gimp/app/tools/gimpiscissorstool.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.
 */

/* This tool is based on a paper from SIGGRAPH '95:
 *  "Intelligent Scissors for Image Composition", Eric N. Mortensen and
 *   William A. Barrett, Brigham Young University.
 *
 * thanks to Professor D. Forsyth for prompting us to implement this tool. */

/* Personal note: Dr. Barrett, one of the authors of the paper written above
 * is not only one of the most brilliant people I have ever met, he is an
 * incredible professor who genuinely cares about his students and wants them
 * to learn as much as they can about the topic.
 *
 * I didn't even notice I was taking a class from the person who wrote the
 * paper until halfway through the semester.
 *                                                   -- Rockwalrus
 */

/* The history of this implementation is lonog and varied.  It was
 * orignally done by Spencer and Peter, and worked fine in the 0.54
 * (motif only) release of the gimp.  Later revisions (0.99.something
 * until about 1.1.4) completely changed the algorithm used, until it
 * bore little resemblance to the one described in the paper above.
 * The 0.54 version of the algorithm was then forwards ported to 1.1.4
 * by Austin Donnelly.
 */

/* Livewire boundary implementation done by Laramie Leavitt */


#include "config.h"

#include <stdlib.h>

#include <gtk/gtk.h>
#include <gdk/gdkkeysyms.h>

#include "libgimpmath/gimpmath.h"
#include "libgimpwidgets/gimpwidgets.h"

#include "tools-types.h"

#include "base/pixel-region.h"
#include "base/temp-buf.h"
#include "base/tile-manager.h"
#include "base/tile.h"

#include "paint-funcs/paint-funcs.h"

#include "core/gimpchannel.h"
#include "core/gimpimage.h"
#include "core/gimpimage-mask.h"
#include "core/gimpimage-mask-select.h"
#include "core/gimpimage-projection.h"
#include "core/gimpscanconvert.h"
#include "core/gimptoolinfo.h"

#include "display/gimpdisplay.h"
#include "display/gimpdisplay-foreach.h"

#include "gimpbezierselecttool.h"
#include "gimpiscissorstool.h"
#include "gimpeditselectiontool.h"
#include "selection_options.h"

#include "libgimp/gimpintl.h"


/*  Other defines...  */
#define  MAX_GRADIENT      179.606  /* == sqrt(127^2 + 127^2) */
#define  GRADIENT_SEARCH   32  /* how far to look when snapping to an edge */
#define  TARGET_HEIGHT     25
#define  TARGET_WIDTH      25
#define  POINT_WIDTH       8   /* size (in pixels) of seed handles */
#define  POINT_HALFWIDTH   (POINT_WIDTH / 2)
#define  EXTEND_BY         0.2 /* proportion to expand cost map by */
#define  FIXED             5   /* additional fixed size to expand cost map */
#define  MIN_GRADIENT      63  /* gradients < this are directionless */

#define  MAX_POINTS        2048

#define  COST_WIDTH         2  /* number of bytes for each pixel in cost map  */
#define  BLOCK_WIDTH       64
#define  BLOCK_HEIGHT      64
#define  CONV_WIDTH        (BLOCK_WIDTH + 2)
#define  CONV_HEIGHT       (BLOCK_HEIGHT + 2)

/* weight to give between gradient (_G) and direction (_D) */
#define  OMEGA_D           0.2
#define  OMEGA_G           0.8

/* sentinel to mark seed point in ?cost? map */
#define  SEED_POINT        9

/*  Functional defines  */
#define  PIXEL_COST(x)     (x >> 8)
#define  PIXEL_DIR(x)      (x & 0x000000ff)


struct _ICurve
{
  gint       x1, y1;
  gint       x2, y2;
  GPtrArray *points;
};


/*  local function prototypes  */

static void   gimp_iscissors_tool_class_init (GimpIscissorsToolClass *klass);
static void   gimp_iscissors_tool_init       (GimpIscissorsTool      *iscissors);

static void   gimp_iscissors_tool_finalize       (GObject           *object);

static void   gimp_iscissors_tool_control        (GimpTool          *tool,
                                                  GimpToolAction     action,
                                                  GimpDisplay       *gdisp);
static void   gimp_iscissors_tool_button_press   (GimpTool          *tool,
                                                  GimpCoords        *coords,
                                                  guint32            time,
                                                  GdkModifierType    state,
                                                  GimpDisplay       *gdisp);
static void   gimp_iscissors_tool_button_release (GimpTool          *tool,
                                                  GimpCoords        *coords,
                                                  guint32            time,
                                                  GdkModifierType    state,
                                                  GimpDisplay       *gdisp);
static void   gimp_iscissors_tool_motion         (GimpTool          *tool,
                                                  GimpCoords        *coords,
                                                  guint32            time,
                                                  GdkModifierType    state,
                                                  GimpDisplay       *gdisp);
static void   gimp_iscissors_tool_oper_update    (GimpTool          *tool,
                                                  GimpCoords        *coords,
                                                  GdkModifierType    state,
                                                  GimpDisplay       *gdisp);
static void   gimp_iscissors_tool_cursor_update  (GimpTool          *tool,
                                                  GimpCoords        *coords,
                                                  GdkModifierType    state,
                                                  GimpDisplay       *gdisp);

static void   gimp_iscissors_tool_reset          (GimpIscissorsTool *iscissors);
static void   gimp_iscissors_tool_draw           (GimpDrawTool      *draw_tool);


static void          iscissors_convert         (GimpIscissorsTool *iscissors,
                                                GimpDisplay       *gdisp);
static TileManager * gradient_map_new          (GimpImage         *gimage);

static void          find_optimal_path         (TileManager       *gradient_map,
						TempBuf           *dp_buf,
						gint               x1,
						gint               y1,
						gint               x2,
						gint               y2,
						gint               xs,
						gint               ys);
static void          find_max_gradient         (GimpIscissorsTool *iscissors,
						GimpImage         *gimage,
						gint              *x,
						gint              *y);
static void          calculate_curve           (GimpTool          *tool,
						ICurve            *curve);
static void          iscissors_draw_curve      (GimpDrawTool      *draw_tool,
						ICurve            *curve);
static void          iscissors_free_icurves    (GSList            *list);
static void          iscissors_free_buffers    (GimpIscissorsTool *iscissors);

static gint          mouse_over_vertex         (GimpIscissorsTool *iscissors,
						gdouble            x,
						gdouble            y);
static gboolean      clicked_on_vertex         (GimpTool          *tool);
static GSList      * mouse_over_curve          (GimpIscissorsTool *iscissors,
						gdouble            x,
						gdouble            y);
static gboolean      clicked_on_curve          (GimpTool          *tool);

static void          precalculate_arrays       (void);
static GPtrArray   * plot_pixels               (GimpIscissorsTool *iscissors,
						TempBuf           *dp_buf,
						gint               x1,
						gint               y1,
						gint               xs,
						gint               ys,
						gint               xe,
						gint               ye);


/*  static variables  */

/*  where to move on a given link direction  */
static gint move[8][2] =
{
  {  1,  0 },
  {  0,  1 },
  { -1,  1 },
  {  1,  1 },
  { -1,  0 },
  {  0, -1 },
  {  1, -1 },
  { -1, -1 },
};

/* IE:
 * '---+---+---`
 * | 7 | 5 | 6 |
 * +---+---+---+
 * | 4 |   | 0 |
 * +---+---+---+
 * | 2 | 1 | 3 |
 * `---+---+---'
 */


