jpeg-load.c

/* The GIMP -- an image manipulation program
 * Copyright (C) 1995 Spencer Kimball and Peter Mattis
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

/* JPEG loading and saving file filter for the GIMP
 *  -Peter Mattis
 *
 * This filter is heavily based upon the "example.c" file in libjpeg.
 * In fact most of the loading and saving code was simply cut-and-pasted
 *  from that file. The filter, therefore, also uses libjpeg.
 */

/* 11-JAN-99 - Added support for JPEG comments and Progressive saves.
 *  -pete whiting <pwhiting@sprint.net>
 *
 * Comments of size up to 32k can be stored in the header of jpeg
 * files.  (They are not compressed.)  The JPEG specs and libraries
 * support the storing of multiple comments.  The behavior of this
 * code is to merge all comments in a loading image into a single
 * comment (putting \n between each) and attach that string as a
 * parasite - gimp-comment - to the image.  When saving, the image is
 * checked to see if it has the gimp-comment parasite - if so, that is
 * used as the default comment in the save dialog.  Further, the other
 * jpeg parameters (quaility, smoothing, compression and progressive)
 * are attached to the image as a parasite.  This allows the
 * parameters to remain consistent between saves.  I was not able to
 * figure out how to determine the quaility, smoothing or compression
 * values of an image as it is being loaded, but the code is there to
 * support it if anyone knows how.  Progressive mode is a method of
 * saving the image such that as a browser (or other app supporting
 * progressive loads - gimp doesn't) loads the image it first gets a
 * low res version displayed and then the image is progressively
 * enhanced until you get the final version.  It doesn't add any size
 * to the image (actually it often results in smaller file size) - the
 * only draw backs are that progressive jpegs are not supported by some
 * older viewers/browsers, and some might find it annoying.
 */

/* 
 * 21-AUG-99 - Added support for JPEG previews, subsampling,
 *             non-baseline JPEGs, restart markers and DCT method choice
 * - Steinar H. Gunderson <sgunderson@bigfoot.com>
 *
 * A small preview appears and changes according to the changes to the
 * compression options. The image itself works as a much bigger preview.
 * For slower machines, the save operation (not the load operation) is
 * done in the background, with a standard GTK+ idle loop, which turned
 * out to be the most portable way. Win32 porters shouldn't have much
 * difficulty porting my changes (at least I hope so...).
 *
 * Subsampling is a pretty obscure feature, but I thought it might be nice
 * to have it in anyway, for people to play with :-) Does anybody have
 * any better suggestions than the ones I've put in the menu? (See wizard.doc
 * from the libjpeg distribution for a tiny bit of information on subsampling.)
 *
 * A baseline JPEG is often larger and/or of worse quality than a non-baseline
 * one (especially at low quality settings), but all decoders are guaranteed
 * to read baseline JPEGs (I think). Not all will read a non-baseline one.
 *
 * Restart markers are useful if you are transmitting the image over an
 * unreliable network. If a file gets corrupted, it will only be corrupted
 * up to the next restart marker. Of course, if there are no restart markers,
 * the rest of the image will be corrupted. Restart markers take up extra
 * space. The libjpeg developers recommend a restart every 1 row for
 * transmitting images over unreliable networks, such as Usenet.
 *
 * The DCT method is said by the libjpeg docs to _only_ influence quality vs.
 * speed, and nothing else. However, I've found that there _are_ size
 * differences. Fast integer, on the other hand, is faster than integer,
 * which in turn is faster than FP. According to the libjpeg docs (and I
 * believe it's true), FP has only a theoretical advantage in quality, and
 * will be slower than the two other methods, unless you're blessed with
 * very a fast FPU. (In addition, images might not be identical on
 * different types of FP hardware.)
 *
 * ...and thus ends my additions to the JPEG plug-in. I hope. *sigh* :-)
 */

/* 
 * 21-AUG-99 - Bunch O' Fixes.
 * - Adam D. Moss <adam@gimp.org>
 *
 * We now correctly create an alpha-padded layer for our preview --
 * having a non-background non-alpha layer is a no-no in GIMP.
 *
 * I've also tweaked the GIMP undo API a little and changed the JPEG
 * plugin to use gimp_image_{freeze,thaw}_undo so that it doesn't store
 * up a whole load of superfluous tile data every time the preview is
 * changed.
 */

/*
 * 22-DEC-99 - volatiles added
 * - Austin Donnelly <austin@gimp.org>
 *
 * When gcc complains a variable may be clobbered by a longjmp or
 * vfork, it means the following: setjmp() was called by the JPEG
 * library for error recovery purposes, but gcc is keeping some
 * variables in registers without updating the memory locations on the
 * stack consistently.  If JPEG error recovery is every invoked, the
 * values of these variable will be inconsistent.  This is almost
 * always a bug, but not one that's commonly seen unless JPEG recovery
 * code is exercised frequently.  The correct solution is to tell gcc
 * to keep the stack version of the affected variables up to date, by
 * using the "volatile" keyword.   Here endeth the lesson.
 */

