[gegl] transform: allow wiggle room in required/invalidated region calculation

[N/A <ell src gnome org>]

commit 5ca329c1a8afe1ce4ba96a6a7a941fe7b6632825
Author: Ell <ell_se yahoo com>
Date:   Tue Dec 12 08:41:30 2017 -0500

    transform: allow wiggle room in required/invalidated region calculation
    
    When computing required/invalidated regions, calculate the bounding
    box based on pixel corners, rather than pixel centers, to allow for
    some error in the sampled input coordinates.  Otherwise, in some
    cases, we sample pixels outside the required input region during
    process() (and, conversely, we probably also miss affected pixels
    when determining the invalidated region.)
    
    Update the reference output of the "clones" test composition, which
    is affected by this change.

 operations/transform/transform-core.c   |   25 +++++++++++++++++--------
 tests/compositions/reference/clones.png |  Bin 723988 -> 723998 bytes
 2 files changed, 17 insertions(+), 8 deletions(-)
---
diff --git a/operations/transform/transform-core.c b/operations/transform/transform-core.c
index 690f9af..e31ddd9 100644
--- a/operations/transform/transform-core.c
+++ b/operations/transform/transform-core.c
@@ -594,14 +594,14 @@ gegl_transform_get_required_for_output (GeglOperation       *op,
   /*
    * Convert indices to absolute positions:
    */
-  need_points [0] = requested_rect.x + (gdouble) 0.5;
-  need_points [1] = requested_rect.y + (gdouble) 0.5;
+  need_points [0] = requested_rect.x;
+  need_points [1] = requested_rect.y;
 
-  need_points [2] = need_points [0] + (requested_rect.width  - (gint) 1);
+  need_points [2] = need_points [0] + requested_rect.width;
   need_points [3] = need_points [1];
 
   need_points [4] = need_points [2];
-  need_points [5] = need_points [3] + (requested_rect.height - (gint) 1);
+  need_points [5] = need_points [3] + requested_rect.height;
 
   need_points [6] = need_points [0];
   need_points [7] = need_points [5];
@@ -666,6 +666,15 @@ gegl_transform_get_invalidated_by_change (GeglOperation       *op,
   /*
    * TODO: Should the result be given extra elbow room all around to
    * allow for round off error (for "safety")?
+   *
+   * ^-- Looks like the answer is "yes": if we cut things too close,
+   * there can indeed be "missing pixels" at the edge of the input
+   * buffer (due to similar logic in get_required_for_output()).
+   * This might suggest that our sampling coordinates are not accurate
+   * enough, but for now, allowing some wiggle room, by computing the
+   * bounding box based on pixel corners, rather that pixel centers
+   * (in contrast to the last sentence of the previous comment) seems
+   * to be enough.
    */
 
   sampler = gegl_buffer_sampler_new_at_level (NULL,
@@ -707,14 +716,14 @@ gegl_transform_get_invalidated_by_change (GeglOperation       *op,
   /*
    * Convert indices to absolute positions:
    */
-  affected_points [0] = region.x + (gdouble) 0.5;
-  affected_points [1] = region.y + (gdouble) 0.5;
+  affected_points [0] = region.x;
+  affected_points [1] = region.y;
 
-  affected_points [2] = affected_points [0] + ( region.width  - (gint) 1);
+  affected_points [2] = affected_points [0] + region.width;
   affected_points [3] = affected_points [1];
 
   affected_points [4] = affected_points [2];
-  affected_points [5] = affected_points [3] + ( region.height - (gint) 1);
+  affected_points [5] = affected_points [3] + region.height;
 
   affected_points [6] = affected_points [0];
   affected_points [7] = affected_points [5];
diff --git a/tests/compositions/reference/clones.png b/tests/compositions/reference/clones.png
index e3edb7e..43914e6 100644
Binary files a/tests/compositions/reference/clones.png and b/tests/compositions/reference/clones.png differ