Merge remote-tracking branch 'upstream/master' into feature/histogramEqualization

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.0.{build}
-image: Visual Studio 2017
+image: Previous Visual Studio 2017
 
 # prevent the double build when a branch has an active PR
 skip_branch_with_pr: true

src/ImageSharp/Common/Helpers/UnitConverter.cs

@@ -0,0 +1,94 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.MetaData;
+using SixLabors.ImageSharp.MetaData.Profiles.Exif;
+
+namespace SixLabors.ImageSharp.Common.Helpers
+{
+    /// <summary>
+    /// Contains methods for converting values between unit scales.
+    /// </summary>
+    internal static class UnitConverter
+    {
+        /// <summary>
+        /// The number of centimeters in a meter.
+        /// 1 cm is equal to exactly 0.01 meters.
+        /// </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.
+        /// </summary>
+        private const double InchesInMeter = 1 / 0.0254D;
+
+        /// <summary>
+        /// Scales the value from centimeters to meters.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static double CmToMeter(double x) => x * CmsInMeter;
+
+        /// <summary>
+        /// Scales the value from meters to centimeters.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static double MeterToCm(double x) => x / CmsInMeter;
+
+        /// <summary>
+        /// Scales the value from meters to inches.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static double MeterToInch(double x) => x / InchesInMeter;
+
+        /// <summary>
+        /// Scales the value from inches to meters.
+        /// </summary>
+        /// <param name="x">The value to scale.</param>
+        /// <returns>The <see cref="double"/>.</returns>
+        [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>
+        /// <param name="profile">The EXIF profile containing the value.</param>
+        /// <returns>The <see cref="PixelResolutionUnit"/></returns>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static PixelResolutionUnit ExifProfileToResolutionUnit(ExifProfile profile)
+        {
+            return profile.TryGetValue(ExifTag.ResolutionUnit, out ExifValue resolution)
+                ? (PixelResolutionUnit)(byte)(((ushort)resolution.Value) - 1) // EXIF is 1, 2, 3
+                : default;
+        }
+    }
+}

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers.Binary;
 using System.IO;
 using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
@@ -59,6 +60,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         private Stream stream;
 
+        /// <summary>
+        /// The metadata
+        /// </summary>
+        private ImageMetaData metaData;
+
         /// <summary>
         /// The file header containing general information.
         /// TODO: Why is this not used? We advance the stream but do not use the values parsed.
@@ -103,7 +109,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             {
                 this.ReadImageHeaders(stream, out bool inverted, out byte[] palette);
 
-                var image = new Image<TPixel>(this.configuration, this.infoHeader.Width, this.infoHeader.Height);
+                var image = new Image<TPixel>(this.configuration, this.infoHeader.Width, this.infoHeader.Height, this.metaData);
 
                 Buffer2D<TPixel> pixels = image.GetRootFramePixelBuffer();
 
@@ -157,7 +163,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         public IImageInfo Identify(Stream stream)
         {
             this.ReadImageHeaders(stream, out _, out _);
-            return new ImageInfo(new PixelTypeInfo(this.infoHeader.BitsPerPixel), this.infoHeader.Width, this.infoHeader.Height, new ImageMetaData());
+            return new ImageInfo(new PixelTypeInfo(this.infoHeader.BitsPerPixel), this.infoHeader.Width, this.infoHeader.Height, this.metaData);
         }
 
         /// <summary>
@@ -518,6 +524,23 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 throw new NotSupportedException($"ImageSharp does not support this BMP file. HeaderSize: {headerSize}.");
             }
 
+            // Resolution is stored in PPM.
+            var meta = new ImageMetaData();
+            meta.ResolutionUnits = PixelResolutionUnit.PixelsPerMeter;
+            if (this.infoHeader.XPelsPerMeter > 0 && this.infoHeader.YPelsPerMeter > 0)
+            {
+                meta.HorizontalResolution = this.infoHeader.XPelsPerMeter;
+                meta.VerticalResolution = this.infoHeader.YPelsPerMeter;
+            }
+            else
+            {
+                // Convert default metadata values to PPM.
+                meta.HorizontalResolution = Math.Round(UnitConverter.InchToMeter(ImageMetaData.DefaultHorizontalResolution));
+                meta.VerticalResolution = Math.Round(UnitConverter.InchToMeter(ImageMetaData.DefaultVerticalResolution));
+            }
+
+            this.metaData = meta;
+
             // skip the remaining header because we can't read those parts
             this.stream.Skip(skipAmount);
         }

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -3,6 +3,8 @@
 
 using System;
 using System.IO;
+using SixLabors.ImageSharp.Common.Helpers;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
 
@@ -50,6 +52,38 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             int bytesPerLine = 4 * (((image.Width * bpp) + 31) / 32);
             this.padding = bytesPerLine - (image.Width * (int)this.bitsPerPixel);
 
+            // Set Resolution.
+            ImageMetaData meta = image.MetaData;
+            int hResolution = 0;
+            int vResolution = 0;
+
+            if (meta.ResolutionUnits != PixelResolutionUnit.AspectRatio)
+            {
+                if (meta.HorizontalResolution > 0 && meta.VerticalResolution > 0)
+                {
+                    switch (meta.ResolutionUnits)
+                    {
+                        case PixelResolutionUnit.PixelsPerInch:
+
+                            hResolution = (int)Math.Round(UnitConverter.InchToMeter(meta.HorizontalResolution));
+                            vResolution = (int)Math.Round(UnitConverter.InchToMeter(meta.VerticalResolution));
+                            break;
+
+                        case PixelResolutionUnit.PixelsPerCentimeter:
+
+                            hResolution = (int)Math.Round(UnitConverter.CmToMeter(meta.HorizontalResolution));
+                            vResolution = (int)Math.Round(UnitConverter.CmToMeter(meta.VerticalResolution));
+                            break;
+
+                        case PixelResolutionUnit.PixelsPerMeter:
+                            hResolution = (int)Math.Round(meta.HorizontalResolution);
+                            vResolution = (int)Math.Round(meta.VerticalResolution);
+
+                            break;
+                    }
+                }
+            }
+
             var infoHeader = new BmpInfoHeader(
                 headerSize: BmpInfoHeader.Size,
                 height: image.Height,
@@ -58,7 +92,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 planes: 1,
                 imageSize: image.Height * bytesPerLine,
                 clrUsed: 0,
-                clrImportant: 0);
+                clrImportant: 0,
+                xPelsPerMeter: hResolution,
+                yPelsPerMeter: vResolution);
 