/*
 * 4-SEP-01 - remove the use of GtkText
 * - DindinX <David@dindinx.org>
 *
 * The new version of GTK+ does not support GtkText anymore.
 * I've just replaced the one used for the image comment by
 * a GtkTextView/GtkTextBuffer couple;
 */

#include "config.h"   /* configure cares about HAVE_PROGRESSIVE_JPEG */

#include <glib.h>     /* We want glib.h first because of some
		       * pretty obscure Win32 compilation issues.
		       */
#include <setjmp.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
#include <jpeglib.h>
#include <jerror.h>

#include <libgimp/gimp.h>
#include <libgimp/gimpui.h>

#include "libgimp/stdplugins-intl.h"

#define SCALE_WIDTH         125

/* if you are not compiling this from inside the gimp tree, you have to  */
/* take care yourself if your JPEG library supports progressive mode     */
/* #undef HAVE_PROGRESSIVE_JPEG   if your library doesn't support it     */
/* #define HAVE_PROGRESSIVE_JPEG  if your library knows how to handle it */

#define DEFAULT_QUALITY     0.75
#define DEFAULT_SMOOTHING   0.0
#define DEFAULT_OPTIMIZE    1
#define DEFAULT_PROGRESSIVE 0
#define DEFAULT_BASELINE    1
#define DEFAULT_SUBSMP      0
#define DEFAULT_RESTART     0
#define DEFAULT_DCT         0
#define DEFAULT_PREVIEW     1

#define DEFAULT_COMMENT     "Created with The GIMP"

/* sg - these should not be global... */
static gint32 volatile  image_ID_global       = -1;
static gint32           orig_image_ID_global  = -1;
static gint32           drawable_ID_global    = -1;
static gint32           layer_ID_global       = -1;
static GtkWidget       *preview_size          = NULL;
static GimpDrawable    *drawable_global       = NULL;
 
typedef struct
{
  gdouble quality;
  gdouble smoothing;
  gint    optimize;
  gint    progressive;
  gint    baseline;
  gint    subsmp;
  gint    restart;
  gint    dct;
  gint    preview;
} JpegSaveVals;

typedef struct
{
  gint  run;
} JpegSaveInterface;

typedef struct 
{
  struct jpeg_compress_struct cinfo;
  gint          tile_height;
  FILE         *outfile;
  gboolean      has_alpha;
  gint          rowstride;
  guchar       *temp;
  guchar       *data;
  guchar       *src;
  GimpDrawable *drawable;
  GimpPixelRgn  pixel_rgn;
  gchar        *file_name;
  gboolean      abort_me;
} PreviewPersistent;

static gboolean *abort_me = NULL;


/* Declare local functions.
 */
static void     query                     (void);
static void     run                       (gchar            *name,
					   gint              nparams,
					   GimpParam        *param,
					   gint             *nreturn_vals,
					   GimpParam       **return_vals);
static gint32   load_image                (gchar            *filename, 
					   GimpRunModeType   runmode, 
					   gboolean          preview);
static gboolean save_image                (gchar            *filename,
					   gint32            image_ID,
					   gint32            drawable_ID,
					   gint32            orig_image_ID,
					   gboolean          preview);

static gboolean save_dialog                (void);

static void     save_close_callback        (GtkWidget       *widget,
					    gpointer         data);
static void     save_ok_callback           (GtkWidget       *widget,
					    gpointer         data);
static void     save_restart_toggle_update (GtkWidget       *toggle,
					    GtkAdjustment   *adjustment);
static void     save_restart_update        (GtkAdjustment   *adjustment,
					    GtkWidget       *toggle);

static void     make_preview               (void);
static void     destroy_preview            (void);

static void     menu_callback              (GtkWidget       *widget, 
					    gpointer         data);


GimpPlugInInfo PLUG_IN_INFO =
{
  NULL,  /* init_proc  */
  NULL,  /* quit_proc  */
  query, /* query_proc */
  run,   /* run_proc   */
};

static JpegSaveVals jsvals =
{
  DEFAULT_QUALITY,
  DEFAULT_SMOOTHING,
  DEFAULT_OPTIMIZE,
  DEFAULT_PROGRESSIVE,
  DEFAULT_BASELINE,
  DEFAULT_SUBSMP,
  DEFAULT_RESTART,
  DEFAULT_DCT,
  DEFAULT_PREVIEW
};

static JpegSaveInterface jsint =
{
  FALSE   /*  run  */
};


static gchar     *image_comment         = NULL;
static GtkWidget *restart_markers_scale = NULL;
static GtkWidget *restart_markers_label = NULL;


