Can now preserve correct resolution for jpeg and gif.

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -450,8 +450,8 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 {
                     int index = Unsafe.Add(ref indicesRef, i);
 
-                    if (this.graphicsControlExtension.TransparencyFlag == false ||
+                    if (!this.graphicsControlExtension.TransparencyFlag
-                        this.graphicsControlExtension.TransparencyIndex != index)
+                        || this.graphicsControlExtension.TransparencyIndex != index)
                     {
                         ref TPixel pixel = ref Unsafe.Add(ref rowRef, x);
                         rgba.Rgb = colorTable[index];
@@ -516,14 +516,42 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// <param name="stream">The stream containing image data. </param>
         private void ReadLogicalScreenDescriptorAndGlobalColorTable(Stream stream)
         {
-            this.metaData = new ImageMetaData();
-
             this.stream = stream;
 
             // Skip the identifier
             this.stream.Skip(6);
             this.ReadLogicalScreenDescriptor();
 
+            var meta = new ImageMetaData();
+
+            // The Pixel Aspect Ratio is defined to be the quotient of the pixel's
+            // width over its height.  The value range in this field allows
+            // specification of the widest pixel of 4:1 to the tallest pixel of
+            // 1:4 in increments of 1/64th.
+            //
+            // Values :        0 -   No aspect ratio information is given.
+            //            1..255 -   Value used in the computation.
+            //
+            // Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
+            if (this.logicalScreenDescriptor.PixelAspectRatio > 0)
+            {
+                meta.ResolutionUnits = ResolutionUnits.AspectRatio;
+                float ratio = (this.logicalScreenDescriptor.PixelAspectRatio + 15) / 64F;
+
+                if (ratio > 1)
+                {
+                    meta.HorizontalResolution = ratio;
+                    meta.VerticalResolution = 1;
+                }
+                else
+                {
+                    meta.VerticalResolution = 1 / ratio;
+                    meta.HorizontalResolution = 1;
+                }
+            }
+
+            this.metaData = meta;
+
             if (this.logicalScreenDescriptor.GlobalColorTableFlag)
             {
                 int globalColorTableLength = this.logicalScreenDescriptor.GlobalColorTableSize * 3;

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -226,11 +226,41 @@ namespace SixLabors.ImageSharp.Formats.Gif
         {
             byte packedValue = GifLogicalScreenDescriptor.GetPackedValue(useGlobalTable, this.bitDepth - 1, false, this.bitDepth - 1);
 
+            // The Pixel Aspect Ratio is defined to be the quotient of the pixel's
+            // width over its height.  The value range in this field allows
+            // specification of the widest pixel of 4:1 to the tallest pixel of
+            // 1:4 in increments of 1/64th.
+            //
+            // Values :        0 -   No aspect ratio information is given.
+            //            1..255 -   Value used in the computation.
+            //
+            // Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
+            ImageMetaData meta = image.MetaData;
+            byte ratio = 0;
+
+            if (meta.ResolutionUnits == ResolutionUnits.AspectRatio)
+            {
+                double hr = meta.HorizontalResolution;
+                double vr = meta.VerticalResolution;
+                if (hr != vr)
+                {
+                    if (hr > vr)
+                    {
+                        ratio = (byte)((hr * 64) - 15);
+                    }
+                    else
+                    {
+                        ratio = (byte)(((1 / vr) * 64) - 15);
+                    }
+                }
+            }
+
             var descriptor = new GifLogicalScreenDescriptor(
                 width: (ushort)image.Width,
                 height: (ushort)image.Height,
                 packed: packedValue,
-                backgroundColorIndex: unchecked((byte)transparencyIndex));
+                backgroundColorIndex: unchecked((byte)transparencyIndex),
+                ratio);
 
             descriptor.WriteTo(this.buffer);
 

src/ImageSharp/Formats/Jpeg/Components/Decoder/JFifMarker.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using SixLabors.ImageSharp.MetaData;
 
 namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 {
@@ -31,7 +32,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
             this.MajorVersion = majorVersion;
             this.MinorVersion = minorVersion;
-            this.DensityUnits = densityUnits;
+            this.DensityUnits = (ResolutionUnits)densityUnits;
             this.XDensity = xDensity;
             this.YDensity = yDensity;
         }
@@ -52,7 +53,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         ///  01 : Pixels per inch (2.54 cm)
         ///  02 : Pixels per centimeter
         /// </summary>
-        public byte DensityUnits { get; }
+        public ResolutionUnits DensityUnits { get; }
 
         /// <summary>
         /// Gets the horizontal pixel density. Must not be zero.

