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

[Mukund Sivaraman <muks src gnome org>]

commit 3e5234992ec30ef7df5b6764525442fcd989649e
Author: Robert Sasu <sasu robert gmail com>
Date:   Tue Aug 30 20:12:33 2011 +0530

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

 operations/workshop/color-to-alpha.c |  174 ++++++++++++++++++++++++++++++++++
 1 files changed, 174 insertions(+), 0 deletions(-)
---
diff --git a/operations/workshop/color-to-alpha.c b/operations/workshop/color-to-alpha.c
new file mode 100644
index 0000000..f167f55
--- /dev/null
+++ b/operations/workshop/color-to-alpha.c
@@ -0,0 +1,174 @@
+/* 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
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * 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 <http://www.gnu.org/licenses/>.
+ *
+ * Color To Alpha plug-in v1.0 by Seth Burgess, sjburges gimp org 1999/05/14
+ *  with algorithm by clahey
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ * Copyright (C) 2011 Robert Sasu <sasu robert gmail com>
+ */
+
+#include "config.h"
+#include <glib/gi18n-lib.h>
+
+#ifdef GEGL_CHANT_PROPERTIES
+
+gegl_chant_color (value, _("Color"), "black",
+                  _("The color to render (defaults to 'black')"))
+
+#else
+
+#define GEGL_CHANT_TYPE_FILTER
+#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
+             matter
+  <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
+             color.
+  <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];
+       else 
+          temp[i] = 0.0;
+
+   if (temp[0] > temp[1])
+     {
+       if (temp[0] > temp[2])
+          src[offset+3] = temp[0];
+       else
+          src[offset+3] = temp[2];
+     }
+   else if (temp[1] > temp[2])
+          src[offset+3] = temp[1];
+        else
+          src[offset+3] = temp[2];
+
+   if (src[offset+3] < 0.0001)
+      return;
+
+   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.");
+}
+
+#endif