Guide to writing image loaders for xli

Include files

#include "xli.h"

Identify function

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: CW 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

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. CW 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: CW 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:

/ 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: CW void freeImageData(image)
Image image;

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

File input resources:

/
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.
/
zgetc(zf)
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: CW #define memToVal(PTR,LEN)

#define valToMem(VAL,PTR,LEN)

Little Endian CW #define memToValLSB(PTR,LEN)

#define valToMemLSB(VAL,PTR,LEN)

Memory allocation:

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:

/ Zero a block of memory /
bzero(P,N)

/ Fill a block of memory with a value /
bfill(P,N,C)

If all else fails:

Table of Contents

• Guide to writing image loaders for xli
• Include files
• Identify function
• Loading the image
• Useful info in image_ops:
• Useful things to set in the image structure:
• Creating and destroying Image structures:
• File input resources:
• Memory allocation:
• Other facilities:
• If all else fails:

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