MAIN ()

static void
query (void)
{
  static GimpParamDef load_args[] =
  {
    { GIMP_PDB_INT32,    "run_mode",     "Interactive, non-interactive" },
    { GIMP_PDB_STRING,   "filename",     "The name of the file to load" },
    { GIMP_PDB_STRING,   "raw_filename", "The name of the file to load" }
  };
  static GimpParamDef load_return_vals[] =
  {
    { GIMP_PDB_IMAGE,   "image",         "Output image" }
  };
  static gint nload_args        = sizeof (load_args) / sizeof (load_args[0]);
  static gint nload_return_vals = (sizeof (load_return_vals) /
				   sizeof (load_return_vals[0]));

  static GimpParamDef save_args[] =
  {
    { GIMP_PDB_INT32,    "run_mode",     "Interactive, non-interactive" },
    { GIMP_PDB_IMAGE,    "image",        "Input image" },
    { GIMP_PDB_DRAWABLE, "drawable",     "Drawable to save" },
    { GIMP_PDB_STRING,   "filename",     "The name of the file to save the image in" },
    { GIMP_PDB_STRING,   "raw_filename", "The name of the file to save the image in" },
    { GIMP_PDB_FLOAT,    "quality",      "Quality of saved image (0 <= quality <= 1)" },
    { GIMP_PDB_FLOAT,    "smoothing",    "Smoothing factor for saved image (0 <= smoothing <= 1)" },
    { GIMP_PDB_INT32,    "optimize",     "Optimization of entropy encoding parameters (0/1)" },
    { GIMP_PDB_INT32,    "progressive",  "Enable progressive jpeg image loading - ignored if not compiled with HAVE_PROGRESSIVE_JPEG (0/1)" },
    { GIMP_PDB_STRING,   "comment",      "Image comment" },
    { GIMP_PDB_INT32,    "subsmp",       "The subsampling option number" },
    { GIMP_PDB_INT32,    "baseline",     "Force creation of a baseline JPEG (non-baseline JPEGs can't be read by all decoders) (0/1)" },
    { GIMP_PDB_INT32,    "restart",      "Frequency of restart markers (in rows, 0 = no restart markers)" },
    { GIMP_PDB_INT32,    "dct",          "DCT algorithm to use (speed/quality tradeoff)" }
  };
  static gint nsave_args = sizeof (save_args) / sizeof (save_args[0]);

  gimp_install_procedure ("file_jpeg_load",
                          "loads files in the JPEG file format",
                          "loads files in the JPEG file format",
                          "Spencer Kimball, Peter Mattis & others",
                          "Spencer Kimball & Peter Mattis",
                          "1995-1999",
			  "<Load>/Jpeg",
			  NULL,
                          GIMP_PLUGIN,
                          nload_args, nload_return_vals,
                          load_args, load_return_vals);

  gimp_install_procedure ("file_jpeg_save",
                          "saves files in the JPEG file format",
                          "saves files in the lossy, widely supported JPEG format",
                          "Spencer Kimball, Peter Mattis & others",
                          "Spencer Kimball & Peter Mattis",
                          "1995-1999",
                          "<Save>/JPEG",
			  "RGB*, GRAY*",
                          GIMP_PLUGIN,
                          nsave_args, 0,
                          save_args, NULL);

  gimp_register_magic_load_handler ("file_jpeg_load",
				    "jpg,jpeg",
				    "",
				    "6,string,JFIF,6,string,Exif");
  gimp_register_save_handler       ("file_jpeg_save",
				    "jpg,jpeg",
				    "");
}

static void
run (gchar      *name,
     gint        nparams,
     GimpParam  *param,
     gint       *nreturn_vals,
     GimpParam **return_vals)
{
  static GimpParam      values[2];
  GimpRunModeType       run_mode;
  GimpPDBStatusType     status = GIMP_PDB_SUCCESS;
  gint32                image_ID;
  gint32                drawable_ID;
  gint32                orig_image_ID;
  gint32                display_ID = -1;
  GimpParasite         *parasite;
  gboolean              err;
  GimpExportReturnType  export = GIMP_EXPORT_CANCEL;

  run_mode = param[0].data.d_int32;

  *nreturn_vals = 1;
  *return_vals  = values;
  values[0].type          = GIMP_PDB_STATUS;
  values[0].data.d_status = GIMP_PDB_EXECUTION_ERROR;

  if (strcmp (name, "file_jpeg_load") == 0)
    {
      INIT_I18N_UI();

      image_ID = load_image (param[1].data.d_string, run_mode, FALSE);

      if (image_ID != -1)
	{
	  *nreturn_vals = 2;
	  values[1].type         = GIMP_PDB_IMAGE;
	  values[1].data.d_image = image_ID;
	}
      else
	{
	  status = GIMP_PDB_EXECUTION_ERROR;
	}
    }
  else if (strcmp (name, "file_jpeg_save") == 0)
    {
      INIT_I18N_UI();

      image_ID = orig_image_ID = param[1].data.d_int32;
      drawable_ID = param[2].data.d_int32;

