Added code to display some info about the IHDR, pHYs, and tEXt chunks.

pngchunk/pngchunk.vb

@@ -24,10 +24,13 @@
 
         If Not FileIO.FileSystem.FileExists(filename) Then
             Console.WriteLine("File {0} not found!", filename)
+            Exit Sub
         End If
         Dim fs As New IO.FileStream(filename, IO.FileMode.Open, IO.FileAccess.Read, IO.FileShare.Read)
         Dim fi As New IO.FileInfo(filename)
 
+        Dim sizedigits As Integer = Math.Ceiling(Math.Log10(fs.Length))
+
         While Not fs.Position = fs.Length
             Dim startposition As Long = fs.Position
             FindHeader(fs)
@@ -59,40 +62,54 @@
             End If
 
 
-            Dim buf() As Byte
+            Dim lenbuf(3), nameanddatabuf(), databuf(), crcbuf(3) As Byte
             Dim len As Integer
 
             Dim chunklength As UInt32
             Dim chunktype As String = String.Empty
 
             While chunktype <> "IEND" AndAlso fs.Position <> fs.Length
-                ReDim buf(3)
+                Console.ForegroundColor = ConsoleColor.White
-                len = fs.Read(buf, 0, 4)
+
+                'read chunk length
+                len = fs.Read(lenbuf, 0, 4)
                 If len <> 4 Then
                     Console.WriteLine("Incomplete chunk length header")
                     Exit While
                 End If
 
-                'big endian
+                'read and print chunk length and offset in the file
-                chunklength = &H1000000L * buf(0) + &H10000L * buf(1) + &H100L * buf(2) + buf(3)
+                chunklength = BigEndianUInt32(lenbuf)
-                Console.Write("{0,10}:", chunklength)
+                Console.Write("{0," & sizedigits & "} @ {1,-" & sizedigits & "}:", chunklength, fs.Position - 4)
 
-                ReDim buf(chunklength + 3)
+                'read in the chunk data and type
-                len = fs.Read(buf, 0, chunklength + 4)
+                ReDim nameanddatabuf(chunklength + 3)
+                ReDim databuf(chunklength - 1)
+                len = fs.Read(nameanddatabuf, 0, chunklength + 4)
+
+                'rewind and read only the chunk data
+                fs.Seek(-chunklength, IO.SeekOrigin.Current)
+                fs.Read(databuf, 0, chunklength)
+
+                'check for incomplete reads
                 If len <> chunklength + 4 Then
                     Console.WriteLine("Incomplete chunk data")
                     Exit While
                 End If
-                chunktype = System.Text.Encoding.ASCII.GetString(buf, 0, 4)
-                Console.Write(chunktype & " ")
-                Dim datachecksum As UInt32 = Crc32.ComputeChecksum(buf)
 
-                ReDim buf(3)
+                'get chunk type as string
-                len = fs.Read(buf, 0, 4)
+                chunktype = System.Text.Encoding.ASCII.GetString(nameanddatabuf, 0, 4)
+                Console.Write(chunktype & " ")
+
+                'compute checksum of the chunk
+                Dim datachecksum As UInt32 = Crc32.ComputeChecksum(nameanddatabuf)
+
+                'read reported checksum and compare it to the file
+                len = fs.Read(crcbuf, 0, 4)
                 If len <> 4 Then
                     Console.WriteLine("Incomplete CRC data")
                 End If
-                Dim chunkchecksum As UInt32 = &H1000000L * buf(0) + &H10000L * buf(1) + &H100L * buf(2) + buf(3)
+                Dim chunkchecksum As UInt32 = BigEndianUInt32(crcbuf)
                 If chunkchecksum = datachecksum Then
                     Console.Write("ok")
                 Else
@@ -100,6 +117,10 @@
                 End If
                 Console.WriteLine()
 
+                'print info for some chunks
+                Console.ForegroundColor = ConsoleColor.Gray
+                PrintChunkInfo(chunktype, databuf)
+
             End While
 
             If extract Then
@@ -110,6 +131,10 @@
         End While
 
         fs.Close()
+
+#If CONFIG = "Debug" Then
+        Console.ReadKey()
+#End If
     End Sub
 
     Public Sub FindHeader(fs As IO.FileStream)
@@ -128,6 +153,10 @@
         End While
     End Sub
 
+    Private Function BigEndianUInt32(ByRef buf As Byte(), Optional offset As Integer = 0) As UInt32
+        Return &H1000000L * buf(0 + offset) + &H10000L * buf(1 + offset) + &H100L * buf(2 + offset) + buf(3 + offset)
+    End Function
+
     Public Sub CopyPart(srcfile As String, destfile As String, offset As Long, bytes As Long)
         Dim srcfs As New IO.FileStream(srcfile, IO.FileMode.Open, IO.FileAccess.Read, IO.FileShare.Read)
         Dim destfs As New IO.FileStream(destfile, IO.FileMode.Create, IO.FileAccess.Write, IO.FileShare.Read)
@@ -139,6 +168,110 @@
         destfs.Close()
     End Sub
 
