[gegl] Bug 578932 – Alpha weighted blend composting operation

[Martin Nordholts <martinn src gnome org>]

commit 9ef5947b2b2769727d67886657578bf27c7343b1
Author: James Legg <lankyleggy gmail com>
Date:   Fri Apr 17 19:41:10 2009 +0200

    Bug 578932 â?? Alpha weighted blend composting operation
    
    Add new gegl:weighted-blend operation that blends two RGBA buffers
    together, using their alpha values as weights.
---
 operations/common/weighted_blend.c |  126 ++++++++++++++++++++++++++++++++++++
 1 files changed, 126 insertions(+), 0 deletions(-)

diff --git a/operations/common/weighted_blend.c b/operations/common/weighted_blend.c
new file mode 100644
index 0000000..4822518
--- /dev/null
+++ b/operations/common/weighted_blend.c
@@ -0,0 +1,126 @@
+/* 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/>.
+ *
+ * Copyright 2006 �yvind Kolås <pippin gimp org>
+ *           2008 James Legg
+ */
+#include "config.h"
+#include <glib/gi18n-lib.h>
+
+
+#ifdef GEGL_CHANT_PROPERTIES
+
+gegl_chant_double (value, _("Value"), -G_MAXDOUBLE, G_MAXDOUBLE, 0.0, _("global value used if aux doesn't contain data"))
+
+#else
+
+#define GEGL_CHANT_TYPE_POINT_COMPOSER
+#define GEGL_CHANT_C_FILE       "weighted_blend.c"
+
+#include "gegl-chant.h"
+
+static void prepare (GeglOperation *operation)
+{
+  Babl *format = babl_format ("RGBA float");
+
+  gegl_operation_set_format (operation, "input", format);
+  gegl_operation_set_format (operation, "aux", format);
+  gegl_operation_set_format (operation, "output", format);
+}
+
+static gboolean
+process (GeglOperation       *op,
+         void                *in_buf,
+         void                *aux_buf,
+         void                *out_buf,
+         glong                n_pixels,
+         const GeglRectangle *roi)
+{
+  gfloat *in  = in_buf;
+  gfloat *out = out_buf;
+  gfloat *aux = aux_buf;
+  gint    i;
+
+  if (aux == NULL)
+    {
+      /* there is no auxilary buffer.
+       * output the input buffer.
+       * gfloat value = GEGL_CHANT_PROPERTIES (op)->value;
+       */
+      for (i = 0; i < n_pixels; i++)
+        {
+          gint j;
+          for (j = 0; j < 4; j++)
+            {
+              out[j] = in[j];
+            }
+        }
+    }
+  else
+    {
+      for (i=0; i<n_pixels; i++)
+        {
+          gint   j;
+          gfloat c;
+          gfloat value;
+          gfloat total_alpha;
+          /* find the proportion between alpha values */
+          total_alpha = in[3] + aux[3];
+          if (!total_alpha)
+            {
+              /* no coverage from any source pixel */
+              for (j = 0; j < 4; j++)
+                {
+                  out[j] = 0.0;
+                }
+            }
+          else
+            {
+              /* the total alpha is non-zero, therefore we may find a colour from a weighted blend */
+              gfloat in_weight = in[3] / total_alpha;
+              gfloat aux_weight = 1.0 - in_weight;
+              for (j = 0; j < 3; j++)
+                {
+                  out[j] = in_weight * in[j] + aux_weight * aux[j];
+                }
+              out[3] = total_alpha;
+            }
+          in  += 4;
+          aux += 4;
+          out += 4;
+        }
+    }
+
+  return TRUE;
+}
+
+static void
+gegl_chant_class_init (GeglChantClass *klass)
+{
+  GeglOperationClass              *operation_class;
+  GeglOperationPointComposerClass *point_composer_class;
+
+  operation_class      = GEGL_OPERATION_CLASS (klass);
+  point_composer_class = GEGL_OPERATION_POINT_COMPOSER_CLASS (klass);
+
+  point_composer_class->process = process;
+  operation_class->prepare      = prepare;
+
+  operation_class->name        = "gegl:weighted-blend";
+  operation_class->categories  = "compositors:blend";
+  operation_class->description =
+    _("blend two images using alpha values as weights");
+}
+#endif