/* Image format-dependent operations. */

typedef struct {

	jas_image_t *(*decode)(jas_stream_t *in, char *opts);
	/* Decode image data from a stream. */

	int (*encode)(jas_image_t *image, jas_stream_t *out, char *opts);
	/* Encode image data to a stream. */

	int (*validate)(jas_stream_t *in);
	/* Determine if stream data is in a particular format. */

} jas_image_fmtops_t;

/* Image format information. */

typedef struct {

	int id;
	/* The ID for this format. */

	char *name;
	/* The name by which this format is identified. */

	char *ext;
	/* The file name extension associated with this format. */

	char *desc;
	/* A brief description of the format. */

	jas_image_fmtops_t ops;
	/* The operations for this format. */

} jas_image_fmtinfo_t;

　　这段代码是从jasper的源代码中摘抄出来的，jas_image_fmtops_t 这个结构体定义了三个函数指针，是对图像的基本操作。s_image_fmtinfo_t 描述了一个图片的最基本的属性。包括id号，格式定义等。刚刚开始的时候我不明白为什么id不定义成枚举类型，而要定义成int。当我看到下面的代码的时候，我知道了。

我们接着看下面的代码

jas_init

/* Initialize the image format table. */
int jas_init()
{
	jas_image_fmtops_t fmtops;
	int fmtid;

	fmtid = 0;

#if !defined(EXCLUDE_MIF_SUPPORT)
	fmtops.decode = mif_decode;
	fmtops.encode = mif_encode;
	fmtops.validate = mif_validate;
	jas_image_addfmt(fmtid, "mif", "mif", "My Image Format (MIF)", &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_PNM_SUPPORT)
	fmtops.decode = pnm_decode;
	fmtops.encode = pnm_encode;
	fmtops.validate = pnm_validate;
	jas_image_addfmt(fmtid, "pnm", "pnm", "Portable Graymap/Pixmap (PNM)",
	  &fmtops);
	jas_image_addfmt(fmtid, "pnm", "pgm", "Portable Graymap/Pixmap (PNM)",
	  &fmtops);
	jas_image_addfmt(fmtid, "pnm", "ppm", "Portable Graymap/Pixmap (PNM)",
	  &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_BMP_SUPPORT)
	fmtops.decode = bmp_decode;
	fmtops.encode = bmp_encode;
	fmtops.validate = bmp_validate;
	jas_image_addfmt(fmtid, "bmp", "bmp", "Microsoft Bitmap (BMP)", &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_RAS_SUPPORT)
	fmtops.decode = ras_decode;
	fmtops.encode = ras_encode;
	fmtops.validate = ras_validate;
	jas_image_addfmt(fmtid, "ras", "ras", "Sun Rasterfile (RAS)", &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_JP2_SUPPORT)
	fmtops.decode = jp2_decode;
	fmtops.encode = jp2_encode;
	fmtops.validate = jp2_validate;
	jas_image_addfmt(fmtid, "jp2", "jp2",
	  "JPEG-2000 JP2 File Format Syntax (ISO/IEC 15444-1)", &fmtops);
	++fmtid;
	fmtops.decode = jpc_decode;
	fmtops.encode = jpc_encode;
	fmtops.validate = jpc_validate;
	jas_image_addfmt(fmtid, "jpc", "jpc",
	  "JPEG-2000 Code Stream Syntax (ISO/IEC 15444-1)", &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_JPG_SUPPORT)
	fmtops.decode = jpg_decode;
	fmtops.encode = jpg_encode;
	fmtops.validate = jpg_validate;
	jas_image_addfmt(fmtid, "jpg", "jpg", "JPEG (ISO/IEC 10918-1)", &fmtops);
	++fmtid;
#endif

#if !defined(EXCLUDE_PGX_SUPPORT)
	fmtops.decode = pgx_decode;
	fmtops.encode = pgx_encode;
	fmtops.validate = pgx_validate;
	jas_image_addfmt(fmtid, "pgx", "pgx", "JPEG-2000 VM Format (PGX)", &fmtops);
	++fmtid;
#endif

	/* We must not register the JasPer library exit handler until after
	at least one memory allocation is performed.  This is desirable
	as it ensures that the JasPer exit handler is called before the
	debug memory allocator exit handler. */
	atexit(jas_cleanup);

	return 0;
}

int jas_image_addfmt(int id, char *name, char *ext, char *desc,
  jas_image_fmtops_t *ops)
{
	jas_image_fmtinfo_t *fmtinfo;
	assert(id >= 0 && name && ext && ops);
	if (jas_image_numfmts >= JAS_IMAGE_MAXFMTS) {
		return -1;
	}
	fmtinfo = &jas_image_fmtinfos[jas_image_numfmts];
	fmtinfo->id = id;
	if (!(fmtinfo->name = jas_strdup(name))) {
		return -1;
	}
	if (!(fmtinfo->ext = jas_strdup(ext))) {
		jas_free(fmtinfo->name);
		return -1;
	}
	if (!(fmtinfo->desc = jas_strdup(desc))) {
		jas_free(fmtinfo->name);
		jas_free(fmtinfo->ext);
		return -1;
	}
	fmtinfo->ops = *ops;
	++jas_image_numfmts;
	return 0;
}

　　jas_image_fmtinfos 是jas_image_fmtinfo_t类型的数组。

答案即将揭晓：
从上面的代码可以看出，作者使用了表格驱动编程。全局变量jas_image_fmtinfos 保存了所有支持的图片格式的全部信息。而每次要对某种格式进行添加或者删除的时候，只用修改jas_init的代码就可以了，而且格式的编号是动态的。每次进行使用的时候去该数组中查找就可以了。
下面看看使用的代码，这段函数是根据后缀名查询编号的。
int jas_image_fmtfromname(char *name)

int jas_image_fmtfromname(char *name)
{
	int i;
	char *ext;
	jas_image_fmtinfo_t *fmtinfo;
	/* Get the file name extension. */
	if (!(ext = strrchr(name, '.'))) {
		return -1;
	}
	++ext;
	/* Try to find a format that uses this extension. */	
	for (i = 0, fmtinfo = jas_image_fmtinfos; i < jas_image_numfmts; ++i,
	  ++fmtinfo) {
		/* Do we have a match? */
		if (!strcmp(ext, fmtinfo->ext)) {
			return fmtinfo->id;
		}
	}
	return -1;
}

　　从上面的代码可以看出，每次查询后缀对应的格式的时候，去比较当前的后缀名和jas_image_fmtinfos 数组中对应的后缀名，来确定id好，扩展性极好。