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -210,7 +210,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             int componentCount = 3;
 
             // Write the Start Of Image marker.
-            this.WriteApplicationHeader((short)image.MetaData.HorizontalResolution, (short)image.MetaData.VerticalResolution);
+            this.WriteApplicationHeader(
+                (byte)image.MetaData.ResolutionUnits,
+                (short)image.MetaData.HorizontalResolution,
+                (short)image.MetaData.VerticalResolution);
 
             this.WriteProfiles(image);
 
@@ -425,9 +428,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <summary>
         /// Writes the application header containing the JFIF identifier plus extra data.
         /// </summary>
+        /// <param name="resolutionUnits">The resolution unit of measurement.</param>
         /// <param name="horizontalResolution">The resolution of the image in the x- direction.</param>
         /// <param name="verticalResolution">The resolution of the image in the y- direction.</param>
-        private void WriteApplicationHeader(short horizontalResolution, short verticalResolution)
+        private void WriteApplicationHeader(byte resolutionUnits, short horizontalResolution, short verticalResolution)
         {
             // Write the start of image marker. Markers are always prefixed with with 0xff.
             this.buffer[0] = JpegConstants.Markers.XFF;
@@ -445,7 +449,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.buffer[10] = 0x00; // = "JFIF",'\0'
             this.buffer[11] = 0x01; // versionhi
             this.buffer[12] = 0x01; // versionlo
-            this.buffer[13] = 0x01; // xyunits as dpi
+            this.buffer[13] = resolutionUnits; // xyunits
 
             // Resolution. Big Endian
             this.buffer[14] = (byte)(horizontalResolution >> 8);

src/ImageSharp/Formats/Jpeg/PdfJsPort/PdfJsJpegDecoderCore.cs

@@ -412,6 +412,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort
             {
                 this.MetaData.HorizontalResolution = this.jFif.XDensity;
                 this.MetaData.VerticalResolution = this.jFif.YDensity;
+                this.MetaData.ResolutionUnits = this.jFif.DensityUnits;
             }
             else if (this.isExif)
             {
@@ -423,10 +424,15 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort
                     ? ((Rational)verticalTag.Value).ToDouble()
                     : 0;
 
+                byte units = this.MetaData.ExifProfile.TryGetValue(ExifTag.ResolutionUnit, out ExifValue resolutionTag)
+                    ? (byte)(((ushort)resolutionTag.Value) - 1) // EXIF is 1,2,3
+                    : byte.MinValue;
+
                 if (horizontalValue > 0 && verticalValue > 0)
                 {
                     this.MetaData.HorizontalResolution = horizontalValue;
                     this.MetaData.VerticalResolution = verticalValue;
+                    this.MetaData.ResolutionUnits = (ResolutionUnits)units;
                 }
             }
 

src/ImageSharp/MetaData/ImageMetaData.cs

@@ -47,6 +47,7 @@ namespace SixLabors.ImageSharp.MetaData
         {
             this.HorizontalResolution = other.HorizontalResolution;
             this.VerticalResolution = other.VerticalResolution;
+            this.ResolutionUnits = other.ResolutionUnits;
             this.RepeatCount = other.RepeatCount;
 
             foreach (ImageProperty property in other.Properties)
@@ -99,6 +100,11 @@ namespace SixLabors.ImageSharp.MetaData
             }
         }
 
+        /// <summary>
+        /// Gets or sets unit of measure used when reporting resolution.
+        /// </summary>
+        public ResolutionUnits ResolutionUnits { get; set; } = ResolutionUnits.PixelsPerInch;
+
         /// <summary>
         /// Gets or sets the Exif profile.
         /// </summary>

src/ImageSharp/Formats/DensityUnits.cs → src/ImageSharp/MetaData/ResolutionUnits.cs

@@ -1,12 +1,12 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-namespace SixLabors.ImageSharp.Formats
+namespace SixLabors.ImageSharp.MetaData
 {
     /// <summary>
     /// Provides enumeration of available pixel density units.
     /// </summary>
-    public enum DensityUnits : byte
+    public enum ResolutionUnits : byte
     {
         /// <summary>
         /// No units; width:height pixel aspect ratio = Ydensity:Xdensity

tests/ImageSharp.Tests/Formats/Jpg/JFifMarkerTests.cs

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
-
+using SixLabors.ImageSharp.MetaData;
 using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Formats.Jpg
@@ -32,7 +32,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             Assert.True(isJFif);
             Assert.Equal(1, marker.MajorVersion);
             Assert.Equal(1, marker.MinorVersion);
-            Assert.Equal(1, marker.DensityUnits);
+            Assert.Equal(ResolutionUnits.PixelsPerInch, marker.DensityUnits);
             Assert.Equal(96, marker.XDensity);
             Assert.Equal(96, marker.YDensity);
         }