Properly handle png resolution.

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

@@ -425,7 +425,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort
                     : 0;
 
                 byte units = this.MetaData.ExifProfile.TryGetValue(ExifTag.ResolutionUnit, out ExifValue resolutionTag)
-                    ? (byte)(((ushort)resolutionTag.Value) - 1) // EXIF is 1,2,3
+                    ? (byte)(((ushort)resolutionTag.Value) - 1) // ExifTag.ResolutionUnit values are 1, 2, 3
                     : byte.MinValue;
 
                 if (horizontalValue > 0 && verticalValue > 0)

src/ImageSharp/Formats/Png/PngConstants.cs

@@ -41,5 +41,11 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// The header bytes as a big endian coded ulong.
         /// </summary>
         public const ulong HeaderValue = 0x89504E470D0A1A0AUL;
+
+        /// <summary>
+        /// The number of inches in a meter. Used for converting PPM to PPI in <see cref="PngChunkType.Physical"/>.
+        ///  One inch is equal to exactly 0.0254 meters.
+        /// </summary>
+        public const double InchesInMeter = 1 / 0.0254D;
     }
 }

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -233,7 +233,7 @@ 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 +307,7 @@ 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 +396,34 @@ 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.
+            // Conversion note: one inch is equal to exactly 0.0254 meters.
+            int hResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(0, 4));
+            int vResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(4, 4));
+            byte unit = data[8];
+
+            if (unit == byte.MinValue)
+            {
+                metadata.HorizontalResolution = hResolution;
+                metadata.VerticalResolution = vResolution;
+                metadata.ResolutionUnits = ResolutionUnits.AspectRatio;
+                return;
+            }
+
+            // Use PPI for its commonality.
+            const double inchesInMeter = PngConstants.InchesInMeter;
+            metadata.HorizontalResolution = hResolution / inchesInMeter;
+            metadata.VerticalResolution = vResolution / inchesInMeter;
+            metadata.ResolutionUnits = ResolutionUnits.PixelsPerInch;
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -8,6 +8,7 @@ using System.Linq;
 using SixLabors.ImageSharp.Advanced;
 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 +599,46 @@ 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.
+            // Conversion note: one inch is equal to exactly 0.0254 meters.
+            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 ResolutionUnits.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.HorizontalResolution));
+                    break;
+
+                case ResolutionUnits.PixelsPerCentimeter:
 
-                this.chunkDataBuffer[8] = 1;
+                    this.chunkDataBuffer[8] = 1;
+                    const int CmInMeter = 100;
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution * CmInMeter));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.HorizontalResolution * CmInMeter));
+                    break;
 
-                this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, 9);
+                default:
+
+                    this.chunkDataBuffer[8] = 1;
+                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution * PngConstants.InchesInMeter));
+                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.HorizontalResolution * PngConstants.InchesInMeter));
+                    break;
             }
+
+            this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, 9);
         }
 
         /// <summary>