[gegl] workshop: add saturation hue constant

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 5da528f6549222ab1155946467238576a5ca14d5
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri Jun 28 15:29:10 2019 +0200

    workshop: add saturation hue constant
    
    This implements saturation towards luminance gray directly in RGB without using
    CIE Lab - thus avoiding hue shifts as reported in issue #171, since the
    documentation of gegl:saturation doesn't document using CIE Lab under the hood,
    after evaluation we might consider swapping this operation in for
    gegl:saturation
    
    It can be worhwhile investigating a different grayscale to linearly interpolate
    with.
    
    This new operation pushed under a different name to workshop for testing and
    further evalutation.

 operations/workshop/Makefile.am               |   1 +
 operations/workshop/saturation-hue-constant.c | 104 ++++++++++++++++++++++++++
 po/POTFILES.in                                |   1 +
 3 files changed, 106 insertions(+)
---
diff --git a/operations/workshop/Makefile.am b/operations/workshop/Makefile.am
index da4c8e447..f9513a12c 100644
--- a/operations/workshop/Makefile.am
+++ b/operations/workshop/Makefile.am
@@ -26,6 +26,7 @@ ops = \
        integral-image.la \
        rawbayer-load.la  \
        segment-kmeans.la \
+       saturation-hue-constant.la \
        selective-hue-saturation.la
 
 if HAVE_CXX14
diff --git a/operations/workshop/saturation-hue-constant.c b/operations/workshop/saturation-hue-constant.c
new file mode 100644
index 000000000..1a9900737
--- /dev/null
+++ b/operations/workshop/saturation-hue-constant.c
@@ -0,0 +1,104 @@
+/* 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 <https://www.gnu.org/licenses/>.
+ *
+ * Copyright 2019 Øyvind Kolås
+ */
+
+#include "config.h"
+#include <glib/gi18n-lib.h>
+
+#ifdef GEGL_PROPERTIES
+
+property_double (scale, _("Scale"), 1.0)
+    description(_("Scale, strength of effect"))
+    value_range (0.0, 10.0)
+    ui_range (0.0, 2.0)
+
+#else
+
+#define GEGL_OP_POINT_FILTER
+#define GEGL_OP_NAME     saturation_hue_constant
+#define GEGL_OP_C_SOURCE saturation-hue-constant.c
+
+#include "gegl-op.h"
+
+static void prepare (GeglOperation *operation)
+{
+  const Babl *space = gegl_operation_get_source_space (operation, "input");
+  const Babl *format;
+
+  format = babl_format_with_space ("RGBA float", space);
+
+  gegl_operation_set_format (operation, "input", format);
+  gegl_operation_set_format (operation, "output", format);
+}
+
+static gboolean
+process (GeglOperation       *operation,
+         void                *in_buf,
+         void                *out_buf,
+         glong                n_pixels,
+         const GeglRectangle *roi,
+         gint                 level)
+{
+  GeglProperties *o = GEGL_PROPERTIES (operation);
+  const Babl *space = gegl_operation_get_source_space (operation, "input");
+  gfloat *in = in_buf;
+  gfloat *out = out_buf;
+  glong i;
+  float scale = o->scale;
+  float rscale = 1.0f - o->scale;
+  double red_luminance, green_luminance, blue_luminance;
+
+  babl_space_get_rgb_luminance (space,
+    &red_luminance, &green_luminance, &blue_luminance);
+
+  for (i = 0; i < n_pixels; i++)
+    {
+      gfloat desaturated = (in[0] * red_luminance +
+                            in[1] * green_luminance +
+                            in[2] * green_luminance) * rscale;
+      for (int c = 0; c < 3; c ++)
+        out[c] = desaturated + in[c] * scale;
+      out[3] = in[3];
+
+      in += 4;
+      out += 4;
+    }
+
+  return TRUE;
+}
+
+static void
+gegl_op_class_init (GeglOpClass *klass)
+{
+  GeglOperationClass *operation_class = GEGL_OPERATION_CLASS (klass);
+  GeglOperationPointFilterClass *point_filter_class =
+    GEGL_OPERATION_POINT_FILTER_CLASS (klass);
+
+  operation_class->prepare = prepare;
+  operation_class->opencl_support = FALSE;
+
+  point_filter_class->process = process;
+
+  gegl_operation_class_set_keys (operation_class,
+    "name"       , "gegl:saturation-hue-constant",
+    "title",       _("Saturation with constant hue"),
+    "categories" , "color",
+    "description", _("Changes the saturation"),
+    NULL);
+}
+
+#endif
diff --git a/po/POTFILES.in b/po/POTFILES.in
index 51e9a622a..d9c71cda2 100644
--- a/po/POTFILES.in
+++ b/po/POTFILES.in
@@ -278,4 +278,5 @@ operations/workshop/integral-image.c
 operations/workshop/rawbayer-load.c
 operations/workshop/segment-kmeans.c
 operations/workshop/selective-hue-saturation.c
+operations/workshop/saturation-hue-constant.c
 operations/workshop/voronoi-diagram.cc