[gegl] value-propagate: issue #129

[Thomas Manni <tmanni src gnome org>]

commit 8ecb55461a2d17adfb526537e38ffb83457c463f
Author: woob <thetoastcaper gmail com>
Date:   Mon Feb 18 11:44:30 2019 +0100

    value-propagate: issue #129
    
    Reverse offsets to propagate in the correct directions.

 operations/common-gpl3+/value-propagate.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/operations/common-gpl3+/value-propagate.c b/operations/common-gpl3+/value-propagate.c
index 8e4f574f4..1181acf8b 100644
--- a/operations/common-gpl3+/value-propagate.c
+++ b/operations/common-gpl3+/value-propagate.c
@@ -471,10 +471,14 @@ prepare (GeglOperation *operation)
 
   params = (VPParamsType *) o->user_data;
 
-  params->offset_left   = o->left   ? -1 : 0;
-  params->offset_top    = o->top    ? -1 : 0;
-  params->offset_right  = o->right  ?  1 : 0;
-  params->offset_bottom = o->bottom ?  1 : 0;
+  /* The op's directional paramaters are used in terms of which direction to
+   * propagate _to_, but offset is used for what neighbors a pixel may
+   * propagate _from_. Therefore, directions are reversed between the two.
+   */
+  params->offset_left   = o->right  ? -1 : 0;
+  params->offset_top    = o->bottom ? -1 : 0;
+  params->offset_right  = o->left   ?  1 : 0;
+  params->offset_bottom = o->top    ?  1 : 0;
 
   area->left = area->right = area->top = area->bottom = 1;