static gint
get_orientation (j_decompress_ptr cinfo)
{
/* This function looks through the meta data in the libjpeg decompress structure to
determine if an EXIF Orientation tag is present and if so return its value (1-8).
If no EXIF Orientation tag is found 0 (zero) is returned. */
guint i; /* index into working buffer */
guint orient_tag_id; /* endianed version of orientation tag ID */
guint ret; /* Return value */
guint offset; /* de-endianed offset in various situations */
guint tags; /* number of tags in current ifd */
guint type; /* de-endianed type of tag used as index into types[] */
guint count; /* de-endianed count of elements in a tag */
guint tiff = 0; /* offset to active tiff header */
guint endian = 0; /* detected endian of data */
jpeg_saved_marker_ptr exif_marker; /* Location of the Exif APP1 marker */
jpeg_saved_marker_ptr cmarker; /* Location to check for Exif APP1 marker */
/* check for Exif marker (also called the APP1 marker) */
exif_marker = NULL;
cmarker = cinfo->marker_list;
while (cmarker) {
if (cmarker->marker == EXIF_JPEG_MARKER) {
/* The Exif APP1 marker should contain a unique
identification string ("Exif\0\0"). Check for it. */
if (!memcmp (cmarker->data, EXIF_IDENT_STRING, 6)) {
exif_marker = cmarker;
}
}
cmarker = cmarker->next;
}
/* Did we find the Exif APP1 marker? */
if (exif_marker == NULL)
return 0;
/* Do we have enough data? */
if (exif_marker->data_length < 32)
return 0;
/* Check for TIFF header and catch endianess */
i = 0;
/* Just skip data until TIFF header - it should be within 16 bytes from marker start.
Normal structure relative to APP1 marker -
0x0000: APP1 marker entry = 2 bytes
0x0002: APP1 length entry = 2 bytes
0x0004: Exif Identifier entry = 6 bytes
0x000A: Start of TIFF header (Byte order entry) - 4 bytes
- This is what we look for, to determine endianess.
0x000E: 0th IFD offset pointer - 4 bytes
exif_marker->data points to the first data after the APP1 marker
and length entries, which is the exif identification string.
The TIFF header should thus normally be found at i=6, below,
and the pointer to IFD0 will be at 6+4 = 10.
*/
while (i < 16) {
/* Little endian TIFF header */
if (memcmp (&exif_marker->data[i], leth, 4) == 0){
endian = G_LITTLE_ENDIAN;
}
/* Big endian TIFF header */
else if (memcmp (&exif_marker->data[i], beth, 4) == 0){
endian = G_BIG_ENDIAN;
}
/* Keep looking through buffer */
else {
i++;
continue;
}
/* We have found either big or little endian TIFF header */
tiff = i;
break;
}
/* So did we find a TIFF header or did we just hit end of buffer? */
if (tiff == 0)
return 0;
/* Endian the orientation tag ID, to locate it more easily */
orient_tag_id = ENDIAN16_IT(0x112);
/* Read out the offset pointer to IFD0 */
offset = de_get32(&exif_marker->data[i] + 4, endian);
i = i + offset;
/* Check that we still are within the buffer and can read the tag count */
if ((i + 2) > exif_marker->data_length)
return 0;
/* Find out how many tags we have in IFD0. As per the TIFF spec, the first
two bytes of the IFD contain a count of the number of tags. */
tags = de_get16(&exif_marker->data[i], endian);
i = i + 2;
/* Check that we still have enough data for all tags to check. The tags
are listed in consecutive 12-byte blocks. The tag ID, type, size, and
a pointer to the actual value, are packed into these 12 byte entries. */
if ((i + tags * 12) > exif_marker->data_length)
return 0;
/* Check through IFD0 for tags of interest */
while (tags--){
type = de_get16(&exif_marker->data[i + 2], endian);
count = de_get32(&exif_marker->data[i + 4], endian);
