Langue: en

Version: 65618 (mandriva - 22/10/07)

Section: 5 (Format de fichier)

Guide to writing image loaders for xli

This is a introduction on how an image load routine should be written, and what resources are available for use by the writer.

Include files

The include file xli.h is generally sufficient to make all the xli resources available: ie:

      #include "xli.h"

Identify function

The identification function is used to implement the -identify option of xli. It is generally of the form :
 int xyyzIdent(fullname, name)
      char *fullname, *name;

where this is an xyyz type image loader file.

fullname is the name including path, while name is the short name of the file that is used as a title, or to refer to the file in messages.

Return 0 if the image is NOT identified.

Return 1 if the image IS identified.

If the file is identified, use a message something like:

 printf("%s is a %dx%d xyyz image with %d colors\n",
          name, width, height, ncolors);

Try and be quick about identifying the image - don't read the whole image just to identify it !

Use printf even if something is wrong with the file. Use perror for a system error - ie. unable to open file.

Loading the image

The load function is used to identify and load the file if it is recognized. It is of the form:
 Image xyyzLoad(fullname, image_ops, verbose)
      ImageOptions *image_ops;
      char         *fullname;
      boolean       verbose;

Identify the image as quickly as possible.

Return NULL if the image is not identified.

Otherwise Return a pointer to an image structure with the image in it.

If the image is properly identified, but there is something wrong with it before anything useful is read in, report the error - ie.

 fprintf(stderr,"xyyzLoad: %s - corrupted color map\n",name);

clean up (ie. release the image structure and any allocated memory) and return NULL.

If the image is properly identified, and there is something wrong with it, but something useful has been read in (ie. color maps etc. have been loaded, and part of the image is loaded), report the error using fprintf (as above) and return what there is of the image.

Remember to release and temporary memory allocated on returning, whether the image is loaded ok, or if something goes wrong.

Remember to read any trailing options before returning a successfully loaded image, and before closing the file. Trailing options are read by calling the function:

 read_trail_opt(image_ops, zf, image, verbose)
     ImageOptions *image_ops;
     ZFILE        *zf;
     Image        *image;
     boolean       verbose;

Remember to close the file before returning.

Remember to add any additional info to the image structure - ie. name, gamma if known etc.

NEVER EVER exit(). There may be other image loaders and other images to load.

Use printf to give verbose output on image details.

Use fprintf(stderr," ... "); to give error warning if the file is corrupted.

use perror for a system error - ie. unable to open file.

Useful info in image_ops:

         char         *name;         /* Short name of image */

Useful things to set in the image structure:

         char         *title;  /* name of image */
         RGBMap        rgb;    / RGB map of image if IRGB type */
         byte         *data;   /* data, rounded to full byte for each row */
         float             gamma;        /* gamma of display the image is adjusted */
                                         /* for, if know. */

Creating and destroying Image structures:

The image structure is created by a call to one of three routines:
 /* For a monochrome bitmap */
 Image *newBitImage(width, height)
      unsigned int width, height;
 /* For a color image with a color map */
 Image *newRGBImage(width, height, depth)
      unsigned int width, height, depth;
 /* For a true color image */
 Image *newTrueImage(width, height)
      unsigned int width, height;

And the image can be freed with:

 void freeImageData(image)
      Image *image;

Examine another image loader (ie. gif.c) to see how to set up a color map.

File input resources:

A set of file input functions are available that automatically handle compressed files, and also by default buffer the file in memory to speed up the detection of the image type. The following functions are available:
   Open a file by name and return a pointer to
   a file descriptor.
 ZFILE *zopen(name)
      char *name;
   Read some data into a buffer
 int zread(zf, buf, len)
      ZFILE *zf;
      byte *buf;
      int len;
   Macro that returns a character from the file.
      ZFILE *zf;
   Function that reads a string into a buffer
 char *zgets(buf, size, zf)
      char         *buf;
      unsigned int  size;
      ZFILE        *zf;
 /* Return the files EOF status */
 int zeof(zf)
      ZFILE *zf;
   Clear any errors (ie. EOF)
 void zclearerr(zf)
      ZFILE *zf;
   Turn off caching when an image has been identified
   and there is no need to re-open it. The cached part
   of the file is still kept, but any reads past this
   point are not cached.
 void znocache(zf)
      ZFILE *zf;
   Rewind the cached file. This won't work if the file
   is stdin and we have read past the cached part of
   the file. Return TRUE on success.
 boolean zrewind(zf)
      ZFILE *zf;
   Return a block of data back to the input stream.
   Useful for a load routine that does its own buffering
   and wants to return what is left after it has read
   an image. (This is needed to allow trailing options
   to be recognized.) Can also be used instead of ungetc().
 void zunread(zf, buf, len)
      ZFILE *zf;
      byte *buf;
      int len;
   Close a file.
   The file is not actually closed until zreset is called,
   in case another loader wants to re-open it.
 void zclose(zf)
      ZFILE *zf;
   Really close a file. Not used by imageLoad functions.
 void zreset(filename)
      char *filename;

Some macros are provided to facilitate reading and writing elements larger than a character from/to a data buffer according to the desired endianness:

Big endian:

 #define memToVal(PTR,LEN)
 #define valToMem(VAL,PTR,LEN)

Little Endian

 #define memToValLSB(PTR,LEN)
 #define valToMemLSB(VAL,PTR,LEN)

Memory allocation:

The built in memory allocation routines are guaranteed not to fail.
 /* Duplicate a string. Good for making up the image title etc. */
 char *dupString(s)
      char *s;
 byte *lmalloc(size)
      unsigned int size;
 byte *lcalloc(size)
      unsigned int size;
 void lfree(area)
      byte *area;

Be careful not to call lfree with NULL. It may cause a fault.

Other facilities:

Memory manipulation
 /* Copy a block of memory */
 /* Zero a block of memory */
 /* Fill a block of memory with a value */

If all else fails:

Read the source. xli.h and image.h are a good place to start. Look at one of the existing loaders ie. pcx.c Grep for anything else!