       /*  eventually export the image */ 
      switch (run_mode)
	{
	case GIMP_RUN_INTERACTIVE:
	case GIMP_RUN_WITH_LAST_VALS:
	  gimp_ui_init ("jpeg", FALSE);
	  export = gimp_export_image (&image_ID, &drawable_ID, "JPEG", 
				      (GIMP_EXPORT_CAN_HANDLE_RGB |
				       GIMP_EXPORT_CAN_HANDLE_GRAY));
	  switch (export)
	    {
	    case GIMP_EXPORT_EXPORT: 
	      display_ID = gimp_display_new (image_ID);
	      gimp_image_set_filename (image_ID, _("Export Preview"));
	      gimp_displays_flush ();
	      break;
	    case GIMP_EXPORT_IGNORE:
	      break;
	    case GIMP_EXPORT_CANCEL:
	      values[0].data.d_status = GIMP_PDB_CANCEL;
	      return;
	      break;
	    }
	  break;
	default:
	  break;
	}
      
      g_free (image_comment);
      image_comment = NULL;
    
      parasite = gimp_image_parasite_find (orig_image_ID, "gimp-comment");
      if (parasite) 
	{
	  image_comment = g_strdup (parasite->data);
	  gimp_parasite_free (parasite);
	}

      if (!image_comment) 
	image_comment = g_strdup (DEFAULT_COMMENT);

      jsvals.quality     = DEFAULT_QUALITY;
      jsvals.smoothing   = DEFAULT_SMOOTHING;
      jsvals.optimize    = DEFAULT_OPTIMIZE;
      jsvals.progressive = DEFAULT_PROGRESSIVE;
      jsvals.baseline    = DEFAULT_BASELINE;
      jsvals.subsmp      = DEFAULT_SUBSMP;
      jsvals.restart     = DEFAULT_RESTART;
      jsvals.dct         = DEFAULT_DCT;
      jsvals.preview     = DEFAULT_PREVIEW;

      switch (run_mode)
	{
	case GIMP_RUN_INTERACTIVE:
	  /*  Possibly retrieve data  */
	  gimp_get_data ("file_jpeg_save", &jsvals);

	  /* load up the previously used values */
	  parasite = gimp_image_parasite_find (orig_image_ID,
					       "jpeg-save-options");
	  if (parasite)
	    {
	      jsvals.quality     = ((JpegSaveVals *)parasite->data)->quality;
	      jsvals.smoothing   = ((JpegSaveVals *)parasite->data)->smoothing;
	      jsvals.optimize    = ((JpegSaveVals *)parasite->data)->optimize;
	      jsvals.progressive = ((JpegSaveVals *)parasite->data)->progressive;
	      jsvals.baseline    = ((JpegSaveVals *)parasite->data)->baseline;
	      jsvals.subsmp      = ((JpegSaveVals *)parasite->data)->subsmp;
	      jsvals.restart     = ((JpegSaveVals *)parasite->data)->restart;
	      jsvals.dct         = ((JpegSaveVals *)parasite->data)->dct;
	      jsvals.preview     = ((JpegSaveVals *)parasite->data)->preview;
	      gimp_parasite_free (parasite);
	    }

	  /* we start an undo_group and immediately freeze undo saving
	     so that we can avoid sucking up tile cache with our unneeded
	     preview steps. */
      	  gimp_undo_push_group_start (image_ID);
      	  gimp_image_undo_freeze (image_ID);

	  /* prepare for the preview */
	  image_ID_global = image_ID;
	  orig_image_ID_global = orig_image_ID;
	  drawable_ID_global = drawable_ID;
 
	  /*  First acquire information with a dialog  */
	  err = save_dialog ();
 
	  /* thaw undo saving and end the undo_group. */
	  gimp_image_undo_thaw (image_ID);
	  gimp_undo_push_group_end (image_ID); 

          if (!err)
	    status = GIMP_PDB_CANCEL;
	  break;

	case GIMP_RUN_NONINTERACTIVE:
	  /*  Make sure all the arguments are there!  */
	  /*  pw - added two more progressive and comment */
	  /*  sg - added subsampling, preview, baseline, restarts and DCT */
	  if (nparams != 14)
	    {
	      status = GIMP_PDB_CALLING_ERROR;
	    }
	  else
	    {
	      jsvals.quality     = param[5].data.d_float;
	      jsvals.smoothing   = param[6].data.d_float;
	      jsvals.optimize    = param[7].data.d_int32;
#ifdef HAVE_PROGRESSIVE_JPEG
	      jsvals.progressive = param[8].data.d_int32;
#endif /* HAVE_PROGRESSIVE_JPEG */
	      jsvals.baseline    = param[11].data.d_int32;
	      jsvals.subsmp      = param[10].data.d_int32;
	      jsvals.restart     = param[12].data.d_int32;
	      jsvals.dct         = param[13].data.d_int32;
	      jsvals.preview     = FALSE;

