[gegl] transform: round numerical errors in transformation matrix

[Ell <ell src gnome org>]

commit 5b2c7dd489ab341ba52cdffa597d49aec3d77f5a
Author: Ell <ell_se yahoo com>
Date:   Thu Jul 9 09:56:56 2020 +0300

    transform: round numerical errors in transformation matrix
    
    In the transform ops, round numerical errors in the generated
    matrix to the nearest integer.  This is especially important when
    using the NEAREST sampler, which is sensitive to boundary
    conditions.
    
    In particular, a perfect 90-deg rotation matrix cannot be generated
    naively using the trigonometric functions (since there is no double
    x such that cos(x) = 1 exactly), which can cause a single row or
    column to be missing from the output in some cases.

 operations/transform/transform-core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/operations/transform/transform-core.c b/operations/transform/transform-core.c
index 97ad47caa..c5d8ac63b 100644
--- a/operations/transform/transform-core.c
+++ b/operations/transform/transform-core.c
@@ -306,7 +306,11 @@ gegl_transform_create_matrix (OpTransform *transform,
   gegl_matrix3_identity (matrix);
 
   if (OP_TRANSFORM_GET_CLASS (transform))
-    OP_TRANSFORM_GET_CLASS (transform)->create_matrix (transform, matrix);
+    {
+      OP_TRANSFORM_GET_CLASS (transform)->create_matrix (transform, matrix);
+
+      gegl_matrix3_round_error (matrix);
+    }
 }
 
 static void