Visualizza wikitesto di Ottenere Metadati Exif da un'immagine JPEG con le risorse del API di libexif

Il formato di file JPEG consente di incorporare informazioni aggiuntive chiamate "metadati" nell'intestazione del file. Lo scopo di questi metadati è di fornire informazioni aggiuntive e utili insieme all'immagine.

E' possibile estrarre tali Metadati, eventualmente contenuti in un file immagine JPEG usando le risorse della libreria ''Libexif''.

E' necessario avere installata nel sistemae richiamare in gambas la seguente libreria condivisa: "''libexif.so.12.3.3''"


Mostriamo un semplice esempio:
 Private content_count As Integer
 Private ifd As Integer
 Private entry_count As Integer
 
 
 Library "libexif:12.3.3"
 
 Public Struct ExifData
   ifd[5] As Pointer
   data As Pointer
   size As Integer
   priv As Pointer
 End Struct
 
 Public Struct ExifEntry
   tag As Integer
   format_ As Integer
   components As Long
   data As Pointer
   size As Integer
   parent As Pointer
   priv As Pointer
 End Struct
 
 <FONT Color=gray>' ''ExifData *exif_data_new_from_file (const char *path)''
 ' ''Allocate a new #Pointer and load EXIF data from a JPEG file.''</font>
 Private Extern exif_data_new_from_file(path As String) As ExifData
 
 <FONT Color=gray>' ''ExifByteOrder exif_data_get_byte_order  (ExifData *data)''
 ' ''Return the byte order in use by this EXIF structure.''</font>
 Private Extern exif_data_get_byte_order(data As ExifData) As Integer
 
 <FONT Color=gray>' ''const char *exif_byte_order_get_name (ExifByteOrder order)''
 ' ''Return a short, localized, textual name for the given byte order.''</font>
 Private Extern exif_byte_order_get_name(order As Integer) As String
 
 <FONT Color=gray>' ''void exif_data_foreach_content (ExifData *data, PointerForeachContentFunc func, void *user_data)''
 ' ''Execute a function on each IFD in turn.''</font>
 Private Extern exif_data_foreach_content(data As ExifData, func As Pointer, user_data As Pointer)
 
 <FONT Color=gray>' ''void exif_content_foreach_entry (ExifContent *content, ExifContentForeachEntryFunc func, void *user_data)''
 ' ''Executes function on each EXIF tag in this IFD in turn.''</font>
 Private Extern exif_content_foreach_entry(content As Pointer, func As Pointer, user_data As Pointer)
 
 <FONT Color=gray>' ''ExifIfd exif_content_get_ifd (ExifContent *c)''
 ' ''Return the IFD number in which the given #ExifContent is found.''</font>
 Private Extern exif_content_get_ifd(c As Pointer) As Integer
 
 <FONT Color=gray>' ''const char *exif_entry_get_value (ExifEntry *entry, char *val, unsigned int maxlen)''
 ' ''Return a localized textual representation of the value of the EXIF entry.''</font>
 Private Extern exif_entry_get_value(entry As ExifEntry, _val As Pointer, maxlen As Integer) As String
 
 <FONT Color=gray>' ''const char *exif_tag_get_name_in_ifd(ExifTag tag, ExifIfd ifd)''
 ' ''Return a textual name of the given tag when found in the given IFD.''</font>
 Private Extern exif_tag_get_name_in_ifd(tag As Integer, ifd As Integer) As String
 
 <FONT Color=gray>' ''const char *exif_format_get_name (ExifFormat format)''
 ' ''Return a textual representation of the given EXIF data type.''</font>
 Private Extern exif_format_get_name(format_ As Integer) As String
 
 <FONT Color=gray>' ''void exif_data_unref (ExifData *data)''</font>
 Private Extern exif_data_unref(data As ExifData)
 
 
 '''Public''' Sub Main()
  
  Dim ed As ExifData
  Dim jpg As String
  Dim callback_data As Pointer
  
  jpg = "<FONT Color=gray>''/percorso/del/file.jpg''</font>"
  Print "File immagine:   "; jpg
  
  ed = exif_data_new_from_file(jpg)
  If IsNull(ed) Then Error.Raise("Errore !")
  
  Print "Ordine dei byte: "; exif_byte_order_get_name(exif_data_get_byte_order(ed))
  
  exif_data_foreach_content(ed, data_foreach_func, callback_data)
  
  exif_data_unref(ed)
   
 '''End'''
 
 
 '''Private''' Function data_foreach_func(content As Pointer, callback_data As Pointer)
  
  ifd = exif_content_get_ifd(content)
  Print "  Content :"; content_count, "ifd= "; ifd
  
  exif_content_foreach_entry(content, content_foreach_func, callback_data)
  
  Inc content_count
   
'''End'''
 
 
 '''Private''' Function content_foreach_func(entry As Pointer, callback_data As Pointer)
  
  Dim buf As New Byte[2000]
  Dim ent As ExifEntry
  
  ent = entry
  
  exif_entry_get_value(entry, buf.Data, buf.Count)
  
  Print "   Entry"; entry_count; ": "; exif_tag_get_name_in_ifd(ent.tag, ifd); "("; exif_format_get_name(ent.format_); ")"
  Print "      Size, Comps: "; ent.size; ", "; ent.components
  Print "      Value: "; exif_entry_get_value(ent, buf.Data, buf.Count)
  Inc entry_count
  
 '''End'''



=Riferimenti=
* https://libexif.github.io/
* https://libexif.github.io/internals/index.html
* http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
* https://web.archive.org/web/20171002174521/http://www.metadataworkinggroup.org/specs/
* https://www.exiv2.org/tags.html
* http://exifdangers.blogspot.com/