/*  temporary convolution buffers --  */
static guchar  maxgrad_conv0[TILE_WIDTH * TILE_HEIGHT * 4] = "";
static guchar  maxgrad_conv1[TILE_WIDTH * TILE_HEIGHT * 4] = "";
static guchar  maxgrad_conv2[TILE_WIDTH * TILE_HEIGHT * 4] = "";


static gint horz_deriv[9] =
{
  1, 0, -1,
  2, 0, -2,
  1, 0, -1,
};

static gint vert_deriv[9] =
{
  1, 2, 1,
  0, 0, 0,
  -1, -2, -1,
};

static gint blur_32[9] = 
{
  1, 1, 1,
  1, 24, 1,
  1, 1, 1,
};

static gfloat    distance_weights[GRADIENT_SEARCH * GRADIENT_SEARCH];

static gint      diagonal_weight[256];
static gint      direction_value[256][4];
static gboolean  initialized = FALSE;
static Tile     *cur_tile = NULL;


static GimpSelectionToolClass *parent_class = NULL;


void
gimp_iscissors_tool_register (GimpToolRegisterCallback  callback,
                              Gimp                     *gimp)
{
  (* callback) (GIMP_TYPE_ISCISSORS_TOOL,
                selection_options_new,
                FALSE,
                "gimp-iscissors-tool",
                _("Intelligent Scissors"),
                _("Select shapes from image"),
                N_("/Tools/Selection Tools/Intelligent Scissors"),
                "I",
                NULL, "tools/iscissors.html",
                GIMP_STOCK_TOOL_ISCISSORS,
                gimp);
}

GType
gimp_iscissors_tool_get_type (void)
{
  static GType tool_type = 0;

  if (! tool_type)
    {
      static const GTypeInfo tool_info =
      {
        sizeof (GimpIscissorsToolClass),
	(GBaseInitFunc) NULL,
	(GBaseFinalizeFunc) NULL,
	(GClassInitFunc) gimp_iscissors_tool_class_init,
	NULL,           /* class_finalize */
	NULL,           /* class_data     */
	sizeof (GimpIscissorsTool),
	0,              /* n_preallocs    */
	(GInstanceInitFunc) gimp_iscissors_tool_init,
      };

      tool_type = g_type_register_static (GIMP_TYPE_SELECTION_TOOL,
					  "GimpIscissorsTool",
                                          &tool_info, 0);
    }

  return tool_type;
}

static void
gimp_iscissors_tool_class_init (GimpIscissorsToolClass *klass)
{
  GObjectClass      *object_class;
  GimpToolClass     *tool_class;
  GimpDrawToolClass *draw_tool_class;

  object_class    = G_OBJECT_CLASS (klass);
  tool_class      = GIMP_TOOL_CLASS (klass);
  draw_tool_class = GIMP_DRAW_TOOL_CLASS (klass);

  parent_class = g_type_class_peek_parent (klass);

  object_class->finalize     = gimp_iscissors_tool_finalize;

  tool_class->control        = gimp_iscissors_tool_control;
  tool_class->button_press   = gimp_iscissors_tool_button_press;
  tool_class->button_release = gimp_iscissors_tool_button_release;
  tool_class->motion         = gimp_iscissors_tool_motion;
  tool_class->oper_update    = gimp_iscissors_tool_oper_update;
  tool_class->cursor_update  = gimp_iscissors_tool_cursor_update;

  draw_tool_class->draw      = gimp_iscissors_tool_draw;
}

static void
gimp_iscissors_tool_init (GimpIscissorsTool *iscissors)
{
  GimpTool *tool;

  tool = GIMP_TOOL (iscissors);

  iscissors->op           = ISCISSORS_OP_NONE;
  iscissors->dp_buf       = NULL;
  iscissors->curves       = NULL;
  iscissors->draw         = DRAW_NOTHING;
  iscissors->state        = NO_ACTION;
  iscissors->mask         = NULL;
  iscissors->gradient_map = NULL;
  iscissors->livewire     = NULL;

  tool->control = gimp_tool_control_new  (TRUE,                       /* scroll_lock */
                                          FALSE,                      /* auto_snap_to */
                                          FALSE,                      /* preserve */
                                          FALSE,                      /* handle_empty_image */
                                          GIMP_MOTION_MODE_HINT,      /* motion_mode */
                                          GIMP_MOUSE_CURSOR,          /* cursor */
                                          GIMP_SCISSORS_TOOL_CURSOR,  /* tool_cursor */
                                          GIMP_CURSOR_MODIFIER_NONE,  /* cursor_modifier */
                                          GIMP_MOUSE_CURSOR,          /* toggle_cursor */
                                          GIMP_TOOL_CURSOR_NONE,      /* toggle_tool_cursor */
                                          GIMP_CURSOR_MODIFIER_NONE   /* toggle_cursor_modifier */);

  gimp_iscissors_tool_reset (iscissors);
}

static void
gimp_iscissors_tool_finalize (GObject *object)
{
  GimpIscissorsTool *iscissors;

  iscissors = GIMP_ISCISSORS_TOOL (object);

  gimp_iscissors_tool_reset (iscissors);

  G_OBJECT_CLASS (parent_class)->finalize (object);
}

static void
gimp_iscissors_tool_control (GimpTool       *tool,
                             GimpToolAction  action,
                             GimpDisplay    *gdisp)
{
  GimpIscissorsTool *iscissors;
  IscissorsDraw      draw;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  switch (iscissors->state)
    {
    case SEED_PLACEMENT:
      draw = DRAW_CURVE | DRAW_CURRENT_SEED;
      break;

    case SEED_ADJUSTMENT:
      draw = DRAW_CURVE | DRAW_ACTIVE_CURVE;
      break;

    default:
      draw = DRAW_CURVE;
      break;
    }

  iscissors->draw = draw;

  GIMP_TOOL_CLASS (parent_class)->control (tool, action, gdisp);

  switch (action)
    {
    case PAUSE: 
      break;

    case RESUME:
      break;

    case HALT:
      gimp_iscissors_tool_reset (iscissors);
      break;

    default:
      break;
    }
}

static void
gimp_iscissors_tool_button_press (GimpTool        *tool,
                                  GimpCoords      *coords,
                                  guint32          time,
                                  GdkModifierType  state,
                                  GimpDisplay     *gdisp)
{
  GimpIscissorsTool *iscissors;
  SelectionOptions  *sel_options;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  sel_options = (SelectionOptions *) tool->tool_info->tool_options;

  iscissors->x = coords->x;
  iscissors->y = coords->y;

  /*  If the tool was being used in another image...reset it  */

  if (gimp_tool_control_is_active(tool->control) && gdisp != tool->gdisp)
    {
      gimp_draw_tool_stop (GIMP_DRAW_TOOL (tool));
      gimp_iscissors_tool_reset (iscissors);
    }

  gimp_tool_control_activate(tool->control);
  tool->gdisp = gdisp;

  switch (iscissors->state)
    {
    case NO_ACTION:
      iscissors->state = SEED_PLACEMENT;
      iscissors->draw  = DRAW_CURRENT_SEED;

      if (! (state & GDK_SHIFT_MASK))
	find_max_gradient (iscissors,
                           gdisp->gimage,
			   &iscissors->x,
                           &iscissors->y);

      iscissors->x = CLAMP (iscissors->x, 0, gdisp->gimage->width - 1);
      iscissors->y = CLAMP (iscissors->y, 0, gdisp->gimage->height - 1);

      iscissors->ix = iscissors->x;
      iscissors->iy = iscissors->y;

