Commit 3e523499 authored by Robert Sasu's avatar Robert Sasu Committed by Mukund Sivaraman

color-to-alpha: Add the color-to-alpha op from soc-2011-ops branch

parent 505b20ad
/* This file is an image processing operation for GEGL
* GEGL is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 3 of the License, or (at your option) any later version.
* GEGL is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* Lesser General Public License for more details.
* You should have received a copy of the GNU Lesser General Public
* License along with GEGL; if not, see <>.
* Color To Alpha plug-in v1.0 by Seth Burgess, 1999/05/14
* with algorithm by clahey
* Copyright (C) 1995 Spencer Kimball and Peter Mattis
* Copyright (C) 2011 Robert Sasu <>
#include "config.h"
#include <glib/gi18n-lib.h>
gegl_chant_color (value, _("Color"), "black",
_("The color to render (defaults to 'black')"))
#define GEGL_CHANT_C_FILE "color-to-alpha.c"
#include "gegl-chant.h"
#include <stdio.h>
#include <math.h>
#define PRV_WIDTH 40
#define PRV_HEIGHT 20
static void prepare (GeglOperation *operation)
gegl_operation_set_format (operation, "input",
babl_format ("RGBA float"));
gegl_operation_set_format (operation, "output",
babl_format ("RGBA float"));
* An excerpt from a discussion on #gimp that sheds some light on the ideas
* behind the algorithm that is being used here.
<clahey> so if a1 > c1, a2 > c2, and a3 > c2 and a1 - c1 > a2-c2, a3-c3,
then a1 = b1 * alpha + c1 * (1-alpha)
So, maximizing alpha without taking b1 above 1 gives
a1 = alpha + c1(1-alpha) and therefore alpha = (a1-c1) / (1-c1).
<sjburges> clahey: btw, the ordering of that a2, a3 in the white->alpha didn't
<clahey> sjburges: You mean that it could be either a1, a2, a3 or
a1, a3, a2?
<sjburges> yeah
<sjburges> because neither one uses the other
<clahey> sjburges: That's exactly as it should be. They are both just
getting reduced to the same amount, limited by the the darkest
<clahey> Then a2 = b2 * alpha + c2 * (1- alpha). Solving for b2 gives
b2 = (a1-c2)/alpha + c2.
<sjburges> yeah
<clahey> That gives us are formula for if the background is darker than the
foreground? Yep.
<clahey> Next if a1 < c1, a2 < c2, a3 < c3, and c1-a1 > c2-a2, c3-a3, and
by our desired result a1 = b1 * alpha + c1 * (1-alpha),
we maximize alpha without taking b1 negative gives
alpha = 1 - a1 / c1.
<clahey> And then again, b2 = (a2-c2) / alpha + c2 by the same formula.
(Actually, I think we can use that formula for all cases, though
it may possibly introduce rounding error.
<clahey> sjburges: I like the idea of using floats to avoid rounding error.
Good call.
static void
color_to_alpha (gfloat *color,
gfloat *src,
gint offset)
gint i;
gfloat temp[4];
temp[3] = src[offset + 3];
src[offset+3] = temp[3];
for (i=0; i<3; i++)
if (color[i] < 0.0001)
temp[i] = src[offset+i];
else if (src[offset+i] > color[i])
temp[i] = (src[offset+i] - color[i]) / (1.0 - color[i]);
else if (src[offset+i] < color[i])
temp[i] = (color[i] - src[offset+i]) / color[i];
temp[i] = 0.0;
if (temp[0] > temp[1])
if (temp[0] > temp[2])
src[offset+3] = temp[0];
src[offset+3] = temp[2];
else if (temp[1] > temp[2])
src[offset+3] = temp[1];
src[offset+3] = temp[2];
if (src[offset+3] < 0.0001)
for (i=0; i<3; i++)
src[offset+i] = (src[offset+i] - color[i]) / src[offset+3] + color[i];
src[offset+3] *=temp[3];
static gboolean
process (GeglOperation *operation,
GeglBuffer *input,
GeglBuffer *output,
const GeglRectangle *result)
GeglChantO *o = GEGL_CHANT_PROPERTIES (operation);
Babl *format = babl_format ("RGBA float");
gfloat *src_buf, color[4];
gint x;
src_buf = g_new0 (gfloat, result->width * result->height * 4);
gegl_buffer_get (input, 1.0, result, format, src_buf, GEGL_AUTO_ROWSTRIDE);
gegl_color_get_rgba4f (o->value, color);
for (x = 0; x < result->width * result->height; x++)
color_to_alpha (color, src_buf, 4 * x);
gegl_buffer_set (output, result, format, src_buf, GEGL_AUTO_ROWSTRIDE);
g_free (src_buf);
return TRUE;
static void
gegl_chant_class_init (GeglChantClass *klass)
GeglOperationClass *operation_class;
GeglOperationFilterClass *filter_class;
operation_class = GEGL_OPERATION_CLASS (klass);
filter_class = GEGL_OPERATION_FILTER_CLASS (klass);
filter_class->process = process;
operation_class->prepare = prepare;
operation_class->categories = "color";
operation_class->name = "gegl:color-to-alpha";
operation_class->description = _("Performs color-to-alpha on the image.");
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