             var fileHeader = new BmpFileHeader(
                 type: 19778, // BM

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 ||
-                        this.graphicsControlExtension.TransparencyIndex != index)
+                    if (!this.graphicsControlExtension.TransparencyFlag
+                        || 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 = PixelResolutionUnit.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 == PixelResolutionUnit.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
 {
@@ -28,10 +29,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         {
             Guard.MustBeGreaterThan(xDensity, 0, nameof(xDensity));
             Guard.MustBeGreaterThan(yDensity, 0, nameof(yDensity));
+            Guard.MustBeBetweenOrEqualTo(densityUnits, 0, 2, nameof(densityUnits));
 
             this.MajorVersion = majorVersion;
             this.MinorVersion = minorVersion;
-            this.DensityUnits = densityUnits;
+            this.DensityUnits = (PixelResolutionUnit)densityUnits;
             this.XDensity = xDensity;
             this.YDensity = yDensity;
         }
@@ -52,7 +54,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         ///  01 : Pixels per inch (2.54 cm)
         ///  02 : Pixels per centimeter
         /// </summary>
-        public byte DensityUnits { get; }
+        public PixelResolutionUnit DensityUnits { get; }
 
         /// <summary>
         /// Gets the horizontal pixel density. Must not be zero.

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -2,11 +2,13 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers.Binary;
 using System.IO;
 using System.Runtime.CompilerServices;
-
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Jpeg.Components;
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.MetaData.Profiles.Exif;
 using SixLabors.ImageSharp.MetaData.Profiles.Icc;
 using SixLabors.ImageSharp.PixelFormats;
@@ -210,7 +212,7 @@ 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(image.MetaData);
 
             this.WriteProfiles(image);
 
@@ -425,9 +427,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <summary>
         /// Writes the application header containing the JFIF identifier plus extra data.
         /// </summary>
-        /// <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)
+        /// <param name="meta">The image meta data.</param>
+        private void WriteApplicationHeader(ImageMetaData meta)
         {
             // Write the start of image marker. Markers are always prefixed with with 0xff.
             this.buffer[0] = JpegConstants.Markers.XFF;
@@ -445,13 +446,25 @@ 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
 
             // Resolution. Big Endian
-            this.buffer[14] = (byte)(horizontalResolution >> 8);
-            this.buffer[15] = (byte)horizontalResolution;
-            this.buffer[16] = (byte)(verticalResolution >> 8);
-            this.buffer[17] = (byte)verticalResolution;
+            Span<byte> hResolution = this.buffer.AsSpan(14, 2);
+            Span<byte> vResolution = this.buffer.AsSpan(16, 2);
+
+            if (meta.ResolutionUnits == PixelResolutionUnit.PixelsPerMeter)
+            {
+                // Scale down to PPI
+                this.buffer[13] = (byte)PixelResolutionUnit.PixelsPerInch; // xyunits
+                BinaryPrimitives.WriteInt16BigEndian(hResolution, (short)Math.Round(UnitConverter.MeterToInch(meta.HorizontalResolution)));
+                BinaryPrimitives.WriteInt16BigEndian(vResolution, (short)Math.Round(UnitConverter.MeterToInch(meta.VerticalResolution)));
+            }
+            else
+            {
+                // We can simply pass the value.
+                this.buffer[13] = (byte)meta.ResolutionUnits; // xyunits
+                BinaryPrimitives.WriteInt16BigEndian(hResolution, (short)Math.Round(meta.HorizontalResolution));
+                BinaryPrimitives.WriteInt16BigEndian(vResolution, (short)Math.Round(meta.VerticalResolution));
+            }
 
             // No thumbnail
             this.buffer[18] = 0x00; // Thumbnail width

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

@@ -7,7 +7,7 @@ using System.Collections.Generic;
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
-
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Jpeg.Components;
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
 using SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort.Components;
@@ -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)
             {
@@ -427,6 +428,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort
                 {
                     this.MetaData.HorizontalResolution = horizontalValue;
                     this.MetaData.VerticalResolution = verticalValue;
+                    this.MetaData.ResolutionUnits = UnitConverter.ExifProfileToResolutionUnit(this.MetaData.ExifProfile);
                 }
             }
 

