[gimp] app: add GIMP_LAYER_MODE_FLAG_SUBTRACTIVE

[N/A <ell src gnome org>]

commit 40c0ea4fe14dda3201f7d7c6a102bbf87b464199
Author: Ell <ell_se yahoo com>
Date:   Wed Mar 8 06:36:39 2017 -0500

    app: add GIMP_LAYER_MODE_FLAG_SUBTRACTIVE
    
    Most modes only modify the *color* of overlapping dest/src regions,
    however, erase and color-erase may also reduce their *alpha*, i.e.,
    eliminate some of the overlapping content.  Flag these modes with
    the new SUBTRACTIVE flag, as they require more general compositing
    code.  The next commit adds the said code.

 app/operations/layer-modes/gimp-layer-modes.c |   17 +++++++++++++++--
 app/operations/layer-modes/gimp-layer-modes.h |    2 ++
 app/operations/operations-enums.h             |    1 +
 3 files changed, 18 insertions(+), 2 deletions(-)
---
diff --git a/app/operations/layer-modes/gimp-layer-modes.c b/app/operations/layer-modes/gimp-layer-modes.c
index 30161e0..485554e 100644
--- a/app/operations/layer-modes/gimp-layer-modes.c
+++ b/app/operations/layer-modes/gimp-layer-modes.c
@@ -405,7 +405,8 @@ static const GimpLayerModeInfo layer_mode_infos[] =
     .function             = gimp_operation_color_erase_process,
     .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE     |
                             GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE |
-                            GIMP_LAYER_MODE_FLAG_COMPOSITE_MODE_IMMUTABLE,
+                            GIMP_LAYER_MODE_FLAG_COMPOSITE_MODE_IMMUTABLE  |
+                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE,
     .context              = GIMP_LAYER_MODE_CONTEXT_PAINT |
                             GIMP_LAYER_MODE_CONTEXT_FADE,
     .paint_composite_mode = GIMP_LAYER_COMPOSITE_SRC_ATOP,
@@ -804,7 +805,8 @@ static const GimpLayerModeInfo layer_mode_infos[] =
 
     .op_name              = "gimp:erase",
     .function             = gimp_operation_erase_process,
-    .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE,
+    .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE |
+                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE,
     .context              = GIMP_LAYER_MODE_CONTEXT_FADE,
     .paint_composite_mode = GIMP_LAYER_COMPOSITE_SRC_ATOP,
     .composite_mode       = GIMP_LAYER_COMPOSITE_SRC_ATOP,
@@ -1149,6 +1151,17 @@ gimp_layer_mode_is_composite_mode_mutable (GimpLayerMode mode)
   return (info->flags & GIMP_LAYER_MODE_FLAG_COMPOSITE_MODE_IMMUTABLE) == 0;
 }
 
+gboolean
+gimp_layer_mode_is_subtractive (GimpLayerMode mode)
+{
+  const GimpLayerModeInfo *info = gimp_layer_mode_info (mode);
+
+  if (! info)
+    return FALSE;
+
+  return (info->flags & GIMP_LAYER_MODE_FLAG_SUBTRACTIVE) != 0;
+}
+
 GimpLayerColorSpace
 gimp_layer_mode_get_blend_space (GimpLayerMode mode)
 {
diff --git a/app/operations/layer-modes/gimp-layer-modes.h b/app/operations/layer-modes/gimp-layer-modes.h
index c5a79f0..9b49fb6 100644
--- a/app/operations/layer-modes/gimp-layer-modes.h
+++ b/app/operations/layer-modes/gimp-layer-modes.h
@@ -31,6 +31,8 @@ gboolean          gimp_layer_mode_is_blend_space_mutable     (GimpLayerMode
 gboolean          gimp_layer_mode_is_composite_space_mutable (GimpLayerMode        mode);
 gboolean          gimp_layer_mode_is_composite_mode_mutable  (GimpLayerMode        mode);
 
+gboolean                 gimp_layer_mode_is_subtractive      (GimpLayerMode        mode);
+
 GimpLayerColorSpace      gimp_layer_mode_get_blend_space     (GimpLayerMode        mode);
 GimpLayerColorSpace      gimp_layer_mode_get_composite_space (GimpLayerMode        mode);
 GimpLayerCompositeMode   gimp_layer_mode_get_composite_mode  (GimpLayerMode        mode);
diff --git a/app/operations/operations-enums.h b/app/operations/operations-enums.h
index b2810cb..c5bf617 100644
--- a/app/operations/operations-enums.h
+++ b/app/operations/operations-enums.h
@@ -173,6 +173,7 @@ typedef enum  /*< pdb-skip, skip >*/
   GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE     =  1 << 1,
   GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE =  1 << 2,
   GIMP_LAYER_MODE_FLAG_COMPOSITE_MODE_IMMUTABLE  =  1 << 3,
+  GIMP_LAYER_MODE_FLAG_SUBTRACTIVE               =  1 << 4
 } GimpLayerModeFlags;