      /*  Initialize the selection core only on starting the tool  */
      gimp_draw_tool_start (GIMP_DRAW_TOOL (tool), gdisp);
      break;

    default:
      /*  Check if the mouse click occured on a vertex or the curve itself  */
      if (clicked_on_vertex (tool))
	{
	  iscissors->nx    = iscissors->x;
	  iscissors->ny    = iscissors->y;
	  iscissors->state = SEED_ADJUSTMENT;

	  iscissors->draw = DRAW_CURVE | DRAW_ACTIVE_CURVE;
	  gimp_draw_tool_resume (GIMP_DRAW_TOOL (tool));
	}
      /*  If the iscissors is connected, check if the click was inside  */
      else if (iscissors->connected && iscissors->mask &&
	       gimp_channel_value (iscissors->mask,
                                   iscissors->x,
                                   iscissors->y))
	{
	  /*  Undraw the curve  */
	  gimp_tool_control_halt(tool->control);    /* sets paused_count to 0 -- is this ok? */

	  iscissors->draw = DRAW_CURVE;
	  gimp_draw_tool_stop (GIMP_DRAW_TOOL (tool));

          gimp_image_mask_select_channel (gdisp->gimage,
                                          iscissors->mask,
                                          0, 0,
                                          GIMP_SELECTION_TOOL (tool)->op,
                                          sel_options->feather,
                                          sel_options->feather_radius,
                                          sel_options->feather_radius);

	  gimp_iscissors_tool_reset (iscissors);

	  gdisplays_flush ();
	}
      else if (! iscissors->connected)
	{
          /*  if we're not connected, we're adding a new point  */

          /*  pause the tool, but undraw nothing  */
          iscissors->draw = DRAW_NOTHING;
	  gimp_draw_tool_pause (GIMP_DRAW_TOOL (tool));

	  iscissors->state = SEED_PLACEMENT;
	  iscissors->draw  = DRAW_CURRENT_SEED;

          if (sel_options->interactive)
            iscissors->draw |= DRAW_LIVEWIRE;

	  gimp_draw_tool_resume (GIMP_DRAW_TOOL (tool));
	}
      break;
    }
}


static void
iscissors_convert (GimpIscissorsTool *iscissors,
		   GimpDisplay       *gdisp)
{
  GimpScanConvert *sc;
  GimpVector2     *points;
  guint            n_points;
  GSList          *list;
  ICurve          *icurve;
  guint            packed;
  gint             i;
  gint             index;

  sc = gimp_scan_convert_new (gdisp->gimage->width, gdisp->gimage->height, 1);

  /* go over the curves in reverse order, adding the points we have */
  list = iscissors->curves;
  index = g_slist_length (list);
  while (index)
    {
      index--;
      icurve = (ICurve *) g_slist_nth_data (list, index);

      n_points = icurve->points->len;
      points   = g_new (GimpVector2, n_points);

      for (i = 0; i < n_points; i ++)
	{
	  packed = GPOINTER_TO_INT (g_ptr_array_index (icurve->points, i));
	  points[i].x = packed & 0x0000ffff;
	  points[i].y = packed >> 16;
	}

      gimp_scan_convert_add_points (sc, n_points, points);
      g_free (points);
    }

  if (iscissors->mask)
    g_object_unref (G_OBJECT (iscissors->mask));

  iscissors->mask = gimp_scan_convert_to_channel (sc, gdisp->gimage);
  gimp_scan_convert_free (sc);

  gimp_channel_invalidate_bounds (iscissors->mask);    
}

static void
gimp_iscissors_tool_button_release (GimpTool        *tool,
                                    GimpCoords      *coords,
                                    guint32          time,
                                    GdkModifierType  state,
                                    GimpDisplay     *gdisp)
{
  GimpIscissorsTool *iscissors;
  SelectionOptions  *sel_options;
  ICurve            *curve;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  sel_options = (SelectionOptions *) tool->tool_info->tool_options;

  /* Make sure X didn't skip the button release event -- as it's known
   * to do
   */
  if (iscissors->state == WAITING)
    return;

  /*  Undraw everything  */
  switch (iscissors->state)
    {
    case SEED_PLACEMENT:
      iscissors->draw = DRAW_CURVE | DRAW_CURRENT_SEED;
      if (sel_options->interactive)
	iscissors->draw |= DRAW_LIVEWIRE;
      break;
    case SEED_ADJUSTMENT:
      iscissors->draw = DRAW_CURVE | DRAW_ACTIVE_CURVE;
      break;
    default:
      break;
    }

  gimp_draw_tool_pause (GIMP_DRAW_TOOL (tool));

  /*  First take care of the case where the user "cancels" the action  */
  if (! (state & GDK_BUTTON3_MASK))
    {
      /*  Progress to the next stage of intelligent selection  */
      switch (iscissors->state)
	{
	case SEED_PLACEMENT:
	  /*  Add a new icurve  */
	  if (!iscissors->first_point)
	    {
	      /*  Determine if we're connecting to the first point  */
	      if (iscissors->curves)
		{
		  curve = (ICurve *) iscissors->curves->data;

		  if (abs (iscissors->x - curve->x1) < POINT_HALFWIDTH &&
		      abs (iscissors->y - curve->y1) < POINT_HALFWIDTH)
		    {
		      iscissors->x = curve->x1;
		      iscissors->y = curve->y1;
		      iscissors->connected = TRUE;
                    }
		}

	      /*  Create the new curve segment  */
	      if (iscissors->ix != iscissors->x ||
		  iscissors->iy != iscissors->y)
		{
		  curve = g_new (ICurve, 1);

		  curve->x1 = iscissors->ix;
		  curve->y1 = iscissors->iy;
		  iscissors->ix = curve->x2 = iscissors->x;
		  iscissors->iy = curve->y2 = iscissors->y;
		  curve->points = NULL;
		  iscissors->curves = g_slist_append (iscissors->curves, curve);
		  calculate_curve (tool, curve);
		}
	    }
	  else /* this was our first point */
	    {
	      iscissors->first_point = FALSE;
	    }
	  break;

	case SEED_ADJUSTMENT:
	  /*  recalculate both curves  */
	  if (iscissors->curve1)
	    {
	      iscissors->curve1->x1 = iscissors->nx;
	      iscissors->curve1->y1 = iscissors->ny;
	      calculate_curve (tool, iscissors->curve1);
	    }
	  if (iscissors->curve2)
	    {
	      iscissors->curve2->x2 = iscissors->nx;
	      iscissors->curve2->y2 = iscissors->ny;
	      calculate_curve (tool, iscissors->curve2);
	    }
	  break;

	default:
	  break;
	}
    }

  iscissors->state = WAITING;

  /*  Draw only the boundary  */
  iscissors->draw = DRAW_CURVE;
  gimp_draw_tool_resume (GIMP_DRAW_TOOL (tool));