/* Is this the orientation tag? */
if (memcmp (&exif_marker->data[i], (char *) &orient_tag_id, 2) == 0){
/* Check that type and count fields are OK. The orientation field
will consist of a single (count=1) 2-byte integer (type=3). */
if (type != 3 || count != 1) return 0;
/* Return the orientation value. Within the 12-byte block, the
pointer to the actual data is at offset 8. */
ret = de_get16(&exif_marker->data[i + 8], endian);
return ret <= 8 ? ret : 0;
}
/* move the pointer to the next 12-byte tag field. */
i = i + 12;
}
return 0; /* No EXIF Orientation tag found */
}
static gboolean
jpeg_parse_exif_app1 (JpegExifContext *context, jpeg_saved_marker_ptr marker)
{
guint i;
guint ret = FALSE;
guint offset;
guint tags; /* number of tags in current ifd */
guint endian = 0; /* detected endian of data */
const char leth[] = {0x49, 0x49, 0x2a, 0x00}; // Little endian TIFF header
const char beth[] = {0x4d, 0x4d, 0x00, 0x2a}; // Big endian TIFF header
/* do we have enough data? */
if (marker->data_length < 4)
goto out;
/* unique identification string */
if (memcmp (marker->data, "Exif", 4) != 0)
goto out;
/* do we have enough data? */
if (marker->data_length < 32)
goto out;
/* Just skip data until TIFF header - it should be within 16 bytes from marker start.
Normal structure relative to APP1 marker -
0x0000: APP1 marker entry = 2 bytes
0x0002: APP1 length entry = 2 bytes
0x0004: Exif Identifier entry = 6 bytes
0x000A: Start of TIFF header (Byte order entry) - 4 bytes
- This is what we look for, to determine endianess.
0x000E: 0th IFD offset pointer - 4 bytes
marker->data points to the first data after the APP1 marker
and length entries, which is the exif identification string.
The TIFF header should thus normally be found at i=6, below,
and the pointer to IFD0 will be at 6+4 = 10.
*/
for (i=0; i<16; i++) {
/* little endian TIFF header */
if (memcmp (&marker->data[i], leth, 4) == 0) {
endian = G_LITTLE_ENDIAN;
ret = TRUE;
break;
}
/* big endian TIFF header */
if (memcmp (&marker->data[i], beth, 4) == 0) {
endian = G_BIG_ENDIAN;
ret = TRUE;
break;
}
}
/* could not find header */
if (!ret)
goto out;
/* read out the offset pointer to IFD0 */
offset = de_get32(&marker->data[i] + 4, endian);
i = i + offset;
/* check that we still are within the buffer and can read the tag count */
if ((i + 2) > marker->data_length) {
ret = FALSE;
goto out;
}
/* find out how many tags we have in IFD0. As per the TIFF spec, the first
two bytes of the IFD contain a count of the number of tags. */
tags = de_get16(&marker->data[i], endian);
i = i + 2;
/* check that we still have enough data for all tags to check. The tags
are listed in consecutive 12-byte blocks. The tag ID, type, size, and
a pointer to the actual value, are packed into these 12 byte entries. */
if ((i + tags * 12) > marker->data_length) {
ret = FALSE;
goto out;
}
/* check through IFD0 for tags */
while (tags--){
guint tag = de_get16(&marker->data[i + 0], endian);
guint type = de_get16(&marker->data[i + 2], endian);
guint count = de_get32(&marker->data[i + 4], endian);
/* values of types small enough to fit are stored directly in the (first) bytes of the Value Offset field */
guint short_value = de_get16(&marker->data[i + 8], endian);
/* orientation tag? */
if (tag == 0x112){
/* The orientation field should consist of a single 2-byte integer */
if (type != 0x3 || count != 1)
continue;
/* get the orientation value */
context->orientation = short_value <= 8 ? short_value : 0;
}
/* move the pointer to the next 12-byte tag field. */
i = i + 12;
}
out:
return ret;
}
