Creating Alpha-Channel for PNG-Images with tRNS-Chunk

[PureLust]

Hi folks,

in THIS German Thread we find out, that PBs PNG-Decodes does not handle PNG Images correctly, which have stored the transparency information by using the tRNS-chunk (instead of by the sRGB way).
This is commonly done by small, icon-like Images with small amount of colors to save some Diskspace or to reduce the Download-Traffic.

So I've written a little routine to add the Alpha-channel to such an incorrect loaded Image - so you'll get the right transparency.
(Within the Code you will find an example and some more Infos how to use it:)

Code: Select all

EnableExplicit

Procedure CreateAlphaFromTRNSChunk(ImageNr,ImageFile.s,*ImageAdress=0)
   Protected *ImageMemPos, FileNum, ReceivedBytes, Transcol.l, *TransColP.RGBQUAD, ImageCopy.l, x, y

   ; This Procedure creates the Alpha-Channel for a loaded PNG-Image where the Transpaent-Information
   ; is stored in a tRNS-Chunk instead of in the sRGB-Information.
   ;
   ; Parameters:
   ; ImageNR      -> Number of the PureBasic-Image (not the ImageID)
   ; ImageFile.s  -> If the Image was loaded from a file, place the Filename here - otherwise leave this blank (= "")
   ; *ImageAdress -> If you've catched the Image from Memory, put the Label-Adress in here and leave ImageFile.s blank (= "")
   ;
   ; How it works:
   ; The Procedure checks the first 50 Bytes of the File or the Memory-Position to check out
   ; - if it is a PGN File
   ; - if it has got the tRNS Chunk
   ;   - if not, it will return without any Changes
   ; - it reads the transparent Color from the tRNS Chunk
   ; - creats a copy of the original Image
   ; - frees the old original Image and recreates a 32bit Alpha-Image with the same ImageNr.
   ; - copies the old RGB-Information back to the new created 32-bit Image
   ; - creates the Alpha-Channel from the Transparent Color which was read from the tRNS-Chunk
   ; - cleans up Memory and frees Backup-Image
   ;
   ; ReturnValue:
   ; It returns the ImageID of the new (or old, if not changed) Image
   ; Because it does NOT CHANGE the ImageNr. used by PB, there is no need to check the Result if you use PBs ImageNr.
   ; If you've used the ImageID (API-ImageID) of the Image before, you should use the ImageID which will be returned
   ;
   ; Examples of Usage:
   ;
   ; Using PBs ImageNumber and loading Image from File:
   ; - MyImage = LoadImage(#PB_Any, "C:\Temp\SpeakerOn.png")
   ; - Dummy = CreateAlphaFromTRNSChunk(MyImage,"C:\Temp\SpeakerOn.png")
   ; 
   ; Using API ImageIDs and catching the Image from Memory:
   ; - ImageID = CatchImage(0, ?Image0)
   ; - ImageID = CreateAlphaFromTRNSChunk(0,"",?Image0)

   If Len(ImageFile)
      *ImageMemPos = AllocateMemory(50)
      If Not *ImageMemPos : ProcedureReturn ImageID(ImageNr) : EndIf
      Filenum   = ReadFile(#PB_Any,ImageFile)
      If Not Filenum : FreeMemory(*ImageMemPos) : ProcedureReturn ImageID(ImageNr) : EndIf
      ReceivedBytes = ReadData(FileNum, *ImageMemPos, 50)
      CloseFile(FileNum)
      If ReceivedBytes < 49 : FreeMemory(*ImageMemPos) : ProcedureReturn ImageID(ImageNr) : EndIf
      *ImageAdress = *ImageMemPos
   ElseIf Not *ImageAdress
      ProcedureReturn ImageID(ImageNr)
   EndIf
   If UCase(PeekS(*ImageAdress+1,3,#PB_Ascii)) = "PNG"
      If UCase(PeekS(*ImageAdress+37,4,#PB_Ascii)) = "TRNS"
         *TransColP = @Transcol
         *TransColP\rgbBlue = PeekB(*ImageAdress+42)
         *TransColP\rgbGreen = PeekB(*ImageAdress+44)
         *TransColP\rgbRed = PeekB(*ImageAdress+46)
         ImageCopy = CopyImage(ImageNr,#PB_Any)
         FreeImage(ImageNr)
         CreateImage(ImageNr,ImageWidth(ImageCopy), ImageHeight(ImageCopy),32)
         If StartDrawing(ImageOutput(ImageNr))
            DrawImage(ImageID(ImageCopy),0,0)
            DrawingMode(#PB_2DDrawing_AlphaChannel)
            Box(0, 0, ImageWidth(ImageCopy), ImageHeight(ImageCopy), $FF000000)
            ; Setzen der Transparenten Punkte durch LANGSAME Point/Plot-Befehle
            For x = 0 To ImageWidth(ImageCopy)-1
                For y = 0 To ImageHeight(ImageCopy)-1
                  If (Point(x,y) & $FFFFFF) = Transcol
                     Plot(x,y,0)
                  EndIf
               Next y
            Next x
            StopDrawing()
         EndIf
         FreeImage(ImageCopy)
      EndIf
   EndIf
   If *ImageMemPos : FreeMemory(*ImageMemPos) : EndIf
   ProcedureReturn ImageID(ImageNr)
EndProcedure

#ImageFileName = "C:\Temp\SpeakerOn.png"  ; change this to fit your Image-File

UsePNGImageDecoder()
Global Image0=CatchImage(0, ?Image0)
Global Image1=CatchImage(1, ?Image0)
Global Image2=LoadImage(2, #ImageFileName)

Image1 = CreateAlphaFromTRNSChunk(1,"",?Image0)
CreateAlphaFromTRNSChunk(2,#ImageFileName)

DataSection
Image0:
  IncludeBinary #ImageFileName
EndDataSection

If OpenWindow(0, 100,100,280,100, "Transparenz-Test",#PB_Window_SystemMenu | #PB_Window_TitleBar )
     ImageGadget(0, 40, 20, 40, 35, Image0)
     ImageGadget(1, 120, 20, 40, 35, Image1)
     ImageGadget(2, 200, 20, 40, 35, ImageID(2))
            
   Repeat 
      Define Event=WaitWindowEvent() 
   Until Event=#PB_Event_CloseWindow 
EndIf

You can download the Demo-PNG you need for the included example-code: HERE
Maybe someone will find it usefull. (Feedback welcome. )

[Remark:] Because I do not have other PNG-Sample Images with tRNS-Chunk i was not able to test it on other Images - so maybe I've fiddled around with the RGB-values a bit (could be BGR instead).
So if you find an Image where the Transparency will not restored correctly, please post a note and a link to the Image if possible.

Thanks and greets,
Albert.

(Sorry for my bad english. Here is the Link to this Thread in german Language.)

[c4s]

Thanks, this looks useful to me as I'm trying to fix this issue for myself: http://www.purebasic.fr/english/viewtop ... =4&t=45344