  /*  convert the curves into a region  */
  if (iscissors->connected)
    iscissors_convert (iscissors, gdisp);
}

static void
gimp_iscissors_tool_motion (GimpTool        *tool,
                            GimpCoords      *coords,
                            guint32          time,
                            GdkModifierType  state,
                            GimpDisplay     *gdisp)
{
  GimpIscissorsTool *iscissors;
  SelectionOptions  *sel_options;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  sel_options = (SelectionOptions *) tool->tool_info->tool_options;

  if (!gimp_tool_control_is_active(tool->control) || iscissors->state == NO_ACTION)
    return;

  if (iscissors->state == SEED_PLACEMENT)
    {
      iscissors->draw = DRAW_CURRENT_SEED;

      if (sel_options->interactive)
	iscissors->draw |= DRAW_LIVEWIRE;
    }
  else if (iscissors->state == SEED_ADJUSTMENT)
    {
      iscissors->draw = DRAW_ACTIVE_CURVE;
    }

  gimp_draw_tool_pause (GIMP_DRAW_TOOL (tool));

  iscissors->x = coords->x;
  iscissors->y = coords->y;
  
  switch (iscissors->state)
    {
    case SEED_PLACEMENT:
      /*  Hold the shift key down to disable the auto-edge snap feature  */
      if (! (state & GDK_SHIFT_MASK))
	find_max_gradient (iscissors, gdisp->gimage,
			   &iscissors->x, &iscissors->y);

      iscissors->x = CLAMP (iscissors->x, 0, gdisp->gimage->width  - 1);
      iscissors->y = CLAMP (iscissors->y, 0, gdisp->gimage->height - 1);

      if (iscissors->first_point)
	{
	  iscissors->ix = iscissors->x;
	  iscissors->iy = iscissors->y;
	}
      break;

    case SEED_ADJUSTMENT:
      /*  Move the current seed to the location of the cursor  */
      if (! (state & GDK_SHIFT_MASK))
	find_max_gradient (iscissors, gdisp->gimage,
			   &iscissors->x, &iscissors->y);

      iscissors->x = CLAMP (iscissors->x, 0, gdisp->gimage->width  - 1);
      iscissors->y = CLAMP (iscissors->y, 0, gdisp->gimage->height - 1);

      iscissors->nx = iscissors->x;
      iscissors->ny = iscissors->y;
      break;

    default:
      break;
    }

  gimp_draw_tool_resume (GIMP_DRAW_TOOL (tool));
}

static void
gimp_iscissors_tool_draw (GimpDrawTool *draw_tool)
{
  GimpTool          *tool;
  GimpIscissorsTool *iscissors;
  GimpDisplay       *gdisp;
  ICurve            *curve;
  GSList            *list;

  tool      = GIMP_TOOL (draw_tool);
  iscissors = GIMP_ISCISSORS_TOOL (draw_tool);

  gdisp = tool->gdisp;

  /*  Draw the crosshairs target if we're placing a seed  */
  if (iscissors->draw & DRAW_CURRENT_SEED)
    {
      gimp_draw_tool_draw_handle (draw_tool,
                                  GIMP_HANDLE_CROSS,
                                  iscissors->x,
                                  iscissors->y,
                                  TARGET_WIDTH,
                                  TARGET_WIDTH,
                                  GTK_ANCHOR_CENTER,
                                  FALSE);

      /* Draw a line boundary */
      if (! iscissors->first_point && ! (iscissors->draw & DRAW_LIVEWIRE))
	{
          gimp_draw_tool_draw_line (draw_tool,
                                    iscissors->ix, iscissors->iy,
                                    iscissors->x, iscissors->y,
                                    FALSE);
	}
    }

  /* Draw the livewire boundary */
  if ((iscissors->draw & DRAW_LIVEWIRE) && ! iscissors->first_point)
    {
      /* See if the mouse has moved.  If so, create a new segment... */
      if (! iscissors->livewire ||
	  (iscissors->livewire &&
	   (iscissors->ix != iscissors->livewire->x1 ||
	    iscissors->x  != iscissors->livewire->x2  ||
	    iscissors->iy != iscissors->livewire->y1 ||
	    iscissors->y  != iscissors->livewire->y2)))
        {
          curve = g_new (ICurve, 1);

          curve->x1 = iscissors->ix;
          curve->y1 = iscissors->iy;
          curve->x2 = iscissors->x;
          curve->y2 = iscissors->y;
          curve->points = NULL;

          if (iscissors->livewire)
            {
              if (iscissors->livewire->points)
                g_ptr_array_free (iscissors->livewire->points, TRUE);
                
              g_free (iscissors->livewire);

              iscissors->livewire = NULL;
	    }

          iscissors->livewire = curve;
          calculate_curve (tool, curve);
          curve = NULL;
        }

      /*  plot the curve  */
      iscissors_draw_curve (draw_tool, iscissors->livewire);
    }

  if ((iscissors->draw & DRAW_CURVE) && ! iscissors->first_point)
    {
      /*  Draw a point at the init point coordinates  */
      if (! iscissors->connected)
        {
          gimp_draw_tool_draw_handle (draw_tool,
                                      GIMP_HANDLE_FILLED_CIRCLE,
                                      iscissors->ix,
                                      iscissors->iy,
                                      POINT_WIDTH,
                                      POINT_WIDTH,
                                      GTK_ANCHOR_CENTER,
                                      FALSE);
        }

      /*  Go through the list of icurves, and render each one...  */
      for (list = iscissors->curves; list; list = g_slist_next (list))
	{
	  curve = (ICurve *) list->data;

          if (iscissors->draw & DRAW_ACTIVE_CURVE)
            {
              /*  don't draw curve1 at all  */
              if (curve == iscissors->curve1)
                continue;
            }

          gimp_draw_tool_draw_handle (draw_tool,
                                      GIMP_HANDLE_FILLED_CIRCLE,
                                      curve->x1,
                                      curve->y1,
                                      POINT_WIDTH,
                                      POINT_WIDTH,
                                      GTK_ANCHOR_CENTER,
                                      FALSE);

          if (iscissors->draw & DRAW_ACTIVE_CURVE)
            {
              /*  draw only the start handle of curve2  */
              if (curve == iscissors->curve2)
                continue;
            }

	  /*  plot the curve  */
	  iscissors_draw_curve (draw_tool, curve);
	}
    }

  if (iscissors->draw & DRAW_ACTIVE_CURVE)
    {
      /*  plot both curves, and the control point between them  */
      if (iscissors->curve1)
	{
          gimp_draw_tool_draw_line (draw_tool,
                                    iscissors->curve1->x2, 
                                    iscissors->curve1->y2,
                                    iscissors->nx,
                                    iscissors->ny,
                                    FALSE);
	}
      if (iscissors->curve2)
	{
          gimp_draw_tool_draw_line (draw_tool,
                                    iscissors->curve2->x1, 
                                    iscissors->curve2->y1,
                                    iscissors->nx,
                                    iscissors->ny,
                                    FALSE);
	}

      gimp_draw_tool_draw_handle (draw_tool,
                                  GIMP_HANDLE_FILLED_CIRCLE,
                                  iscissors->nx,
                                  iscissors->ny,
                                  POINT_WIDTH,
                                  POINT_WIDTH,
                                  GTK_ANCHOR_CENTER,
                                  FALSE);
    }
}


static void
iscissors_draw_curve (GimpDrawTool *draw_tool,
		      ICurve       *curve)
{
  gpointer *point;
  guint     len;
  gint      npts = 0;
  guint32   coords;
  guint32   coords_2;