+    Private Sub PrintChunkInfo(chunktype As String, ByRef data As Byte())
+        Select Case chunktype
+            Case "IHDR"
+                'image width
+                Console.WriteLine("width: {0} px", BigEndianUInt32(data, 0))
+
+                'image height
+                Console.WriteLine("height: {0} px", BigEndianUInt32(data, 4))
+
+                'bit depth
+                Dim bitdepth As Byte = data(8)
+                If {1, 2, 4, 8, 16}.Contains(bitdepth) Then
+                    Console.WriteLine("bit depth: {0} bits/sample", bitdepth)
+                Else
+                    Console.WriteLine("bit depth: {0} bits/sample INVALID", bitdepth)
+                End If
+
+                'color types
+                Dim colortype As Byte = data(9)
+                Dim colortypename As String
+                Select Case colortype
+                    Case 0
+                        colortypename = "greyscale"
+                        If Not {1, 2, 4, 8, 16}.Contains(bitdepth) Then
+                            colortypename &= ", INVALID BIT DEPTH"
+                        End If
+                    Case 2
+                        colortypename = "truecolor"
+                        If Not {8, 16}.Contains(bitdepth) Then
+                            colortypename &= ", INVALID BIT DEPTH"
+                        End If
+                    Case 3
+                        colortypename = "indexed"
+                        If Not {1, 2, 4, 8}.Contains(bitdepth) Then
+                            colortypename &= ", INVALID BIT DEPTH"
+                        End If
+                    Case 4
+                        colortypename = "greyscale with alpha"
+                        If Not {8, 16}.Contains(bitdepth) Then
+                            colortypename &= ", INVALID BIT DEPTH"
+                        End If
+                    Case 6
+                        colortypename = "truecolor with alpha"
+                        If Not {8, 16}.Contains(bitdepth) Then
+                            colortypename &= ", INVALID BIT DEPTH"
+                        End If
+                    Case Else
+                        colortypename = "UNKNOWN"
+                End Select
+                Console.WriteLine("color type: {0} ({1})", colortype, colortypename)
+
+                'compression method
+                If data(10) = 0 Then
+                    Console.WriteLine("compression method: 0 (deflate)")
+                Else
+                    Console.WriteLine("compression method: {0} (UNKNOWN)", data(10))
+                End If
+
+                'filter method
+                If data(11) = 0 Then
+                    Console.WriteLine("filter method: 0 (adaptive)")
+                Else
+                    Console.WriteLine("filter method: {0} (UNKNOWN)", data(11))
+                End If
+
+                'interlace method
+                If data(12) = 0 Then
+                    Console.WriteLine("interlace method: 0 (no interlace)")
+                ElseIf data(12) = 1 Then
+                    Console.WriteLine("interlace method: 1 (Adam7)")
+                Else
+                    Console.WriteLine("interlace method: {0} (UNKNOWN)", data(12))
+                End If
+
+                'end IHDR
+
+            Case "pHYs"
+                Dim unitsaremetres As Boolean = (data(8) = 1)
+                If unitsaremetres Then
+                    Console.WriteLine("pixels per unit, x: {0} px ({1:g} ppi)", BigEndianUInt32(data, 0), BigEndianUInt32(data, 0) * 0.0254)
+                    Console.WriteLine("pixels per unit, y: {0} px ({1:g} ppi)", BigEndianUInt32(data, 4), BigEndianUInt32(data, 0) * 0.0254)
+                Else
+                    Console.WriteLine("pixels per unit, x: {0} px", BigEndianUInt32(data, 0))
+                    Console.WriteLine("pixels per unit, y: {0} px", BigEndianUInt32(data, 4))
+                End If
+                If data(8) = 0 Then
+                    Console.WriteLine("units: 0 (unknown)")
+                ElseIf data(8) = 1 Then
+                    Console.WriteLine("units: 1 (metre)")
+                Else
+                    Console.WriteLine("units: {0} (UNKNOWN)", data(8))
+                End If
+
+            Case "tEXt"
+                Dim keywordlength As Integer = Array.FindIndex(data, Function(x) x = &H0) - 1
+                Dim keyword(keywordlength - 1) As Byte
+                Dim textstring(data.Length - keywordlength - 1 - 1) As Byte
+                Array.Copy(data, 0, keyword, 0, keywordlength)
+                Array.Copy(data, keywordlength + 1, textstring, 0, data.Length - keywordlength - 1)
+                Console.WriteLine("keyword: {0}", System.Text.Encoding.GetEncoding("ISO-8859-1").GetString(keyword))
+                Console.WriteLine("value: {0}", System.Text.Encoding.GetEncoding("ISO-8859-1").GetString(textstring))
+        End Select
+    End Sub
+
     Public Class Crc32
         Shared table As UInteger()