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -10,6 +10,7 @@ using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
 using SixLabors.ImageSharp.MetaData;
@@ -233,7 +234,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                                     this.ValidateHeader();
                                     break;
                                 case PngChunkType.Physical:
-                                    this.ReadPhysicalChunk(metadata, chunk.Data.Array);
+                                    this.ReadPhysicalChunk(metadata, chunk.Data.GetSpan());
                                     break;
                                 case PngChunkType.Data:
                                     if (image == null)
@@ -307,7 +308,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                                 this.ValidateHeader();
                                 break;
                             case PngChunkType.Physical:
-                                this.ReadPhysicalChunk(metadata, chunk.Data.Array);
+                                this.ReadPhysicalChunk(metadata, chunk.Data.GetSpan());
                                 break;
                             case PngChunkType.Data:
                                 this.SkipChunkDataAndCrc(chunk);
@@ -396,9 +397,26 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="data">The data containing physical data.</param>
         private void ReadPhysicalChunk(ImageMetaData metadata, ReadOnlySpan<byte> data)
         {
-            // 39.3700787 = inches in a meter.
-            metadata.HorizontalResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(0, 4)) / 39.3700787d;
-            metadata.VerticalResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(4, 4)) / 39.3700787d;
+            // The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:
+            // Pixels per unit, X axis: 4 bytes (unsigned integer)
+            // Pixels per unit, Y axis: 4 bytes (unsigned integer)
+            // Unit specifier:          1 byte
+            //
+            // The following values are legal for the unit specifier:
+            //   0: unit is unknown
+            //   1: unit is the meter
+            //
+            // When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
+            int hResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(0, 4));
+            int vResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(4, 4));
+            byte unit = data[8];
+
+            metadata.ResolutionUnits = unit == byte.MinValue
+                ? PixelResolutionUnit.AspectRatio
+                : PixelResolutionUnit.PixelsPerMeter;
+
+            metadata.HorizontalResolution = hResolution;
+            metadata.VerticalResolution = vResolution;
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -6,8 +6,10 @@ using System.Buffers.Binary;
 using System.IO;
 using System.Linq;
 using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
 using SixLabors.Memory;
@@ -598,19 +600,52 @@ namespace SixLabors.ImageSharp.Formats.Png
         private void WritePhysicalChunk<TPixel>(Stream stream, Image<TPixel> image)
             where TPixel : struct, IPixel<TPixel>
         {
-            if (image.MetaData.HorizontalResolution > 0 && image.MetaData.VerticalResolution > 0)
+            // The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:
+            // Pixels per unit, X axis: 4 bytes (unsigned integer)
+            // Pixels per unit, Y axis: 4 bytes (unsigned integer)
+            // Unit specifier:          1 byte
+            //
+            // The following values are legal for the unit specifier:
+            //   0: unit is unknown
+            //   1: unit is the meter
+            //
+            // When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
+            ImageMetaData meta = image.MetaData;
+            Span<byte> hResolution = this.chunkDataBuffer.AsSpan(0, 4);
+            Span<byte> vResolution = this.chunkDataBuffer.AsSpan(4, 4);
+
+            switch (meta.ResolutionUnits)
             {
-                // 39.3700787 = inches in a meter.
-                int dpmX = (int)Math.Round(image.MetaData.HorizontalResolution * 39.3700787D);
-                int dpmY = (int)Math.Round(image.MetaData.VerticalResolution * 39.3700787D);
+                case PixelResolutionUnit.AspectRatio:
 
-                BinaryPrimitives.WriteInt32BigEndian(this.chunkDataBuffer.AsSpan(0, 4), dpmX);
-                BinaryPrimitives.WriteInt32BigEndian(this.chunkDataBuffer.AsSpan(4, 4), dpmY);
+                    this.chunkDataBuffer[8] = 0;
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.VerticalResolution));
+                    break;
+
+                case PixelResolutionUnit.PixelsPerInch:
+
+                    this.chunkDataBuffer[8] = 1; // Per meter
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(UnitConverter.InchToMeter(meta.HorizontalResolution)));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(UnitConverter.InchToMeter(meta.VerticalResolution)));
+                    break;
+
+                case PixelResolutionUnit.PixelsPerCentimeter:
 
-                this.chunkDataBuffer[8] = 1;
+                    this.chunkDataBuffer[8] = 1; // Per meter
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(UnitConverter.CmToMeter(meta.HorizontalResolution)));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(UnitConverter.CmToMeter(meta.VerticalResolution)));
+                    break;
+
+                default:
 
-                this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, 9);
+                    this.chunkDataBuffer[8] = 1; // Per meter
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.VerticalResolution));
+                    break;
             }
+
+            this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, 9);
         }
 
         /// <summary>

src/ImageSharp/MetaData/ImageMetaData.cs

@@ -14,13 +14,13 @@ namespace SixLabors.ImageSharp.MetaData
     {
         /// <summary>
         /// The default horizontal resolution value (dots per inch) in x direction.
-        /// <remarks>The default value is 96 dots per inch.</remarks>
+        /// <remarks>The default value is 96 <see cref="PixelResolutionUnit.PixelsPerInch"/>.</remarks>
         /// </summary>
         public const double DefaultHorizontalResolution = 96;
 
         /// <summary>
         /// The default vertical resolution value (dots per inch) in y direction.
-        /// <remarks>The default value is 96 dots per inch.</remarks>
+        /// <remarks>The default value is 96 <see cref="PixelResolutionUnit.PixelsPerInch"/>.</remarks>
         /// </summary>
         public const double DefaultVerticalResolution = 96;
 
@@ -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 +100,15 @@ namespace SixLabors.ImageSharp.MetaData
             }
         }
 
+        /// <summary>
+        /// Gets or sets unit of measure used when reporting resolution.
+        ///  00 : No units; width:height pixel aspect ratio = Ydensity:Xdensity
+        ///  01 : Pixels per inch (2.54 cm)
+        ///  02 : Pixels per centimeter
+        ///  03 : Pixels per meter
+        /// </summary>
+        public PixelResolutionUnit ResolutionUnits { get; set; } = PixelResolutionUnit.PixelsPerInch;
+
         /// <summary>
         /// Gets or sets the Exif profile.
         /// </summary>

src/ImageSharp/MetaData/PixelResolutionUnit.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.MetaData
+{
+    /// <summary>
+    /// Provides enumeration of available pixel density units.
+    /// </summary>
+    public enum PixelResolutionUnit : byte
+    {
+        /// <summary>
+        /// No units; width:height pixel aspect ratio.
+        /// </summary>
+        AspectRatio = 0,
+
+        /// <summary>
+        /// Pixels per inch (2.54 cm).
+        /// </summary>
+        PixelsPerInch = 1,
+
+        /// <summary>
+        /// Pixels per centimeter.
+        /// </summary>
+        PixelsPerCentimeter = 2,
+
+        /// <summary>
+        /// Pixels per meter (100 cm).
+        /// </summary>
+        PixelsPerMeter = 3
+    }
+}