	      /* free up the default -- wasted some effort earlier */
	      g_free (image_comment);
	      image_comment = g_strdup (param[9].data.d_string);

	      if (jsvals.quality < 0.0 || jsvals.quality > 1.0)
		status = GIMP_PDB_CALLING_ERROR;
	      else if (jsvals.smoothing < 0.0 || jsvals.smoothing > 1.0)
		status = GIMP_PDB_CALLING_ERROR;
	      else if (jsvals.subsmp < 0 || jsvals.subsmp > 2)
		status = GIMP_PDB_CALLING_ERROR;
	      else if (jsvals.dct < 0 || jsvals.dct > 2)
		status = GIMP_PDB_CALLING_ERROR;
	    }
	  break;

	case GIMP_RUN_WITH_LAST_VALS:
	  /*  Possibly retrieve data  */
	  gimp_get_data ("file_jpeg_save", &jsvals);

	  parasite = gimp_image_parasite_find (orig_image_ID,
					       "jpeg-save-options");
	  if (parasite)
	    {
	      jsvals.quality     = ((JpegSaveVals *)parasite->data)->quality;
	      jsvals.smoothing   = ((JpegSaveVals *)parasite->data)->smoothing;
	      jsvals.optimize    = ((JpegSaveVals *)parasite->data)->optimize;
	      jsvals.progressive = ((JpegSaveVals *)parasite->data)->progressive;
	      jsvals.baseline    = ((JpegSaveVals *)parasite->data)->baseline;
	      jsvals.subsmp      = ((JpegSaveVals *)parasite->data)->subsmp;
	      jsvals.restart     = ((JpegSaveVals *)parasite->data)->restart;
	      jsvals.dct         = ((JpegSaveVals *)parasite->data)->dct;
	      jsvals.preview     = FALSE;
	      gimp_parasite_free (parasite);
	    }
	  break;

	default:
	  break;
	}

      if (status == GIMP_PDB_SUCCESS)
	{
	  if (save_image (param[3].data.d_string,
			  image_ID,
			  drawable_ID,
			  orig_image_ID,
			  FALSE))
	    {
	      /*  Store mvals data  */
	      gimp_set_data ("file_jpeg_save", &jsvals, sizeof (JpegSaveVals));
	    }
	  else
	    {
	      status = GIMP_PDB_EXECUTION_ERROR;
	    }
	}

      if (export == GIMP_EXPORT_EXPORT)
	{
	  /* If the image was exported, delete the new display. */
	  /* This also deletes the image.                       */

	  if (display_ID > -1)
	    gimp_display_delete (display_ID);
	  else
	    gimp_image_delete (image_ID);
	}

      /* pw - now we need to change the defaults to be whatever
       * was used to save this image.  Dump the old parasites
       * and add new ones. */
      
      gimp_image_parasite_detach (orig_image_ID, "gimp-comment");
      if (image_comment && strlen (image_comment)) 
	{
	  parasite = gimp_parasite_new ("gimp-comment",
					GIMP_PARASITE_PERSISTENT,
					strlen (image_comment) + 1,
					image_comment);
	  gimp_image_parasite_attach (orig_image_ID, parasite);
	  gimp_parasite_free (parasite);
	}
      gimp_image_parasite_detach (orig_image_ID, "jpeg-save-options");
      
      parasite = gimp_parasite_new ("jpeg-save-options",
				    0, sizeof (jsvals), &jsvals);
      gimp_image_parasite_attach (orig_image_ID, parasite);
      gimp_parasite_free (parasite);
    }
  else
    {
      status = GIMP_PDB_CALLING_ERROR;
    }

  values[0].data.d_status = status;
}

/* Read next byte */
static guint
jpeg_getc (j_decompress_ptr cinfo)
{
  struct jpeg_source_mgr *datasrc = cinfo->src;

  if (datasrc->bytes_in_buffer == 0) 
    {
      if (! (*datasrc->fill_input_buffer) (cinfo))
	ERREXIT (cinfo, JERR_CANT_SUSPEND);
    }
  datasrc->bytes_in_buffer--;

  return *datasrc->next_input_byte++;
}

/* We need our own marker parser, since early versions of libjpeg
 * don't keep a conventient list of the marker's that have been
 * seen. */

/*
 * Marker processor for COM markers.
 * This replaces the library's built-in processor, which just skips the marker.
 * Note this code relies on a non-suspending data source.
 */
static GString *local_image_comments = NULL;

static boolean
COM_handler (j_decompress_ptr cinfo)
{
  gint  length;
  guint ch;

  length = jpeg_getc (cinfo) << 8;
  length += jpeg_getc (cinfo);
  if (length < 2)
    return FALSE;
  length -= 2;			/* discount the length word itself */

  if (!local_image_comments)
    local_image_comments = g_string_new (NULL);
  else
    g_string_append_c (local_image_comments, '\n');

  while (length-- > 0)
    {
      ch = jpeg_getc (cinfo);
      g_string_append_c (local_image_comments, ch);
    }

  return TRUE;
}

typedef struct my_error_mgr
{
  struct jpeg_error_mgr pub;            /* "public" fields */

  jmp_buf               setjmp_buffer;  /* for return to caller */
} *my_error_ptr;

