libxif:使用IPTC/XMP将新的出口写入映像

Question

我正在尝试使用libexif将EXIF元数据写入JPEG图像。我遇到过没有EXIF但有IPTC/XMP元数据的图像问题。用IPTC/XMP将EXIF写入图像后，我无法读取EXIF。"exif"命令行实用程序(也使用libxif)报告exif损坏。如果我对没有任何元数据的图像做同样的操作，一切都可以正常工作。

下面是一个演示问题的小测试应用程序(它假设我们正在使用没有EXIF的图像)。

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <libexif/exif-data.h>
#include <libexif/exif-loader.h>
#include <jpeglib.h>
#include "jpegexif/jpeg-data.h"
#include "jpegexif/jpeg-marker.h"
#include "jpegexif/jpeg-data.c"
#include "jpegexif/jpeg-marker.c"

static ExifEntry *create_ascii_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();
    // Create a new ExifEntry using our allocator.
    entry = exif_entry_new_mem(mem);
    // Allocate memory to use for holding the tag data.
    buf = exif_mem_alloc(mem, len);
    // Fill in the entry.
    entry->data = (unsigned char*) buf;
    entry->size = len;
    entry->tag = tag;
    entry->components = len;
    entry->format = EXIF_FORMAT_ASCII;
    // 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 create_exif_and_set_software(const char* const filename, const char* const tagvalue) {
    // For simplification we assume here that file has no EXIF, so we create new.
    ExifData* exif = exif_data_new();
    exif_data_set_option(exif, EXIF_DATA_OPTION_FOLLOW_SPECIFICATION);
    exif_data_set_data_type(exif, EXIF_DATA_TYPE_COMPRESSED);
    exif_data_set_byte_order(exif, EXIF_BYTE_ORDER_INTEL);
    exif_data_fix(exif);
    // Create entry for "Software" and set it's data.
    ExifEntry* entry = create_ascii_tag(exif, EXIF_IFD_0, EXIF_TAG_SOFTWARE, strlen(tagvalue) + 1);
    memcpy(entry->data, tagvalue, strlen(tagvalue) + 1);
    // Write exif to JPEG.
    JPEGData* image = jpeg_data_new_from_file(filename);
    jpeg_data_set_exif_data(image, exif);
    jpeg_data_save_file(image, filename);
    // Unreference exif.
    exif_data_unref(exif);
    return 0;
}

int main(int argc, char ** argv) {
    char* filename = argv[1];
    // Create EXIF, set software and write EXIF to file.
    create_exif_and_set_software(filename, "Awesome image editor.");
    // Check that EXIF exists in file.
    ExifData* exif = exif_data_new_from_file(filename);
    printf("EXIF exists: %dn", exif != NULL);
    return 0;
}

在这里您可以下载项目和示例图像。我使用以下命令在mac上通过macports安装libexif来构建它:/opt/local/include -L/opt/local/lib -lexif

Answer 1

最简单的方法是从头开始编写JPEG:首先，编写没有APP1节的JPEG，然后添加Exif节，然后添加所有其他XMP APP1节。

请注意，IFD偏移量应适当更新