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -10,17 +10,25 @@ using Xunit;
 
 namespace SixLabors.ImageSharp.Tests
 {
+    using SixLabors.ImageSharp.MetaData;
     using static TestImages.Bmp;
 
     public class BmpDecoderTests
     {
-        public const PixelTypes CommonNonDefaultPixelTypes =
-            PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.RgbaVector;
+        public const PixelTypes CommonNonDefaultPixelTypes = PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.RgbaVector;
 
         public static readonly string[] AllBmpFiles =
-            {
-                Car, F, NegHeight, CoreHeader, V5Header, RLE, RLEInverted, Bit8, Bit8Inverted, Bit16, Bit16Inverted
-            };
+        {
+            Car, F, NegHeight, CoreHeader, V5Header, RLE, RLEInverted, Bit8, Bit8Inverted, Bit16, Bit16Inverted
+        };
+
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Bmp.Car, 3780, 3780 , PixelResolutionUnit.PixelsPerMeter },
+            { TestImages.Bmp.V5Header, 3780, 3780 , PixelResolutionUnit.PixelsPerMeter },
+            { TestImages.Bmp.RLE, 2835, 2835, PixelResolutionUnit.PixelsPerMeter }
+        };
 
         [Theory]
         [WithFileCollection(nameof(AllBmpFiles), PixelTypes.Rgba32)]
@@ -64,5 +72,39 @@ namespace SixLabors.ImageSharp.Tests
                 Assert.Equal(expectedPixelSize, Image.Identify(stream)?.PixelType?.BitsPerPixel);
             }
         }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Decode_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new BmpDecoder();
+                using (Image<Rgba32> image = decoder.Decode<Rgba32>(Configuration.Default, stream))
+                {
+                    ImageMetaData meta = image.MetaData;
+                    Assert.Equal(xResolution, meta.HorizontalResolution);
+                    Assert.Equal(yResolution, meta.VerticalResolution);
+                    Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                }
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Identify_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new BmpDecoder();
+                IImageInfo image = decoder.Identify(Configuration.Default, stream);
+                ImageMetaData meta = image.MetaData;
+                Assert.Equal(xResolution, meta.HorizontalResolution);
+                Assert.Equal(yResolution, meta.VerticalResolution);
+                Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+            }
+        }
     }
 }

tests/ImageSharp.Tests/Formats/Bmp/BmpEncoderTests.cs

@@ -1,7 +1,9 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System.IO;
 using SixLabors.ImageSharp.Formats.Bmp;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 using Xunit;
@@ -12,11 +14,19 @@ namespace SixLabors.ImageSharp.Tests
     public class BmpEncoderTests : FileTestBase
     {
         public static readonly TheoryData<BmpBitsPerPixel> BitsPerPixel =
-            new TheoryData<BmpBitsPerPixel>
-                {
-                    BmpBitsPerPixel.Pixel24,
-                    BmpBitsPerPixel.Pixel32
-                };
+        new TheoryData<BmpBitsPerPixel>
+        {
+            BmpBitsPerPixel.Pixel24,
+            BmpBitsPerPixel.Pixel32
+        };
+
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Bmp.Car, 3780, 3780 , PixelResolutionUnit.PixelsPerMeter },
+            { TestImages.Bmp.V5Header, 3780, 3780 , PixelResolutionUnit.PixelsPerMeter },
+            { TestImages.Bmp.RLE, 2835, 2835, PixelResolutionUnit.PixelsPerMeter }
+        };
 
         public BmpEncoderTests(ITestOutputHelper output)
         {
@@ -25,6 +35,31 @@ namespace SixLabors.ImageSharp.Tests
 
         private ITestOutputHelper Output { get; }
 
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Encode_PreserveRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var options = new BmpEncoder();
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        ImageMetaData meta = output.MetaData;
+                        Assert.Equal(xResolution, meta.HorizontalResolution);
+                        Assert.Equal(yResolution, meta.VerticalResolution);
+                        Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                    }
+                }
+            }
+        }
+
         [Theory]
         [WithTestPatternImages(nameof(BitsPerPixel), 24, 24, PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.Rgb24)]
         public void Encode_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
@@ -44,10 +79,10 @@ namespace SixLabors.ImageSharp.Tests
         {
             TestBmpEncoderCore(provider, bitsPerPixel);
         }
-        
+
         private static void TestBmpEncoderCore<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
             where TPixel : struct, IPixel<TPixel>
-        {   
+        {
             using (Image<TPixel> image = provider.GetImage())
             {
                 // there is no alpha in bmp!

