Added support for writing DPI back to physical dimensions chunk.

lib/chunky_png/chunk.rb

@@ -335,12 +335,22 @@ def initialize(ppux, ppuy, unit = :unknown)
 
       def dpix
         raise ChunkyPNG::UnitsUnknown, 'the PNG specifies its physical aspect ratio, but does not specify the units of its pixels\' physical dimensions' unless unit == :meters
-        ppux * INCHES_PER_METER
+        ppux * METERS_PER_INCH
+      end
+
+      def dpix=(value)
+        @unit = :meters
+        @ppux = (value / METERS_PER_INCH).round
       end
 
       def dpiy
         raise ChunkyPNG::UnitsUnknown, 'the PNG specifies its physical aspect ratio, but does not specify the units of its pixels\' physical dimensions' unless unit == :meters
-        ppuy * INCHES_PER_METER
+        ppuy * METERS_PER_INCH
+      end
+
+      def dpiy=(value)
+        @unit = :meters
+        @ppuy = (value / METERS_PER_INCH).round
       end
 
       def self.read(type, content)
@@ -357,7 +367,7 @@ def content
         [ppux, ppuy, unit == :meters ? 1 : 0].pack('NNC')
       end
 
-      INCHES_PER_METER = 0.0254
+      METERS_PER_INCH = 0.0254
     end
 
     # The Text (iTXt) chunk contains keyword/value metadata about the PNG

lib/chunky_png/datastream.rb

@@ -126,6 +126,7 @@ def each_chunk
       other_chunks.each { |chunk| yield(chunk) }
       yield(palette_chunk)      if palette_chunk
       yield(transparency_chunk) if transparency_chunk
+      yield(physical_chunk)     if physical_chunk
       data_chunks.each  { |chunk| yield(chunk) }
       yield(end_chunk)
     end

spec/chunky_png/datastream_spec.rb

@@ -55,5 +55,14 @@
       expect{physical_chunk.dpix}.to raise_error(ChunkyPNG::UnitsUnknown)
       expect{physical_chunk.dpiy}.to raise_error(ChunkyPNG::UnitsUnknown)
     end
+
+    it 'should set units and ppu correctly when setting dpi' do
+      physical_chunk = ChunkyPNG::Chunk::Physical.new(9001, 9001, :unknown)
+      physical_chunk.dpix = 72
+      physical_chunk.dpiy = 72
+      expect(physical_chunk.unit).to eql :meters
+      expect(physical_chunk.ppux).to eql 2835
+      expect(physical_chunk.ppuy).to eql 2835
+    end
   end
 end