Libexif , appending new exif data

Question

I have a task to edit exif tags and add to them application specific values. if the exif tags exist libexif is more than happy to edit them . but if the exif tags don't exist, i will have to create them and append them to file. libexif uses the C fopen so i don't think there is going to be an easy way without some IO manipulation. I am thinking to read the raw image data put them in memory , fopen(newfile, 'w') add the exif data and then append the image data.
only if someone knows an easier way , ( i am restricted with libexif, libexiv2 might create a liscence conflict) .

Velour

Answer 1

for the common good i am going to answer my own question, exif application has a modified libjpeg that enable the manipulation of the jpeg raw data. it has functions like

jpeg_data_load_data (JPEGData *data, const unsigned char *d,unsigned int size); and

jpeg_data_set_exif_data(myJPEGImage,exif); jpeg_data_save_file(myJPEGImage,"gangrene1.jpg");

That can be used, also free available programs like imagemagick have their own libjpeg , libexif implementation to do manipulate exif and jpeg data. Hopes this helps

Velour

Answer 2

I have just gone down the same road as you with choosing between libexif and libexiv2. I went with libexif due to the licensing.

Back to the question at hand,

libexif doesn't support directly loading JPG's in. You'll need another package to read in the JPG and extract the EXIF header (or you could write something yourself).

There is an excellent Github project called exifyay that uses libexif and has two extra libs that handle reading in JPGS. It is a python project but the sources for the libraries are C. You can find exifyay here (note I am not involved in any way with exifyay or libexif)

I have just recently compiled libexif and merged sources from exifyay into a VS2010 project here. There is an example in the folder 'contrib\examples\LibexifExample'. If you don't like downloading random links here is a sample of the code I got working:

/*
 * write-exif.c
 *
 * Placed into the public domain by Daniel Fandrich
 *
 * Create a new EXIF data block and write it into a JPEG image file.
 *
 * The JPEG image data used in this example is fixed and is guaranteed not
 * to contain an EXIF tag block already, so it is easy to precompute where
 * in the file the EXIF data should be. In real life, a library like
 * libjpeg (included with the exif command-line tool source code) would
 * be used to write to an existing JPEG file.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <libexif/exif-data.h>
#include <libjpeg/jpeg-data.h>
#include <JpegEncoderEXIF/JpegEncoderEXIF.h>


/* byte order to use in the EXIF block */
#define FILE_BYTE_ORDER EXIF_BYTE_ORDER_INTEL

/* comment to write into the EXIF block */
#define FILE_COMMENT "libexif demonstration image"

/* special header required for EXIF_TAG_USER_COMMENT */
#define ASCII_COMMENT "ASCII\0\0\0"

static ExifEntry *create_tag(ExifData *exif, ExifIfd ifd, ExifTag tag, size_t len)
{
    void *buf;
    ExifEntry *entry;

    /* Create a memory allocator to manage this ExifEntry */
    ExifMem *mem = exif_mem_new_default();
    assert(mem != NULL); /* catch an out of memory condition */

    /* Create a new ExifEntry using our allocator */
    entry = exif_entry_new_mem (mem);
    assert(entry != NULL);

    /* Allocate memory to use for holding the tag data */
    buf = exif_mem_alloc(mem, len);
    assert(buf != NULL);

    /* Fill in the entry */
    entry->data = (unsigned char*)buf;
    entry->size = len;
    entry->tag = tag;
    entry->components = len;
    entry->format = EXIF_FORMAT_UNDEFINED;

    /* Attach the ExifEntry to an IFD */
    exif_content_add_entry (exif->ifd[ifd], entry);

    /* The ExifMem and ExifEntry are now owned elsewhere */
    exif_mem_unref(mem);
    exif_entry_unref(entry);

    return entry;
}

int main(int argc, char **argv)
{

    ExifEntry *entry;

    //Input JPG
    char mInputFilename[]="example.jpg";

    //Load JPG
    JPEGData * mJpegData = jpeg_data_new_from_file(mInputFilename);

    //Load Exif data from JPG
    ExifData * mExifData = jpeg_data_get_exif_data(mJpegData);

    //Set some Exif options
    exif_data_set_option(mExifData, EXIF_DATA_OPTION_FOLLOW_SPECIFICATION);
    exif_data_set_data_type(mExifData, EXIF_DATA_TYPE_COMPRESSED);
    exif_data_set_byte_order(mExifData, FILE_BYTE_ORDER);

    entry = create_tag(mExifData, EXIF_IFD_EXIF, EXIF_TAG_USER_COMMENT, 
            sizeof(ASCII_COMMENT) + sizeof(FILE_COMMENT) - 2);
    /* Write the special header needed for a comment tag */
    memcpy(entry->data, ASCII_COMMENT, sizeof(ASCII_COMMENT)-1);
    /* Write the actual comment text, without the trailing NUL character */
    memcpy(entry->data+8, FILE_COMMENT, sizeof(FILE_COMMENT)-1);
    /* create_tag() happens to set the format and components correctly for
     * EXIF_TAG_USER_COMMENT, so there is nothing more to do. */

    /* Create a EXIF_TAG_SUBJECT_AREA tag */
    entry = create_tag(mExifData, EXIF_IFD_EXIF, EXIF_TAG_SUBJECT_AREA,
               4 * exif_format_get_size(EXIF_FORMAT_SHORT));
    entry->format = EXIF_FORMAT_SHORT;
    entry->components = 4;

    //Write back exif data
    jpeg_data_set_exif_data(mJpegData,mExifData);

    //Save to JPG
    jpeg_data_save_file(mJpegData,"test.jpg");

    return 0;
}

Diacid