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -4,7 +4,6 @@
 using System.Text;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.Primitives;
 using Xunit;
 using System.IO;
 using SixLabors.ImageSharp.Advanced;
@@ -13,7 +12,7 @@ using SixLabors.ImageSharp.Advanced;
 namespace SixLabors.ImageSharp.Tests.Formats.Gif
 {
     using System.Collections.Generic;
-
+    using SixLabors.ImageSharp.MetaData;
     using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 
     public class GifDecoderTests
@@ -21,31 +20,43 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
         private const PixelTypes TestPixelTypes = PixelTypes.Rgba32 | PixelTypes.RgbaVector | PixelTypes.Argb32;
 
         public static readonly string[] MultiFrameTestFiles =
-            {
-                TestImages.Gif.Giphy, TestImages.Gif.Kumin
-            };
+        {
+            TestImages.Gif.Giphy, TestImages.Gif.Kumin
+        };
 
         public static readonly string[] BasicVerificationFiles =
-            {
-                TestImages.Gif.Cheers,
-                TestImages.Gif.Rings,
+        {
+            TestImages.Gif.Cheers,
+            TestImages.Gif.Rings,
 
-                // previously DecodeBadApplicationExtensionLength:
-                TestImages.Gif.Issues.BadAppExtLength,
-                TestImages.Gif.Issues.BadAppExtLength_2,
+            // previously DecodeBadApplicationExtensionLength:
+            TestImages.Gif.Issues.BadAppExtLength,
+            TestImages.Gif.Issues.BadAppExtLength_2,
 
-                // previously DecodeBadDescriptorDimensionsLength:
-                TestImages.Gif.Issues.BadDescriptorWidth
-            };
+            // previously DecodeBadDescriptorDimensionsLength:
+            TestImages.Gif.Issues.BadDescriptorWidth
+        };
+
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Gif.Rings, (int)ImageMetaData.DefaultHorizontalResolution, (int)ImageMetaData.DefaultVerticalResolution , PixelResolutionUnit.PixelsPerInch},
+            { TestImages.Gif.Ratio1x4, 1, 4 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Gif.Ratio4x1, 4, 1, PixelResolutionUnit.AspectRatio }
+        };
 
         private static readonly Dictionary<string, int> BasicVerificationFrameCount =
-            new Dictionary<string, int>
-                {
-                    [TestImages.Gif.Cheers] = 93,
-                    [TestImages.Gif.Issues.BadDescriptorWidth] = 36,
-            };
+        new Dictionary<string, int>
+        {
+            [TestImages.Gif.Cheers] = 93,
+            [TestImages.Gif.Issues.BadDescriptorWidth] = 36,
+        };
 
-        public static readonly string[] BadAppExtFiles = { TestImages.Gif.Issues.BadAppExtLength, TestImages.Gif.Issues.BadAppExtLength_2 };
+        public static readonly string[] BadAppExtFiles =
+        {
+            TestImages.Gif.Issues.BadAppExtLength,
+            TestImages.Gif.Issues.BadAppExtLength_2
+        };
 
         [Theory]
         [WithFileCollection(nameof(MultiFrameTestFiles), PixelTypes.Rgba32)]
@@ -59,6 +70,40 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
             }
         }
 
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Decode_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new GifDecoder();
+                using (Image<Rgba32> image = decoder.Decode<Rgba32>(Configuration.Default, stream))
+                {
+                    ImageMetaData meta = image.MetaData;
+                    Assert.Equal(xResolution, meta.HorizontalResolution);
+                    Assert.Equal(yResolution, meta.VerticalResolution);
+                    Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                }
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Identify_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new GifDecoder();
+                IImageInfo image = decoder.Identify(Configuration.Default, stream);
+                ImageMetaData meta = image.MetaData;
+                Assert.Equal(xResolution, meta.HorizontalResolution);
+                Assert.Equal(yResolution, meta.VerticalResolution);
+                Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+            }
+        }
+
         [Theory]
         [WithFile(TestImages.Gif.Trans, TestPixelTypes)]
         public void GifDecoder_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider)
@@ -88,7 +133,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
                 image.CompareFirstFrameToReferenceOutput(ImageComparer.Exact, provider);
             }
         }
-        
+
         [Fact]
         public void Decode_IgnoreMetadataIsFalse_CommentsAreRead()
         {
@@ -169,7 +214,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
         [InlineData(TestImages.Gif.Trans, 8)]
         public void DetectPixelSize(string imagePath, int expectedPixelSize)
         {
-            TestFile testFile = TestFile.Create(imagePath);
+            var testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
                 Assert.Equal(expectedPixelSize, Image.Identify(stream)?.PixelType?.BitsPerPixel);

tests/ImageSharp.Tests/Formats/Gif/GifEncoderTests.cs

@@ -17,6 +17,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
         private const PixelTypes TestPixelTypes = PixelTypes.Rgba32 | PixelTypes.RgbaVector | PixelTypes.Argb32;
         private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.001F);
 
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Gif.Rings, (int)ImageMetaData.DefaultHorizontalResolution, (int)ImageMetaData.DefaultVerticalResolution , PixelResolutionUnit.PixelsPerInch},
+            { TestImages.Gif.Ratio1x4, 1, 4 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Gif.Ratio4x1, 4, 1, PixelResolutionUnit.AspectRatio }
+        };
+
         [Theory]
         [WithTestPatternImages(100, 100, TestPixelTypes)]
         public void EncodeGeneratedPatterns<TPixel>(TestImageProvider<TPixel> provider)
@@ -43,6 +51,34 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
             }
         }
 
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Encode_PreserveRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var options = new GifEncoder()
+            {
+                IgnoreMetadata = false
+            };
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        ImageMetaData meta = output.MetaData;
+                        Assert.Equal(xResolution, meta.HorizontalResolution);
+                        Assert.Equal(yResolution, meta.VerticalResolution);
+                        Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                    }
+                }
+            }
+        }
+
         [Fact]
         public void Encode_IgnoreMetadataIsFalse_CommentsAreWritten()
         {

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(PixelResolutionUnit.PixelsPerInch, marker.DensityUnits);
             Assert.Equal(96, marker.XDensity);
             Assert.Equal(96, marker.YDensity);
         }