/*
 * Here's the routine that will replace the standard error_exit method:
 */

static void
my_error_exit (j_common_ptr cinfo)
{
  /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
  my_error_ptr myerr = (my_error_ptr) cinfo->err;

  /* Always display the message. */
  /* We could postpone this until after returning, if we chose. */
  (*cinfo->err->output_message) (cinfo);

  /* Return control to the setjmp point */
  longjmp (myerr->setjmp_buffer, 1);
}

static gint32
load_image (gchar           *filename, 
	    GimpRunModeType  runmode, 
	    gboolean         preview)
{
  GimpPixelRgn     pixel_rgn;
  GimpDrawable    *drawable;
  gint32 volatile  image_ID;
  gint32           layer_ID;
  struct jpeg_decompress_struct cinfo;
  struct my_error_mgr           jerr;
  FILE    *infile;
  guchar  *buf;
  guchar  * volatile padded_buf = NULL;
  guchar **rowbuf;
  gchar   *name;
  gint     image_type;
  gint     layer_type;
  gint     tile_height;
  gint     scanlines;
  gint     i, start, end;

  JpegSaveVals local_save_vals;
  GimpParasite * volatile comment_parasite = NULL;
  GimpParasite * volatile vals_parasite    = NULL;

  /* We set up the normal JPEG error routines. */
  cinfo.err = jpeg_std_error (&jerr.pub);
  jerr.pub.error_exit = my_error_exit;

  if ((infile = fopen (filename, "rb")) == NULL)
    {
      g_message (_("can't open \"%s\"\n"), filename);
      gimp_quit ();
    }

  if (runmode != GIMP_RUN_NONINTERACTIVE)
    {
      name = g_strdup_printf (_("Loading %s:"), filename);
      gimp_progress_init (name);
      g_free (name);
    }

  image_ID = -1;
  /* Establish the setjmp return context for my_error_exit to use. */
  if (setjmp (jerr.setjmp_buffer))
    {
      /* If we get here, the JPEG code has signaled an error.
       * We need to clean up the JPEG object, close the input file, and return.
       */
      jpeg_destroy_decompress (&cinfo);
      if (infile)
	fclose (infile);
      if (image_ID != -1 && !preview)
	gimp_image_delete (image_ID);
      if (preview)
	destroy_preview();
      gimp_quit ();
    }
  /* Now we can initialize the JPEG decompression object. */
  jpeg_create_decompress (&cinfo);

  /* Step 2: specify data source (eg, a file) */

  jpeg_stdio_src (&cinfo, infile);

  /* pw - step 2.1 let the lib know we want the comments. */

  jpeg_set_marker_processor (&cinfo, JPEG_COM, COM_handler);

  /* Step 3: read file parameters with jpeg_read_header() */

  (void) jpeg_read_header (&cinfo, TRUE);
  /* We can ignore the return value from jpeg_read_header since
   *   (a) suspension is not possible with the stdio data source, and
   *   (b) we passed TRUE to reject a tables-only JPEG file as an error.
   * See libjpeg.doc for more info.
   */

  if (!preview) 
    {
      /* if we had any comments then make a parasite for them */
      if (local_image_comments && local_image_comments->len) 
	{
	  gchar *string = local_image_comments->str;
	  g_string_free (local_image_comments, FALSE);
	  local_image_comments = NULL;
	  comment_parasite = gimp_parasite_new ("gimp-comment",
						GIMP_PARASITE_PERSISTENT,
						strlen (string) + 1, string);
	} 
      else 
	{
	  comment_parasite = NULL;
	}
      
      /* pw - figuring out what the saved values were is non-trivial.
       * They don't seem to be in the cinfo structure. For now, I will
       * just use the defaults, but if someone figures out how to derive
       * them this is the place to store them. */
      
      local_save_vals.quality     = DEFAULT_QUALITY;
      local_save_vals.smoothing   = DEFAULT_SMOOTHING;
      local_save_vals.optimize    = DEFAULT_OPTIMIZE;
      
#ifdef HAVE_PROGRESSIVE_JPEG 
      local_save_vals.progressive = cinfo.progressive_mode;
#else
      local_save_vals.progressive = 0;
#endif /* HAVE_PROGRESSIVE_JPEG */
      local_save_vals.baseline    = DEFAULT_BASELINE;
      local_save_vals.subsmp      = DEFAULT_SUBSMP;   /* sg - this _is_ there, but I'm too lazy */ 
      local_save_vals.restart     = DEFAULT_RESTART;
      local_save_vals.dct         = DEFAULT_DCT;
      local_save_vals.preview     = DEFAULT_PREVIEW;
      
      vals_parasite = gimp_parasite_new ("jpeg-save-options", 0,
					 sizeof (local_save_vals),
					 &local_save_vals);
    } 
  
  /* Step 4: set parameters for decompression */

  /* In this example, we don't need to change any of the defaults set by
   * jpeg_read_header(), so we do nothing here.
   */

