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* Color To Alpha plug-in v1.0 by Seth Burgess, sjburges@gimp.org 1999/05/14
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* with algorithm by clahey
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/* GIMP - The GNU Image Manipulation Program
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* Copyright (C) 1995 Spencer Kimball and Peter Mattis
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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#include <libgimp/gimp.h>
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#include <libgimp/gimpui.h>
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#include "libgimp/stdplugins-intl.h"
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#define PLUG_IN_PROC "plug-in-colortoalpha"
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#define PLUG_IN_BINARY "colortoaplha"
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/* Declare local functions.
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static void query (void);
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static void run (const gchar *name,
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const GimpParam *param,
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GimpParam **return_vals);
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static inline void color_to_alpha (GimpRGB *src,
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const GimpRGB *color);
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static void to_alpha_func (const guchar *src,
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static gboolean color_to_alpha_dialog (GimpDrawable *drawable);
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static void color_to_alpha_preview (GimpPreview *preview,
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GimpDrawable *drawable);
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const GimpPlugInInfo PLUG_IN_INFO =
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query, /* query_proc */
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static C2AValues pvals =
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{ 1.0, 1.0, 1.0, 1.0 } /* white default */
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static const GimpParamDef args[] =
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{ GIMP_PDB_INT32, "run-mode", "Interactive, non-interactive" },
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{ GIMP_PDB_IMAGE, "image", "Input image (unused)" },
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{ GIMP_PDB_DRAWABLE, "drawable", "Input drawable" },
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{ GIMP_PDB_COLOR, "color", "Color to remove" }
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gimp_install_procedure (PLUG_IN_PROC,
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N_("Convert a specified color to transparency"),
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"This replaces as much of a given color as possible "
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"in each pixel with a corresponding amount of alpha, "
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"then readjusts the color accordingly.",
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"Seth Burgess <sjburges@gimp.org>",
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N_("Color to _Alpha..."),
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G_N_ELEMENTS (args), 0,
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gimp_plugin_menu_register (PLUG_IN_PROC,
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"<Image>/Colors/Modify");
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gimp_plugin_menu_register (PLUG_IN_PROC,
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"<Image>/Layer/Transparency/Modify");
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run (const gchar *name,
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const GimpParam *param,
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GimpParam **return_vals)
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static GimpParam values[1];
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GimpDrawable *drawable;
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GimpPDBStatusType status = GIMP_PDB_SUCCESS;
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GimpRunMode run_mode;
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run_mode = param[0].data.d_int32;
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*return_vals = values;
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values[0].type = GIMP_PDB_STATUS;
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values[0].data.d_status = status;
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image_ID = param[1].data.d_image;
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drawable = gimp_drawable_get (param[2].data.d_drawable);
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case GIMP_RUN_INTERACTIVE:
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gimp_get_data (PLUG_IN_PROC, &pvals);
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if (! color_to_alpha_dialog (drawable))
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gimp_drawable_detach (drawable);
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case GIMP_RUN_NONINTERACTIVE:
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status = GIMP_PDB_CALLING_ERROR;
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if (status == GIMP_PDB_SUCCESS)
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pvals.color = param[3].data.d_color;
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case GIMP_RUN_WITH_LAST_VALS:
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gimp_get_data (PLUG_IN_PROC, &pvals);
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if (status == GIMP_PDB_SUCCESS &&
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gimp_drawable_is_rgb (drawable->drawable_id) &&
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gimp_drawable_is_layer (drawable->drawable_id))
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gimp_image_undo_group_start (image_ID);
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/* Add alpha if not present */
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gimp_layer_add_alpha (drawable->drawable_id);
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/* Reget the drawable, bpp might have changed */
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drawable = gimp_drawable_get (drawable->drawable_id);
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/* Unset 'Lock alpha' */
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lock_alpha = gimp_layer_get_lock_alpha (drawable->drawable_id);
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gimp_layer_set_lock_alpha (drawable->drawable_id, FALSE);
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gimp_progress_init (_("Removing color"));
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gimp_rgn_iterate2 (drawable, 0 /* unused */, to_alpha_func, NULL);
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gimp_layer_set_lock_alpha (drawable->drawable_id, lock_alpha);
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gimp_image_undo_group_end (image_ID);
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if (run_mode != GIMP_RUN_NONINTERACTIVE)
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gimp_displays_flush ();
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gimp_drawable_detach (drawable);
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if (run_mode == GIMP_RUN_INTERACTIVE)
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gimp_set_data (PLUG_IN_PROC, &pvals, sizeof (pvals));
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values[0].data.d_status = status;
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color_to_alpha (GimpRGB *src,
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const GimpRGB *color)
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if (color->r < 0.0001)
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else if (src->r > color->r)
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alpha.r = (src->r - color->r) / (1.0 - color->r);
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else if (src->r < color->r)
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alpha.r = (color->r - src->r) / color->r;
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if (color->g < 0.0001)
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else if (src->g > color->g)
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alpha.g = (src->g - color->g) / (1.0 - color->g);
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else if (src->g < color->g)
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alpha.g = (color->g - src->g) / (color->g);
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if (color->b < 0.0001)
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else if (src->b > color->b)
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alpha.b = (src->b - color->b) / (1.0 - color->b);
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else if (src->b < color->b)
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alpha.b = (color->b - src->b) / (color->b);
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if (alpha.r > alpha.g)
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if (alpha.r > alpha.b)
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else if (alpha.g > alpha.b)
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src->r = (src->r - color->r) / src->a + color->r;
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src->g = (src->g - color->g) / src->a + color->g;
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src->b = (src->b - color->b) / src->a + color->b;
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* An excerpt from a discussion on #gimp that sheds some light on the ideas
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* behind the algorithm that is being used here.
