blend mask feathering by a guided filter

Feathering adjusts the blend mask by taking into account the image content
such that edges of the mask match those in the image.

data/kernels/blendop.cl

@@ -1007,6 +1007,37 @@ blendop_rgb (__read_only image2d_t in_a, __read_only image2d_t in_b, __read_only
   write_imagef(out, (int2)(x, y), o);
 }
 
+__kernel void
+blendop_mask_enhance_contrast(__read_only image2d_t mask_in, __write_only image2d_t mask_out,
+                              const int width, const int height,
+                              const float e, const float brightness)
+{
+  const int x = get_global_id(0);
+  const int y = get_global_id(1);
+
+  if ( x>= width || y >= height) return;
+
+  float opacity = read_imagef(mask_in, sampleri, (int2)(x, y)).x;
+  float scaled_opacity = 2.f * opacity - 1.f;
+  if (1.f - brightness <= 0.f)
+    scaled_opacity = opacity <= FLT_EPSILON ? -1.f : 1.f;
+  else if (1.f + brightness <= 0.f)
+    scaled_opacity = opacity >= 1.f - FLT_EPSILON ? 1.f : -1.f;
+  else if (brightness > 0.f)
+  {
+    scaled_opacity = (scaled_opacity + brightness) / (1.f - brightness);
+    scaled_opacity = fmin(scaled_opacity, 1.f);
+  }
+  else
+  {
+    scaled_opacity = (scaled_opacity + brightness) / (1.f + brightness);
+    scaled_opacity = fmax(scaled_opacity, -1.f);
+  }
+  opacity = (scaled_opacity * e / (1.f + (e - 1.f) * fabs(scaled_opacity))) / 2.f + 0.5f;
+  write_imagef(mask_out, (int2)(x, y), opacity);
+}
+
+
 __kernel void
 blendop_set_mask (__write_only image2d_t mask, const int width, const int height, const float value)
 {

src/CMakeLists.txt

@@ -29,6 +29,7 @@ FILE(GLOB SOURCE_FILES
   "common/fswatch.c"
   "common/gaussian.c"
   "common/grouping.c"
+  "common/guided_filter.c"
   "common/history.c"
   "common/gpx.c"
   "common/image.c"