  /* Step 5: Start decompressor */

  jpeg_start_decompress (&cinfo);

  /* We may need to do some setup of our own at this point before reading
   * the data.  After jpeg_start_decompress() we have the correct scaled
   * output image dimensions available, as well as the output colormap
   * if we asked for color quantization.
   * In this example, we need to make an output work buffer of the right size.
   */
  /* temporary buffer */
  tile_height = gimp_tile_height ();
  buf = g_new (guchar, 
	       tile_height * cinfo.output_width * cinfo.output_components);
  
  if (preview)
    padded_buf = g_new (guchar, tile_height * cinfo.output_width *
			(cinfo.output_components + 1));

  rowbuf = g_new (guchar*, tile_height);

  for (i = 0; i < tile_height; i++)
    rowbuf[i] = buf + cinfo.output_width * cinfo.output_components * i;

  /* Create a new image of the proper size and associate the filename with it.

     Preview layers, not being on the bottom of a layer stack, MUST HAVE
     AN ALPHA CHANNEL!
   */
  switch (cinfo.output_components)
    {
    case 1:
      image_type = GIMP_GRAY;
      layer_type = preview ? GIMP_GRAYA_IMAGE : GIMP_GRAY_IMAGE;
      break;

    case 3:
      image_type = GIMP_RGB;
      layer_type = preview ? GIMP_RGBA_IMAGE : GIMP_RGB_IMAGE;
      break;

    default:
      g_message ("don't know how to load JPEGs\nwith %d color channels",
		 cinfo.output_components);
      gimp_quit ();
      break;
    }

  if (preview) 
    {
      image_ID = image_ID_global;
    } 
  else 
    {
      image_ID = gimp_image_new (cinfo.output_width, cinfo.output_height,
				 image_type);
      gimp_image_set_filename (image_ID, filename);
    }

  if (preview) 
    {
      layer_ID_global = layer_ID = 
	gimp_layer_new (image_ID, _("JPEG preview"),
			cinfo.output_width,
			cinfo.output_height,
			layer_type, 100, GIMP_NORMAL_MODE);
    }
  else 
    {
      layer_ID = gimp_layer_new (image_ID, _("Background"),
				 cinfo.output_width,
				 cinfo.output_height,
				 layer_type, 100, GIMP_NORMAL_MODE);
    }

  drawable_global = drawable = gimp_drawable_get (layer_ID);
  gimp_pixel_rgn_init (&pixel_rgn, drawable, 0, 0,
		       drawable->width, drawable->height, TRUE, FALSE);

  /* Step 5.1: if the file had resolution information, set it on the image */
  if (!preview && cinfo.saw_JFIF_marker)
    {
      gdouble xresolution;
      gdouble yresolution;
      gdouble asymmetry;

      xresolution = cinfo.X_density;
      yresolution = cinfo.Y_density;

      switch (cinfo.density_unit)
	{
	case 0: /* unknown -> set the aspect ratio but use the default
		*  image resolution
		*/
	  asymmetry = xresolution / yresolution;
	  gimp_image_get_resolution (image_ID, &xresolution, &yresolution);
	  xresolution *= asymmetry;
	  break;

	case 1: /* dots per inch */
	  break;

	case 2: /* dots per cm */
	  xresolution *= 2.54;
	  yresolution *= 2.54;
	  gimp_image_set_unit (image_ID, GIMP_UNIT_MM);
	  break;

	default:
	  g_message ("unknown density unit %d\nassuming dots per inch",
		     cinfo.density_unit);
	  break;
	}

      gimp_image_set_resolution (image_ID, xresolution, yresolution);
    }

  /* Step 6: while (scan lines remain to be read) */
  /*           jpeg_read_scanlines(...); */

  /* Here we use the library's state variable cinfo.output_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   */
  while (cinfo.output_scanline < cinfo.output_height)
    {
      start = cinfo.output_scanline;
      end   = cinfo.output_scanline + tile_height;
      end   = MIN (end, cinfo.output_height);
      scanlines = end - start;

      for (i = 0; i < scanlines; i++)
	jpeg_read_scanlines (&cinfo, (JSAMPARRAY) &rowbuf[i], 1);

      if (preview) /* Add a dummy alpha channel -- convert buf to padded_buf */
	{
	  guchar *dest = padded_buf;
	  guchar *src  = buf;
	  gint    num  = drawable->width * scanlines;

	  switch (cinfo.output_components)
	    {
	    case 1:
	      for (i=0; i<num; i++)
		{
		  *(dest++) = *(src++);
		  *(dest++) = 255;
		}
	      break;

	    case 3:
	      for (i=0; i<num; i++)
		{
		  *(dest++) = *(src++);
		  *(dest++) = *(src++);
		  *(dest++) = *(src++);
		  *(dest++) = 255;
		}
	      break;

	    default:
	      g_warning ("JPEG - shouldn't have gotten here.  Report to adam@gimp.org");
	      break;
	    }
	}

      gimp_pixel_rgn_set_rect (&pixel_rgn, preview ? padded_buf : buf,
			       0, start, drawable->width, scanlines);

      if (runmode != GIMP_RUN_NONINTERACTIVE)
	{
	  gimp_progress_update ((gdouble) cinfo.output_scanline / 
				(gdouble) cinfo.output_height);
	}
    }