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.MetaData.cs

@@ -15,30 +15,39 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
     using System.Runtime.CompilerServices;
 
     using SixLabors.ImageSharp.Formats.Jpeg;
+    using SixLabors.ImageSharp.MetaData;
 
     public partial class JpegDecoderTests
     {
         // TODO: A JPEGsnoop & metadata expert should review if the Exif/Icc expectations are correct. 
         // I'm seeing several entries with Exif-related names in images where we do not decode an exif profile. (- Anton)
         public static readonly TheoryData<bool, string, int, bool, bool> MetaDataTestData =
-            new TheoryData<bool, string, int, bool, bool>
-                {
-                    { false, TestImages.Jpeg.Progressive.Progress, 24, false, false },
-                    { false, TestImages.Jpeg.Progressive.Fb, 24, false, true },
-                    { false, TestImages.Jpeg.Baseline.Cmyk, 32, false, true },
-                    { false, TestImages.Jpeg.Baseline.Ycck, 32, true, true },
-                    { false, TestImages.Jpeg.Baseline.Jpeg400, 8, false, false },
-                    { false, TestImages.Jpeg.Baseline.Snake, 24, true, true },
-                    { false, TestImages.Jpeg.Baseline.Jpeg420Exif, 24, true, false },
-
-                    { true, TestImages.Jpeg.Progressive.Progress, 24, false, false },
-                    { true, TestImages.Jpeg.Progressive.Fb, 24, false, true },
-                    { true, TestImages.Jpeg.Baseline.Cmyk, 32, false, true },
-                    { true, TestImages.Jpeg.Baseline.Ycck, 32, true, true },
-                    { true, TestImages.Jpeg.Baseline.Jpeg400, 8, false, false },
-                    { true, TestImages.Jpeg.Baseline.Snake, 24, true, true },
-                    { true, TestImages.Jpeg.Baseline.Jpeg420Exif, 24, true, false },
-                };
+        new TheoryData<bool, string, int, bool, bool>
+        {
+            { false, TestImages.Jpeg.Progressive.Progress, 24, false, false },
+            { false, TestImages.Jpeg.Progressive.Fb, 24, false, true },
+            { false, TestImages.Jpeg.Baseline.Cmyk, 32, false, true },
+            { false, TestImages.Jpeg.Baseline.Ycck, 32, true, true },
+            { false, TestImages.Jpeg.Baseline.Jpeg400, 8, false, false },
+            { false, TestImages.Jpeg.Baseline.Snake, 24, true, true },
+            { false, TestImages.Jpeg.Baseline.Jpeg420Exif, 24, true, false },
+
+            { true, TestImages.Jpeg.Progressive.Progress, 24, false, false },
+            { true, TestImages.Jpeg.Progressive.Fb, 24, false, true },
+            { true, TestImages.Jpeg.Baseline.Cmyk, 32, false, true },
+            { true, TestImages.Jpeg.Baseline.Ycck, 32, true, true },
+            { true, TestImages.Jpeg.Baseline.Jpeg400, 8, false, false },
+            { true, TestImages.Jpeg.Baseline.Snake, 24, true, true },
+            { true, TestImages.Jpeg.Baseline.Jpeg420Exif, 24, true, false },
+        };
+
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Jpeg.Baseline.Ratio1x1, 1, 1 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Jpeg.Baseline.Snake, 300, 300 , PixelResolutionUnit.PixelsPerInch},
+            { TestImages.Jpeg.Baseline.GammaDalaiLamaGray, 72, 72, PixelResolutionUnit.PixelsPerInch }
+        };
 
         [Theory]
         [MemberData(nameof(MetaDataTestData))]
@@ -76,14 +85,49 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 iccProfilePresent);
         }
 
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Decode_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new JpegDecoder();
+                using (Image<Rgba32> image = decoder.Decode<Rgba32>(Configuration.Default, stream))
+                {
+                    ImageMetaData meta = image.MetaData;
+                    Assert.Equal(xResolution, meta.HorizontalResolution);
+                    Assert.Equal(yResolution, meta.VerticalResolution);
+                    Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                }
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Identify_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new JpegDecoder();
+                IImageInfo image = decoder.Identify(Configuration.Default, stream);
+                ImageMetaData meta = image.MetaData;
+                Assert.Equal(xResolution, meta.HorizontalResolution);
+                Assert.Equal(yResolution, meta.VerticalResolution);
+                Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+            }
+        }
+
         private static void TestImageInfo(string imagePath, IImageDecoder decoder, bool useIdentify, Action<IImageInfo> test)
         {
             var testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
                 IImageInfo imageInfo = useIdentify
-                                           ? ((IImageInfoDetector)decoder).Identify(Configuration.Default, stream)
-                                           : decoder.Decode<Rgba32>(Configuration.Default, stream);
+                ? ((IImageInfoDetector)decoder).Identify(Configuration.Default, stream)
+                : decoder.Decode<Rgba32>(Configuration.Default, stream);
+
                 test(imageInfo);
             }
         }
@@ -141,7 +185,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                         }
                     });
         }
-        
+
         [Theory]
         [InlineData(false)]
         [InlineData(true)]
@@ -166,7 +210,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 }
             }
         }
-        
+
         [Theory]
         [InlineData(false)]
         [InlineData(true)]

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

@@ -3,6 +3,7 @@
 
 using System.IO;
 using SixLabors.ImageSharp.Formats.Jpeg;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
@@ -14,16 +15,24 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
     public class JpegEncoderTests
     {
         public static readonly TheoryData<JpegSubsample, int> BitsPerPixel_Quality =
-            new TheoryData<JpegSubsample, int>
-                {
-                    { JpegSubsample.Ratio420, 40 },
-                    { JpegSubsample.Ratio420, 60 },
-                    { JpegSubsample.Ratio420, 100 },
+        new TheoryData<JpegSubsample, int>
+        {
+            { JpegSubsample.Ratio420, 40 },
+            { JpegSubsample.Ratio420, 60 },
+            { JpegSubsample.Ratio420, 100 },
 
