Added method to update the profile data (based on #96)

src/ImageSharp.Formats.Jpeg/JpegEncoderCore.cs

@@ -706,6 +706,7 @@ namespace ImageSharp.Formats
         private void WriteProfiles<TColor>(Image<TColor> image)
             where TColor : struct, IPackedPixel, IEquatable<TColor>
         {
+            image.MetaData.SyncProfiles();
             this.WriteProfile(image.MetaData.ExifProfile);
         }
 

src/ImageSharp/MetaData/ImageMetaData.cs

@@ -136,5 +136,34 @@ namespace ImageSharp
         /// <remarks>0 means to repeat indefinitely.</remarks>
         /// </summary>
         public ushort RepeatCount { get; set; }
+
+        /// <summary>
+        /// Synchronizes the profiles with the current meta data.
+        /// </summary>
+        internal void SyncProfiles()
+        {
+            this.SyncExifProfile();
+        }
+
+        private void SyncExifProfile()
+        {
+            if (this.ExifProfile == null)
+            {
+                return;
+            }
+
+            this.SyncExifResolution(ExifTag.XResolution, this.HorizontalResolution);
+            this.SyncExifResolution(ExifTag.YResolution, this.VerticalResolution);
+        }
+
+        private void SyncExifResolution(ExifTag tag, double resolution)
+        {
+            ExifValue value = this.ExifProfile.GetValue(tag);
+            if (value != null)
+            {
+                Rational newResolution = new Rational(resolution, false);
+                this.ExifProfile.SetValue(tag, newResolution);
+            }
+        }
     }
 }

tests/ImageSharp.Tests/MetaData/ImageMetaDataTests.cs

@@ -38,5 +38,38 @@ namespace ImageSharp.Tests
             Assert.Equal(24, clone.Quality);
             Assert.Equal(1, clone.RepeatCount);
         }
+
+        [Fact]
+        public void SyncProfiles()
+        {
+            ExifProfile exifProfile = new ExifProfile();
+            exifProfile.SetValue(ExifTag.XResolution, new Rational(200));
+            exifProfile.SetValue(ExifTag.YResolution, new Rational(300));
+
+            Image image = new Image(1, 1);
+            image.MetaData.ExifProfile = exifProfile;
+            image.MetaData.HorizontalResolution = 200;
+            image.MetaData.VerticalResolution = 300;
+
+            image.MetaData.HorizontalResolution = 100;
+
+            Assert.Equal(200, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.XResolution).Value).ToDouble());
+            Assert.Equal(300, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.YResolution).Value).ToDouble());
+
+            image.MetaData.SyncProfiles();
+
+            Assert.Equal(100, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.XResolution).Value).ToDouble());
+            Assert.Equal(300, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.YResolution).Value).ToDouble());
+
+            image.MetaData.VerticalResolution = 150;
+
+            Assert.Equal(100, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.XResolution).Value).ToDouble());
+            Assert.Equal(300, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.YResolution).Value).ToDouble());
+
+            image.MetaData.SyncProfiles();
+
+            Assert.Equal(100, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.XResolution).Value).ToDouble());
+            Assert.Equal(150, ((Rational)image.MetaData.ExifProfile.GetValue(ExifTag.YResolution).Value).ToDouble());
+        }
     }
 }

tests/ImageSharp.Tests/MetaData/Profiles/Exif/ExifProfileTests.cs

@@ -150,6 +150,9 @@ namespace ImageSharp.Tests
 
             image.MetaData.ExifProfile.SetValue(ExifTag.XResolution, new Rational(150.0));
 
+            // We also need to change this value because this overrides XResolution when the image is written.
+            image.MetaData.HorizontalResolution = 150.0;
+
             value = image.MetaData.ExifProfile.GetValue(ExifTag.XResolution);
             TestValue(value, new Rational(150, 1));