Visualizza wikitesto di Modificare il colore di sfondo di un'immagine GIF con le funzioni del API di GifLib

La libreria ''GifLib'', scritta da Eric Steven Raymond, consente di caricare, gestire, manipolare, creare ex novo e salvare file immagine in fomato GIF.

E' possibile con alcune sue funzioni caricare un'immagine di tipo GIF, modificarne il colore di sfondo e quindi salvare il risultato in un nuovo file d tipo ''.gif'' .

Per poter utilizzare al meglio le risorse della libreria ''GifLib'' con Gambas, suggeriamo di utilizzare la relativa libreria condivisa: "''libgif.so.7.1.0'' "

Mostriamo un semplice esempio:
 <FONT Color=gray>''The GIFLIB distribution is Copyright (c) 1997  Eric S. Raymond
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:
 
 The above copyright notice and this permission notice shall be included in
 all copies or substantial portions of the Software.
 
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.</font>
 
 Library "libgif:7.1.0"
 
 Private Const GIF_FONT_HEIGHT As Integer = 8
 
 <FONT Color=gray>' ''GifFileType * DGifOpenFileName(const char *FileName, int *Error)''
 ' ''Open a new GIF file for read, given by its name.''</font>
 Private Extern DGifOpenFileName(FileName As String, ErrorP As Pointer) As Pointer
 
 <FONT Color=gray>' ''int DGifSlurp(GifFileType *GifFile)''
 ' ''Reads an entire GIF into core, hanging all its state info off the GifFileType pointer.''</font>
 Private Extern DGifSlurp(GifFileType As Pointer) As Integer
 
 <FONT Color=gray>' ''GifFileType * EGifOpenFileName(const char *FileName, const bool TestExistence, int *Error)''
 ' ''Open a new GIF file for write, specified by name.''</font>
 Private Extern EGifOpenFileName(FileName As String, TestExistence As Boolean, ErrorP As Pointer) As Pointer
 
 <FONT Color=gray>' ''ColorMapObject * GifMakeMapObject (int ColorCount, const GifColorType * ColorMap)''
 ' ''Allocate a color map of given size.''</font>
 Private Extern GifMakeMapObject(ColorCount As Integer, ColorMap As Pointer) As Pointer
 
 <FONT Color=gray>' ''SavedImage * GifMakeSavedImage(GifFileType *GifFile, const SavedImage *CopyFrom)''
 ' ''Append an image block to the SavedImages array.''</font>
 Private Extern GifMakeSavedImage(GifFileType As Pointer, CopyFrom As Pointer) As Pointer
 
 <FONT Color=gray>' ''int EGifSpew(GifFileType *GifFileOut)''</font>
 Private Extern EGifSpew(GifFileType As Pointer) As Integer
 
 <FONT Color=gray>' ''int DGifCloseFile(GifFileType *GifFile, int *ErrorCode)''
 ' ''Close the GIF file.''</font>
 Private Extern DGifCloseFile(GifFileType As Pointer, ErrorP As Pointer) As Integer
 
 
 '''Public''' Sub Main()
 
  Dim fileGIFIn, fileGIFOut, p1, p2, p3, SavedImage As Pointer
  Dim SWidth, SHeight As Integer
  Dim SColorResolution, SBackGroundColor, ColorCount As Integer
  Dim stIn, colori, rgb, stOut As Stream
  Dim b As Byte
 
 <FONT Color=gray>' ''Apre il file .gif in entrata:''</font>
  fileGIFIn = DGifOpenFileName("''/percorso/del/file.gif''", 0)
  If fileGIFIn == 0 Then Error.Raise("Impossibile creare il file GIF !")
 
  DGifSlurp(fileGIFIn)
 
 <FONT Color=gray>' ''Apre il file .gif in uscita:''</font>
  fileGIFOut = EGifOpenFileName("''/percorso/del/nuovo/file.gif''", 0, 0)
  If fileGIFOut == 0 Then Error.Raise("Impossibile creare il file GIF !")
   
 <FONT Color=gray>' ''Legge i valori RGB del colore di fondo dell'immagine gif caricata:''</font>
 
  stIn = Memory fileGIFIn For Read Write      <FONT Color=gray>' ''struct GifFileType''</font>
  Read #stIn, SWidth
  Read #stIn, SHeight
  Read #stIn, SColorResolution
  Read #stIn, SBackGroundColor
  Seek #stIn, 24                            <FONT Color=gray>' ''struct ColorMapObject *SColorMap''</font>
  Read #stIn, p1
       
  Seek #stIn, 72
  Read #stIn, SavedImage                    <FONT Color=gray>' ''struct SavedImage''</font>
  stIn.Close
  Print "Dimensione: "; Null, SWidth; " pixel x "; SHeight; " pixel"
  Print "Risoluzione: "; Null, SColorResolution
  colori = Memory p1 For Read Write         <FONT Color=gray>' ''struct ColorMapObject *SColorMap''</font>
  Read #colori, ColorCount
  Seek #colori, 16
  Read #colori, p2
  colori.Close
  rgb = Memory p2 For Read                   <FONT Color=gray>' ''struct GifColorType *Colors''</font>
  Print "\nColori dello sfondo:"
  Read #rgb, b
  Print "Rosso: "; Null, Null, b
  Read #rgb, b
  Print "Verde: "; Null, Null, b
  Read #rgb, b
  Print "Verde: "; Null, Null, b
  rgb.Close
   
 <FONT Color=gray>' ''Scrive i nuovi valori RGB del colore di fondo dell'immagine gif caricata:''</font>
  stIn = Memory fileGIFIn For Read
  Seek #stIn, 24
  Read #stIn, p1
  stIn.Close
  colori = Memory p1 For Read
  Seek #colori, 16
  Read #colori, p2
  colori.Close
  rgb = Memory p2 For Write
  Write #rgb, 255 As Byte
  Write #rgb, 255 As Byte
  Write #rgb, 0 As Byte
  rgb.Close
    
  stOut = Memory fileGIFOut For Read Write
  Write #stOut, SWidth As Integer
  Write #stOut, SHeight As Integer
  Write #stOut, SColorResolution As Integer
  Write #stOut, SBackGroundColor As Integer
      
  p3 = Alloc(SizeOf(gb.Byte), 24)
  colori = Memory p3 For Write               <FONT Color=gray>' ''struct ColorMapObject''</font>
  Seek #colori, 16
  Write #colori, GifMakeMapObject(ColorCount, p2)
  Seek #stOut, 24                           <FONT Color=gray>' ''struct ColorMapObject *SColorMap''</font>
  Write #stOut, p3
  colori.Close
  stOut.Close
 
  GifMakeSavedImage(fileGIFOut, SavedImage)
  
 <FONT Color=gray>' ''Va in chiusura:''</font>
  Free(p3)
  EGifSpew(fileGIFOut)
  DGifCloseFile(fileGIFIn, 0)
 
 '''End'''



=Riferimenti=
* http://giflib.sourceforge.net/gif_lib.html
* http://fossies.org/dox/giflib-5.1.1/index.html