AquaGeek · August 23, 2017 19:35 · Feb 1, 2013 · Feb 1, 2013
diff --git a/png.rb b/png.rb
@@ -76,7 +76,8 @@ def normalize(file_path)
           (0...@height).each do |y|
             i = new_data.length
 
-            # With filter method 0, the only one currently defined, we have to prepend a filter type byte to each scan line. Currently, we just copy what was there before (though this could be wrong).
+            # With filter method 0, the only one currently defined, we have to prepend a filter type byte to each scan line.
+            # Currently, we just copy what was there before (though this could be wrong).
             new_data += uncompressed[i]
 
             (0...@width).each do |x|

diff --git a/png.rb b/png.rb
@@ -0,0 +1,139 @@
+# Helper method to fix Apple's stupid png optimizations
+# Adapted from:
+#   http://www.axelbrz.com.ar/?mod=iphone-png-images-normalizer
+#   https://github.com/peperzaken/iPhone-optimized-PNG-reverse-script/blob/master/Peperzaken/Ios/DecodeImage.php
+# PNG spec: http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
+
+require 'zlib'
+require 'logger'
+
+module PNG
+  extend self
+
+  attr_accessor :logger
+
+  PNGHEADER = "\x89PNG\r\n\x1A\n".force_encoding('ASCII-8BIT')
+
+  # TODO: Accept output file path?
+  def normalize(file_path)
+    File.open(file_path, 'rb') do |f|
+      header_data = f.read(8)
+
+      # Check if it's a PNG
+      if header_data != PNGHEADER
+        logger.error "File is not a PNG" if logger
+        # TODO: Raise exception?
+        return nil
+      end
+
+      chunks = []
+      idat_data_chunks = []
+      iphone_compressed = false
+
+      while !f.eof?
+        # Unpack the chunk
+        chunk = {}
+        chunk['length'] = f.read(4).unpack("L>").first
+        chunk['type'] = f.read(4)
+        data = f.read(chunk['length'])  # Can be 0...
+        chunk['crc'] = f.read(4).unpack("L>").first
+
+        logger.debug "Chunk found :: length: #{chunk['length']}, type: #{chunk['type']}" if logger
+
+        # This chunk is first when it's an iPhone compressed image
+        if chunk['type'] == 'CgBI'
+          iphone_compressed = true
+        end
+
+        # Extract the header
+        #   Width:              4 bytes
+        #   Height:             4 bytes
+        #   Bit depth:          1 byte
+        #   Color type:         1 byte
+        #   Compression method: 1 byte
+        #   Filter method:      1 byte
+        #   Interlace method:   1 byte
+        if chunk['type'] == 'IHDR' && iphone_compressed
+          @width = data[0, 4].unpack("L>").first
+          @height = data[4, 4].unpack("L>").first
+          @bit_depth = data[8, 1].unpack("C").first
+          @filter_method = data[11, 1].unpack("C").first
+          logger.info "Image size: #{@width}x#{@height} (#{@bit_depth}-bit)" if logger
+        end
+
+        # Extract and mutate the data chunk if needed (can be multiple)
+        if chunk['type'] == 'IDAT' && iphone_compressed
+          idat_data_chunks << data
+          next
+        elsif idat_data_chunks.length > 0
+          # All the IDAT chunks must be consecutive. Consequently, if we reach this point, we've
+          # already seen at least one.
+          idat_data = idat_data_chunks.join('')
+          uncompressed = zlib_inflate(idat_data)
+
+          # Let's swap some colors
+          new_data = ''
+          (0...@height).each do |y|
+            i = new_data.length
+
+            # With filter method 0, the only one currently defined, we have to prepend a filter type byte to each scan line. Currently, we just copy what was there before (though this could be wrong).
+            new_data += uncompressed[i]
+
+            (0...@width).each do |x|
+              i = new_data.length
+
+              # Swap BGRA to RGBA
+              new_data += uncompressed[i + 2]  # Red
+              new_data += uncompressed[i + 1]  # Green
+              new_data += uncompressed[i + 0]  # Blue
+              new_data += uncompressed[i + 3]  # Alpha
+            end
+          end
+
+          # Compress the data again after swapping (this time with the headers, CRC, etc)
+          # TODO: Split into multiple IDAT chunks
+          idat_data = zlib_deflate(new_data)
+          idat_chunk = {
+            'type' => 'IDAT',
+            'length' => idat_data.length,
+            'data' => idat_data,
+            'crc' => Zlib::crc32('IDAT' + idat_data)
+          }
+          chunks << idat_chunk
+        end
+
+        chunk['data'] = data
+        chunks << chunk
+      end  # EOF
+
+      # Rebuild the image without the CgBI chunk
+      out = header_data
+      chunks.each do |chunk|
+        next if chunk['type'] == 'CgBI'
+        logger.debug "Writing #{chunk['type']}" if logger
+
+        out += [chunk['length']].pack("L>")
+        out += chunk['type']
+        out += chunk['data']
+        out += [chunk['crc']].pack("L>")
+      end
+      out
+    end  # File.open
+  end
+
+  private
+    def zlib_inflate(string)
+      zstream = Zlib::Inflate.new(-Zlib::MAX_WBITS)
+      buf = zstream.inflate(string)
+      # zstream.finish
+      zstream.close
+      buf
+    end
+
+    def zlib_deflate(string, level = Zlib::DEFAULT_COMPRESSION)
+      zstream = Zlib::Deflate.new(level)
+      buf = zstream.deflate(string, Zlib::FINISH)
+      zstream.close
+      buf
+    end
+end