  /* Uh, this shouldn't happen, but it does.  So we ignore it.
   * Quality code, baby.
   */
  if (! curve->points)
    return;

  point = curve->points->pdata + 1;
  len   = curve->points->len - 1;

  while (len--)
    {
      coords   = GPOINTER_TO_INT (*point);
      coords_2 = GPOINTER_TO_INT (*(point - 1));
      point++;

      if (npts < MAX_POINTS)
	{
          gimp_draw_tool_draw_line (draw_tool,
                                    (coords & 0x0000ffff),
                                    (coords >> 16),
                                    (coords_2 & 0x0000ffff),
                                    (coords_2 >> 16),
                                    FALSE);
	  npts++;
	}
      else
	{
	  g_warning ("too many points in ICurve segment!");
	  return;
	}
    }
}

static void
gimp_iscissors_tool_oper_update (GimpTool        *tool,
                                 GimpCoords      *coords,
                                 GdkModifierType  state,
                                 GimpDisplay     *gdisp)
{
  GimpIscissorsTool *iscissors;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  GIMP_TOOL_CLASS (parent_class)->oper_update (tool, coords, state, gdisp);

  if (mouse_over_vertex (iscissors, coords->x, coords->y))
    {
      iscissors->op = ISCISSORS_OP_MOVE_POINT;
    }
  else if (mouse_over_curve (iscissors, coords->x, coords->y))
    {
      iscissors->op = ISCISSORS_OP_ADD_POINT;
    }
  else if (iscissors->connected && iscissors->mask)
    {
      if (gimp_channel_value (iscissors->mask, coords->x, coords->y))
        {
          iscissors->op = ISCISSORS_OP_SELECT;
        }
      else
        {
          iscissors->op = ISCISSORS_OP_IMPOSSIBLE;
        }
    }
  else
    {
      switch (iscissors->state)
	{
	case WAITING:
          iscissors->op = ISCISSORS_OP_ADD_POINT;
	  break;

	case SEED_PLACEMENT:
	case SEED_ADJUSTMENT:
	default:
          iscissors->op = ISCISSORS_OP_NONE;
          break;
	}
    }
}

static void
gimp_iscissors_tool_cursor_update (GimpTool        *tool,
                                   GimpCoords      *coords,
                                   GdkModifierType  state,
                                   GimpDisplay     *gdisp)
{
  GimpIscissorsTool  *iscissors;
  GdkCursorType       cursor    = GIMP_MOUSE_CURSOR;
  GimpCursorModifier  cmodifier = GIMP_CURSOR_MODIFIER_NONE;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  switch (iscissors->op)
    {
    case ISCISSORS_OP_SELECT:
      GIMP_TOOL_CLASS (parent_class)->cursor_update (tool, coords, state, gdisp);
      return;
    case ISCISSORS_OP_MOVE_POINT:
      cmodifier = GIMP_CURSOR_MODIFIER_MOVE;
      break;
    case ISCISSORS_OP_ADD_POINT:
      cmodifier = GIMP_CURSOR_MODIFIER_PLUS;
      break;
    case ISCISSORS_OP_IMPOSSIBLE:
      cursor = GIMP_BAD_CURSOR;
      break;
    default:
      break;
    }

  gimp_tool_set_cursor (tool, gdisp,
                        cursor,
                        GIMP_SCISSORS_TOOL_CURSOR /* tool->tool_cursor*/,
                        cmodifier);
}


static void
gimp_iscissors_tool_reset (GimpIscissorsTool *iscissors)
{
  /*  Free and reset the curve list  */
  if (iscissors->curves)
    {
      iscissors_free_icurves (iscissors->curves);
      g_slist_free (iscissors->curves);
      iscissors->curves = NULL;
    }

  /*  free mask  */
  if (iscissors->mask)
    {
      g_object_unref (G_OBJECT (iscissors->mask));
      iscissors->mask = NULL;
    }

  /* free the gradient map */
  if (iscissors->gradient_map)
    {
      /* release any tile we were using */
      if (cur_tile)
	{
	  tile_release (cur_tile, FALSE);
          cur_tile = NULL;
        }

      tile_manager_destroy (iscissors->gradient_map);
      iscissors->gradient_map = NULL;
    }

  iscissors->curve1      = NULL;
  iscissors->curve2      = NULL;
  iscissors->first_point = TRUE;
  iscissors->connected   = FALSE;
  iscissors->draw        = DRAW_NOTHING;
  iscissors->state       = NO_ACTION;

  /*  Reset the dp buffers  */
  iscissors_free_buffers (iscissors);

  /*  If they haven't already been initialized, precalculate the diagonal
   *  weight and direction value arrays
   */
  if (!initialized)
    {
      precalculate_arrays ();
      initialized = TRUE;
    }
}


static void
iscissors_free_icurves (GSList *list)
{
  ICurve * curve;

  while (list)
    {
      curve = (ICurve *) list->data;
      if (curve->points)
	g_ptr_array_free (curve->points, TRUE);

      g_free (curve);
      list = g_slist_next (list);
    }
}


static void
iscissors_free_buffers (GimpIscissorsTool *iscissors)
{
  if (iscissors->dp_buf)
    temp_buf_free (iscissors->dp_buf);

  iscissors->dp_buf = NULL;
}


/* XXX need some scan-conversion routines from somewhere.  maybe. ? */

static gint
mouse_over_vertex (GimpIscissorsTool *iscissors,
		   gdouble            x,
		   gdouble            y)
{
  GSList *list;
  ICurve *curve;
  gint    curves_found = 0;

  /*  traverse through the list, returning non-zero if the current cursor
   *  position is on an existing curve vertex.  Set the curve1 and curve2
   *  variables to the two curves containing the vertex in question
   */

  iscissors->curve1 = iscissors->curve2 = NULL;

  list = iscissors->curves;

  while (list && curves_found < 2)
    {
      curve = (ICurve *) list->data;

      if (gimp_draw_tool_on_handle (GIMP_DRAW_TOOL (iscissors),
                                    GIMP_TOOL (iscissors)->gdisp,
                                    x, y,
                                    GIMP_HANDLE_CIRCLE,
                                    curve->x1, curve->y1,
                                    POINT_WIDTH, POINT_WIDTH,
                                    GTK_ANCHOR_CENTER,
                                    FALSE))
	{
	  iscissors->curve1 = curve;

	  if (curves_found++)
	    return curves_found;
	}
      else if (gimp_draw_tool_on_handle (GIMP_DRAW_TOOL (iscissors),
                                         GIMP_TOOL (iscissors)->gdisp,
                                         x, y,
                                         GIMP_HANDLE_CIRCLE,
                                         curve->x2, curve->y2,
                                         POINT_WIDTH, POINT_WIDTH,
                                         GTK_ANCHOR_CENTER,
                                         FALSE))
	{
	  iscissors->curve2 = curve;

	  if (curves_found++)
	    return curves_found;
	}

      list = g_slist_next (list);
    }

  return curves_found;
}

static gboolean
clicked_on_vertex (GimpTool *tool)
{
  GimpIscissorsTool *iscissors;
  gint               curves_found = 0;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  curves_found = mouse_over_vertex (iscissors, iscissors->x, iscissors->y);

  if (curves_found > 1)
    {
      /*  undraw the curve  */
      iscissors->draw = DRAW_CURVE;
      gimp_draw_tool_pause (GIMP_DRAW_TOOL (tool));

      return TRUE;
    }

  /*  if only one curve was found, the curves are unconnected, and
   *  the user only wants to move either the first or last point
   *  disallow this for now.
   */
  if (curves_found == 1)
    return FALSE;

  return clicked_on_curve (tool);
}


static GSList *
mouse_over_curve (GimpIscissorsTool *iscissors,
		  gdouble            x,
		  gdouble            y)
{
  GSList   *list;
  gpointer *pt;
  gint      len;
  ICurve   *curve;
  guint32   coords;
  gint      tx, ty;

