Versione attuale delle 20:30, 30 nov 2021

Come si sa, salvare un'immagine in un file di formato .GIF (Graphics Interchange Format), in Gambas non è attualmente possibile. Gambas possiede, sì, risorse per caricare file immagine di tipo .gif, ma non per salvarle.

Un aiuto, però, per la generazione di file immagine di tipo GIF può giungerci dalle risorse della libreria GIFLIB, la quale consente di caricare, gestire, maniplare, creare ex novo e salvare file immagine in fomato 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 di seguito un breve esempio per generare ex novo un semplice file immagine trasparente di tipo .gif:

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.

Private Const QuantitasColori As Integer = 256
Private Const DimMappaColori As Integer = 8


Library "libgif:7.1.0"

Private Enum GIF_ERROR = 0, GIF_OK

' ColorMapObject * GifMakeMapObject (int ColorCount, const GifColorType * ColorMap)
' Allocate a color map of given size.
Private Extern GifMakeMapObject(ColorCount As Integer, ColorMap As Pointer) As Pointer

' GifFileType * EGifOpenFileName(const char *FileName, const bool TestExistence, int *Error)
' Open a new GIF file for write, specified by name.
Private Extern EGifOpenFileName(FileName As String, TestExistence As Boolean, ErrorP As Pointer) As Pointer

' int EGifPutScreenDesc(GifFileType *GifFile, const int Width, const int Height, const int ColorRes, const int BackGround, const ColorMapObject *ColorMap)
' This routine should be called immediately following the GIF file opening.
Private Extern EGifPutScreenDesc(GifFileType As Pointer, wI As Integer, hI As Integer, ColorRes As Integer, BackGround As Integer, ColorMap As Pointer) As Integer

' int EGifPutImageDesc(GifFileType *GifFile, const int Left, const int Top, const int Width, const int Height, const bool Interlace, const ColorMapObject *ColorMap)
' This routine should be called before any attempt to dump an image - any call to any of the pixel dump routines.
Private Extern EGifPutImageDesc(GifFileType As Pointer, LeftI As Integer, Top As Integer, wI As Integer, hI As Integer, Interlace As Boolean, ColorMap As Pointer) As Integer

' int EGifCloseFile(GifFileType *GifFile, int *ErrorCode)
' Closes the GIF file.
Private Extern EGifCloseFile(GifFileType As Pointer, ErrorCode As Pointer) As Integer


Public Sub Main()

 Dim Width, Height, err As Integer
 Dim MappaColori, fileGIF As Pointer
 
' Impostiamo a piacere le dimensioni in pixel dell'immagine da creare:
 Width = 640
 Height = 400

 MappaColori = GifMakeMapObject(QuantitasColori, Null)
 If MappaColori == 0 Then Error.Raise("Impossibile allocare memoria !")
  
 fileGIF = EGifOpenFileName("/percorso/del/file.gif", 0, 0)
 If fileGIF == 0 Then Error.Raise("Impossibile creare il file GIF !")

 err = EGifPutScreenDesc(fileGIF, Width, Height, DimMappaColori, 0, MappaColori)
 If err = GIF_ERROR Then Error.Raise("Errore alla funzione 'EGifPutScreenDesc()' !")
  
 err = EGifPutImageDesc(fileGIF, 0, 0, Width, Height, False, Null)
 If err = GIF_ERROR Then Error.Raise("Errore alla funzione 'EGifPutImageDesc()' !")
  
 If EGifCloseFile(fileGIF, 0) = GIF_ERROR Then Error.Raise("Errore nella chiusura del file GIF !")
  
End

Riferimenti

• http://giflib.sourceforge.net/gif_lib.html
• http://wwwcdf.pd.infn.it/libgif/index.html
• http://fossies.org/dox/giflib-5.1.1/index.html