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<clahey> so if a1 > c1, a2 > c2, and a3 > c2 and a1 - c1 > a2-c2, a3-c3,
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then a1 = b1 * alpha + c1 * (1-alpha)
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So, maximizing alpha without taking b1 above 1 gives
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a1 = alpha + c1(1-alpha) and therefore alpha = (a1-c1) / (1-c1).
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<sjburges> clahey: btw, the ordering of that a2, a3 in the white->alpha didn't
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<clahey> sjburges: You mean that it could be either a1, a2, a3 or
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<sjburges> because neither one uses the other
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<clahey> sjburges: That's exactly as it should be. They are both just
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getting reduced to the same amount, limited by the the darkest
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<clahey> Then a2 = b2 * alpha + c2 * (1- alpha). Solving for b2 gives
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b2 = (a1-c2)/alpha + c2.
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<clahey> That gives us are formula for if the background is darker than the
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<clahey> Next if a1 < c1, a2 < c2, a3 < c3, and c1-a1 > c2-a2, c3-a3, and
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by our desired result a1 = b1 * alpha + c1 * (1-alpha),
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we maximize alpha without taking b1 negative gives
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<clahey> And then again, b2 = (a2-c2) / alpha + c2 by the same formula.
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(Actually, I think we can use that formula for all cases, though
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it may possibly introduce rounding error.
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<clahey> sjburges: I like the idea of using floats to avoid rounding error.
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to_alpha_func (const guchar *src,
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gimp_rgba_set_uchar (&color, src[0], src[1], src[2], 255);
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gimp_rgba_set_uchar (&color, src[0], src[1], src[2], src[3]);
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color_to_alpha (&color, &pvals.color);
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gimp_rgba_get_uchar (&color, &dest[0], &dest[1], &dest[2], &dest[3]);
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color_to_alpha_preview (GimpPreview *preview,
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GimpDrawable *drawable)
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GimpPixelRgn src_rgn;
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gimp_preview_get_position (preview, &x, &y);
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gimp_preview_get_size (preview, &width, &height);
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dest = g_new (guchar, width * height * 4);
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src = g_new (guchar, width * height * bpp);
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gimp_pixel_rgn_init (&src_rgn, drawable,
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gimp_pixel_rgn_get_rect (&src_rgn, src, x, y, width, height);
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for (i = 0; i < width * height; i++)
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to_alpha_func (src + i * bpp, dest + i * 4, bpp, NULL);
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/* Our code assumes that the drawable has an alpha channel (and adds
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* one later if the effect is actually performed). For that reason
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* we have to take care when drawing the preview.
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gimp_preview_draw_buffer (preview, dest, width * 4);
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/* This is not correct because we ignore the selection, but it
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* is the best we can easily do.
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gimp_preview_area_draw (GIMP_PREVIEW_AREA (preview->area),
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GIMP_RGBA_IMAGE, dest, width * 4);
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color_to_alpha_dialog (GimpDrawable *drawable)
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GtkWidget *main_vbox;
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gimp_ui_init (PLUG_IN_BINARY, TRUE);
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dialog = gimp_dialog_new (_("Color to Alpha"), PLUG_IN_BINARY,
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gimp_standard_help_func, PLUG_IN_PROC,
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GTK_STOCK_CANCEL, GTK_RESPONSE_CANCEL,
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GTK_STOCK_OK, GTK_RESPONSE_OK,
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gtk_dialog_set_alternative_button_order (GTK_DIALOG (dialog),
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gimp_window_set_transient (GTK_WINDOW (dialog));
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main_vbox = gtk_vbox_new (FALSE, 12);
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gtk_container_set_border_width (GTK_CONTAINER (main_vbox), 12);
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gtk_container_add (GTK_CONTAINER (GTK_DIALOG (dialog)->vbox), main_vbox);
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gtk_widget_show (main_vbox);
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preview = gimp_drawable_preview_new (drawable, NULL);
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gtk_box_pack_start_defaults (GTK_BOX (main_vbox), preview);
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gtk_widget_show (preview);
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g_signal_connect (preview, "invalidated",
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G_CALLBACK (color_to_alpha_preview),
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hbox = gtk_hbox_new (FALSE, 6);
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gtk_box_pack_start (GTK_BOX (main_vbox), hbox, FALSE, FALSE, 0);
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gtk_widget_show (hbox);
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label = gtk_label_new (_("From:"));
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gtk_box_pack_start (GTK_BOX (hbox), label, FALSE, FALSE, 0);
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gtk_widget_show (label);
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button = gimp_color_button_new (_("Color to Alpha Color Picker"),
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PRV_WIDTH, PRV_HEIGHT,
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GIMP_COLOR_AREA_FLAT);
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gimp_color_button_set_update (GIMP_COLOR_BUTTON (button), TRUE);
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gtk_box_pack_start (GTK_BOX (hbox), button, TRUE, TRUE, 0);
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gtk_widget_show (button);
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g_signal_connect (button, "color-changed",
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G_CALLBACK (gimp_color_button_get_color),
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g_signal_connect_swapped (button, "color-changed",
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G_CALLBACK (gimp_preview_invalidate),
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label = gtk_label_new (_("to alpha"));
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gtk_box_pack_start (GTK_BOX (hbox), label, FALSE, FALSE, 0);
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gtk_widget_show (label);
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gtk_widget_show (dialog);
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run = (gimp_dialog_run (GIMP_DIALOG (dialog)) == GTK_RESPONSE_OK);
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gtk_widget_destroy (dialog);
added
removed
plug-ins/common/colortoalpha.c