-                    { JpegSubsample.Ratio444, 40 },
-                    { JpegSubsample.Ratio444, 60 },
-                    { JpegSubsample.Ratio444, 100 },
-                };
+            { JpegSubsample.Ratio444, 40 },
+            { JpegSubsample.Ratio444, 60 },
+            { JpegSubsample.Ratio444, 100 },
+        };
+
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Jpeg.Baseline.Ratio1x1, 1, 1 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Jpeg.Baseline.Snake, 300, 300 , PixelResolutionUnit.PixelsPerInch},
+            { TestImages.Jpeg.Baseline.GammaDalaiLamaGray, 72, 72, PixelResolutionUnit.PixelsPerInch }
+        };
 
         [Theory]
         [WithFile(TestImages.Png.CalliphoraPartial, nameof(BitsPerPixel_Quality), PixelTypes.Rgba32)]
@@ -82,10 +91,10 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 image.Mutate(c => c.MakeOpaque());
 
                 var encoder = new JpegEncoder()
-                                  {
-                                      Subsample = subsample,
-                                      Quality = quality
-                                  };
+                {
+                    Subsample = subsample,
+                    Quality = quality
+                };
                 string info = $"{subsample}-Q{quality}";
                 ImageComparer comparer = GetComparer(quality, subsample);
 
@@ -93,7 +102,6 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 image.VerifyEncoder(provider, "jpeg", info, encoder, comparer, referenceImageExtension: "png");
             }
         }
-        
 
         [Theory]
         [InlineData(false)]
@@ -104,7 +112,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             {
                 IgnoreMetadata = ignoreMetaData
             };
-            
+
             using (Image<Rgba32> input = TestFile.Create(TestImages.Jpeg.Baseline.Floorplan).CreateImage())
             {
                 using (var memStream = new MemoryStream())
@@ -126,7 +134,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 }
             }
         }
-        
+
         [Fact]
         public void Quality_0_And_1_Are_Identical()
         {
@@ -172,5 +180,30 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 Assert.NotEqual(memStream0.ToArray(), memStream1.ToArray());
             }
         }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Encode_PreserveRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var options = new JpegEncoder();
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        ImageMetaData meta = output.MetaData;
+                        Assert.Equal(xResolution, meta.HorizontalResolution);
+                        Assert.Equal(yResolution, meta.VerticalResolution);
+                        Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                    }
+                }
+            }
+        }
     }
 }

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.Chunks.cs

@@ -77,8 +77,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
         [Theory]
         [InlineData((uint)PngChunkType.Gamma)] // gAMA
         [InlineData((uint)PngChunkType.PaletteAlpha)] // tRNS
-        [InlineData(
-            (uint)PngChunkType.Physical)] // pHYs: It's ok to test physical as we don't throw for duplicate chunks.
+        [InlineData((uint)PngChunkType.Physical)] // pHYs: It's ok to test physical as we don't throw for duplicate chunks.
         //[InlineData(PngChunkTypes.Text)] //TODO: Figure out how to test this
         public void Decode_IncorrectCRCForNonCriticalChunk_ExceptionIsThrown(uint chunkType)
         {
@@ -112,9 +111,11 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 
         private static void WriteChunk(MemoryStream memStream, string chunkName)
         {
-            memStream.Write(new byte[] { 0, 0, 0, 1 }, 0, 4);
-            memStream.Write(Encoding.GetEncoding("ASCII").GetBytes(chunkName), 0, 4);
-            memStream.Write(new byte[] { 0, 0, 0, 0, 0 }, 0, 5);
+            // Needs a minimum length of 9 for pHYs chunk.
+            memStream.Write(new byte[] { 0, 0, 0, 9 }, 0, 4);
+            memStream.Write(Encoding.GetEncoding("ASCII").GetBytes(chunkName), 0, 4); // 4 bytes chunk header
+            memStream.Write(new byte[] { 0, 0, 0, 0, 0, 0, 0, 0, 0 }, 0, 9); // 9 bytes of chunk data
+            memStream.Write(new byte[] { 0, 0, 0, 0 }, 0, 4); // Junk Crc
         }
 
         private static void WriteDataChunk(MemoryStream memStream)

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -8,6 +8,7 @@ using System.IO;
 using System.Text;
 
 using SixLabors.ImageSharp.Formats.Png;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 
@@ -18,8 +19,6 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
     public partial class PngDecoderTests
     {
         private const PixelTypes PixelTypes = Tests.PixelTypes.Rgba32 | Tests.PixelTypes.RgbaVector | Tests.PixelTypes.Argb32;
-        
-        
 
         public static readonly string[] CommonTestImages =
         {
@@ -67,6 +66,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             TestImages.Png.GrayTrns16BitInterlaced
         };
 
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Png.Splash, 11810, 11810 , PixelResolutionUnit.PixelsPerMeter},
+            { TestImages.Png.Ratio1x4, 1, 4 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Png.Ratio4x1, 4, 1, PixelResolutionUnit.AspectRatio }
+        };
+
         [Theory]
         [WithFileCollection(nameof(CommonTestImages), PixelTypes.Rgba32)]
         public void Decode<TPixel>(TestImageProvider<TPixel> provider)
@@ -218,5 +225,39 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
                 Assert.Equal(expectedPixelSize, Image.Identify(stream)?.PixelType?.BitsPerPixel);
             }
         }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Decode_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new PngDecoder();
+                using (Image<Rgba32> image = decoder.Decode<Rgba32>(Configuration.Default, stream))
+                {
+                    ImageMetaData meta = image.MetaData;
+                    Assert.Equal(xResolution, meta.HorizontalResolution);
+                    Assert.Equal(yResolution, meta.VerticalResolution);
+                    Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                }
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Identify_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var decoder = new PngDecoder();
+                IImageInfo image = decoder.Identify(Configuration.Default, stream);
+                ImageMetaData meta = image.MetaData;
+                Assert.Equal(xResolution, meta.HorizontalResolution);
+                Assert.Equal(yResolution, meta.VerticalResolution);
+                Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+            }
+        }
     }
 }

