Add physical pixel size handling for PNG.

lib/src/formats/png/png_info.dart

@@ -25,6 +25,51 @@ class PngColorType {
 
 enum PngFilterType { none, sub, up, average, paeth }
 
+/// The intended physical pixel size of the image.
+/// See <https://www.w3.org/TR/png-3/#11pHYs>.
+class PngPhysicalPixelDimensions {
+  static const double _inchesPerM = 39.3701;
+
+  /// Unit is unknown.
+  static const int unitUnknown = 0;
+
+  /// Unit is the meter.
+  static const int unitMeter = 1;
+
+  /// Pixels per unit on the X axis.
+  final int xPxPerUnit;
+
+  /// Pixels per unit on the Y axis.
+  final int yPxPerUnit;
+
+  /// Unit specifier, either [unitUnknown] or [unitMeter].
+  final int unitSpecifier;
+
+  /// Constructs a dimension descriptor with the given values.
+  const PngPhysicalPixelDimensions(
+      {required this.xPxPerUnit,
+      required this.yPxPerUnit,
+      required this.unitSpecifier});
+
+  /// Constructs a dimension descriptor specifying x and y resolution in dots
+  /// per inch (DPI). If [yDpi] is unspecified, [xDpi] is used for both x and y
+  /// axes.
+  PngPhysicalPixelDimensions.dpi(int xDpi, [int? yDpi])
+      : xPxPerUnit = (xDpi * _inchesPerM).round(),
+        yPxPerUnit = ((yDpi ?? xDpi) * _inchesPerM).round(),
+        unitSpecifier = unitMeter;
+
+  @override
+  int get hashCode => Object.hash(xPxPerUnit, yPxPerUnit, unitSpecifier);
+
+  @override
+  bool operator ==(Object other) =>
+      other is PngPhysicalPixelDimensions &&
+      other.xPxPerUnit == xPxPerUnit &&
+      other.yPxPerUnit == yPxPerUnit &&
+      other.unitSpecifier == unitSpecifier;
+}
+
 class PngInfo implements DecodeInfo {
   @override
   int width = 0;
@@ -44,6 +89,7 @@ class PngInfo implements DecodeInfo {
   int iccpCompression = 0;
   Uint8List? iccpData;
   Map<String, String> textData = {};
+  PngPhysicalPixelDimensions? pixelDimensions;
 
   // APNG extensions
   @override

lib/src/formats/png_decoder.dart

@@ -75,6 +75,15 @@ class PngDecoder extends Decoder {
           }
           _input.skip(4); //crc
           break;
+        case 'pHYs':
+          final physData = InputBuffer.from(_input.readBytes(chunkSize));
+          final x = physData.readUint32();
+          final y = physData.readUint32();
+          final unit = physData.readByte();
+          _info.pixelDimensions = PngPhysicalPixelDimensions(
+              xPxPerUnit: x, yPxPerUnit: y, unitSpecifier: unit);
+          _input.skip(4); // CRC
+          break;
         case 'IHDR':
           final hdr = InputBuffer.from(_input.readBytes(chunkSize));
           final Uint8List hdrBytes = hdr.toUint8List();

lib/src/formats/png_encoder.dart

@@ -19,7 +19,7 @@ enum PngFilter { none, sub, up, average, paeth }
 class PngEncoder extends Encoder {
   Quantizer? _globalQuantizer;
 
-  PngEncoder({this.filter = PngFilter.paeth, this.level});
+  PngEncoder({this.filter = PngFilter.paeth, this.level, this.pixelDimensions});
 
   int _numChannels(Image image) => image.hasPalette ? 1 : image.numChannels;
 
@@ -74,6 +74,14 @@ class PngEncoder extends Encoder {
       }
     }
 
+    if (pixelDimensions != null) {
+      final phys = OutputBuffer(bigEndian: true)
+        ..writeUint32(pixelDimensions!.xPxPerUnit)
+        ..writeUint32(pixelDimensions!.yPxPerUnit)
+        ..writeByte(pixelDimensions!.unitSpecifier);
+      _writeChunk(output!, 'pHYs', phys.getBytes());
+    }
+
     if (isAnimated) {
       _writeFrameControlChunk(image);
       sequenceNumber++;
@@ -410,4 +418,5 @@ class PngEncoder extends Encoder {
   bool isAnimated = false;
   OutputBuffer? output;
   Map<String, String>? textData;
+  PngPhysicalPixelDimensions? pixelDimensions;
 }

test/formats/png_test.dart

@@ -439,6 +439,23 @@ void main() {
         expect(img2?.textData?["foo"], equals("bar"));
       });
 
+      test('pHYs', () {
+        final img = Image(width: 16, height: 16);
+        const phys1 = PngPhysicalPixelDimensions(
+            xPxPerUnit: 1000,
+            yPxPerUnit: 1000,
+            unitSpecifier: PngPhysicalPixelDimensions.unitMeter);
+        final png1 = PngEncoder(pixelDimensions: phys1).encode(img);
+        final dec1 = PngDecoder()..decode(png1);
+        expect(dec1.info.pixelDimensions, phys1);
+
+        final phys2 = PngPhysicalPixelDimensions.dpi(144, 288);
+        final png2 = PngEncoder(pixelDimensions: phys2).encode(img);
+        final dec2 = PngDecoder()..decode(png2);
+        expect(dec2.info.pixelDimensions, isNot(phys1));
+        expect(dec2.info.pixelDimensions, phys2);
+      });
+
       test('iCCP', () {
         final bytes = File('test/_data/png/iCCP.png').readAsBytesSync();
         final image = PngDecoder().decode(bytes)!;