        [Fact]
+        public void CmToFromMeter()
+        {
+            const double expected = 96D;
+            double actual = UnitConverter.CmToMeter(expected);
+            actual = UnitConverter.MeterToCm(actual);
+
+            Assert.Equal(expected, actual, 15);
+        }
+    }
+}