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -7,6 +7,7 @@ using System.Linq;
 
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Png;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
@@ -61,6 +62,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             80, 100, 120, 230
         };
 
+        public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
+        new TheoryData<string, int, int, PixelResolutionUnit>
+        {
+            { TestImages.Png.Splash, 11810, 11810 , PixelResolutionUnit.PixelsPerMeter},
+            { TestImages.Png.Ratio1x4, 1, 4 , PixelResolutionUnit.AspectRatio},
+            { TestImages.Png.Ratio4x1, 4, 1, PixelResolutionUnit.AspectRatio }
+        };
+
         [Theory]
         [WithFile(TestImages.Png.Palette8Bpp, nameof(PngColorTypes), PixelTypes.Rgba32)]
         [WithTestPatternImages(nameof(PngColorTypes), 48, 24, PixelTypes.Rgba32)]
@@ -256,5 +265,30 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
                 Assert.Equal(expected, data);
             }
         }
+
+        [Theory]
+        [MemberData(nameof(RatioFiles))]
+        public void Encode_PreserveRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        {
+            var options = new PngEncoder();
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        ImageMetaData meta = output.MetaData;
+                        Assert.Equal(xResolution, meta.HorizontalResolution);
+                        Assert.Equal(yResolution, meta.VerticalResolution);
+                        Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                    }
+                }
+            }
+        }
     }
 }

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);
+        }
+    }
+}

tests/ImageSharp.Tests/TestImages.cs

@@ -65,6 +65,9 @@ namespace SixLabors.ImageSharp.Tests
             public const string Banner7Adam7InterlaceMode = "Png/banner7-adam.png";
             public const string Banner8Index = "Png/banner8-index.png";
 
+            public const string Ratio1x4 = "Png/ratio-1x4.png";
+            public const string Ratio4x1 = "Png/ratio-4x1.png";
+
             public static class Bad
             {
                 // Odd chunk lengths
@@ -79,7 +82,7 @@ namespace SixLabors.ImageSharp.Tests
                 Powerpoint, SplashInterlaced, Interlaced,
                 Filter0, Filter1, Filter2, Filter3, Filter4,
                 FilterVar, VimImage1, VimImage2, VersioningImage1,
-                VersioningImage2
+                VersioningImage2, Ratio4x1, Ratio1x4
             };
         }
 
@@ -124,13 +127,14 @@ namespace SixLabors.ImageSharp.Tests
                 public const string Jpeg420Small = "Jpg/baseline/jpeg420small.jpg";
                 public const string Testorig420 = "Jpg/baseline/testorig.jpg";
                 public const string MultiScanBaselineCMYK = "Jpg/baseline/MultiScanBaselineCMYK.jpg";
+                public const string Ratio1x1 = "Jpg/baseline/ratio-1x1.jpg";
 
                 public static readonly string[] All =
-                    {
-                        Cmyk, Ycck, Exif, Floorplan,
-                        Calliphora, Turtle, GammaDalaiLamaGray,
-                        Hiyamugi, Jpeg400, Jpeg420Exif, Jpeg444,
-                    };
+                {
+                    Cmyk, Ycck, Exif, Floorplan,
+                    Calliphora, Turtle, GammaDalaiLamaGray,
+                    Hiyamugi, Jpeg400, Jpeg420Exif, Jpeg444, Ratio1x1
+                };
             }
 
             public static class Issues
@@ -176,15 +180,17 @@ namespace SixLabors.ImageSharp.Tests
             public const string Cheers = "Gif/cheers.gif";
             public const string Trans = "Gif/trans.gif";
             public const string Kumin = "Gif/kumin.gif";
+            public const string Ratio4x1 = "Gif/base_4x1.gif";
+            public const string Ratio1x4 = "Gif/base_1x4.gif";
 
-            public class Issues
+            public static class Issues
             {
                 public const string BadAppExtLength = "Gif/issues/issue405_badappextlength252.gif";
                 public const string BadAppExtLength_2 = "Gif/issues/issue405_badappextlength252-2.gif";
                 public const string BadDescriptorWidth = "Gif/issues/issue403_baddescriptorwidth.gif";
             }
 
-            public static readonly string[] All = { Rings, Giphy, Cheers, Trans, Kumin };
+            public static readonly string[] All = { Rings, Giphy, Cheers, Trans, Kumin, Ratio4x1, Ratio1x4 };
         }
     }
 }

tests/Images/Input/Gif/base_1x4.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:56e2409223f140145db2ba405f5562451dc0fa9d4274830fb02bd78d42552162
+size 1620

tests/Images/Input/Gif/base_4x1.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:24fd7e9dd3c6516ddab7336a30efc5901754b6d43a9d989c6fbb3e06d8944c80
+size 1620

tests/Images/Input/Jpg/baseline/ratio-1x1.jpg

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:1e9410c16bc61f08bbc18ae0e0b13181c9c4f57c66e82a7c6e593a57f40756d7
+size 34674

tests/Images/Input/Png/ratio-1x4.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:438018f19c85e582cb586ac7cca2220008ecb7fc70ce50e8f2a76b494c128a20
+size 404

tests/Images/Input/Png/ratio-4x1.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:a068eaf1f7040490e08eda3259befb6689849dd0ff8bb4cc03c705d117cb2b9f
+size 344