Read/Write bmp resolution

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -59,6 +59,11 @@ 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 +108,7 @@ 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 +162,7 @@ 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 +523,17 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 throw new NotSupportedException($"ImageSharp does not support this BMP file. HeaderSize: {headerSize}.");
             }
 
+            // Resolution is stored in PPM.
+            ImageMetaData meta = new ImageMetaData();
+            if (this.infoHeader.XPelsPerMeter > 0 && this.infoHeader.YPelsPerMeter > 0)
+            {
+                meta.ResolutionUnits = PixelResolutionUnit.PixelsPerMeter;
+                meta.HorizontalResolution = this.infoHeader.XPelsPerMeter;
+                meta.VerticalResolution = this.infoHeader.YPelsPerMeter;
+            }
+
+            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/Jpeg/JpegEncoderCore.cs

@@ -5,7 +5,7 @@ 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;
@@ -448,11 +448,23 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.buffer[12] = 0x01; // versionlo
 
             // Resolution. Big Endian
-            this.buffer[13] = (byte)meta.ResolutionUnits; // xyunits
             Span<byte> hResolution = this.buffer.AsSpan(14, 2);
             Span<byte> vResolution = this.buffer.AsSpan(16, 2);
-            BinaryPrimitives.WriteInt16BigEndian(hResolution, (short)Math.Round(meta.HorizontalResolution));
-            BinaryPrimitives.WriteInt16BigEndian(vResolution, (short)Math.Round(meta.VerticalResolution));
+
+            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/Png/PngDecoderCore.cs

@@ -411,18 +411,12 @@ namespace SixLabors.ImageSharp.Formats.Png
             int vResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(4, 4));
             byte unit = data[8];
 
-            if (unit == byte.MinValue)
-            {
-                metadata.HorizontalResolution = hResolution;
-                metadata.VerticalResolution = vResolution;
-                metadata.ResolutionUnits = PixelResolutionUnit.AspectRatio;
-                return;
-            }
+            metadata.ResolutionUnits = unit == byte.MinValue
+                ? PixelResolutionUnit.AspectRatio
+                : PixelResolutionUnit.PixelsPerMeter;
 
-            // Use PPC since original is in meters.
-            metadata.HorizontalResolution = UnitConverter.MeterToCm(hResolution);
-            metadata.VerticalResolution = UnitConverter.MeterToCm(vResolution);
-            metadata.ResolutionUnits = PixelResolutionUnit.PixelsPerCentimeter;
+            metadata.HorizontalResolution = hResolution;
+            metadata.VerticalResolution = vResolution;
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -623,6 +623,13 @@ namespace SixLabors.ImageSharp.Formats.Png
                     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; // Per meter
@@ -633,8 +640,8 @@ namespace SixLabors.ImageSharp.Formats.Png
                 default:
 
                     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)));
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.VerticalResolution));
                     break;
             }
 

src/ImageSharp/MetaData/PixelResolutionUnit.cs

@@ -9,18 +9,23 @@ namespace SixLabors.ImageSharp.MetaData
     public enum PixelResolutionUnit : byte
     {
         /// <summary>
-        /// No units; width:height pixel aspect ratio = Ydensity:Xdensity
+        /// No units; width:height pixel aspect ratio.
         /// </summary>
         AspectRatio = 0,
 
         /// <summary>
-        /// Pixels per inch (2.54 cm)
+        /// Pixels per inch (2.54 cm).
         /// </summary>
-        PixelsPerInch = 1, // Other image formats would default to this.
+        PixelsPerInch = 1,
 
         /// <summary>
         /// Pixels per centimeter.
         /// </summary>
-        PixelsPerCentimeter = 2
+        PixelsPerCentimeter = 2,
+
+        /// <summary>
+        /// Pixels per meter (100 cm).
+        /// </summary>
+        PixelsPerMeter = 3
     }
 }