  /*  traverse through the list, returning the curve segment's list element
   *  if the current cursor position is on a curve... 
   */

  for (list = iscissors->curves; list; list = g_slist_next (list))
    {
      curve = (ICurve *) list->data;

      pt = curve->points->pdata;
      len = curve->points->len;
      while (len--)
	{
	  coords = GPOINTER_TO_INT (*pt);
	  pt++;
	  tx = coords & 0x0000ffff;
	  ty = coords >> 16;

	  /*  Is the specified point close enough to the curve?  */
	  if (gimp_draw_tool_in_radius (GIMP_DRAW_TOOL (iscissors),
                                        GIMP_TOOL (iscissors)->gdisp,
                                        tx, ty,
                                        x, y,
                                        POINT_HALFWIDTH))
	    {
	      return list;
	    }
	}
    }

  return NULL;
}

static gboolean
clicked_on_curve (GimpTool *tool)
{
  GimpIscissorsTool *iscissors;
  SelectionOptions  *sel_options;
  GSList            *list, *new_link;
  ICurve            *curve, *new_curve;

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  sel_options = (SelectionOptions *) tool->tool_info->tool_options;

  /*  traverse through the list, getting back the curve segment's list
   *  element if the current cursor position is on a curve...
   *  If this occurs, replace the curve with two new curves,
   *  separated by a new vertex.
   */
  list = mouse_over_curve (iscissors, iscissors->x, iscissors->y);

  if (list)
    {
      curve = (ICurve *) list->data;

      /*  undraw the curve  */
      iscissors->draw = DRAW_CURVE;
      gimp_draw_tool_pause (GIMP_DRAW_TOOL (tool));

      /*  Create the new curve  */
      new_curve = g_new (ICurve, 1);

      new_curve->x2 = curve->x2;
      new_curve->y2 = curve->y2;
      new_curve->x1 = curve->x2 = iscissors->x;
      new_curve->y1 = curve->y2 = iscissors->y;
      new_curve->points = NULL;

      /*  Create the new link and supply the new curve as data  */
      new_link = g_slist_alloc ();
      new_link->data = new_curve;

      /*  Insert the new link in the list  */
      new_link->next = list->next;
      list->next = new_link;

      iscissors->curve1 = new_curve;
      iscissors->curve2 = curve;

      return TRUE;
    }

  return FALSE;
}


static void
precalculate_arrays (void)
{
  gint i;

  for (i = 0; i < 256; i++)
    {
      /*  The diagonal weight array  */
      diagonal_weight[i] = (int) (i * G_SQRT2);

      /*  The direction value array  */
      direction_value[i][0] = (127 - abs (127 - i)) * 2;
      direction_value[i][1] = abs (127 - i) * 2;
      direction_value[i][2] = abs (191 - i) * 2;
      direction_value[i][3] = abs (63 - i) * 2;
    }

  /*  set the 256th index of the direction_values to the hightest cost  */
  direction_value[255][0] = 255;
  direction_value[255][1] = 255;
  direction_value[255][2] = 255;
  direction_value[255][3] = 255;
}


static void
calculate_curve (GimpTool *tool,
		 ICurve   *curve)
{
  GimpIscissorsTool *iscissors;
  GimpDisplay       *gdisp;
  gint               x, y, dir;
  gint               xs, ys, xe, ye;
  gint               x1, y1, x2, y2;
  gint               width, height;
  gint               ewidth, eheight;

  /*  Calculate the lowest cost path from one vertex to the next as specified
   *  by the parameter "curve".
   *    Here are the steps:
   *      1)  Calculate the appropriate working area for this operation
   *      2)  Allocate a temp buf for the dynamic programming array
   *      3)  Run the dynamic programming algorithm to find the optimal path
   *      4)  Translate the optimal path into pixels in the icurve data
   *            structure.
   */

  iscissors = GIMP_ISCISSORS_TOOL (tool);

  gdisp = tool->gdisp;

  /*  Get the bounding box  */
  xs = CLAMP (curve->x1, 0, gdisp->gimage->width - 1);
  ys = CLAMP (curve->y1, 0, gdisp->gimage->height - 1);
  xe = CLAMP (curve->x2, 0, gdisp->gimage->width - 1);
  ye = CLAMP (curve->y2, 0, gdisp->gimage->height - 1);
  x1 = MIN (xs, xe);
  y1 = MIN (ys, ye);
  x2 = MAX (xs, xe) + 1;  /*  +1 because if xe = 199 & xs = 0, x2 - x1, width = 200  */
  y2 = MAX (ys, ye) + 1;

  /*  expand the boundaries past the ending points by 
   *  some percentage of width and height.  This serves the following purpose:
   *  It gives the algorithm more area to search so better solutions
   *  are found.  This is particularly helpful in finding "bumps" which
   *  fall outside the bounding box represented by the start and end
   *  coordinates of the "curve".
   */
  ewidth  = (x2 - x1) * EXTEND_BY + FIXED;
  eheight = (y2 - y1) * EXTEND_BY + FIXED;

  if (xe >= xs)
    x2 += CLAMP (ewidth, 0, gdisp->gimage->width - x2);
  else
    x1 -= CLAMP (ewidth, 0, x1);
  if (ye >= ys)
    y2 += CLAMP (eheight, 0, gdisp->gimage->height - y2);
  else
    y1 -= CLAMP (eheight, 0, y1);

  /* blow away any previous points list we might have */
  if (curve->points)
    {
      g_ptr_array_free (curve->points, TRUE);
      curve->points = NULL;
    }

  /*  If the bounding box has width and height...  */
  if ((x2 - x1) && (y2 - y1))
    {
      width = (x2 - x1);
      height = (y2 - y1);

      /* Initialise the gradient map tile manager for this image if we
       * don't already have one. */
      if (!iscissors->gradient_map)
	  iscissors->gradient_map = gradient_map_new (gdisp->gimage);

      /*  allocate the dynamic programming array  */
      iscissors->dp_buf = 
	temp_buf_resize (iscissors->dp_buf, 4, x1, y1, width, height);

      /*  find the optimal path of pixels from (x1, y1) to (x2, y2)  */
      find_optimal_path (iscissors->gradient_map, iscissors->dp_buf,
			 x1, y1, x2, y2, xs, ys);
      
      /*  get a list of the pixels in the optimal path  */
      curve->points = plot_pixels (iscissors, iscissors->dp_buf,
				   x1, y1, xs, ys, xe, ye);
    }
  /*  If the bounding box has no width  */
  else if ((x2 - x1) == 0)
    {
      /*  plot a vertical line  */
      y = ys;  
      dir = (ys > ye) ? -1 : 1;
      curve->points = g_ptr_array_new ();
      while (y != ye)
	{
	  g_ptr_array_add (curve->points, GINT_TO_POINTER ((y << 16) + xs));
	  y += dir;
	}
    }
  /*  If the bounding box has no height  */
  else if ((y2 - y1) == 0)
    {
      /*  plot a horizontal line  */
      x = xs;
      dir = (xs > xe) ? -1 : 1;
      curve->points = g_ptr_array_new ();
      while (x != xe)
	{
	  g_ptr_array_add (curve->points, GINT_TO_POINTER ((ys << 16) + x));
	  x += dir;
	}
    }
}


