Ottenere alcune informazioni su file immagine JPEG con le funzioni del API di libjpeg
Da Gambas-it.org - Wikipedia.
Libjpeg è una libreria scritta in C utilizzata per la lettura e la scrittura di file immagine JPEG.
Utilizzando alcune risorse della libreria, attualmente alla versione "libjpeg.so.8.2.2 " è possibile, fra l'altro, ottenere alcune informazioni relative al file immagine jpeg, come, ad esempio, la dimensione in pixel della larghezza e dell'altezza, nonché il numero di byte usato per definire un pixel.
Mostriamo un semplice esempio al riguardo:
Library "libjpeg:8.2.2"
' struct jpeg_error_mgr * jpeg_std_error(struct jpeg_error_mgr * err)
' Update error manager with error handling routines.
Private Extern jpeg_std_error(jpeg_error_mgr As Pointer) As Pointer
' void jpeg_CreateDecompress(j_decompress_ptr cinfo, int version, size_t structsize)
' Allocate and initialize a JPEG decompression object.
Private Extern jpeg_CreateDecompress(j_decompress_ptr As Pointer, version As Integer, structsize As Integer)
' void jpeg_stdio_src(j_decompress_ptr cinfo, FILE * infile)
' Initialize state for input from stdio stream.
Private Extern jpeg_stdio_src(j_decompress_ptr As Pointer, infile As Pointer)
' int jpeg_read_header(j_decompress_ptr cinfo, boolean require_image)
' Read start of JPEG datastream.
Private Extern jpeg_read_header(j_decompress_ptr As Pointer, require_image As Integer) As Integer
' boolean jpeg_start_decompress(j_decompress_ptr cinfo)
' Initialize a decompression cycle.
Private Extern jpeg_start_decompress(j_decompress_ptr As Pointer) As Integer
' void jpeg_destroy_decompress(j_decompress_ptr cinfo)
' Deallocate a JPEG decompression object
Private Extern jpeg_destroy_decompress(j_decompress_ptr As Pointer)
Library "libc:6"
' FILE *fopen(const char *path, const char *mode)
Private Extern fopen(path As String, mode As String) As Pointer
' int fclose(FILE *stream)
Private Extern fclose(fileStream As Pointer) As Integer
Public Sub Main()
Dim fl, err_mgr, cinfo As Pointer
Dim width, height, bytePerPixel As Integer
Dim st As Stream
fl = fopen("/percorso/del/file.jpg", "rb")
err_mgr = Alloc(SizeOf(gb.Byte), 168)
cinfo = Alloc(SizeOf(gb.Byte), 656)
st = Memory cinfo For Read Write
Write #st, jpeg_std_error(err_mgr) As Pointer
jpeg_CreateDecompress(cinfo, 80, 656)
jpeg_stdio_src(cinfo, fl)
jpeg_read_header(cinfo, 1)
jpeg_start_decompress(cinfo)
Seek #st, 48
Read #st, width
Print "Numero pixel in larghezza: "; width
Read #st, height
Print "Numero pixel in altezza: "; height
Seek #st, 60
Read #st, bytePerPixel
Print "Numero byte per pixel: "; bytePerPixel
' Va a terminare:
jpeg_destroy_decompress(cinfo)
st.Close
Free(cinfo)
Free(err_mgr)
fclose(fl)
End
