Completely revamped, comments below reflect the new version. Backup your previous version if you still want it.
If you use GDI+ to write PNGs, one of the shortcomings is that many chunks/properties/tags are not written even if they exist in the PNG before saving it. Another issue many have is that GDI+ automatically adds a gAMA chunk whether you want it or not, whether you feel it is the correct value or not.
This class is meant as a stop-gap for adding chunks and removing chunks after the image is saved by GDI+ but before it is written to disk or to a byte array. I have added lots of comments throughout the class
Caveats:
1. The class does not provide compression for chunks that can include compressed data. Those chunks are IDAT (pixel data), iCCP (ICM profile), zTxt (compressed text only), iTXt (UTF8 text where compression is optional).
2. All but a few standard PNG chunks are now coded as separate functions, which makes it easier for those not totally familiar with the PNG layout to add chunks during PNG creation. There is also a generic function for adding any chunk one would want. The chunks not directly coded for are:
IDAT: pixel data. GDI+ creates these
sBIT: significant bits and pertains specifically to how the pixel data is interpreted
bKGD: suggested background color for rendering transparency on. This is specific to the bit depth of the written image
hIST: palette histogram, again specific to the written pixel data
tRNS: color to be transparent within the image. This is specific to the bit depth of the written image
sPLT: suggested palette. This is specific to the bit depth of the written image
pHYs: typically used to describe non-square pixel dimensions
The class is a text file uploaded here. Simply rename it as .cls once downloaded. Here's a really short example of usage...
- Assumption is GDI+ is loaded and running before you call any class methods.
Should you want a short routine to review the chunks that exist in any valid PNG file, you can use the following.
And a short example using the above code follows. Note. For simplicity, I used VB's File I/O functions in above code. You may want to use APIs for unicode support
Edited: Link to PNG format specifications
New version uploaded
If you use GDI+ to write PNGs, one of the shortcomings is that many chunks/properties/tags are not written even if they exist in the PNG before saving it. Another issue many have is that GDI+ automatically adds a gAMA chunk whether you want it or not, whether you feel it is the correct value or not.
This class is meant as a stop-gap for adding chunks and removing chunks after the image is saved by GDI+ but before it is written to disk or to a byte array. I have added lots of comments throughout the class
Caveats:
1. The class does not provide compression for chunks that can include compressed data. Those chunks are IDAT (pixel data), iCCP (ICM profile), zTxt (compressed text only), iTXt (UTF8 text where compression is optional).
2. All but a few standard PNG chunks are now coded as separate functions, which makes it easier for those not totally familiar with the PNG layout to add chunks during PNG creation. There is also a generic function for adding any chunk one would want. The chunks not directly coded for are:
IDAT: pixel data. GDI+ creates these
sBIT: significant bits and pertains specifically to how the pixel data is interpreted
bKGD: suggested background color for rendering transparency on. This is specific to the bit depth of the written image
hIST: palette histogram, again specific to the written pixel data
tRNS: color to be transparent within the image. This is specific to the bit depth of the written image
sPLT: suggested palette. This is specific to the bit depth of the written image
pHYs: typically used to describe non-square pixel dimensions
The class is a text file uploaded here. Simply rename it as .cls once downloaded. Here's a really short example of usage...
- Assumption is GDI+ is loaded and running before you call any class methods.
Code:
Private Sub Command1_Click()
Dim hImage As Long, c As IPngWriter
GdipLoadImageFromFile StrPtr("C:\Test Images\LaVolpe.png"), hImage
If hImage Then
Set c = New IPngWriter
' example of adding a tEXt chunk
c.AddChunk_tEXt keySoftware, "Custom PNGWriter Class", BeforeIEND
' example of removing the gAMA, cHRM & sRGB chunks
c.WritePngToFile hImage, "D:\Users\LaVolpe\Desktop\Test.PNG", CHUNK_gAMA, CHUNK_sRGB, CHUNK_cHRM
Set c = Nothing
GdipDisposeImage hImage
Else
MsgBox "Failed to load that image"
End If
End SubCode:
Private Sub pvReadPngChunks(FileName As String)
Dim fnr As Integer, lName As Long, lSize As Long
Dim sName As String * 4&
Dim lPtr As Long, lMax As Long
Dim lPrevName As Long, bFailed As Boolean
On Error Resume Next
fnr = FreeFile()
Open FileName For Binary Access Read As #fnr
If Err Then
MsgBox Err.Description, vbExclamation + vbOKOnly, "Error"
Exit Sub
End If
On Error GoTo 0
lMax = LOF(fnr)
If lMax < 46& Then
bFailed = True: GoTo ExitRoutine
Else
Get #fnr, 1, lName
If lName <> 1196314761 Then bFailed = True: GoTo ExitRoutine
Get #fnr, , lName
If lName <> 169478669 Then bFailed = True: GoTo ExitRoutine
Debug.Print "Processing "; FileName;
lPtr = 9
Do Until lPtr + 8& > lMax
Get #fnr, lPtr, lSize: lSize = pvReverseLong(lSize)
Get #fnr, , lName
Mid$(sName, 4, 1) = Chr$(((lName And &HFF000000) \ &H1000000) And &HFF)
Mid$(sName, 3, 1) = Chr$((lName And &HFF0000) \ &H10000)
Mid$(sName, 2, 1) = Chr$((lName And &HFF00&) \ &H100)
Mid$(sName, 1, 1) = Chr$(lName And &HFF)
If lName = lPrevName Then
Debug.Print ","; sName; "("; CStr(lSize); ")";
Else
lPrevName = lName
Debug.Print vbCrLf; sName; "("; CStr(lSize); ")";
End If
lPtr = lPtr + 12& + lSize
Loop
End If
Debug.Print vbCrLf; "Done..."
ExitRoutine:
Close #fnr
If bFailed Then MsgBox "Failed to process that file. Sure it was a PNG?", vbQuestion + vbOKOnly
End Sub
Private Function pvReverseLong(ByVal inLong As Long) As Long
' fast function to reverse a long value from big endian to little endian
' PNG files contain reversed longs, as do ID3 v3,4 tags
pvReverseLong = _
(((inLong And &HFF000000) \ &H1000000) And &HFF&) Or _
((inLong And &HFF0000) \ &H100&) Or _
((inLong And &HFF00&) * &H100&) Or _
((inLong And &H7F&) * &H1000000)
If (inLong And &H80&) Then pvReverseLong = pvReverseLong Or &H80000000
End FunctionCode:
Call pvReadPngChunks("C:\Test Images\LaVolpe.PNG")New version uploaded