  /* Step 7: Finish decompression */

  jpeg_finish_decompress (&cinfo);
  /* We can ignore the return value since suspension is not possible
   * with the stdio data source.
   */

  /* Step 8: Release JPEG decompression object */

  /* This is an important step since it will release a good deal of memory. */
  jpeg_destroy_decompress (&cinfo);

  /* free up the temporary buffers */
  g_free (rowbuf);
  g_free (buf);
  if (preview)
    g_free (padded_buf);

  /* After finish_decompress, we can close the input file.
   * Here we postpone it until after no more JPEG errors are possible,
   * so as to simplify the setjmp error logic above.  (Actually, I don't
   * think that jpeg_destroy can do an error exit, but why assume anything...)
   */
  fclose (infile);

  /* At this point you may want to check to see whether any corrupt-data
   * warnings occurred (test whether jerr.num_warnings is nonzero).
   */

  /* Tell the GIMP to display the image.
   */
  gimp_image_add_layer (image_ID, layer_ID, 0);
  gimp_drawable_flush (drawable);

  /* pw - Last of all, attach the parasites (couldn't do it earlier -
     there was no image. */

  if (!preview) 
    {
      if (comment_parasite)
	{
	  gimp_image_parasite_attach (image_ID, comment_parasite);
	  gimp_parasite_free (comment_parasite);
	}

      if (vals_parasite)
	{
	  gimp_image_parasite_attach (image_ID, vals_parasite);
	  gimp_parasite_free (vals_parasite);
	}   
    }

  return image_ID;
}

/*
 * sg - This is the best I can do, I'm afraid... I think it will fail
 * if something bad really happens (but it might not). If you have a
 * better solution, send it ;-)
 */ 
static void
background_error_exit (j_common_ptr cinfo)
{
  if (abort_me) 
    *abort_me = TRUE;
  (*cinfo->err->output_message) (cinfo);
}

static gboolean
background_jpeg_save (PreviewPersistent *pp)
{
  guchar *t;
  guchar *s;
  gint    i, j;
  gint    yend;

  if (pp->abort_me || (pp->cinfo.next_scanline >= pp->cinfo.image_height))
    {
      /* clean up... */
      if (pp->abort_me)
	{
	  jpeg_abort_compress (&(pp->cinfo));
	}
      else
	{
	  jpeg_finish_compress (&(pp->cinfo));
	}

      fclose (pp->outfile);
      jpeg_destroy_compress (&(pp->cinfo));

      g_free (pp->temp);
      g_free (pp->data);

      if (pp->drawable) 
	gimp_drawable_detach (pp->drawable);

      /* display the preview stuff */
      if (!pp->abort_me) 
	{
	  struct stat buf;
	  gchar       temp[128];

	  stat (pp->file_name, &buf);
	  g_snprintf (temp, sizeof (temp),
		      _("Size: %lu bytes (%02.01f kB)"),
		      buf.st_size, (float) (buf.st_size) / 1024.0);
	  gtk_label_set_text (GTK_LABEL (preview_size), temp);
	
	  /* and load the preview */
	  load_image (pp->file_name, GIMP_RUN_NONINTERACTIVE, TRUE);
	}

      /* we cleanup here (load_image doesn't run in the background) */
      unlink (pp->file_name);
      g_free (pp->file_name);

      if (abort_me == &(pp->abort_me)) 
	abort_me = NULL;

      g_free (pp);

      gimp_displays_flush ();
      gdk_flush ();

      return FALSE;
    }
  else
    {
      if ((pp->cinfo.next_scanline % pp->tile_height) == 0)
	{
	  yend = pp->cinfo.next_scanline + pp->tile_height;
	  yend = MIN (yend, pp->cinfo.image_height);
	  gimp_pixel_rgn_get_rect (&pp->pixel_rgn, pp->data, 0, 
				   pp->cinfo.next_scanline,
				   pp->cinfo.image_width,
				   (yend - pp->cinfo.next_scanline));
	  pp->src = pp->data;
	}

      t = pp->temp;
      s = pp->src;
      i = pp->cinfo.image_width;

      while (i--)
	{
	  for (j = 0; j < pp->cinfo.input_components; j++)
	    *t++ = *s++;
	  if (pp->has_alpha)  /* ignore alpha channel */
	    s++;
	}

      pp->src += pp->rowstride;
      jpeg_write_scanlines (&(pp->cinfo), (JSAMPARRAY) &(pp->temp), 1);

      return TRUE;
    }
}

static gboolean
save_image (gchar    *filename,
	    gint32    image_ID,
	    gint32    drawable_ID,
	    gint32    orig_image_ID,
	    gboolean  preview)