/* badly need to get a replacement - this is _way_ too expensive */
static gboolean
gradient_map_value (TileManager *map,
		    gint         x,
		    gint         y,
		    guint8      *grad,
		    guint8      *dir)
{
  static gint  cur_tilex;
  static gint  cur_tiley;
  guint8      *p;

  if (!cur_tile ||
      x / TILE_WIDTH != cur_tilex ||
      y / TILE_HEIGHT != cur_tiley)
    {
      if (cur_tile)
	tile_release (cur_tile, FALSE);
      cur_tile = tile_manager_get_tile (map, x, y, TRUE, FALSE);
      if (!cur_tile)
	return FALSE;
      cur_tilex = x / TILE_WIDTH;
      cur_tiley = y / TILE_HEIGHT;	
    }

  p = tile_data_pointer (cur_tile, x % TILE_WIDTH, y % TILE_HEIGHT);
  *grad = p[0];
  *dir  = p[1];

  return TRUE;
}

static gint
calculate_link (TileManager *gradient_map,
		gint         x,
		gint         y,
		guint32      pixel,
		gint         link)
{
  gint   value = 0;
  guint8 grad1, dir1, grad2, dir2;

  if (!gradient_map_value (gradient_map, x, y, &grad1, &dir1))
    {
      grad1 = 0;
      dir1 = 255;
    }

  /* Convert the gradient into a cost: large gradients are good, and
   * so have low cost. */
  grad1 = 255 - grad1;

  /*  calculate the contribution of the gradient magnitude  */
  if (link > 1)
    value += diagonal_weight[grad1] * OMEGA_G;
  else
    value += grad1 * OMEGA_G;

  /*  calculate the contribution of the gradient direction  */
  x += (gint8)(pixel & 0xff);
  y += (gint8)((pixel & 0xff00) >> 8);
  if (!gradient_map_value (gradient_map, x, y, &grad2, &dir2))
    {
      grad2 = 0;
      dir2 = 255;
    }  
  value += (direction_value[dir1][link] + direction_value[dir2][link]) *
    OMEGA_D;

  return value;
}


static GPtrArray *
plot_pixels (GimpIscissorsTool *iscissors,
	     TempBuf           *dp_buf,
	     gint               x1,
	     gint               y1,
	     gint               xs,
	     gint               ys,
	     gint               xe,
	     gint               ye)
{
  gint       x, y;
  guint32    coords;
  gint       link;
  gint       width;
  guint     *data;
  GPtrArray *list;

  width = dp_buf->width;

  /*  Start the data pointer at the correct location  */
  data = (guint *) temp_buf_data (dp_buf) + (ye - y1) * width + (xe - x1);

  x = xe;
  y = ye;

  list = g_ptr_array_new ();

  while (1)
    {
      coords = (y << 16) + x;
      g_ptr_array_add (list, GINT_TO_POINTER (coords));

      link = PIXEL_DIR (*data);
      if (link == SEED_POINT)
	return list;

      x += move[link][0];
      y += move[link][1];
      data += move[link][1] * width + move[link][0];
    }

  /*  won't get here  */
  return NULL;
}


#define PACK(x, y) ((((y) & 0xff) << 8) | ((x) & 0xff))
#define OFFSET(pixel) ((gint8)((pixel) & 0xff) + \
  ((gint8)(((pixel) & 0xff00) >> 8)) * dp_buf->width)


static void
find_optimal_path (TileManager *gradient_map,
		   TempBuf     *dp_buf,
		   gint         x1,
		   gint         y1,
		   gint         x2,
		   gint         y2,
		   gint         xs,
		   gint         ys)
{
  gint     i, j, k;
  gint     x, y;
  gint     link;
  gint     linkdir;
  gint     dirx, diry;
  gint     min_cost;
  gint     new_cost;
  gint     offset;
  gint     cum_cost[8];
  gint     link_cost[8];
  gint     pixel_cost[8];
  guint32  pixel[8];
  guint32 *data;
  guint32 *d;

  /*  initialize the dynamic programming buffer  */
  data = (guint32 *) temp_buf_data_clear (dp_buf);

  /*  what directions are we filling the array in according to?  */
  dirx = (xs - x1 == 0) ? 1 : -1;
  diry = (ys - y1 == 0) ? 1 : -1;
  linkdir = (dirx * diry);

  y = ys;

  for (i = 0; i < dp_buf->height; i++)
    {
      x = xs;

      d = data + (y-y1) * dp_buf->width + (x-x1);

      for (j = 0; j < dp_buf->width; j++)
	{
	  min_cost = G_MAXINT;

	  /* pixel[] array encodes how to get to a neigbour, if possible.
	   * 0 means no connection (eg edge).
	   * Rest packed as bottom two bytes: y offset then x offset.
	   * Initially, we assume we can't get anywhere. */
	  for (k = 0; k < 8; k++)
	    pixel[k] = 0;

	  /*  Find the valid neighboring pixels  */
	  /*  the previous pixel  */
	  if (j)
	    pixel[((dirx == 1) ? 4 : 0)] = PACK (-dirx, 0);

	  /*  the previous row of pixels  */
	  if (i)
	    {
	      pixel[((diry == 1) ? 5 : 1)] = PACK (0, -diry);

	      link = (linkdir == 1) ? 3 : 2;
	      if (j)
		pixel[((diry == 1) ? (link + 4) : link)] = PACK(-dirx, -diry);

	      link = (linkdir == 1) ? 2 : 3;
	      if (j != dp_buf->width - 1)
		pixel[((diry == 1) ? (link + 4) : link)] = PACK (dirx, -diry);
	    }

	  /*  find the minimum cost of going through each neighbor to reach the
	   *  seed point...
	   */
	  link = -1;
	  for (k = 0; k < 8; k ++)
	    if (pixel[k])
	      {
		link_cost[k] = calculate_link (gradient_map,
                                               xs + j*dirx, ys + i*diry,
                                               pixel [k],
                                               ((k > 3) ? k - 4 : k));
		offset = OFFSET (pixel [k]);
		pixel_cost[k] = PIXEL_COST (d[offset]);
		cum_cost[k] = pixel_cost[k] + link_cost[k];
		if (cum_cost[k] < min_cost)
		  {
		    min_cost = cum_cost[k];
		    link = k;
		  }
	      }

	  /*  If anything can be done...  */
	  if (link >= 0)
	    {
	      /*  set the cumulative cost of this pixel and the new direction  */
	      *d = (cum_cost[link] << 8) + link;

	      /*  possibly change the links from the other pixels to this pixel...
	       *  these changes occur if a neighboring pixel will receive a lower
	       *  cumulative cost by going through this pixel.  
	       */
	      for (k = 0; k < 8; k ++)
		if (pixel[k] && k != link)
		  {
		    /*  if the cumulative cost at the neighbor is greater than
		     *  the cost through the link to the current pixel, change the
		     *  neighbor's link to point to the current pixel.
		     */
		    new_cost = link_cost[k] + cum_cost[link];
		    if (pixel_cost[k] > new_cost)
		    {
		      /*  reverse the link direction   /-----------------------\ */
		      offset = OFFSET (pixel[k]);
		      d[offset] = (new_cost << 8) + ((k > 3) ? k - 4 : k + 4);
		    }
		  }
	    } 
	  /*  Set the seed point  */
	  else if (!i && !j)
	    *d = SEED_POINT;

	  /*  increment the data pointer and the x counter  */
	  d += dirx;
	  x += dirx;
	}

