[gimp] app: add gimp_layer_mode_is_alpha_only()

[N/A <ell src gnome org>]

commit 2c585981720981b85bc020a0486808350b6fae0f
Author: Ell <ell_se yahoo com>
Date:   Mon May 7 16:35:42 2018 -0400

    app: add gimp_layer_mode_is_alpha_only()
    
    ... which determines if a layer mode's blend function only affects
    the alpha, maintaining the backdrop's color.  This is currently
    true only for ERASE, SPLIT, and ANTI_ERASE modes.

 app/operations/layer-modes/gimp-layer-modes.c |   22 ++++++++++++++++++----
 app/operations/layer-modes/gimp-layer-modes.h |    1 +
 app/operations/operations-enums.h             |    3 ++-
 3 files changed, 21 insertions(+), 5 deletions(-)
---
diff --git a/app/operations/layer-modes/gimp-layer-modes.c b/app/operations/layer-modes/gimp-layer-modes.c
index f53da44..f4d55ea 100644
--- a/app/operations/layer-modes/gimp-layer-modes.c
+++ b/app/operations/layer-modes/gimp-layer-modes.c
@@ -774,7 +774,8 @@ static const GimpLayerModeInfo layer_mode_infos[] =
 
     .op_name              = "gimp:erase",
     .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE |
-                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE,
+                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE           |
+                            GIMP_LAYER_MODE_FLAG_ALPHA_ONLY,
     .context              = GIMP_LAYER_MODE_CONTEXT_ALL,
     .paint_composite_mode = GIMP_LAYER_COMPOSITE_CLIP_TO_BACKDROP,
     .composite_mode       = GIMP_LAYER_COMPOSITE_CLIP_TO_BACKDROP,
@@ -796,7 +797,8 @@ static const GimpLayerModeInfo layer_mode_infos[] =
     .op_name              = "gimp:split",
     .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE     |
                             GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE |
-                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE,
+                            GIMP_LAYER_MODE_FLAG_SUBTRACTIVE               |
+                            GIMP_LAYER_MODE_FLAG_ALPHA_ONLY,
     .context              = GIMP_LAYER_MODE_CONTEXT_ALL,
     .paint_composite_mode = GIMP_LAYER_COMPOSITE_CLIP_TO_BACKDROP,
     .composite_mode       = GIMP_LAYER_COMPOSITE_CLIP_TO_BACKDROP
@@ -825,8 +827,9 @@ static const GimpLayerModeInfo layer_mode_infos[] =
   { GIMP_LAYER_MODE_ANTI_ERASE,
 
     .op_name              = "gimp:anti-erase",
-    .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE |
-                            GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE,
+    .flags                = GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE     |
+                            GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE |
+                            GIMP_LAYER_MODE_FLAG_ALPHA_ONLY,
     .context              = GIMP_LAYER_MODE_CONTEXT_FADE,
     .paint_composite_mode = GIMP_LAYER_COMPOSITE_UNION,
     .composite_mode       = GIMP_LAYER_COMPOSITE_UNION
@@ -1175,6 +1178,17 @@ gimp_layer_mode_is_subtractive (GimpLayerMode mode)
   return (info->flags & GIMP_LAYER_MODE_FLAG_SUBTRACTIVE) != 0;
 }
 
+gboolean
+gimp_layer_mode_is_alpha_only (GimpLayerMode mode)
+{
+  const GimpLayerModeInfo *info = gimp_layer_mode_info (mode);
+
+  if (! info)
+    return FALSE;
+
+  return (info->flags & GIMP_LAYER_MODE_FLAG_ALPHA_ONLY) != 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 ef80605..b9a67f7 100644
--- a/app/operations/layer-modes/gimp-layer-modes.h
+++ b/app/operations/layer-modes/gimp-layer-modes.h
@@ -32,6 +32,7 @@ gboolean                   gimp_layer_mode_is_composite_space_mutable (GimpLayer
 gboolean                   gimp_layer_mode_is_composite_mode_mutable  (GimpLayerMode           mode);
 
 gboolean                   gimp_layer_mode_is_subtractive             (GimpLayerMode           mode);
+gboolean                   gimp_layer_mode_is_alpha_only              (GimpLayerMode           mode);
 
 GimpLayerColorSpace        gimp_layer_mode_get_blend_space            (GimpLayerMode           mode);
 GimpLayerColorSpace        gimp_layer_mode_get_composite_space        (GimpLayerMode           mode);
diff --git a/app/operations/operations-enums.h b/app/operations/operations-enums.h
index 2b13702..1b44f8a 100644
--- a/app/operations/operations-enums.h
+++ b/app/operations/operations-enums.h
@@ -181,7 +181,8 @@ 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
+  GIMP_LAYER_MODE_FLAG_SUBTRACTIVE               =  1 << 4,
+  GIMP_LAYER_MODE_FLAG_ALPHA_ONLY                =  1 << 5
 } GimpLayerModeFlags;