      /*  increment the y counter  */
      y += diry;
    }
}


/* Called to fill in a newly referenced tile in the gradient map */
static void
gradmap_tile_validate (TileManager *tm,
		       Tile        *tile)
{
  static gboolean first_gradient = TRUE;

  gint         x, y;
  gint         dw, dh;
  gint         sw, sh;
  gint         i, j;
  gint         b;
  gfloat       gradient;
  guint8      *gradmap;
  guint8      *tiledata;
  guint8      *datah, *datav;
  gint8        hmax, vmax;
  Tile        *srctile;
  PixelRegion  srcPR, destPR;
  GimpImage   *gimage;

  gimage = (GimpImage *) tile_manager_get_user_data (tm);

  if (first_gradient)
    {
      gint radius = GRADIENT_SEARCH >> 1;

      /*  compute the distance weights  */
      for (i = 0; i < GRADIENT_SEARCH; i++)
	for (j = 0; j < GRADIENT_SEARCH; j++)
	  distance_weights[i * GRADIENT_SEARCH + j] =
	    1.0 / (1 + sqrt (SQR(i - radius) + SQR(j - radius)));

      first_gradient = FALSE;
    }

  tile_manager_get_tile_coordinates (tm, tile, &x, &y);
  dw = tile_ewidth (tile);
  dh = tile_eheight (tile);

  /* get corresponding tile in the gimage */
  srctile = tile_manager_get_tile (gimp_image_projection (gimage),
				   x, y, TRUE, FALSE);
  if (!srctile)
    return;
  
  sw = tile_ewidth (srctile);
  sh = tile_eheight (srctile);

  srcPR.w         = MIN (dw, sw);
  srcPR.h         = MIN (dh, sh);
  srcPR.bytes     = gimp_image_projection_bytes (gimage);
  srcPR.data      = tile_data_pointer (srctile, 0, 0);
  srcPR.rowstride = srcPR.w * srcPR.bytes;

  /* XXX tile edges? */

  /*  Blur the source to get rid of noise  */
  destPR.rowstride = TILE_WIDTH * 4;
  destPR.data      = maxgrad_conv0;
  convolve_region (&srcPR, &destPR, blur_32, 3, 32, GIMP_NORMAL_CONVOL);

  /*  Set the "src" temp buf up as the new source Pixel Region  */
  srcPR.rowstride = destPR.rowstride;
  srcPR.data      = destPR.data;

  /*  Get the horizontal derivative  */
  destPR.data = maxgrad_conv1;
  convolve_region (&srcPR, &destPR, horz_deriv, 3, 1, GIMP_NEGATIVE_CONVOL);

  /*  Get the vertical derivative  */
  destPR.data = maxgrad_conv2;
  convolve_region (&srcPR, &destPR, vert_deriv, 3, 1, GIMP_NEGATIVE_CONVOL);

  /* calculate overall gradient */
  tiledata = tile_data_pointer (tile, 0, 0);
  for (i = 0; i < srcPR.h; i++)
    {
      datah = maxgrad_conv1 + srcPR.rowstride*i;
      datav = maxgrad_conv2 + srcPR.rowstride*i;
      gradmap = tiledata + tile_ewidth (tile) * COST_WIDTH * i;

      for (j = 0; j < srcPR.w; j++)
	{
	  hmax = datah[0] - 128;
	  vmax = datav[0] - 128;
	  for (b = 1; b < srcPR.bytes; b++)
	    {
	      if (abs (datah[b] - 128) > abs (hmax)) hmax = datah[b] - 128;
	      if (abs (datav[b] - 128) > abs (vmax)) vmax = datav[b] - 128;
	    }

	  if (i == 0 || j == 0 || i == srcPR.h-1 || j == srcPR.w-1)
	  {
	      gradmap[j*COST_WIDTH] = 0;
	      gradmap[j*COST_WIDTH + 1] = 255;
	      goto contin;
	  }

	  /* 1 byte absolute magitude first */
	  gradient = sqrt(SQR(hmax) + SQR(vmax));
	  gradmap[j*COST_WIDTH] = gradient * 255 / MAX_GRADIENT;

	  /* then 1 byte direction */
	  if (gradient > MIN_GRADIENT)
	    {
	      gfloat direction;

	      if (!hmax)
		direction = (vmax > 0) ? G_PI_2 : -G_PI_2;
	      else
		direction = atan ((double) vmax / (double) hmax);

	      /* Scale the direction from between 0 and 254,
	       *  corresponding to -PI/2, PI/2 255 is reserved for
	       *  directionless pixels */
	      gradmap[j*COST_WIDTH + 1] =
		(guint8) (254 * (direction + G_PI_2) / G_PI);
	    }
	  else
	    gradmap[j*COST_WIDTH + 1] = 255; /* reserved for weak gradient */

contin:
	  datah += srcPR.bytes;
	  datav += srcPR.bytes;
	}
    }

  tile_release (srctile, FALSE);
}

static TileManager *
gradient_map_new (GimpImage *gimage)
{
  TileManager *tm;

  tm = tile_manager_new (gimage->width, gimage->height,
			 sizeof (guint8) * COST_WIDTH);
  tile_manager_set_user_data (tm, gimage);
  tile_manager_set_validate_proc (tm, gradmap_tile_validate);

  return tm;
}

static void
find_max_gradient (GimpIscissorsTool *iscissors,
		   GimpImage         *gimage,
		   gint              *x,
		   gint              *y)
{
  PixelRegion  srcPR;
  gint         radius;
  gint         i, j;
  gint         endx, endy;
  gint         sx, sy, cx, cy;
  gint         x1, y1, x2, y2;
  void        *pr;
  guint8      *gradient;
  gfloat       g, max_gradient;

  /* Initialise the gradient map tile manager for this image if we
   * don't already have one. */
  if (!iscissors->gradient_map)
    iscissors->gradient_map = gradient_map_new (gimage);

  radius = GRADIENT_SEARCH >> 1;

  /*  calculate the extent of the search  */
  cx = CLAMP (*x, 0, gimage->width);
  cy = CLAMP (*y, 0, gimage->height);
  sx = cx - radius;
  sy = cy - radius;
  x1 = CLAMP (cx - radius, 0, gimage->width);
  y1 = CLAMP (cy - radius, 0, gimage->height);
  x2 = CLAMP (cx + radius, 0, gimage->width);
  y2 = CLAMP (cy + radius, 0, gimage->height);
  /*  calculate the factor to multiply the distance from the cursor by  */

  max_gradient = 0;
  *x = cx;
  *y = cy;

  /*  Find the point of max gradient  */
  pixel_region_init (&srcPR, iscissors->gradient_map,
		     x1, y1, x2 - x1, y2 - y1, FALSE);

  /* this iterates over 1, 2 or 4 tiles only */
  for (pr = pixel_regions_register (1, &srcPR);
       pr != NULL;
       pr = pixel_regions_process (pr))
    {
      endx = srcPR.x + srcPR.w;
      endy = srcPR.y + srcPR.h;
      for (i = srcPR.y; i < endy; i++)
	{
	  gradient = srcPR.data + srcPR.rowstride * (i - srcPR.y);
	  for (j = srcPR.x; j < endx; j++)
	    {
	      g = *gradient;
	      gradient += COST_WIDTH;
	      g *= distance_weights [(i-y1) * GRADIENT_SEARCH + (j-x1)];
	      if (g > max_gradient)
		{
		  max_gradient = g;
		  *x = j;
		  *y = i;
		}
	    }
	}
    }
}